inter ca laci on

8/2/2019 Inter CA Laci On

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5.4. Infrared studies of the lattice modes 1655.5. Inelastic neutron scattering studies 169

6. Tec hn ological app lications of inter calated gr ap hite 1716.1. Introduction 1716.2. Battery and electrode materials 1716.3. Chemical catalytic applications 1726.4. Conductivity applications 172

6.5. Carbon bres 1736.6. Other applications 175

Acknowledgements 176

References 176

1. Introduction

Research on the preparation and properties of graphite intercalation compounds

has recently undergone a resurgence of interest and our fundamental knowledge of

the physics of these remarkable materials has increased substantially. It seems

appropriate to review this subject now but, with research activity and interest in

the eld remaining high, this review may well require updating in the near future.This is a broad review (an outgrowth of lecture notes developed for graduate

students in our research group at MIT) of a wide range of topics from the basic

chemistry and physics of intercalated graphite to engineering applications. It has not

been possible therefore to include reference to all of the important papers that have

been written in this eld (and we apologize to authors whose work would otherwise

have merited inclusion).

Graphite intercalation compounds are formed by the insertion of atomic or

molecular layers of a dierent chemical species called the intercalant between layers

in a graphite host material, as shown in gure 1. The intercalation compounds occur

in highly anisotropic layered structures where the intraplanar binding forces are

large in comparison with the interplanar binding forces. The most common examplesof host materials for intercalation compounds are graphite and the transition metal

dichalcogenides. Of the various types of intercalation compounds, the graphite

compounds are of particular physical interest because of their relatively high degree

of structural ordering. The most important and characteristic ordering property of

graphite intercalation compounds is the staging phenomenon, which is characterized

by intercalate layers that are periodically arranged in a matrix of graphite layers.

Graphite intercalation compounds are thus classied by a stage index n denoting the

number of graphite layers between adjacent intercalate layers, as is illustrated in

gure 2. This staging phenomenon is a general phenomenon in graphite intercalation

compounds, even in those samples with very dilute intercalate concentrations

(n 10).

Intercalation provides to the host material a means for controlled variation of

many physical properties over wide ranges. Because the free carrier concentration of

the graphite host is very low (104 free carriers/atom at room temperature),

intercalation with dierent chemical species and concentrations permits wide vari-

ation of the free carrier concentration and thus of the electrical, thermal and

magnetic properties of the host material. Of these properties, the eect of

intercalation on the electrical conductivity has probably attracted the greatest

amount of attention because of the fabrication of an intercalation compound

(CXAsF5) with a reported room temperature conductivity exceeding that of copper

M. S. Dresselhaus and G. Dresselhaus2


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(Foley et al. 1977). Perhaps even more striking is the range of electrical conductivity

behaviour, ranging from almost insulating behaviour for the c-axis conductivity in

certain acceptor compounds to superconducting in-plane behaviour below 1.0 K forthe rst stage alkali metal donor compounds C8K where neither of the parent

chemical species individually exhibit superconductivity (Hanney et al. 1965, Koike et

al. 1978). The large increase in conductivity in intercalated graphite results from a

charge transfer from the intercalate layer where the carriers have a low mobility to

the graphite layers where the mobility is high. Since the most signicant modica-

tions to the graphite involve graphite layers adjacent to the intercalate layer, it is

convenient to distinguish between the graphite bounding layers adjacent to the

intercalant, and the graphite interior layers that have only graphite nearest-neighbour

layers.

The synthesis of a graphite intercalation compound was rst reported by

Schaa utl (1841). However, the rst systematic studies of these compounds began

in the early 1930s with the introduction of X-ray diraction techniques for stage

index determinations (Homan and Frenzel 1931, Schleede and Wellman 1932).

Though the systematic study of their physical properties began in the late 1940s, it is

only in recent years that research on graphite intercalation compounds has become a

eld of intense activity internationally.

A large number (100) of reagents can be intercalated into graphite. These

intercalants are commonly classied according to whether they form donor or

acceptor compounds. The most common and most widely studied of the donor

compounds are the alkali metal compounds with K, Rb, Cs and Li, though other

Intercalation compounds of graphite 3

Figure 1. Model for C8K according to Ru dor and Schulze (1954) showing the stacking ofgraphite layers (networks of small solid balls) and of potassium layers (networks oflarge hollow balls). The graphite and intercalate layers are arranged in anAA-AA stacking sequence, where A refers to the graphite layers and the Greekletters to the intercalate layers.


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donor intercalants are known, such as alkaline earth metals, lanthanides and metal

alloys of these with each other or with alkali metals. Ternary donor intercalationcompounds have also been prepared using alkali metals with hydrogen or polar

molecules, such as ammonia and tetrahydrofuran, and aromatic molecules, such as

benzene. A very large variety of acceptor compounds have also been prepared, and

are often based on Lewis acid intercalants such as the halogen Br2 or halogen

mixtures, metal chlorides, bromides, uorides and oxyhalides, acidic oxides such as

N2O5 and SO3 and strong Bro nsted acids such as H2SO4 and HNO3. In the

intercalation process, the molecular intercalants generally remain molecular in form.

From gures 1 and 2 it is seen that intercalation causes crystal dilatation along the c-

axis: the larger the molecular intercalants, the larger the dilatation for compounds of

comparable stage. In general, both chemical anities and geometric constraints

associated with intercalant size and intercalant bonding distances determine whether

or not a given chemical species will intercalate.

Many of these compounds are unstable in air, with donor compounds being

easily oxidized and acceptors being easily desorbed. For this reason, most intercala-

tion compounds require encapsulation to ensure chemical stability, though some

compounds, such as graphiteFeCl3 and graphiteSbCl5, are relatively stable in air.

In addition to the large number of chemical species that can be intercalated, a

number of dierent types of graphite host materials are used for each of the various

applications. From a structural point of view, the simplest host material is a single

crystal graphite ake, such as those separated from the limestone rocks found in the


Figure 2. Schematic diagram illustrating the staging phenomenon in graphitepotassium

compounds for stages 1 n 4. The potassium layers are indicated by dashed linesand the graphite layers by solid lines connecting open circles, and indicatingschematically a projection of the carbon atom positions. The . . . ABAB . . . graphitelayer stacking for stages n 2 is maintained between intercalate layers, although arhombohedral stacking arrangement appears across intercalate layers. The stackingordering is well-conrmed by X-ray diraction (00l) patterns. For each stage, thedistance Ic between adjacent intercalate layers is indicated. For rst stage C8K,the unit cell includes intercalate layers with stacking indices , -, , (see gures 1and 20).


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Ticonderoga mines of New York State. Because ake dimensions are 1mm in

diameter and only several hundredths of a millimetre in thickness, these materials

often cannot be conveniently used for carrying out physical properties meas-

urements. In such cases, samples of large physical dimensions based on highly

oriented pyrolytic graphite (HOPG) are used (Moore 1973). HOPG is a synthetic

graphite formed by cracking a hydrocarbon at high temperature and subsequent

heat treatment, often combined with the application of pressure. The resultingmaterial is highly oriented along the c-axis (orientational deviations less than 18) but

in the layer planes consists of a randomly ordered collection of crystallites of1 mm

average diameter. For many physical measurements the greater exibility in sample

size provided by the HOPG host material is of greater importance than the more

perfect ordering of the single crystal akes. In fact, HOPG has been the most

common host material for graphite intercalation compounds during the recent

period of active research.

Another type of graphite host material is kish graphite, obtained by the

crystrallization of carbon from molten steel during the steel manufacturing process.

Kish graphite samples typically contain several large single crystallites, exhibiting

much higher structural ordering than HOPG, but not quite as ordered or aschemically pure as natural single crystal akes. On the other hand, kish graphite

samples are normally an order of magnitude greater in area and in thickness when

compared with single crystal akes. Though little use has so far been made of

this host material, intercalation compounds based on kish graphite can also be

prepared.

Carbon bres represent yet another class of synthetic graphite with great

mechanical strength, because the bre axis is along the graphite a-axis where the

iteratomic bonding is extremely strong. The intercalation of carbon bres is under

consideration as a method for variation of the electrical, mechanical and adhesive

properties of this commercially important class of bre materials.

In recent years several excellent review articles have appeared on the synthesisand structure of graphite intercalation compounds (Ebert 1986, He rold 1979), as well

as on the electronic properties (Fischer 1979), the lattice properties (Dresselhaus and

Dresselhaus 1979), and on applications areas (Whittingham and Ebert 1979). In

addition, a large collection of research and review articles on these subjects have

appeared in the Proceedings of the Franco-American Conference of La Napoule

(1977, Mater. Sci. Engng, 31, 1).

This review article on graphite intercalation compounds covers the eld broadly

in an eort to interrelate many of the factors which aect the structural, electronic

and lattice properties of these materials. Section 2 is devoted to the preparation and

characterization of materials, as well as related topics such as intercalation kinetics

and staging. Sections 3, 4 and 5 deal respectively with the structural electronic and

lattice properties of graphite intercalation compounds, while some applications areas

for these materials are briey discussed in section 6.

2. Materials preparation and characterizatio n

2.1. Intercalation methods

2.1.1. Introduction

A number of general methods have been developed for the preparation of

graphite intercalation compounds (Ebert 1976, He rold 1977, 1979) including the



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two-zone vapour transport technique, the liquid intercalation method, the electro-

chemical method and cointercalation techniques. For the various techniques

employed, the parameters of signicance are temperature, vapour pressure, the

chemical and physical properties of the intercalant and the characteristics of the

graphite host material. Small, thin samples intercalate more quickly and often yield

better-staged, more homogeneous material than large, thick samples. Also single

crystal graphite akes intercalate more readily than a highly oriented pyrolyticgraphite (HOPG) specimen (Moore 1973), or than a carbon bre (Hooley 1977a).

Although a given intercalation compound can often be prepared by alternative

growth techniques, the physical and chemical properties of the intercalant play an

important role in favouring one intercalation method over another. Excellent

reviews of methods used for the preparation of specic intercalation compounds

are given in the articles by Ebert (1976) and He rold (1977, 1979).

Intercalation can be achieved starting from solid, liquid or gaseous reagents

(Croft 1960, Holey 1977a, He rold 1977, 1979) though preparation using vapour

transport with the two-zone method is the most common for the preparation of well-

staged specimens (Fredenhagen and Cadenbach 1926, He rold 1955). Intercalation

occurs for many types of reagents (more than 100), ranging from simple ionic speciessuch as alkali metals, diatomic molecules such as the halogens, metal chlorides,

bromides, oxides and sulphides to large organic molecules such as benzene (Croft

1956, E bert 1976, Stumpp 1977, He rold 1977, 1979). T he simpler binary and ternary

compounds are usually prepared by direct synthesis, and the more complicated

materials by a variety of stepwise intercalation procedures.

Because of the high reactivity of most graphite intercalation compounds, they are

commonly prepared and stored in ampoules, containing either intercalate vapour, an

overpressure of an inert gas or vacuum, depending on the intercalate species.

Cooling samples (for example, to liquid nitrogen temperatures) greatly increases

their stability.

Certain intercalation compounds (for example, graphiteBr2), when removedfrom their encapsulating ampoules and exposed to the normal room temperature

environment, will desorb for a period of time, after which there is negligible

desorption. The resulting material, called a residue compound, is chemically stable

and contains an intercalate concentration (usually small), dependent on the

desorption temperature and on the intercalate concentration of the compound prior

to desorption (Hennig 1952b). Lamellar compounds, denoting intercalation com-

pounds that have not been desorbed, may be single-staged, multi-staged or have a

random arrangement of the intercalate layers.

2.1.2. The two-zone vapour transport methodIn the two-zone vapour transport method, the intercalant is typically heated to

some temperature Ti and the graphite, which is some distance away, is heated to a

higher temperature Tg as illustrated in gure 3. The stage of the compound, or the


Figure 3. Schematic diagram of the t wo-zone vapour transport method where Tg and Tiindicate the temperature of the graphite and intercalant respectively.


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depend on many parameters, including geometrical factors (sample size, such as

thickness and cross-sectional area and ampoule size and shape) and the accuracy of

temperature control (Hooley et al. 1965, Hooley 1977a). For the preparation of

dilute alkali metal compounds, shaping the ampoule to minimize the free volume for

intercalate vapour around the graphite, and necking down the connection between

the graphite and intercalate zones increases the stage index of the sample (Underhill

et al. 1980). The intercalation ofsmall graphite samples promotes the formation of a

single stage over the sample volume. For a given geometrical arrangement, the

preparation of a dilute compound of specied stage is greatly aided by use of a

growth diagram such as gure 4. An upper limit for Tg is imposed in practice by the

softening of the encapsulating glass and by its increased reactivity with the alkali

metal intercalants at high temperatures. A lower limit on Ti is imposed by the

requirement of some minimum acceptable reaction rate and of the condition p > pt,

where pt is the threshold pressure below which intercalation does not occur. It should

be mentioned that although alkali metal compounds are typically grown under lowvapour pressure (less than 1 torr) conditions, the intercalation rates are relatively

rapid; for example, single-staged graphiteRb samples for n 8 can be prepared inless than 24 hours (Underhill et al. 1980).

Preparation of a variety of single-staged acceptor compounds is also carried out

using the two-zone technique, though the detailed growth conditions are quite

dierent from the donor compounds and vary from one class of acceptors to

another. In the case of acceptors, the intercalant is in molecular form, and normally

a much larger c-axis expansion of the graphite host is required to accommodate the

intercalant. For growth of acceptor compounds by the two-zone method, the

graphite temperature Tg is typically held constant, and the intercalant, at a lower

temperature Ti is varied to produce the desired stage (Hooley 1973). The acceptor

compounds typically have a high threshold vapour pressure and therefore are

prepared under high vapour pressure conditions.

The metal chloride acceptor compounds with AlCl3 and FeCl3 are both prepared

in a similar way (Hooley 1973). The metal chloride intercalant is rst produced in situ

by direct reaction of the heated metal wire with Cl2 gas, as shown in gure 5. Prior to

intercalation, the two-zone ampoule is back-lled with Cl2 gas to encourage staging

(Dzurus and Hennig 1957b, Metz and Hohlwein 1975b); without the Cl2 gas, only

weight uptake is achieved and staging is inhibited (Ru dor and Zeller 1955, Hooley

1972, 1973, Underhill et al. 1979). The presence of Cl2 gas is also necessary for the


Table 1. Ti and Tg for preparation of alkali metal compoundswith stages 1 n 3.a

K Rb CsTi 2508C Ti 2088C Ti 1948C

Stage Tg8C Tg8C Tg8C

1 225320 215330 2004252 350400 375430 4755303 450480 450480 550

a From Nixon (1966).


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preparation of many other metal chloride intercalation compounds (Stumpp 1977),

generally increasing the amount of intercalant uptake, reducing the threshold

pressure, and encouraging good staging to occur. Whether or not there is a

signicant uptake of excess chloride in such reactions has not been fully resolved

(He rold 1979).

Once intercalation has been completed, the reaction chamber and its contents are

quenched. To prevent exfoliation during this cooling process, the reaction chamber is

rst quenched on the intercalant side, away from the graphite, so that the vapourpressure is suitably reduced and condensation of metal chloride vapour on the

sample is avoided (Hooley 1973). Once cool, the ampoule containing the intercala-

tion compound is sealed o. Reducing Tg Ti in the presence of a high intercalate

vapour pressure can result in very rapid intercalate uptake, the formation of mixed

stages, and the introduction of large strains which are usually relieved by rapid

(explosive) exfoliation of the sample. Exfoliation can also cause problems in

handling samples. In some cases, exposure of intercalated graphite to vacuum causes

exfoliation, which is prevented by use of an inert gas atmosphere in the encapsulating

ampoule. For example, graphiteAsF5 samples are encapsulated in dry nitrogen gas,

rather than in a vacuum (Falardeau et al. 1978).

For the case of halogen growth, typical growth temperatures are close to room

temperature, owing to the high threshold vapour pressure of the halogens. The only

diatomic homopolar halogen molecule that intercalates readily into graphite is Br2,

though intercalation with ICl and IBr can also be carried out. For Br2 intercalation,

the lowest stage that has been reported is a stage 2 compound. GraphiteBr2compounds are readily prepared using the growth conditions Tg 208C and

308C < Ti < 208C, by inserting liquid bromine into a temperature-controlled

(refrigerated) alcohol bath (Sasa et al. 1971). To illustrate the staging conditions

(regions of stability) for stage 2, 3, 4 and 5 compounds, the adsorption isotherm for

graphiteBr2 is shown in gure 6. Also evident in this gure is the threshold vapour


Figure 5. Schematic diagram of a system used for the preparation of graphiteFeCl3.(a) System used to prepare crystalline FeCl3 in situ by passing Cl2 gas over a heated

Fe wire. This closed system is advantageous because FeCl3 is highly hygroscopic.(b) Two-zone ampoule containing a highly oriented pyrolytic graphite sample(HOPG) in one zone and the crystalline FeCl3 to be intercalated in the other zone.


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pressure for intercalation, pt 0:1p0, where p0 is the pressure where the saturationcompound (second stage C16Br2) is formed. The open circles give the experimental

points of Sasa et al. (1971) for bromination (adsorption) and the solid circles for

debromination (desorption). Because of the instability of these compounds (seen as

hysteresis in the adsorptiondesorption curves), they must be encapsulated in

ampoules for storage and properties measurements. We note from this gure that

in the absence of an ambient Br2 vapour pressure (p=p0 0), about 30% of the Br2

uptake at saturation remains in the resulting residue compound.The two-zone vapour transport method is widely applied to the growth of well-

staged intercalation compounds and for many systems is the preferred growth

method. However, for specic systems, other intercalation methods are advanta-

geous as described below.

2.1.3. Other intercalation methods

Isothermal vapour transport is another method used for the preparation of

acceptor compounds. This method is for example applied to the preparation of the

high conductivity graphiteAsF5 compounds (Falardeau et al. 1978). For this

system, the growth takes place at room temperature using an AsF5

overpressure

of 3atm. The growth time is the principal parameter used to control the stage

index, and well-staged compounds have been prepared for 1 n 5. During the

growth process, Falardeau et al. monitored the stage index visually by measurement

of the increase in c-axis sample thickness, which exhibits distinct steps for each of

these low stage compounds that is produced (gure 7).

Another modication of the basic vapour transport technique to control the

stage index is the use of restricted amounts of intercalant in the reaction chamber.

For example, a number of well-staged compounds have been prepared using a two-

zone vapour transport system but with restricted quantities of crystalline AlCl3, the

smaller the quantity, the higher the stage that results (Gualberto et al. 1980).


Figure 6. Isotherms of bromine uptake by highly oriented pyrolytic graphite (HOPG) at208C as measured by Sasa et al. (1971) for the bromination cycle (open circles) and

the debromination cycle (closed circles). Below the threshold pressure pt, nointercalation occurs. Note also that when p 0 on the debromination cycle, a non-vanishing bromine uptake remains, forming a `residue compound.


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Post-intercalation methods have also been used successfully to change theintercalate concentration or oxidation state. For example, using the wash method,

the preparation of dilute compounds from a more concentrated material can be

carried out by washing with a solvent, as for example using acetone to wash

graphiteFeCl3 (Hooley and Soniassy 1970). While this method can provide very

dilute compounds, the resulting samples are not well staged.

Chemical reactions with the intercalate species to change the oxidation state is in

some cases also possible after intercalation is completed. For example, FeCl3 is

readily intercalated into graphite by the two-zone method, but FeCl2 has not been

successfully intercalated directly. However, FeCl3 can be reduced to FeCl2 in the

intercalation compound using H2 at 3758C (Hooley et al. 1968). In this case, the

oxidation state of the intercalant was identied using Mo ssbauer spectroscopy,which shows distinctly dierent spectra for FeCl3 and FeCl2. It is also of interest that

the reduction to FeCl2 could be carried out essentially completely, so that no

characteristic Mo ssbauer spectrum for FeCl3 was observable.

It has also been claimed that complete reduction of a metal chloride can be

carried out to obtain two-dimensional metal sheets between graphite layers (Klotz

and Schneider 1962, Novikov et al. 1971, Volpin et al. 1975). Though a number of

workers have found evidence for the presence of-Fe upon reduction of graphiteFeCl3, using X-ray, Mo ssbauer and other techniques, it has not been established that

staged intercalate metal layers can be prepared by reduction methods (Bewer et al.

1977). Studies of the dierences of H2 and CO adsorption isotherms on unactivated

and activated (by heat treatment in H2) stage 1 graphiteFeCl3 have been interpreted

to indicate that Fe metal is formed on the sample surface, but is not intercalated

(Parkash et al. 1978). Another interesting chemical reaction that has been carried out

within an intercalation compound is the reduction of various graphite metal

chlorides with alkali metals to produce a catalyst for NH3 synthesis (Ichikawa et

al. 1972a). The electron diraction patterns taken by Evans and Thomas (1975) of

the graphiteFeCl3K system show evidence for the presence of KCl and free iron, in

agreement with Mo ssbauer studies (Tricker et al. 1974), also showing free iron. It has

been proposed (Evans and Thomas 1975) that the catalytic activity of this system is

due to the presence of dispersed free iron on the graphite surface.


Figure 7. Plot of c-axis expansion ratio, t=t0, versus reaction time for a typicalintercalation of AsF5 into graphite (from Falardeau et al. 1978). Note the stabilityregions oft=t0 corresponding to each of the stages 1 n 5:


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Somewhat related to the use of chemical rections to change the oxidation state of

the intercalant is the state of the intercalant before and after intercalation. For

example in the case of graphiteHNO3 compounds, Forsman et al. (1978) have

concluded that the intercalant is present in the form of neutral HNO 3 molecules

admixed with charged NO3 ions by identifying NO2 as a by-product of the

intercalation process. It is believed that similar phenomenon may occur in other

acceptor compounds such as with the intercalants AsF5 and SbCl5.Another useful technique for the preparation of certain intercalation compounds

is the use of liquid intercalants. For the case of the alkali metal donor lithium, rst

stage C6Li samples can be prepared by degassing the graphite and then immersing it

in molten lithium in a stainless steel crucible in an argon atmosphere containing less

than 1 ppm H2O and O2 (Zanini et al. 1978a, Basu et al. 1979). Using a solution of

molten lithium and sodium (3.8 wt% Li) stage 2 samples can be produced, with only

Li being intercalated. After intercalation, the external sample surfaces containing a

metal lm are removed by cleavage. Graphitelithium compounds with stages

1 n 4 have also been prepared by compressing lithium powder (1020 kbar)

with crushed natural graphite in an argon atmosphere (Gue rard and He rold 1975).

Somewhat related to the method of intercalation from the melt is intercalationfrom solution. This method can for example be applied to the preparation of

graphitebromine compounds using CCl4 as a solvent (Hennig 1952a, Hennig and

McClelland 1955, Saunders et al. 1963). In this case the intercalate concentration in

the compound is controlled by the intercalate concentration in solution, its

temperature and the immersion time. Though intercalation from solution provides

a convenient intercalation method, it is dicult to prepare well-staged compounds

using this technique. Some other examples of intercalant/solvent mixtures are FeCl3in acetone and FeCl3 in nitro-methane (Hooley 1972), and in nitroethane (Ginderow

and Setton 1963), and other metal chlorides in SOCl2, SO2Cl2 and CCl4 (Stumpp

1977). Gas solutions have also been used to intercalate species that do not alone

intercalate readily. For example, metal chloride vapours such as AlCl3 and FeCl3have been used successfully to intercalate other metal chloride species such as CoCl2and NiCl2 into graphite (Stumpp 1977).

In the case of donor intercalants, liquid ammonia has been used by Ru dor et al.

(1955) and other workers as a solvent for the intercalation of metals M such as Li,

Na, K, Rb, Cs, Co, Sr, Ba, though in this solution growth process a ternary

compound results, as for example stage 1 compounds with stoichiometry

C12M(NH3)2. Other workers (Stein et al. 1966, Ginderow and Setton 1970, Beguin

and Setton 1975) have used organic solvents for intercalating metals, and some dilute

donor compounds have been prepared with this technique.

The preparation of ternary intercalation compounds by either cointercalation or

sequential intercalation has also been pursued. Some examples of interest are

cointercalation of alloys of the alkali metals K, Cs, Rb with Na and of mixed

halogen compounds of Br2 with I2 and Cl2. Preparation of such alloys and mixtures

provides a method for the insertion of materials that do not readily intercalate by

themselves. For example, Na has been intercalated into graphite as a binary

compound only at very high stages (Asher 1959), but Na can be readily intercalated

as an alloy with Cs and K (Billaud and He rold 1974, Billaud et al. 1980). Likewise,

chlorine does not intercalate as a binary compound but can be intercalated as a

mixture with bromine (Furdin et al. 1970, Furdin and He rold 1972) and with iodine

(Bach and He rold 1963, 1968). One explanation that has been proposed for the



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failure of Na and Cl2 to intercalate in low stage compounds is the lattice mismatch of

layers of graphite and of solid Na (Dresselhaus and Dresselhaus 1979), and of solid

Cl2 (Hooley 1973). In order to prepare a compound by this type of cointercalation

process it is necessary for one of the elements to intercalate by itself and for the two

intercalants to be miscible (He rold 1979). In these cases, cointercalation improves the

lattice match, thereby promoting the intercalation process.

Another example of cointercalation is the introduction of intercalants withdierent chemical properties. For example, graphiteAlBr3 Br2 can be prepared

as a stage 1 compound with stoichiometry C9AlBr3 Br2, containing a higher

bromine density than the saturated stage 2 compound C 16Br2 (Sasa et al. 1972).

In general, the preparation of metal bromide compounds is similar to that of metal

chloride compounds in that the presence of Br2 gas is usually required for

intercalation and the preparation of well-staged samples (Stumpp 1977). However,

for the case of the metal bromides, the presence of Br2 gas results in the growth of

ternary compounds because of Br2 uptake.

Whereas the alloy compounds mentioned above are intercalated at the same

time, sequential intercalation is also commonly used to insert materials that are not

readily intercalated as binary compounds. For example, hydrogen which does notintercalate by itself, can be inserted into an alkali metal compound. The introduction

of hydrogen into a rst stage C8K compound forms an ordered second stage

compound C16K2H4=3 in which the intercalant resides in a triple layer sandwich

formed by two highly electropositive K layers between which is inserted a less

electropositive H layer (Gue rard et al. 1977b, Lagrange and He rold 1978). A model

proposed by Lagrange and He rold (1978) for the intercalation process that takes

2C8K into C16K2H4=3 is shown in gure 8(a), and the corresponding layer

arrangement is shown in gure 8(b). The intercalate sandwich, including a hydrogen

layer between two potassium layers, has a thickness of 5.18 A . Subsequent

intercalation of C16 K2H4=3 with an alkali metal M K, Rb or Cs yields another

stage 2 compound with stoichiometry C8K2H4=3, C8M and a uni t cell of Ic 8:53 A

IM as shown in gure 8(b). The intercalation of higher stage alkali

metal compounds can also be carried out, and use of such compounds for H2 storage

and as molecular sieves for H2 has been discussed (Lagrange and He rold 1978).

Ternary compounds with HgK and HgRb have been similarly prepared by

Lagrange et al. (1980). Large organic molecules can also be intercalated using the

sequential intercalation technique starting with a graphiteK binary compound. For

example, benzene can be intercalated into stage 2 or stage 3 graphitepotassium, but

not into rst stage C8K (Merle et al. 1977, Bonnetain et al. 1977).

The use of cointercalation and sequential intercalation techniques greatly

expands the number of possible intercalation compounds that can be prepared.

These techniques furthermore form a basis for the use of graphite intercalation

compounds as catalysts for carrying out chemical reactions. Such applications have

been made to the preparation of a number of organic compounds (Setton 1977) and

to the use of graphite layers to promote polymerization (Gole 1977).

Electrochemical techniques are also useful for the preparation of graphite

intercalation compounds, particularly for strong acid intercalants such as sulphuric,

perchloric, nitric and triuoroacetic acids. The graphite is oxidized anodically by

placing the graphite specimen in a platinum cap suspended in concentrated acid, and

using a second platinum counterelectrode. In this case, the stage formation is

controlled by the electrode voltage (Ru dor and Siecke 1958, Bottomley et al.



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1963b, Horn and Boehm 1977, McRae et al. 1980a). The electrochemical preparation

method provides an excellent technique for study of the thermodynamic changes

accompanying a change in stage during the growth process. This technique has been

applied to both the system of donor compounds graphiteK (Aronson et al. 1968)and the acceptor compounds based on the intercalant H 2SO4 (Aronson et al. 1971),

as discussed in section 2.3. Because of the colour change associated with dierent

stages of certain graphite intercalation compounds (for example, alkali metal

compounds), it is possible to use electrochemical means to produce a change in

stage (and colour), thereby yielding an electrochromic eect. Such an electrochromic

eect has been proposed for display device applications (Puger et al. 1979).

2.1.4. Some prototype intercalants

For various physical measurements, specic intercalation compounds are of

particular interest from the point of view of chemical stability, the simplicity of the

intercalate structure, and specic properties such as high electrical conductivity or

anisotropy, magnetic ordering and superconductivity. We discuss below some

prototype intercalation compounds that are particularly suitable for specic applica-

tions.

Because of their stability in air (Lazo and Hooley 1956), graphiteFeCl 3 samples

may be safely removed from their encapsulating ampoules for short periods of time,

and therefore provide prototype materials for properties measurements and ex-

ploratory investigations. Curiously, pristine FeCl3 is itself highly reactive and

hygroscopic, in contrast with the behaviour of its intercalation compounds. It has,

however, not yet been demonstrated that one can prepare a stage 1 graphiteFeCl3


Figure 8. Model for ternary graphitealkali metal compounds containing hydrogen asproposed by Lagrange et al. (1978). (a) Schematic model for the transformation ofrst stage 2C8K to second stage C16K2H4=3. (b) Layer arrangement for pristinegraphite, rst stage C8K, second stage C16K2H4=3 and second stage C8K2H4=3, C8M

where the thickness of the alkali metal sandwich is IM 1:97, 2.27, 2.58 A dependingon whether M K, Rb or Cs.


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sample without inclusions of pristine graphite, as was long ago noted by Cowley and

Ibers (1956). On the other hand, stage 1 graphiteAlCl3 is readily prepared, although

it is found to be very unstable when removed from its growth ampoule and exposed

to air (Ru dor and Zeller 1955, Dzurus and Hennig 1957a). Because of the chemical

similarity between graphiteFeCl3 and graphiteAlCl3, stage 1 samples of graphite

AlCl3 are sometimes used in conjunction with higher stage graphiteFeCl3 com-

pounds for properties measurements. Although stage 1 graphiteAlCl3 is highlyreactive and requires encapsulation, high stage (n 4) graphiteAlCl3 samples are of

comparable stability to graphiteFeCl3 samples of similar (high) stage. Environ-

mental stability has also been reported for graphiteSbCl5 compounds for n 2 by

Murthy et al. (1980).

Alkali metal compounds are used frequently as prototype materials for many

property measurements. Of all intercalation compounds these materials are most

easily prepared, exhibit the highest degree of order and are best understood from a

structural point of view. Compounds formed with the heavy alkali metals K, Rb and

Cs form one class of compounds with many similar properties, which can be

compared to compounds formed with the alkali metal donor Li. The graphiteLi

compounds dier from the heavy alkali metal compounds with regard to structural

ordering and intercalate ionic size, and therefore exhibit somewhat dierent proper-

ties. Other donor intercalants exhibiting the same in-plane ordering as Li are the

alkaline earth metals Ba and Sr (Gue rard and He rold 1974) and the lanthanides Eu,

Yb, Sm and Tm (Lagrange et al. 1980).

Among the acceptors, the simplest compounds are the graphiteBr2 compounds,

where the intercalant is a simple homopolar diatomic molecule having its molecular

axis aligned within the intercalate layer. Another simplifying feature of the graphite

Br2 system is the formation of commensurate compounds, in which the ordering of

the bromine intercalate layer is in registry with that in the graphite layers (Eeles and

Turnbull 1965). The graphiteBr2 system however does not form stage 1 compounds(Sasa et al. 1971). On the other hand, the halogen ICl does form stage 1 compounds,

and has the further advantage of having a dipole moment unlike Br2, thereby

providing increased coupling of the intercalant to an applied electromagnetic eld

probe. The failure to prepare stage 1 compounds is common for many acceptor

compounds, as for example with certain metal halides (Stumpp 1977), and the reason

for this diculty is not generally understood. There is also much evidence in the

literature for diculties in the preparation of certain low stage compounds with

specic intercalants, as for example PdCl2 which formed only stage 3 compounds

(Novikov et al. 1973).

Intercalation in some cases permits study of a chemical species that is otherwise

unstable. For example, TlBr2 does not exist by itself but must be kept in liquid Br2.However, stable graphite intercalation compounds can be formed with TlBr3 (Niess

and Stumpp 1978).

Low stage acceptor compounds prepared with the highly reactive Lewis acid

AsF5 have been prototype materials for conductivity studies because of reports of a

room temperature in-plane conductivity exceeding that of copper and an anisotropy

between in-plane and c-axis conductivity of106 (Foley et al. 1977, Falardeau et al.

1977). Signicant advances in materials preparation of graphiteAsF5 compounds

(Falardeau et al. 1978) have also contributed to the widespread interest in these

compounds.



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The graphiteFeCl3 compounds are prototype magnetic materials with anti-

ferromagnetic ordering reported below 3.6 K for stage 1 and ferromagnetic ordering

below 8.5 K for stage 2 (Karimov et al. 1971, Ohhashi and Tsujikawa 1974a) while

the stage 1 alkali metal compound C8K is an intercalation compound in which

superconductivity can be studied below 1 K (Hannay et al. 1965, Koike et al. 1978).

As graphite intercalation compounds nd new application areas, it is likely that

other intercalation compounds will become prototypes for these applications.

2.2. Materials characterization

A number of techniques are exploited for sample characterization of graphite

intercalation compounds, including visual inspection, weight uptake, chemical

analysis, c-axis dilatation, diraction measurements and electron microscopy.

Diraction studies yield the stage index and information on staging delity and

on in-plane order, and electron microscopy provides information on the micro-

structure and submicrostructure. In this section we review some of the most

important sample characterization techniques.

Several simple characterization methods are used to provide useful qualitative

information. The sample colour observed by visual inspection gives qualitativeinformation on the stage: for example, for the alkali metal compounds a yellow,

gold or red colour is characteristic of stage 1 compounds, steel blue for stage 2, dark

blue for stage 3 and graphitemetallic for higher stages (Hennig 1959, He rold 1979).

For acceptor compounds, stage 1 is often blue and higher stage compounds

graphitemetallic. Direct chemical analysis gives the chemical formula for the

intercalation compound. Weight uptake gives information on the sample stage if

the chemical formula for the compound is known and stoichiometry is assumed.

However, this information is qualitative because of sample inhomogeneity, the

presence of intercalate vacancies and the preferential accumulation of intercalant

in the vicinity of crystal defects.

Because of sample expansion along the c-axis due to the intercalation process,dilatation along the c-axis, observed using a travelling microscope, also provides

qualitative information on the sample stage. This method is subject to large

systematic errors because of sample inhomogeneities, the formation of microcracks

and the tendency for exfoliation especially near the sample edges where the length

measurements are usually made. However, for the case of graphiteAsF5, c-axis

sample expansion provides a good indicator of stage, as shown in gure 7. For this

system, c-axis expansion is used as a diagnostic for stopping the growth process at a

given stage (Falardeau et al. 1978).

Since many properties measurements are strongly dependent on the stage index,

it is important to characterize samples to be used for property measurements with

regard to stage index and stage delity. This type of characterization of graphite

intercalation compounds is provided by X-ray diraction using (00l) reections.

Sample characterization for stage index n and repeat distance Ic has been carried out

for large numbers of compounds (more than 50) and the results are given in various

review articles (Hennig 1959, Ebert 1976, Stumpp 1977) and in table 6. It has become

common for property measurement studies on graphite intercalation compounds to

include staging information as obtained by (00l) diractograms. Accurate staging

determinations can be made with a system such as is shown in gure 9 (Leung et al.

1981a). Mo K radiation is used in order to minimize the X-ray absorption by the

glass encapsulating the samples. A typical set of (00l) diractrograms is shown in



17/186

gure 10 for graphite and graphiteRb samples of stages 1, 3 and 6. From these

diractograms, the diraction angles l corresponding to the various (00l) reections

are determined, and using Braggs law

l 2Ic sin l; 2:1

Ic, the repeat distance (see gure 2), is accurately evaluated. Since the graphite

interlayer separation is essentially unaected by intercalation, the stage index n is

found from the relation

Ic nc0 di n 1c0 ds; 2:2

where c0 is the distance between adjacent graphite layers (c0 3:35 A ) andds c0 di is the distance separating two graphite layers between which an inter-calate layer is sandwiched. Analysis of (00l) diractograms show that for a given

intercalant, ds and c0 are essentially independent of stage (Hennig 1959, Ru dor

1959), as illustrated in table 2. The validity of (2.2) provides strong evidence that the

n graphite layers remain essentially unperturbed by the intercalation process. From

(2.2) it follows that the increase in length along the c-direction t relative to the

length prior to intercalation t0 is given by


Figure 9. X-ray system for (00l) diractometer scans. K radiation from a Mo X-ray sourceis incident on the sample and the diracted beam is detected by a cooled Li-driftedsilicon detector. This detector permits high resolution energy discrimination of thediracted beam. The energy windows of the single channel analyser are set so thatonly signals corresponding to K1 and K2 radiation are processed. The multi-channel analyser is used for data acquisition of (00l) diractograms (from Leung etal. 1981a).


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tt0 di

nc0: 2:3

Since the separation of neighbouring (00l) diraction peaks becomes smaller as

the stage index increases, the energy discrimination technique provides an invaluable

tool for stage determination of dilute compounds (for example, stage n > 6). In the

analysis of staging for dilute compounds, the greatest sensitivity for determining the

stage index is achieved with low index reections (small 2l). However, the intensities

of the (00l) reections for small l due to the superlattice structure in high stage

compounds are usually very low because of the mismatch of these 2l values with the

maximum (at 2 128) in the envelope function (see gure 10) due to the strong


Figure 10. X-ray stage characterization using (00l) diractograms for stages 1, 3 and 6graphiteRb compounds and for pristine graphite (from Leung et al. 1980a). Notethe correlation in 2 of the high intensity reections for the high stage intercalationcompounds with the occurrence of (00l) reections in graphite.

Table 2. Identity period or repeat distanceI

c for graphitepotassium compounds.

Stage n 1 2 3 4

Ic (measured) A Ru dora

5.41 8.77 12.12 15.49(measured) A Parry

b5.35 8.72 12.10 15.45

(measured) A Underhillc

5.32 8.74 12.07 15.44Ic (calculated) 5:41 3:35n 1 A

5.41 8.76 12.11 15.46

References: a Rudor and Schulze (1954);b Nixon and Parry (1968); c Private communication.


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graphite (002) reection. However, for high stage compounds (n 6), highintensities are again obtained for 2 < 18, through matching with the envelopeprovided by the strong graphite (000) peak. Stage determination for very dilute

compounds is made possible by the narrow linewidths of the diracted patterns and

the ability to resolve diraction peaks within 0 :58 of the direct beam (Underhill et al.1980). For example, the full widths at half maximum (FWHM) intensity for the

alkali metal intercalation compounds are roughly equivalent to those of pristinegraphite; for 2 128 (the position of the (002) graphite peak which is the mostintense peak), typical FWHM values for graphite are 0.28.

Using the two-zone vapour transport growth technique described in the previous

section, it is possible to prepare single-staged alkali metal compounds as dilute as

n 8 (Underhill et al. 1980), and using a diractometer system such as in gure 9 tocharacterize them for stage index and stage delity. Stage delity is established by the

absence of satellite diraction peaks due to small quantities of admixed stages, and

by accurate distinction between stages n and n 1 in the analysis of the diract-ometer scans. The absence of broadening of the diraction lines at large diraction

angles provides strong evidence that the samples are single staged, and not an

average over a distribution of stages (Metz and Hohlwein 1975b). The X-ray patternobtained from most intercalation compounds during a staging phase transition is of

the type discussed by Metz and Hohlwein and can be explained in terms of the

domain model for staging proposed by Daumas and He rold (1969) and discussed in

section 2.4.

In discussing stage indelities it is useful to distinguish two types of stage

indelity. A mixed stage sample contains macroscopic regions that exhibit stage n

and other macroscopic regions with dierent stages, usually stages n 1 or n 1. Inthis case the X-ray (00l) diractograms show reections that are broadened relative

to those from a single-staged sample, some reections that can be identied with

each constituent stage, and others that are shifted because of unresolved 2lcomponents. In contrast, a randomly staged material will contain a randomarrangement of regions with various stages. If one of these stages is dominant, a

well-dened diraction pattern will result but with reections showing extensive line

broadening (Metz and Hohlwein 1975b). The random stacking of intercalate layers

has been studied in detail by Metz and Hohlwein (1975a, b) for the graphiteFeCl3system, but no systematic study of this phenomenon has been carried out for any

other intercalant.

Further sample characterization is provided by analysis of the intensity of the

(00l) reections, by relating the square root of the observed intensities II00l to the

magnitude of the structure factor F00l

jF00lj fC sin n"lsin "l

1lX

X

fX exp 2i"Xl-----

-----; 2:4

where fC and fX are atomic scattering factors for carbon and for the intercalant on

layer X, " c0=Ic, and "X zX=Ic, where zX is the distance of an intercalate layerrelative to the centre of the intercalate sandwich, and 1= is the intercalate density.Since the structure factor determines the relative intensity of the (00l) reections, the

index ll of the reection with maximum peak intensity (00ll) approximately

determines the stage n according to the relation

ll n mi; 2:5



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wherem

i is the nearest integer to the ratiod

i=c

0 (Leunget al.

1980b). For multilayerintercalants (for example, FeCl3 and AlCl3), structure factor calculations can

provide valuable information for determining the intercalant stacking arrangement

when compared to experimental II00l measurements. An example of such a deter-

mination is the identication of the Cl3Al2Cl3 stacking arrangement of the

intercalate layer sandwich, as shown in gure 11 (Leung et al. 1980b).

Integrated intensity measurements of (00l) reections also provides an important

characterization technique through determination of the intercalate in-plane density

1=. While gravimetric measurements depend on the total weight uptake duringintercalation, the structure factor depends only on the fraction of intercalate

arranged in ordered stages. For compounds where the in-plane structure is known,

the in-plane intercalate density can be used to obtain the fractional site occupation inthe intercalate layer (Leung et al. 1979). Such an analysis applied to a rst stage

graphiteRb sample, which forms a p2 2R08 superlattice (Ru dor and Schulze1954, Nixon and Parry 1968, Kambe et al. 1980a) and assuming the stoichiometry,

C8Rb, shows 15% vacancies at room temperature (Leung et al. 1979). Real spaceelectron micrographs of thin graphiteRb specimens show the presence of more than

one phase at 300 K for compounds with stages n 2 (Kambe et al. 1980a). Theseexperiments suggest that even in single-stage d materials with long-range c-axis

ordering, defects and multiple phases within the intercalate layer are common for

n 2 compounds. As discussed in section 3, diraction measurements also providethe principal technique for structural studies of the in-plane ordering in graphite

intercalation compounds.

The electron microscope has also provided useful information on the micro-

structure and ultramicrostructur e of graphite intercalation compounds. An espe-

cially vivid application of this technique was made by Evans and Thomas (1975) and

by Thomas et al. (1976), who applied high resolution transmission electron

microscopy (T.E.M.) in the lattice imaging mode to `see individual graphite and

intercalant lattice planes directly. Such a bright eld lattice image of a graphite

FeCl3 samples is shown in gure 12(a) and the schematic diagram illustrating the

interpretation given by Thomas et al. (1976) of this photograph is presented in gure

12(b). This work demonstrates the staging phenomenon directly with real image


Figure 11. Plot of relative X-ray integrated intensity versus x for stage 2 graphiteAlCl3,where x is the dimensionless quantity x c0l=Ic (see text). The solid and dashedcurves represent Cl3Al2Cl3 and AlCl6Al intercalate stacking respectively. The tto the experimental measurements (closed circles) of Gualberto et al. (1980) is muchbetter for the Cl3Al2Cl3 intercalate stacking arrangement than for the AlCl6Alstacking (Leung et al. 1980b).


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(a)

(b)

Figure 12. High resolution transmission electron microscopy applied to lattice imaging ofindividual graphite and intercalate lattice planes in graphiteFeCl3 by Thomas et al.(1976). (a) Bright-eld lattice image of a graphiteFeCl3 sample (average stage n 2).The light striations on the micrograph delineate the sheets of FeCl 3 intercalant, andfrom their separation the stage indices may be identied. (b) Schematic diagramillustrating the interpretation of (a) in terms of the various stages that are containedin the photograph.


22/186

photographs and gives evidence for regions of material with large repeat distances

(sixth stage). These authors were also able to relate the observed layer thicknesses in

the intercalation compound with those in the parent materials. Furthermore, the

sample region shown in gures 12(a) and (b) indicates a predominance of stage 2

with some admixtures of stage 1 and trace amounts of stages 5 and 6. In view of the

diculty in preparing a-face samples (see section 4.3), the staging delity shown in

gure 12(a) is impressive. Similar latttice image results have also been presented forpristine graphite and for graphiteK compounds (Evans and Thomas 1975). The

T.E.M. technique is dicult to apply in practice because very thin samples (600 A )are required to permit penetration by the electron beam and because the intercalate

tends to desorb after several hours under the high vacuum conditions of the electron

microscope column. Nevertheless, the use of high resolution transmission electron

microscopy under dynamic conditions could provide extremely valuable information

on the intercalation mechanism.

Very high resolution studies using scanning transmission electron microscopy

(S.T.E.M.) have been carried out on the heavy metal atom migration on thin

amorphous carbon lms, allowing the motion of single atoms to be studied (Isaacson

et al. 1979). Such studies could also provide very interesting information onintercalated graphite. Also of interest is the S.T.E.M. study of commercially

produced `graphimet intercalates by Fischer et al. (1979) showing very thin metal

clusters of about 2060 A in diameter.

Transmission electron microscopy has also been applied very successfully by

Heerschap et al. (1964) and Heerschap and Delavignette (1967) to study the

microstructure associated with in-plane imperfections (stacking faults and disloca-

tions) in graphiteBr2, graphiteICl and graphiteFeCl2 compounds. Their real

image micrographs provide evidence for the existence of isolated dislocations

bounding the edge of an intercalated layer (see gure 13) in the case of all three

intercalant systems. The usual hexagonal layer stacking of pristine graphite is ABAB

as shown in gure 14, while rhombohedral graphite exhibits the ABCABC stacking

shown in gure 15. The Burgers vectors for the dislocations in the graphiteBr 2 and

graphiteICl systems are equivalent to the Burgers vector of the normal partial

dislocations in hexagonal pristine graphite. This result is interpreted in terms of an

interlayer shift of the graphite bounding layers to achieve identical crystallographic

positions, shown in gure 14 as an `A over A stacking arrangement of the graphite

layers about the intercalant (AjA), in agreement with X-ray diraction results for


Figure 13. Schematic cross-section of a graphite crystal containing a dislocation boundingan intercalated layer of a reactant. Intercalation often causes a shift of the adjacentcarbon layers to yield a fully symmetrical arrangement relative to the intercalant layerX. For clarication of the A, B, C interlayer arrangement of graphite layers seegures 14 and 15.


23/186

these materials (Ru dor 1941). On the other hand, for the graphiteFeCl3compounds the Burgers vector is found to be perpendicular to the graphite planes

so that the graphite preserves its ABAB stacking upon intercalation with FeCl3, in

agreement with X-ray results of Ru dor and Schulz (1940). It is reasonable to expect

commensurate intercalate structures to exhibit a symmetrical AjA stacking, whileincommensurate structures have no symmetry reason for disturbing the normal

graphite stacking.

The pinning of dislocation loops of1mm diameter at crystal imperfections isalso demonstrated in the micrographs obtained by Heerschap and Delavignette

(1967). Based on these observations it is concluded that upon intercalate desorption

to form residue compounds (Hennig 1959), intercalate islands can be trapped in


Figure 14. Structure of hexagonal graphite showing ABAB stacking (from Wycko 1964).

Figure 15. Structure of rhombohedral graphite showing ABCABC stacking (from Wycko1964).


24/186

dislocation loops, as shown in gure 16. Evidence for these trapped intercalate

islands is also provided by in-plane X-ray and electron diraction patterns from

residue compounds (Maire and Mering 1959, Chung et al. 1977). The preferential

trapping of the intercalant at crystal defects is observed in the T.E.M. micrographs

of Heerschap et al. (1964) and has also been identied by X-ray dispersive analysis

using scanning transmission electron microscopy (Chung 1977).

A combination of electron diraction patterns and real image electron micro-

graphs have shown that epitaxial layers form on the graphite surface under certain

growth conditions. For example, when a graphiteBr2 compound is exposed to

potassium vapour, Evans and Thomas (1975) have shown that KBr forms epitaxially

on the surface, and they have interpreted their results in terms of an extraction of

bromine from the intercalation compound. The formation of epitaxial layers of

potassium and caesium on the surface of graphiteK and graphiteCs compounds

when heated (to 2008C) has also been noted by Halpin and Jenkins (1970) and by

Chung et al. (1977), and this surface growth has been related to intercalate

desorption from the bulk.

Scanning electron microscopy (S.E.M.) also provides information on the

microstructures of the intercalation compounds. Such observations on an a-face

(containing both an a-axis and a c-axis) give evidence for microcrack formationresulting from intercalation (Chung 1977). These S.E.M. obervations further show

that microcracks perpendicular to the c-direction are common in the HOPG host

material and in single crystal akes separated by acid treatment from the carbonate

ore-bearing rocks, but such microcracks are almost absent in single crystal akes

that are separated mechanically.

2.3. Kinetics, thermodynamic s and the intercalation process

Important insight into the intercalation process has been provided by studies

made while intercalation is in progress. Such time-dependent studies involve both

kinetic and thermodynamic considerations as described below.

An important set of experiments relevant to intercalation kinetics was carried out

by Hooley et al. (1965) on the graphitebromine system using a cylindrical HOPG

host sample. In the rst of these experiments, it was found that no intercalation

occurred for bromine vapour pressures less than the threshold pressure for

intercalation pt, where pt 170 torr at 208C for Br2 intercalation. As p was increased

above pt, intercalation rst began into the layers near the terminal free surfaces. By

coating the outer cylindrical surfaces of the graphite sample with impervious

material but leaving the end planes uncoated, and then coating the end planes but

leaving the cylindrical surfaces uncoated, Hooley et al. (1965) concluded that

interaction of the intercalant with both the exposed basal plane surfaces and the


Figure 16. Model of a residue compound showing intercalate islands trapped betweendislocation loops.


25/186

exposed cylindrical surfaces is necessary for the initiation of the intercalation

process. From these experiments Hooley et al. inferred that intercalation sets up a

tension between the outer surface region and the inner core region. If these tension

forces are small, the reaction front proceeds to the inner core and the samples

become fully intercalated. If, however, these forces are large, then the intercalation

process stops and the tension breaks up the cylinder into discs. The results of the

Hooley experiments have been interpreted to indicate that a strain energy isintroduced by separating two graphite layers through the insertion of an intercalant.

The connection of a strain mechanism with staging is discussed in the following

section.

As shown in gure 6, intercalation does not begin unless the intercalant vapour

pressure exceeds the threshold pressure pt. It is believed that a threshold pressure is

required to unpin lattice dislocations and to relieve lattice strain since the

intercalation process changes the atomic stacking sequence and requires the motion

of dislocations (Ubbelohde 1968a, Dowell 1977). The intercalation threshold

depends sensitively on intercalate species, on temperature and on the characteristics

of the graphite host materials (Hooley et al. 1965). With regard to sample size, pt is

lower for smaller sample thicknesses, so that thin samples intercalate more readilythan thick samples. With regard to sample perfection, pt is lowest for single crystal

akes, higher for HOPG host materials and yet higher for carbon bres (Hooley and

Bartlett 1967, Hooley 1977a). Measurement of intercalation isotherms for graphite

bres by Hooley and Dietz (1978) with the intercalants Br2 and ICl show that the

high initial value for pt is signicantly reduced (for example, by a factor of2) afteran intercalationdeintercalation cycle, and the intercalation rate is correspondingly

increased. This behaviour is attributed to the formation of microscopic cracks in the

interlocking amorphous carbon networks between the graphitized domains of the

bres, and this explanation is consistent with the observed dependence of pt on the

graphitization temperature of the bres.

Values for pt relative to p0, the pressure at maximum intercalate uptake (loweststage that can be prepared), are shown in gure 6. Values for pt=p0 can be varied overmore than three orders of magnitude with pt=p0 < 10

4 for materials that

intercalate easily (K, Rb, Cs, AlCl3) and pt=p0 > 0:1 for materials that are dicultto intercalate (MoCl5, WCl6, HgCl2). Intercalants with high (pt=p0) values tend tohave lower intercalate uptake (higher stages) at saturation. For example, the lowest

stage that has been prepared with WCl6 having pt=p0 0:5 is stage n 5 (Hooley1977a). Furthermore pt can in some cases be reduced by the presence of other

chemical species, as for example in the case of graphiteFeCl3, the presence of Cl2gas signicantly lowers pt. A decrease in pt is generally achieved by raising the

temperature of the graphite host material.

The initiation of intercalation at the edges of the graphite crystal as discussed

above is also supported by a number of other experiments, including studies of

successive sorptiondesorption cycles in a bromine vapour atmosphere by Marchand

et al. (1973) which were explained by a simple two-dimensional diusion process

along the basal planes of the graphitic crystallites. The importance of the diusion

mechanism was previously recognized by Aronson (1963), who studied the exchange

of normal gaseous Br2 gas with radioactive bromine (Br82 with a half-life of 35.7

hours) that has previously been intercalated into natural graphite powders. The

average concentration of radioactive bromine in the intercalation compound and in

the exchange gas was determined from the decay. From measurement of the



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bromine exchange rate at 303 and 321 K, Aronson concluded that self-diusion of

the bromine in the graphite rather than an exchange between adsorbed and gaseous

bromine at the graphite surface was dominant in the intercalation process. Because

of the distribution of particle sizes and shapes, a fully quantitative study of the

diusion process could not be carried out.

Although intercalation is initiated at the edges of a graphite crystal, kinetic

studies on the graphiteFeCl3 system by Metz and Siemsglu ss (1978) usinggravimetric (weight uptake) techniques have indicated that an approximately

constant macroscopic distribution of intercalant is achieved in the intercalate layers

when the total intercalate uptake is only 20 to 30% of its saturation value. This

absence of a concentration gradient in the intercalated layers has been interpreted in

terms of intercalate nucleation near the edge of the graphite crystal, rapid growth of

the nucleus to an island having approximately the nal structure of the intercalate

layer at saturation intercalate uptake. Thus Metz and Siemsglu ss concluded that

even at these low intercalant uptake values, the intercalate islands are uniformly

distributed on a macroscopic scale between two sequential graphite layers. The slope

of an Arrhenius plot of the intercalation rate constant is interpreted as an activation

energy for nucleus formation at the edges of the graphite crystal (25 kcal/mol for thegraphiteFeCl3 system) and this activation process limits diusion into the bulk

graphite (Metz and Siemsglu ss 1978). The activation energy for nucleus formation is

much larger than the activation energy for diusion which was found to be 23 kcal/

mol by Barker and Croft (1953). Aronson (1963) also noted that more energy was

required to initiate intercalation between two graphite host layers than to sustain

subsequent diusion into the host.

The main conclusions of the MetzSiemsglu ss study on the graphiteFeCl3system are in agreement with kinetic diusion studies by Dowell and Badorrek

(1978) on the intercalation of HNO3, Br2 and PdCl2 into an HOPG host material. In

this study the diusion coecient D was determined by measuring the weight gain

Mt=M1 and its time derivative dMt=M1=dt as a function of time, and using theequation for diusion into a cylinder

Mt

M1

4

1=2Dt

r2

1=2

Dt

r2

1

31=2Dt

r2

3=2; 2:6

where r is the radius of the cylinder, and Mt and M1 are respectively the weight gain

at time t and at very long times. These measurements, taken at constant vapour

pressure on samples with dierent radii and as a function of temperature, show that

the results for both Mt=M1 and dMt=M1=dt can be explained by equation (2.6)in terms of a single diusion coecient which is operative over the entire

composition range. The absence of change in diusion coecient as the sample

passes from one stage to another, as for example from stage 4 to stage 3 on the way

to the saturated stage 2 compound, implies that only minor crystal rearrangements

occur upon stage transformation.

Dowell and Badorrek showed that the magnitudes of D vary widely from one

intercalant to another. For example, for Br2 at 303 K and p 258 torr, D 1:47 106 cm2/min, while for HNO3 at the same temperature and p 81 torr,D 203 106 cm2/min. The value for D obtained by Dowell and Badorrek (1978)for bromine diusion into an HOPG host material of specied cylindrical shape is in

qualitative agreement wi th measurements by Aronson (1963) on intercalated natural



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graphite powders. The high diusion coecient for HNO3 has been attributed by

Dowell and Badorrek (1978) to the liquid-like HNO3 intercalate phase at room

temperature, whereas the Br2 intercalant at room temperature is arranged in an

ordered phase. An activation energy of 5 kcal/mol for Br2 diusion in a HOPG host

material has been measured by Mukaibo and Takahashi (1963), and 12 kcal/molfor Br2 diusion in natural graphite by Aronson (1963).

The kinetics of the intercalation process depends intimately on thermodynamicconsiderations. A detailed determination of the thermodynamic parameters con-

nected with changes in stage has been carried out for the case of the graphitealkali

metal compounds with the intercalants K, Rb and Cs (Salzano and Aronson 1966a,

b, c and Aronson et al. 1968). Relatively little attention has been given to the

corresponding thermodynami c data for the acceptor compounds, though some work

has been done on the graphiteH2SO4 system (Aronson et al. 1971).

Referring to the isobaric growth diagram for the graphiteK system presented in

gure 4, we identify temperature (Tg Ti) regions of phase stability where a singlestage is dominant (for example, for n 1, 2, 3), and other temperature regions wheretwo stages are in equilibrium. The transition from one stage to another occurs in

these equilbrium regions and the thermodynamic studies to be described measure thechanges in free energy, enthalpy and entropy associated with a stage transformation .

Thermodynamic data can be obtained directly from calorimetric measurements,

calculations based on the temperatur e dependence of equilibrium pressures, and

electrochemical measurements. For the conditions used to prepare most intercala-

tion compounds, energy is absorbed upon the stage transition n ! n 1 and theentropy decreases.

While calorimetric measurements provide the most direct determination of

thermodynamic data, this method is dicult to apply in practice to the study of

stage transitions in intercalated graphite. This diculty arises from the sensitivity of

the intercalation process to crystalline faults as demonstrated by the kinetic studies

of Hooley and Dietz (1978) and Dowell (1977) on graphite host materials of varyingperfection. On the other hand, direct calorimetric methods can be readily applied to

the study of structural phase transitions or orderdisorder transformations in

graphite intercalation compounds as discussed in section 3.4.

Using a solid state electrochemical cell method on the graphiteK system

(Aronson et al. 1968), and a Knudsen eusion method in conjunction with a

radioactive tracer technique for the graphiteRb and graphiteCs systems (Salzano

and Aronson 1966a, b, c), values for the enthalpy and entropy changes associated

with stage transformations were obtained and the results are given in table 3.

In the electrochemical method, Aronson et al. employed the e.m.f. cell shown in

gure 17, which consisted of a liquid potassium Km

anode and an intercalated

graphite cathode. These electrodes were coupled by a solid potassiumglass

electrolyte which provided transport of K ions from anode to cathode. The

electrochemical reaction at the anode was therefore

Km e K

; 2:7

while at the cathode a stage change n ! n 1 occurred. For example, for thetransformation from stage 3 to stage 2, the reaction at the cathode was

2C36L K e 3C24K 2:8

to yield an overall reaction



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2C36K Km 3C24K: 2:9

The free energy of the reaction Gcell is then obtained from the measured e.m.f. of

the cell, E,

Gcell mFE; 2:10

where F is Faradays constant (F 96 450 C) and m is the number of equivalents ofpotassium that are transferred. Using the thermodynami c relation between the

Gibbs free energy, enthalpy and entropy,

Gcell Hcell TScell; 2:11


Table 3. Thermodynamic properties of graphitepotassium, graphiterubidium and graphitecaesium compounds.a

Graphite Graphite Graphite

potassium rubidium caesium

H8 S8 H8 S8 H8 S8

(cal/mol (cal/mol (cal/mol (cal/mol (cal/mol (cal/mol

Equilibrium reaction K) K 8K) Rb) Rb 8K) Cs) Cs 8K)

1a 1=3C24Ms Ms C8 Ms 27 400 25.7

1b 4C10Ms Mg 5C8Ms 38 000 44 33 900 42.2 43 800 43.42 5=7C24Ms Mg 12=7C10Ms 24 000 24 25 300 19.2 29 600 19.6

3 2C36Ms Mg 3C24 Ms 27 800 20.6 27 200 17.0 32 700 18.74 3C48Ms Mg 4C36 Ms 30 000 20.7 29 500 17.6 34 200 18.6

5 4C60Ms Mg 5C48 Ms 30 600 20.8 31 100 18.3 34 900 18.66 5C72Ms Mg 6C60 Ms 31 800 18.1 35 800 18.8

7 6C84Ms Mg 7C72 Ms8 7C96Ms Mg 8C84 Ms 31 700 20.9

a

From Aronson et al. (1968).

Figure 17. Apparatus used by Aronson et al. (1968) for e.m.f. measurements on cells of thetype K (molten) jKglass j graphiteK. Based on the temperature dependence ofthese e.m.f. measurements the enthalpy and entropy changes associated with a stagetransformation were determined.


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the entropy term can be found from the temperature dependence ofGcell according

to the thermodynamic relation Scell @Gcell=@T. Using this value ofScell,the enthalpy change is determined by

Hcell

Gcell T

Scell: 2:12

The resulting values for Hcell and Scell are given in table 3 for several stage

transitions that were studied in the graphiteK system.

Using an eusion method, Salzano and Aronson (1966a, b, c) measured the

equilibrium pressure versus temperature for a number of stage transitions in the

graphiteRb and graphiteCs systems and obtained the results shown in gure 18.

From the slopes of these curves, the enthalpy change was calculated and from the

1=T ! 0 intercept, the entropy change was determined. He rold (1979) has pointedout that the stoichiometry of the system changes during the stage transformation so

that improved values for H8 and S8 are obtained by integration over thelimits of the concentration range associated with the change in stage. He rold further

noted that these corrections are especially important for the low stage transitions and

may explain the anomalous H8 and S8 values in table 3 for the low stage

transitions. However, the thermodynamic results obtained by Salzano and Aronson

are believed to be qualitatively correct. The data in table 3 show S8 to beapproximately independent of stage and H8 to decrease in magnitude withdecreasing stage, at least for n 2. It should be noted that at high temperaturesSalzano and Aronson observed an additional rst stage phase C10M which is less

dense than C8M and is purple in colour. This observation illustrated that dierent in-

plane structures and in-plane intercalate densities are possible for a compound of a

given stage.


Figure 18. Phase diagram by Aronson and Salzano (1966a) for several stage transformationsin the graphitecaesium system.


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Listed in table 4 are the overall enthalpies and entropies of formation for each ofthe compounds listed in table 3, based on the starting reactants pristine graphite and

the intercalant in the gas phase. These entropies and enthalpies of formation are

found by summing the corresponding entropies and enthalpies for each previous

stage transition, taking proper account of the number of carbon and metal atoms for

each transition (Aronson et al. 1968, He rold 1979). It is of interest to note that these

values for the enthalpies of formation are in good agreement with the corresponding

values calculated from (1) the measured heats of vaporization for the alkali metals,

and (2) the measured enthalpies of formation of C8K, C8Rb and C8Cs from the

liquid metal using calorimetric techniques (Saehr 1964).

Salzano and Aronson (1966a) also related these thermodynamic studies to a

determination of the energy required to separate two graphite layers to innityagainst the van der Waals interplanar binding forces which hold the graphite layers

together. This separation energy provides an estimate for the potential barrier that

must be overcome when an intercalate layer is introduced into the graphite host. The

value obtained by Salzano and Aronson (1966a) for this separation energy is

1.23 kcal/g mol carbon, which is in rough agreement with a number of other

estimates of this quantity obtained on the basis of totally dierent techniques.

By considering the stage dependence of the thermodynamic data in terms of an

electrostatic model for the binding of the intercalate metal layer to the adjacent

graphite layers, Salzano and Aronson (1966a) concluded that the amount of charge

transfer to the graphite layers is one electron per intercalant atom (f 1) for the K,Rb and Cs systems, at least for stage n 2, and close to f 1 for the rst stagecompounds. This conclusion is consistent with other experiments relevant to the

electronic structure (see section 4). On the other hand, analysis of the thermodyamic

data for the acceptor intercalant H2SO4 (Aronson et al. 1971) indicated that f 0:3,also in agreement with studies of the electronic properties of acceptors (Hennig 1959,

Ubbelohde and Lewis 1960).

Thermodynamic arguments have also been applied to other aspects of the

intercalation process. For example, Dzurus et al. (1960) used thermodynamic

arguments in an attempt to explain why Na does not intercalate into graphite to

form low stage compounds by plotting the free energy of formation for the K, Rb


Table 4. Heats and entropies of f ormation of graphitepotassium, graphiterubidium andgraphitecaesium compounds.a

Graphite Graphite Graphitepotassium rubidium caesium

Hf8 Sf8 Hf8 Sf8 Hf8 S8(cal/mol (cal/mol (cal/mol (cal/mol (cal/mol (cal/mol

Reaction K) K 8K) Rb) Rb 8K) Cs) Cs 8K)

8Cs Mg ! C8 Ms 28 500 24.0 28800 23.3 34200 24.010Cs Mg ! C10Ms 27 000 22.5 27 500 18.5 32 200 19.224Cs Mg ! C24Ms 30 400 20.7 30 600 17.6 34 800 18.736Cs Mg ! C36Ms 31 700 20.7 32 200 18.0 35 800 18.748Cs Mg ! C48Ms 32 300 20.7 33 100 18.1 36 400 18.860Cs Mg ! C60Ms 32 800 20.8 33 600 18.1 36 700 18.8

a FromAronson et al. (1968).


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and Cs intercalation compounds against the ionization potential of these metals and

extrapolating to a positive free energy of formation for a rst stage Na compound. It

would be of interest to explore whether there are thermodynamic constraints which

inhibit the formation of certain low stage compounds, as, for example, rst stage

graphiteBr2. For the graphiteBr2 system, the lowest stage compound that has been

prepared is stage 2.

2.4. The staging phenomenon

A remarkable feature of graphite intercalation compounds is the occurrence of

staging, in contrast to other intercalation compounds such as the transition metal

dichalcogenides, which generally do not exhibit accurate staging. The existence of

staging has been well documented by X-ray (00l) diractograms as discussed in

section 2.2, showing that the resulting superlattice unit cell extends over many

atomic layers; for example, in stage 8 alkali metal compounds, Ic 30 A . Further-

more, the measured linewidth of the diractograms indicates that a single stage

sample can be established over macroscopic dimensions (1000 A ). It should further

be noted that although stacking faults in intercalated graphite are separated by as

little as 30 A (see section 3.4), the staging phenomenon is long range.Staging is very general, occurring in both donor and acceptor compounds,

regardless of whether the intercalate in-plane structure is commensurate or

incommensurate with the graphite host lattice. The existence and extent of staging

does not seem sensitive to the amount of charge transfer between the graphite and

intercalate layers, which is large for the alkali metal donor compounds and

considerably smaller for the molecular acceptor compounds (Ubbelohde 1976).

Furthermore, models for the c-axis charge distribution (Pietronero et al. 1978), the

electrical conductivity (Bok 1978), and the interpretation of Raman and infrared

spectra (Nemanich et al. 1977c, Dresselhaus et al. 1977b, Underhill et al. 1979),

provide strong evidence that a single graphite bounding layer eectively screens the

intercalate from the graphite interior layers, thereby inhibiting long-range electro-static interactions in the intercalation compounds. These observations on the

eective screening of the intercalate layer by the adjacent graphite bounding layers

suggest that the staging phenomenon is related to a long-range lattice strain

interaction rather than an electrostatic eect.

The expansion of the lattice constant in graphiteK compounds as a function of

reciprocal stage (1=n) (Nixon and Parry 1969) and the (1=n) dependence of the latticemode upshift in acceptors and downshifts in donors also strongly suggest a strain

mechanism for staging (Underhill et al. 1979). Raman experiments, indicate that

donor intercalants cause the in-plane graphite layers to expand as electrons are

added, and acceptors to contract as electrons are removed. The Raman experiments

also imply that the electron transfer causes a stage-dependent stiening (acceptors)

or softening (donors) of the carboncarbon bonds in the bounding layer plane.

Whether the intercalate bonding is ionic, covalent or metallic, there is strong

evidence that the long-range forces associated with staging are directly related to

the strain energy of the solids.

The kinetic studies of the intercalation process described in section 2.3 are also

consistent with a strain model for intercalation. These kinetic studies show that once

the exposed surface graphite planes have interacted with the intercalate species, the

intercalant is introduced into the bulk host material as layers close to the exposed

end surfaces, and sequentially into layers increasingly distant from the exposed end



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surface. Initiation of intercalation at these extremal layers minimizes the elastic

energy that must be supplied to produce the dilatation of the graphite host

accompanying the intercalation process. The strain model can also account for the

observation that thin samples intercalate more readily than thick samples because of

the lower elastic impedance of thin samples.

Molecular alignment studies in liquid crystals by de Gennes (1974) show that the

elastic force eld yields a decrease in strain energy by clustering and aligningneighbouring rod-like molecules. Application of this concept to graphite intercala-

tion compounds implies that the strain energy is minimized by the clustering of

intercalate atoms to form platelets between a single set of graphite planes, in which

intraplanar forces are extremely strong. Thus if a random distribution of intercalate

atoms were to be introduced into graphite initially, the strain energy of the system

would be lowered by the clustering of all intercalate atoms between a single set of

graphite planes. However, each intercalate l

inter ca laci on

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