1

CHAPTER - 1

INTRODUCTION

A brief introduction to the actual work done in the present study is provided in

this chapter along with a brief non-comprehensive review of various studies made on

TGS crystals by various researchers in the near past is provided in this chapter.

1.1. Importance of crystals

Man had admired crystals for long, as he had appreciated beauty. The gems

and crystals delivered by mother earth have always attracted our mankind and the

belief in the virtues of gems and some minerals dates back to atleast two thousand

years. During the 15th century man has learned the tactics of cutting, cleaving and

polishing of gemstones to raise the appearance of gems and enhanced the sparkle in

gems, that is the inherent optical effects which arise from their high refractive index

and dispersion. The significance of that beauty for a technological society and for the

development of scientific knowledge has only begun to be realized, however.

Crystallization of salt is mentioned on a Chinese print of 2700 B.C and was

also described on the Egyptian “Papyrus Ebers” of about 1500 B.C and by Aristoteles

(384 - 322 B.C). There were also reports during 300 B.C about the crystallization

processes and the preparation of sugarcane syrup in India mentioned by Ray (1956).

Behind every new solid state device there stands a single crystals and the explosion in

solid state device development, which followed the invention of the transistor in 1948.

This meant that many new crystals had to be grown and fabricated in order to assess

their device properties. The ever increasing application of semiconductor based

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electronics creates an enormous demand for high quality semiconducting,

ferroelectric, piezoelectric oxide single crystals.

There is a vast market for solid state devices using crystalline materials

(crystals) are used in several appliances which are highly useful to mankind. The

requirements of the electronics industry has stimulated many advances in crystal

growth both extending existing techniques and bringing forward new ones. The

modern technology is very much dependent upon crystalline materials such as

semiconductors, polarizers, transducers, radiation detectors, ultrasonic amplifiers,

ferrites, magnetic garnets, solid state lasers and non-linear optic, piezoelectric,

acoustooptic and photo sensitive materials and crystalline films for microelectronics

and computer industries. All these involve research in the preparation (formation) and

characterization of crystalline materials [1-15].

1.2. Triglycine Sulphate (TGS) crystal

This chapter gives a review of various studies made on Triglycine sulphate

(TGS) single crystals in the near past. Some old reports are also added for

comprehensiveness in the properties of TGS crystals.

1.2.1. General and Structural properties

Triglycine Sulphate (NH2CH2COOH)3H2SO4, and its isomorph crystals have

been subjected to numerous studies in the past about 50 years because of its excellent

ferroelectric and pyroelectric properties. The ferroelectric property of TGS was

discovered by Mathias et al [16] in 1956. It has immediately attracted the attention of

many researchers, because it exhibits ferroelectric properties at room temperature,

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and it can be grown easily as large samples. The b-cut / (0 1 0) crystals, this is also a

cleavage plane, are used for infrared detector fabrication [17], earth exploration,

radiation monitoring and astronomical telescopes [18], it is reported to be the best

choice among available materials as a sensitive elements in pyroelectric sensors due

to high pyroelectric coefficients reasonably low dielectric constant and best figure

of merit [19]. TGS crystal shows a ferroelectric phase transition at the curie

point temperature (Tc = 49°C) [20-21]. Its crystallizes in monoclinic structure with

non-centrosymmetric space group P21 below curie temperature (Tc = 49°C) and

changes to centrosymmetrical P21/m group [22-23], in the paraelectric phase above

curie temperature. TGS shown second-order ferroelectric phase transition at the

curie point and it is an order-disorder ferroelectric. The crystal structure of TGS

crystal is shown in the figure 1.

Figure 1: Crystal Structure of Triglycine sulphate

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The crystals structure was determined from full three-dimensional X-Ray

diffraction data using CuKα radiation by [24]. Out of the three glycine molecules in

the crystal, one has the usual zwitter - ion configuration, with the - NH3+ group out of

the plane of the other atoms; the remaining two glycines are mono-protonated, and

planar within experimental error, and are designated as glycinium ions. Thus the

chemical formula is properly written as (NH3+ CH2COO-).(NH3

+ CH2 COOH)2. SO4-,

and the compound is best described by the chemical name glycine-di-glycinium

sulfate. One of the planar glycinium ions lies near but not in the plane y = ¼ which

becomes the mirror plane in the high - temperature phase.

The nitrogen atoms form N-H..O hydrogen bonds of the usual strength,

whereas a quite strong O-H..O hydrogen bond of a distance of 2.43 Å is found

between the oxygen atom of the carboxyl group of the zwitter-ion glycine and that of

the planar glycinium ion which lies near the plane y = 1/2. Above the curie point, at

49°C, mirror symmetry is attained by statistical arrangement of atoms around the

mirror planes at y = 1/4 and ¾. The disorder of the glycinium ions near the mirror

planes, and the above mentioned strong O-H..O bond are of particular importance for

the ferroelectric behaviour of the crystal.

It consist of SO42-(G1), 2(N+H3CH2COOH) (G2), and N+H3CH2COO-(G3)

species held together by hydrogen bonds [25] (hydrogen atoms are not shown), these

bonds are easily broken by the polar molecules of water that explain the

hygroscopicity of TGS – its crystals are easily etched by water. Along the b-axis, the

G1-SO4 and G2-G3 layers are stacked alternately. The nearest two neighbouring

layers with identical chemical composition are rotated 180° around the b-axis against

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each other [23, 26]. Below the curie point TGS exhibits antiparallel 180° domains,

which are generally rod like parallel to the b-axis [27, 28].

1.2.2. Effect of dopants on TGS crystals

Inspite of the promising features of TGS crystal, such as room temperature

detector operation and without any external bias, it has major disadvantage that it

depolarizes by thermal, mechanical and electric means. The depolarization negatively

influences the pyroelectric figure of merit [29]. In order to overcome this difficulty,

several studies attempted with different dopant to achieve effective internal bias and

desired ferroelectric properties of TGS crystals. Investigation on the doping of TGS

crystals by amino acid [30-33] reveal that this drawback may overcome. Many

researchers have investigated the properties of pure TGS single crystals and influence

of amino acids, simple organic and inorganic compounds, metallic impurities, etc., on

the physical and other properties of TGS single crystals. [34-246, 322-336, 338-340]

Alanine dopant molecule is similar to the glycine molecule, except the

CH3 asymmetric group. Doped TGS crystals show a modified habitues versus the

pure ones, mirror external shapes of L-alanine versus D-alanine doped crystals

[80,81] suggest an equivalent substitution of the glycine G1 molecule in the crystal

lattice [82], however, the hysteresis loops and some other characteristic data of TGS

crystals, doped with racemic mixture (L+D- alanine) [83], suggest a non-equivalent

substitution. L-alanine molecule substituted for glycine1 in the crystal lattice cannot

be inverted upon switching because of its chiral structure. Thus L-alanine doped TGS

crystal causes an internal bias field Eb and results in a macroscopic irreversible

polarization in one predetermined sense of the ferroelectric b-axis [84].

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The effect of amino acids doping in TGS has then studied extensively

[30,40,53,55,60,61,85]. In case of lysine doped TGS dielectric permittivity and

spontaneous polarization decrease with improved pyroelectric properties. In addition

lysine doped TGS crystals possess higher mechanical hardness when compared to

pure TGS [53]. It has also reported that L-threonine, DL-threonine, and L-methioine

doped TGS results decrease of dielectric constant when compared to pure TGS. The

L-threonine doped TGS has the maximum pyroelectric coefficient and hence material

is suitable for detector applications [55]. Raghavan et al [60] found that the presence

of optically active dopants of leucine and isoleucine increases the coercive field and

decreases the spontaneous depolarization of TGS crystals, will be useful for

pyroelectric applications. Jayalakshmi and Kumar [61] have recently observed an

increase of coercive field and spontaneous polarization for L-tryptophan added TGS

when compared to that for pure TGS crystal. Meera et al [34] observed an increase of

coercive field for L-cystine added TGS crystal. L-asparagine addition [51] increases

whereas L-tyrosine addition [46] decreases the microhardness of TGS single crystal.

The study of the temperature dependence of the absorption edge of a certain

substance provides a deep understanding of the interaction processes between the

optical excitation (ie, electronic excitations) and the phonon spectra. In particular the

measurements of the absorption edge of ferroelectrics in the vicinity of the phase

transition temperature gives an opportunity to obtain information on the character of

the phase transition and the nature of the chemical bonds between lattice units [86].

The admixture (addition) of transition metal ions essentially modifies the physical

properties of TGS, causing bands in the ultraviolet spectral range. These bands make

these crystals a candidate for short wavelength holographic recording.

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E1-Fadl [43,44] has carried out optical absorption measurements near the

absorption edge in the energy range 3 to 5.5 eV on pure TGS and TGS crystals doped

with Cu2+, Mn2+ or Ni2+ in the temperature range 290 to 360°K. Temperature

dependence of the band gap Eg (T) reveals an anomaly at the phase transition

temperature for both pure and doped TGS crystals. The indirect allowed transition is

the most probable type of transition near the fundamental edge of pure TGS and TGS

crystals doped with divalent ions, and the phonon energy associated with the allowed

indirect transition changes from one dopant to another. Doping of TGS crystals with

small concentration of divalent ions considerably affects the measured optical

parameters. The effect however, varies from one dopant to another according to their

physicochemical activity. I.V. Vavresyuk et al [87] have studied changes in properties

of TGS doped with metal ions

Mihaylova and Byrne [48] have studied TGS crystals doped with Nd by

Raman spectroscopy. Doping of TGS with Nd does not cause a change in the crystal

structure and the symmetries of groups in the unit cell remain unaffected. A new

mode at 1535cm-1 appeared for all crystals doped with Nd, which indicates that Nd is

co-ordinated with the glycine molecule. Doping with Nd affects significantly the

C-CO bending mode at 584 cm-1, The O-C-O bending mode at 665 cm-1, the SO42-

mode at 1086 cm-1 and the CH2 wagging and twisting mode at 1310 cm-1. The Raman

bands at 1374 and 1413 cm-1 indicate the presence of a zwitterion, which supports

Hoshino’s theory for spontaneous polarization reversal in TGS.

CO2+ doped TGS crystals studied by Prokopova et al [88] have been shown to

possess a very good structural quality with stabilized domain structure at room

temperature. The results obtained by Novotny et al [50] on the preparation of TGS

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single crystals doped by palladium (especially on the growth of non-polar (001)

pyramid) could be important for the growth of high-volume crystals for application

purposes. Novotny et al [56] have reported the procedure of single crystal growth of

full faceted TGS and deuterated TGS (DTGS) with varying content of Pt (II) and

Pt (IV) ions and with L-alanine impurities in the growth solution. They measured in

great detail the main physical properties of prepared LATGS / Pt (IV) and

LADTGS / Pt (II) singles crystals such as spontaneous polarization PS, values of

coercive fields EC, Internal electric field Eb, dielectric permittivity εr and dielectric

losses tan δ. Their results suggest that the prepared modified TGS single crystals are

suitable material for preparation of high sensitivity broad area infrared detectors.

Muralidharan et al [38, 52] have reported the rare earth ions dopants La, Ce

and Nd on the growth aspects and ferroelectric properties of TGS single crystals.

The dopants significantly modify the morphology of the crystals. Dielectric

measurement revealed that the dielectric constant of Ce doped TGS increases rapidly

at the transition temperature. Well – structured rectangular hysteresis loop have been

observed for all the doped crystals. Lenticular ferroelectric domain patterns were

observed on the (010) plane of the grown crystals. La doped TGS crystals possess a

high Tc and coercive field values. Among the three dopants, Nd doped TGS possesses

low dielectric constant and high pyroelectric coefficient suggesting that this can be a

potential material for infrared detectors. Sun et al [45] have grown guanidine doped

TGS crystal and found that this crystal has better pyroelectric properties than those of

pure TGS crystal.

Chang et al [42] have reported the growth and properties of TGS crystals

doped with urea. It was found that the normalized growth yield and pyroelectric and

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dielectric constants could be increased significantly by urea addition. TGS crystals

doped with 5 and 10 wt% urea exhibited upto five times higher material figures of

merit for infrared pyroelectric detectors compared with undoped TGS crystals. The

Vicker’s hardness of doped crystals increased with urea content to about three times

the undoped value in the (010) direction at 10% urea. No significant increase in the

hardness was found, however, in the (001) direction.

Based on Devonshire [89] thermodynamic theory the relationship λ/X = Ps/C

can be obtained and can be regarded as a method to increase the pyroelectric material

figure of merit of ferroelectric crystals. Shi et al [47] have grown modified TGS

crystals doped with urea or co-doped with urea and other dopants and investigated the

pyroelectric properties. The pyroelectric figures of merit M(λ/εr) of the doped TGS

crystals are obviously higher than those of pure TGS.

Meera et al [53] have grown and characterized thiourea doped TGS crystals.

Dielectric studies show a shift in the curie temperature because of doping pyroelectric

coefficient was found to be increased due to thiourea addition on TGS. Very recently,

Krishna kumar et al [62] have grown thiourea doped TGS single crystals and

subjected them to X-Ray diffraction, FTIR and Raman spectroscopic and optical

absorption measurements. The powder technique of Kurtz and Perry confirmed the

non-linear optical (NLO) property of the grown crystals. The shift and the appearance

of additional wave number in the IR and Raman spectra of doped crystal establish the

co-ordination of thiourea with TGS in the lattice. The results of the response of the

mechanical behaviour of thiourea doped TGS crystal will have significant effect on

machining the crystal for device purpose. From the results of optical transmission it

was found that the doped crystal exhibit excellent transmission in entire UV-Visible

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range studied than undoped crystal. This property enables the material for the

fabrication of IR detectors and in some optoelectronics device technology. Also

sufficient shift in the lower cut off wavelength towards UV region in doped crystal

makes this material to find application in generation of UV light. Higher second

harmonic generation (SHG) efficiencies were observed for pure (52.5 mV) and

thiourea doped (60.2 mV) TGS crystals when compared to that for potassium

dihydrogen orthophosphate, KDP (15 mV).

Many metallic dopants modified the properties to some extent, but in general

they have been found ineffective for achieving the desired changes in the

properties [90]. Metallic ions like Fe3+ and Cr3+ ions changed the crystal growth

characteristics, but decreased the pyroelectric coefficient and spontaneous

polarization [91]. Li+ and Mn2+ modified the growth habit and produced a high

pyroelectric coefficient, but the high dielectric constant value did not affect the

pyroelectric material figure of merit (P/εr), where P = pyroelectric coefficient,

εr = dielectric constant [92]. Ni2+ ions at a concentration level of 10% by weight in

the solution increased the pyroelectric figure of merit but did not enhance the crystal

growth rate [90], whereas Mg2+ and Cu2+ modified the growth habit but marginally

increased the pyroelectric material figure of merit [91].

Phosphoric acid (H3PO4) as a dopant has been found to yield crystals with a

large ‘ac’ plane area useful for IR detector applications [49,54,93,94]. Even at a very

low concentration level of H3PO4, a significant improvement in the pyroelectric

coefficient, lowering of the dielectric permittivity at transition temperature (Tc) and

increase in the coercive field of the TGS crystal have been reported [95-97]. The

replacement of sulphur by phosphorous in the lattice introduces asymmetry in the

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P-E hysteresis loop [96]. It has also been found that the concentration of H3PO4 is

critical for achieving the desired material properties for commercial IR detector

applications [98].

Very recently, Balu et al [63] have grown ammonium dihydrogen

orthophosphate (ADP) doped TGS single crystals and characterized. X-Ray analysis

showed a slight variation in the lattice parameters for doped crystals. The optical

transmission spectra indicated a wide optical transparency in the entire visible and

near IR region. Dielectric measurements revealed a shift of curie point to lower

temperature with increase in dopant concentration. Microhardness studies have

shown that the hardness decrease with increase in ADP concentration. Nakatani [99]

has studied temperature independent internal bias field in L-α-alanine doped TGS

crystal. Serna et al [100] has studied the positron life time spectra of pure and

L-alanine doped TGS.

Arunmozhi et al [101] have studied the effect, of anti- ferroelectric ADP with

TGS crystals. They shown that inhomogeneous incorporation of dopants gives rise to

a distribution in coercive fields in the different growth sectors. The incorporated

dopant hinders polarization switching, which results in the increase in coercive field.

N.Theresita Shanthi et al [70] have grown sodium bromide doped TGS single crystal

from aqueous solution. They shown that the dopant addition increases the hardness

number for low concentration and decreases the hardness when the concentration is

high, they also show that there is no change in the curie point temperature due to

dopant addition.

Copper sulphate doped TGS crystal were grown by K.Balasubramanian and

P.Selvarajan [72]. They observed that the dielectric parameters increased when the

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TGS crystal are doped with copper sulphate. They also noticed that there was an

increase of microhardness number when TGS crystal was doped with copper sulphate.

Farhana Khanum and Jiban Podder [77] have studied the effect of nickel sulphate

doped TGS crystals. They found that the lattice parameters are slightly distorted

due to the incorporation of nickel ion into the lattice size of the TGS crystal,

M.Costache et al [102] have grown alanine doped TGS crystal in the pyroelectric

phase. They found that remarkable asymmetry of measure pyroelectric coefficient of

d and L-alanine doped TGS crystal. Suggest a non-equivalent substitution of glycine

in host lattice. Farhana Khanum and Jiban Podder [76] have doped potassium bromide

with TGS and they shown that the doped crystal have wide optical transparency in the

entire visible region. K.Balasubramanian et al [71] have grown TGS crystal in

CdS nano particle dispersed water by solution method and studied the effect of

Cd2+ ions. Alexandru and Ann NY [75] has studied the effect of D-alanine in TGS

crystal in the dielectric parameters. They found that the relaxation time is not a real

constant on such large time interval. In a semi long scale, permittivity shows three

stages, probably related to several mechanism of relaxation. A.J.Jeya Prakash

Manoharan et al [78], have studied the effect of amino acid in TGS crystal. They

found that the D.C conductivity increases with temperature as well as dopant

concentration.

Farhana Khanum and Jiban Podder [79] have studied that effect of LiSO4 on

TGS crystal. They found that the D.C conductivity increases with temperature as well

as dopant concentration and also shown that the curie point temperature remain same

for pure and doped crystal. But the dielectric constant and loss factor increases with

doped concentration grown by n bromo succinide added TGS crystal were grown by

Rai and his co workers [74]. They found that, the dielectric constant of NBSTGS

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crystal decrease with the increase in NBS concentration and considerable shift in the

phase transition temperature (Tc) towards the higher temperature.

1.2.3. Various methods of growth of TGS crystal

TGS crystals were grown by several authors by different crystal growth

methods in several environment.

In low gravity

TGS crystals were grown from aqueous solutions in Spacelab 3 in 1985 [103].

A. Natarajah et al [104] have modelled this experiment in two dimensions employing

the finite volume code PHOENICS. Thermal and solutal convection were included in

this model and the crystal growth rate was chosen as the sensitivity parameter for the

response to convective transport. Simulations were carried out for steady, impulsive

and periodic acceleration in order to determine tolerable acceleration levels.

The apparatus consist of a temperature controlled cylindrical growth cell with

a cylindrical pedestal (string) inside. The string was insulated on all sides except the

top, on which the seed crystal was mounted. The temperatures of the container walls

and string were independently controlled. The seed was enclosed by a cap during a

long storage period prior to launch and until the experiment was started on the shuttle.

The cap separated the seed from the nutrient solution with which the container was

filled, preventing premature growth or etching. At the start of the experiment the

nutrient solution was brought to its saturation temperature, following which the cap

was withdrawn. The string and wall temperatures were then increased briefly in order

to dissolve any spurious crystallites, after which the system was cooled at a relatively

rapid rate to induce growth. Hence it was necessary to reduce the temperatures

14

of the walls and the string to avoid damage to the crystal. For the experiments on

Spacelab 3, the cooling was continued for 10hr after which the temperature was held

steady for atleast another 15hr.

In silica gel

Bhatt and Patel [105] have grown TGS single crystals upto 11x7x3 mm3 in

size in silica gel at room temperature by the reaction method. It has been found that

when PH of the gel is 2.5 or higher, only TGS crystals are formed and for PH between

2.0 and 2.4 both TGS and DGS (diglycine sulphate) crystals are formed. When PH of

the gel is less than 2.0, only DGS crystals are produced in the gel.

The growth of the crystals in microgravity holds the promise of producing

large defect free crystals. This is because the absence of gravity eliminates gravity –

driven convection flows at the crystal- liquid interface. Yoo et al [103] studied the

growth of TGS in microgravity. TGS is a non-linear optical material used in infrared

detectors. Yoo et al [103] were unable to match their observed concentration profiles

around the growing crystal to those predicted by a computer model.

The rate of crystal growth is determined by the slowest step in the growth

process. This can either be the diffusion of the materials to the surface or the kinetics

of the surface processes. Both factors need to be understood to successfully predict

growth rates. Both the diffusion and the surface integration kinetics are processes

driven by differences in chemical potential, not supersaturation. The initial

appearance of nuclei is also driven by differences on chemical potential. For diffusion

limited processes the diffusion coefficient is the important parameter. In zero gravity,

it is the exclusive method of transport to the surface. In normal gravity there is a

15

boundary layer near the surface where diffusion is the dominant mode of transport.

Yoo et al [103] used the under saturated values of the diffusion coefficient in their

analysis. Myerson and co-worker [106,107] have shown that for supersaturated

solutions of salts. The diffusion coefficient and the viscosity are strong functions of

concentration in the supersaturated regime. This behavior cannot be predicted by

extrapolating from data in the under saturated region. Myerson and Lo [108] have

also shown that the diffusion, being a function of crystal face, impurity concentration

and other factors. In addition, Yoo et al [103] used concentration in their calculations

as the supersaturated driving force. While concentration supersaturation is normally

used, the true driving force for crystal growth is a difference in the chemical potential.

The variation of the diffusion coefficient with time is believed to be due to the

formation of subcritical solute clusters. Their average size grows larger as the

solution remains in the metastable state. Evidence for such clusters was reported by

Mullin and Leci [109] for citric acid. This effect was not found in under saturated

solution. Larson and Garside [110] explained this phenomenon by assuming the solute

in excess of saturation was joined in clusters. Myerson and Lo [108] repeated this

experiment for glycine. They assumed a distribution of cluster sizes. The number

average cluster size was between 2 and 100 solute molecule. Most of the clusters

were low molecular weight dimers or trimers. Other evidence for the existence of

clusters comes from spectroscopy.

In preparation for the space experiments, Kroes and Reiss [111] measured the

following properties of aqueous solutions of TGS: Viscosity and density as a function

of temperature and concentration, solubility and diffusion coefficient as a function of

temperature. All the data were for unsaturated solutions. The Spherical Void

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Electrodynamic Levitor Trap (SVELT) can be used to measure the activity of highly

supersaturated solutions. The SVELT suspends micron-sized droplets by means of

electrodynamic levitation. In this containerless environment very high supersaturation

can be achieved and the activity of the solvent can be measured. This provide a

technique to measure properties of solutions beyond those that could ordinarily be

measured in bulk solutions. Bohenek et al [112] used this technique to obtain

thermodynamic data for TGS through a large part of the metastable zone. They have

found that the metastable zone is very wide. The spinodal concentration is many

times the saturated concentration. It has also been found that critical cluster size for

nucleation is a small number over most of the metastable region.

Sun at al [113] studied the growth kinetics of (110) and (010) faces of

TGS crystals at high supersaturations under the curie temperature. The analysis of

experimental data proved that the growth of TGS crystals was mainly controlled by

the surface diffusion spiral dislocation mechanism (BCF surface diffusion model)

Earlier, Alexandru and Berbecaru [114] measured the growth rates of the (110) face

of TGS crystal with the supersaturation under 3% and Reiss et al [115] obtained the

growth rates with the supersaturations lower than 4.5%. Later Sun et al [116]

investigated the growth kinetics and mechanism on the (001) and (100) faces of TGS

crystals. They found the growth on the (001) and (100) faces at high supersaturation

was mainly controlled by a BCF surface diffusion mechanism.

Different techniques have been used to grow large crystals of TGS. Their

growth morphology depends on several parameter: PH value of solution, impurities or

additives, degree of supersaturation, and also process parameters such a seed

orientation and manner of attachment of seed [117]. The growth rates of the different

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habit faces of TGS are widely different. Generally, the growth rate of the (010) face

is much higher than that of (001). Therefore the probability of appearance of the

(010) face on the grown crystal is rather small.

G.Sivanesan et al [156] studied that the single crystals of triglycine sulphate

with increased size with respect to the earlier report (Bhatt et al) and improved

morphology were grown in silica gel at room temperature

Slow evaporation technique

To grow crystals of TGS from aqueous solution, researchers have used

different types of crystallizers. The techniques used include the reciprocating-motion

technique [118] and the rotating–disc techniques [119]. The method of seed

attachment is also an important parameter in solution growth. Various types of seed

holders have been used for TGS. Often, the seed is secured to the Lucite holder with a

plastic thumb screw or the top of seed plate is inserted into a bevel in the holder [120].

The seed has a tendency to grow laterally at the point of contact between it and the

holder. This causes considerable difficulty during removal of the grown crystal from

the holder. Another way of holding the seed plate is that of inserting four screws to

support the seed plate. But the disadvantage in this method is that, if there is a slight

dissolution of the seed, it detaches from the grip of the screws.

Seeded Growth

To eliminate the above problems, the cylindrical seed method (CSM) was

developed [121]. In the rotating disc technique for growing TGS crystal, the seed was

recessed in a bed of silicone rubber in a circular Perspex holder. The Perspex holder

was dipped into the solution, and the crystal grow downwards. Satapathy et al [122]

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have reported the growth of TGS crystal by the platform technique, using seeds which

need not be long along the c-axis. Crystals were grown from aqueous solution by

placing seeds of size 5x5x5mm3 in different orientations on the platform. The

variation of growth rate of various habit facets with different orientations of seeds was

determined. An optimum seed orientation was found such that the platform technique

permits one to grow TGS crystals, from which large-area (010) plates can be cut. The

crystals grown by this method exhibit high growth rates and good optical quality.

They demonstrated the use of such a crystal in a laser energy meter.

Solution Growth

Metastable zone width is an essential parameter for the growth of large size

single crystals from solution since it is the direct measure of the stability of the

solution in its supersaturated region [123]. Conventionally, purification of raw

material is the only method being adopted to enhance the metastable zone width of

solutions. Srinivasan et al [124,125] have developed a contemporary method to

enhance the metastable zone width of the solution in their supersaturated region in

order to enhance their stability to grow large size crystals with faster cooling rates. In

which the incorporation of a small quantity (1 wt %) of ethylenediamine tetra

acetic acid (EDTA), a well known gelating agent, enhances the zone width

significantly due to its gelating action. It was observed that this addition reduces the

nucleation rate and enhances the growth rate of the crystal appreciably. However,

Meera et al [126] have found that, by carrying out microhardness measurement, the

EDTA added TGS crystal was softer than pure TGS crystal.

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SR method

Recently, TGS crystals were grown by SR method in both (010) and (001)

direction by so many authors [127-130]. Large single crystal of triglycine sulphate

(dimension 100mm along monoclinic b-axis and 15mm in diameter) was grown using

the unidirectional solution growth technique (SR method) by Justin Raj et al [127].

M.Senthil Pandian and P.Ramasamy et al [128] have grown TGS crystals from the

solution by unidirectional SR method in (010) direction. They shown that there was

improvements in the properties of TGS crystals grown by SR method when compared

to those grown by the conventional slow evaporation technique. M.Senthil Pandian et

al [129] have also grown (001) directional TGS crystal by SR method, they shown

that the TGS crystals grown by SR method have higher hardness than conventional

method grown crystals. Dislocation density (DD) is less in SR grown crystal

compared to conventional method grown TGS crystals. The transmittance of SR

grown TGS is 8% higher than that of the conventional grown crystal. N.Balamurugan

et al [130] have grown TGS crystal by SR method and they prove the suitability of the

modified SR method for oriented TGS crystal.

1.2.4. Physical properties of TGS crystals

X-Ray diffraction studies to determine the crystal structure was carried out by

several authors [24,50,62,63,77] they all shown that the Triglycine Sulphate crystal

belongs to monoclinic system and also they observed slight change in the lattice

parameters for doped crystal. Muralidharan and his co-workers [38,52] have reported,

the rare earth ion dopants La, Ce and Nd on the growth aspect and ferroelectric

properties of TGS single crystals. The dopants significantly modify the morphology

of the crystals. FTIR spectrum recorded by the authors [23,25,26,48] confirm the

20

presence of all the functional groups which were present in glycine and sulphuric

acid.

S.Lijewski et al [131] have studied the electron spin echo technique in

triglycine sulphate (TGS) family crystals from the sound velocities derived from

elastic constant. And they determined the debye temperature of TGS family crystals.

They reported that the debye temperature of TGS crystals as 190ºK. The

microhardness studies were made for pure and several doped TGS crystals by several

authors [46,53,63,70,132]. Therasita Shanti et al [70] and Meera et al [53] have shown

that the microhardness number of TGS crystals increased with increase in dopant

concentration, while Balu et al [63] and Meera et al [46] have grown ADP and

L-tyrosine doped TGS crystal respectively. They shown that the hardness number

decreases with increase in dopant concentration. K.Balasubramanian et al [132] have

grown copper sulphate doped TGS crystal, they found that the doped crystals were

more harder than pure TGS crystal.

Mikhnevich [133] have studied the effect of impurity concentration on the

dielectric properties of TGS crystal with the non isomorphic impurity ions of

chromium and mixed isomorphic crystals of TGS and TGSe. The lectrical

measurement like dielectric constant, A.C. conductivity and D.C. conductivity made

by so many authors for pure and doped TGS crystals [19,30,40,53,55,60,61,72,

74,75,79,85,92,127]. They all shown that the dielectric constant increases with the

temperature and it is maximum at curie point temperature (49°C) and decrease further

with increase in temperature. Also they shown that there was no change in the curie

point temperature due to dopant addition, Justin Raj et al [127] have studied the

frequency response of dielectric constant and they shown that the dielectric constant

value decreases with increase in frequency. Kashevich et al [134] has studied the

21

dielectric properties of TGS crystals with a periodical stratified impurity distribution.

Stankowiska [135] have studied the dielectric properties of TGS crystals admixture

with b-d and dl- phenylalanine.

B.Hilczer and M.Michalczyk [136] have measured the temperature

dependence of D.C conductivity in the three crystallographic directions for pure and

y-irradiated TGS single crystal. An exponential law is obeyed from 20 to 80°C with a

change in activation energy at Tc, the electric conductivity is the highest along the

c-axis, the three activation energies increase slowly with increasing defect

concentration in the paraelectric state. In the ferroelectric phase, they increase

initially upto about 0.5 MR and then decrease. Various radiation induced changer in

the three axes. Many suggest that the electric conductivity in TGS can vary in type

from different crystallographic directions.

Therasita Shanthi and P.Selvarajan [137] have studied the temperature and the

concentration dependence of some halides doped TGS crystal. They found that the

D.C electrical conductivity increases with the increase in both the temperature

and impurity concentration. Farhana khanum et al [79] also shown that the

D.C conductivity increases with both the increase in temperature and impurity

concentration

1.2.5. Ferroelectric and hysteresis studies

Ferroelectric and hysteresis studies were made for pure and doped

TGS crystals by various author, Muralidharan and his co-workers [38] studied the

hysteresis effects of TGS crystals. They reported that lanthanum doped TGS crystals

possess, a maximum coercive force field value. Also they found that ES is high for

dopant ions having less number of 4f electrons.

22

Justin Raj et al [127] recorded the ferroelectric domain pattern using an optical

microscope and by scanning electron microscope, they observed that majority of

domains were elliptical or circular of 2 -10 μm. Also they observed the damaging the

domain pattern due to charging by the electron beam by focusing on the domain for

few seconds.

Arunmozhi et al [101] have found that there is no distinct change in the

spontaneous polarizations due to ADP doping in TGS. They also reported that the

dopant creates non-polar effects in the lattice and ADP doping increase the coercive

force.

Nakatani et al [138] has studied about changes in ferroelectric domain

structure of TGS with passage of time after heat treatment. Zhang and Zhang [139]

have used the order–disorder model to explain the mechanism of the average field of

ferroelectrics. Furthermore, the order-disorder structure is a common character for all

the ferroelectrics. Likodimos et al [140] have reported the surface charges

compensation and ferroelectric domain structure of TGS revealed by voltage-

modulated scanning force microscopy.

Afonskaya et al [211] has studied the re-arrangement of the domain structure

of cobalt and chromium doped TGS crystals. Toshio Kikuta et al [141] have studied

the domain pattern of TGS after exposure to an electric field perpendicular to the

ferroelectric axis, they found that the spontaneous polarization could be observed after

a prolonged application of electric field, the domain pattern appears have become a

rigid straight and the direction of the perpendicular electric field but is parallel to

c-axis. Montemezzani et al [142] have studied the effects of ultraviolet illumination

on Fe3+ doped TGS crystals and aqueous solutions. Montemezzani et al [143] have

23

studied the influence of ultraviolet irradiation on the optical absorption spectra of

iron-doped TGS crystals and aqueous solutions. Montemezzani et al [144] have

studied phase grating in Fe3+ doped TGS crystals recorded in the ultraviolet spectral

region.

1.2.6. Optical properties

Santra et al [145] has studied about the Raman spectroscopy of alanine doped

TGS ferroelectric single crystal. Sahai and Arya [146] have studied the temperature

dependence of Raman scattering spectra of internal modes in TGS. Sheen et al [147]

have studied the low temperature phase transition of TGS from Raman scattering in

the range of temperature between 10 and 300°K.

1.2.7. Pyroelectric properties

Many metallic dopants modified the properties to some extent, but in general,

they have been found ineffective for achieving the desired changes in the

properties [148]. Metallic ions like Fe3+ and Cr3+ ions changed the crystal growth

characterizations, but decreased the pyroelectric coefficient and spontaneous

polarization [91]. Li+ and Mn2+ modified the growth habit and produced a high

dielectric constant value did not affect the pyroelectric figure of merit (p/εr), where

p-pyroelectric coefficient, εr-dielectric constant [149]. Ni2+ ions at a concentration

level of 10% by weight in the solution increased the pyroelectric figure of merit but

did not enhance the crystal growth rate [148], whereas Mg2+ and Cu2+ modified the

growth habit but marginally increased the pyroelectric figure of merit [91].

Phosphoric acid (H3PO4) as a dopant has been found to yield crystals with a

large “ac” plane area used for IR detector application [49,54,94,150]. Even at a very

24

low concentration level of H3PO4, a significant improvement in the pyroelectric

coefficient, lowering of the dielectric permittivity at transition temperature (Tc) and

increase in the coercive field of the TGS crystal have been reported [95-97]. The

replacement of sulphur by phosphorous in the lattice introduces asymmetry in the P.E

hysteresis loop [96] it has also been found that the concentration of H3PO4 is

critical for achieving the desired material properties for commercial IR detector

applications [98].

1.2.8. Device Fabrication

Chavan [151] has reported about the battery formation of solid state by

applying D.C electric field on ferroelectric Triglycine Sulphate at high temperature.

Investigation of functional devices based on molecular materials organic

conductors and biomaterials has developed devices rapidly with the progress of the

nanotechnology. Especially, the fabrication of functional devices based on molecular

materials has been investigated by many researches with much interest [152-154].

Pentacene (C22H14) is one of the molecular materials and has very interesting

properties, for example, Pentacene film prepared by sublimation becomes a highly

oriented film [157]. It is expected from this feature that the highly orderly surface (the

interface between pentacene and another material) will be realized. It is also known

that the iodine (acceptor) or rubidium (donor) is intercalated between pentacene

molecular layers and the intercalation compounds are formed [214,238]. Moreover,

electrical conductivity increases drastically more than 108 times larger than that of

pure pentacene by doping of iodine and rubidium [214,238]. In addition to these

fundamental investigations, the devices based on pentacene have also been

investigated with much interest (153,154,327).

25

Matsuo et al [155] have fabricated a Field Effect Transistor (FET) based on

the ferroelectric TGS single crystal and pentacene film and investigated the electrical

properties. Figure 2 [155] illustrates the FET based on TGS and pentacene.

Figure 2: FET based on TGS and pentacene

It was found that the drain current decreases drastically with the increase of

the gate electric field around 100 V/cm and shows a minimum at 400 V/cm. Which

corresponds to the coercive electric field of TGS. This decrease of the drain current is

caused by the appearance of the depletion layer in the pentacene film. This result

indicates that the FET based on the TGS single crystal and pentacene film operates at

low gate electric field owing to the rapid generation of the surface charge

accompanied by the appearance of the spontaneous polarization in the ferroelectric

TGS insulator. It was also found that the drain current does not return to the initial

value (before the gate electric field of 450 V/cm is applied) for one week even in the

gate electric field is turned from 450 to 0V/cm. This result indicates that the surface

charges of pentacene remains for one week. From these results, it is suggested that the

26

FET based on the TGS single crystal and pentacene film shows a memory effect.

Very recently, P.R.Deepti et al [195] shown that KDP doped TGS crystals is suitable

for optoelectronic applications in their studies. The lattice parameters determined by

various authors for pure TGS crystal grown by different methods are given in table 1.

27

Table 1: Lattice parameters of TGS crystals by different researchers

a (Å)

b (Å)

c (Å)

β ( º )

Reference

9.418

9.417

9.398

9.6010

9.453

9.41

9.15

9.167

9.424

9.15

9.419

9.15

9.419

12.662

12.643

12.634

12.56

12.644

12.61

12.64

12.64

12.643

12.69

12.647

12.69

12.65

5.740

5.735

5.784

5.45

5.735

5.73

5.72

5.729

5.735

5.73

5.727

5.73

5.732

110.31

110.4

109.55

-

106.52

110.23

105.53

105.58

110.36

105.67

110.3

105

110.36

[339]

[159]

[328]

[76]

[330]

[24]

[331]

[39]

[332]

[333]

[334]

[335]

[338]

28

1.3. Present work

Today, crystals are the pillars of the modern technology, without crystals there

would no electronic industry, no photonic industry, no fiber optics communication,

very little modern optical equipment. Pure as well as mixed and doped crystals are

grown in every day in large quantities due to the increased need of crystals in solid

state devices

TGS is the order – disorder type ferroelectric crystal. Since the discovery of its

ferroelectric nature in 1956 by Mathias, TGS is one of the best studied ferroelectric

materials TGS exhibits good pyroelectric properties and finds wide application as

pyroelectric detectors. It shows second – order phase transition at curie point

temperature (Tc = 49°C). [20,21,233]

In the present study, TGS was synthesized from glycine and sulphuric acid, pure,

calcium and lanthanum doped single crystals of TGS were grown by slow evaporation

technique. Totally, eleven crystals (1 pure TGS, 5 calcium doped TGS, and 5

lanthanum doped TGS) were grown in identical condition.

The concentration of calcium in the calcium doped TGS crystal were

determined from Atomic Absorption spectrum and the lanthanum concentration in the

lanthanum doped TGS crystals were determined from EDAX spectrum taken.

Powder X-Ray diffraction data were collected for the eleven samples and they were

indexed using the standard methods, lattice parameters were also determined.

Thermal parameters like debye-waller factor, mean square amplitude of vibration,

debye temperature and debye frequency were determined from X-Ray diffraction

intensity data

29

The microhardness, and melting point of all the grown crystals were

determined by Vicker’s microhardness apparatus and TG/DTA thermal analysis

respectively. Work hardening coefficient was also determined from the hardness

value and the activation energy was calculated from the mass changes with respect to

temperature in the DTA curve. Debye temperatures were also determined from both

microhardness and melting point of the grown crystals.

The D.C electrical conductivity was determined by the conventional two probe

setup at various temperature ranging from 30°C to 120°C. Activation energy was also

determined from the D.C conductivity data.

The dielectric parameters like, dielectric constant and dielectric loss factor

(tanδ) were measured at different frequency viz. 50Hz, 100Hz, 1KHz, 10KHz,

50KHz, 100KHz, 200KHz, at various temperature ranging from 30°C to 120°C.

A.C conductivity was also calculated from the above two values. Activation energy

and debye temperature were also calculated from the A.C conductivity data. The

cole-cole plot also been drawn from the calculated complex dielectric values.

Electrical impedance measurement were carried out on all the eleven grown

crystal in the frequency range 1Hz to 1MHz at two temperature viz. 30°C and 49°C

(curie point temperature). Optical measurements like UV – Visible and FTIR studies

have been carried out for all the eleven grown crystals. Ferroelectric hysteresis curve

were drawn for all the grown crystals and the coercive field were determined.

The results obtained and detailed report of present research work are provided

in this thesis which is organized as follows. The topic of research is introduced in the

first chapter. The detail explanation of the synthesize of sample TGS crystal and their

chemical characterization are discussions in chapter two. Chapter three deals with the

30

structural characterization of the grown crystal along with the results and discussions.

The fourth chapter gives the mechanical characterization of the grown crystals along

with results and discussions. Electrical characterization including D.C conductivity,

dielectric constant, A.C conductivity, Cole-Cole plot and ferroelectric domain

structures are discussed in chapter six along with results and discussions. Electrical

impedance analysis is discussed in chapter seven along with results and discussions.

The optical characterization are discussed in chapter eight along with the result

obtained. Summary, conclusion and future scope are given in chapter nine. References

are provided in chapter ten.