edited by...the transition metal coordination chemistry of hemilabile ligands 233 caroline s. sidne....

PROGRESS IN INORGANIC CHEMISTRY

Edited by

KENNETH D. KARLIN

DEPARTMENT OF CHEMISTRY

JOIINS HOPKINS UNIVERSITY

BALTIMORE, MARYI .AN11

VOLUME 48

AN INTERSCIENCE" PUBLICATIQN JOHN WILEY & SONS, INC. New York Chichester Weinheim Brisbane Singapore Toronto

Progress in Inorganic Chemistry

Volume 48

Advisory Board

JACQUELINE K. BARTON CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

THEODORE J. BROWN UNIVERSITY OF ILLINOIS, URBANA, ILLINOIS

JAMES P. COLLMAN STANFORD UNIVERSITY, STANFORD, CALIFORNIA

F. ALBERT COTTON TEXAS A & M UNIVERSITY, COLLEGE STATION, TEXAS

ALAN H. COWLEY UNIVERSITY OF TEXAS, AUSTIN, TEXAS

RICHARD H. HOLM HARVARD UNIVERSITY, CAMBRIDGE, MASSACHUSETTS

EIICHI KIMURA HIROSHIMA UNIVERSITY, HIROSHIMA, JAPAN

NATHAN S. LEWIS CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

STEPHEN J. LIPPARD MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS

NORTHWESTERN UNIVERSITY, EVANSTON, ILLINOIS

EXXON RESEARCH & ENGINEERING CO., ANNANDALE, NEW JERSEY

TOBIN J. MARKS

EDWARD I. STIEFEL

KARL WIEGHARDT MAX-PLANCK-INSTITUT, MULHEIM, GERMANY

PROGRESS IN INORGANIC CHEMISTRY

Edited by

KENNETH D. KARLIN

DEPARTMENT OF CHEMISTRY

JOIINS HOPKINS UNIVERSITY

BALTIMORE, MARYI .AN11

VOLUME 48

AN INTERSCIENCE" PUBLICATIQN JOHN WILEY & SONS, INC. New York Chichester Weinheim Brisbane Singapore Toronto

Cover Illustration of “a molecular femc wheel” %as adapted from Taft, K. L. and Lippard, S. J., J. Am. Chem. Soc.. 1990, 112, 9629.

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Copyright 0 1999 by John Wiley & Sons. Inc. All rights reserved.

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1 0 9 8 7 6 5 4 3 2 1

Contents

Synthesis, Structure, and Properties of Organic-Inorganic Perovskites and Related Materials

DAVID B. MITZI IBM T J. Watson Research Centel; PO Box 218, Yorktown Heights, New York, USA

1

Transition Metals in Polymeric n-Conjugated Organic Frameworks 123 RICHARD P. KINGSBOROUGH, TIMOTHY M. SWAGER Department of Chemistry, Massachusetts Institute of Technology 77 Massachusetts Avenue, 18-209, Cambridge, Mussachusetts, USA

The Transition Metal Coordination Chemistry of Hemilabile Ligands 233 CAROLINE S. SIDNE. DANA A. WEINBtRGEK, CHAD A. MIRKIN Department of Chemistry, Northwestern UniversiQ, 2 145 Sheridan Road, Evanston, Illinois, USA

Organometallic Fluorides of the Main Group Metals Containing the C-M-F Fragment 35 1

BALAJI R. JAGIRDAR, EAMOKN F. M u m w , HERBERT W. ROESKY Universitat Giittingen, Tammunnstrasse 4, 0-37077 GottinKen, Germany

Coordination Complex Impregnated Molecular Sieves-Synthesis, Characterization, Reactivity, and Catalysis 457

PARTHA P. PAUL Southwest Research Institute. 6220 Culehru Road, PO Drawer 28510, Sun Antonio, Texas, USA

Advances in Metal Boryl and Metal-Mediated B-X Activation Chemistry 505

MILTON R. SMITH I11 Department of Chemistry, Michigan State University, East Lansing, Michigan, USA

v

CONTENTS vi

Subject Index

Cumulative Index, Volumes 1-48

569

589

Progress in Inorganic Chemistry

Volume 48

Synthesis, Structure, and Properties of Organic-Inorganic Perovskites and Related Materials

DAVID B. MITZI

IBM T J. Watson Research Center I? 0. Box218 Yorktown Heights, NY

CONTENI’S

1 . INI’RODUCTION

A. Organic-Inorganic Hybrids B. Inorganic Perovskites c scope

11. ORGANIC-INORGANIC PEROVSKITE STRIJCTI:’RES

A. Three-Dimensional Systems R. 1,ayered (100) Oriented Perovskites

1. Transition Metal Halides 2. Group 14 (IVA) Metal Halides 3. Rare Earth Metal Halides

C. Structural Transitions D. More Complex Organic Cations E. Polymerized Organic Layer F. Multilayer Perovskite Structures (i. Layered (1 10) Oriented Perovskites € I . One-Dimensional Systems I. Zero-Dimensional Systems J. Summary of Structures

111. SYNTHESIS AND CRYSTAL GROWTH

A. Self-Assembling Structures €3. Solid-State Reactivity and Melt Processing C. Solution Chemistv

1. Simple Organic Cations

Progre.r.c in Inorganic. Chernisrty Kd. 48, Edited by Kenneth D. Karlin. ISBN 0-471-32623-2 0 1999 John Wiley Rr Sons, Inc.

2 DAVID B. MITZI

2. Less Stable Metal Oxidation States 3. More Complex Organic Cations 4. Gel Techniques and Layered Growth 5. Multilayer Structures 6. Cyanamide Chemistry and the (1 10) Oriented Compounds

D. Polymerization Reactions E. Thin-Film Growth

1. Spin Coating 2. Thermal Evaporation 3. Dip Coating

IV. PROPERTIES

A. Magnetism 1. Transition Metal Halides 2. Rare Earth Metal Halides

1. Transition Metal Halides 2. Group 14 (IVA) Metal Halides 3. Rare Earth Metal Halides

B. Photoluminescence

C. Electrical Transport D. Electroluminescence E. Other Properties

V. CONCL.USION

ACKNOWLEDGMENTS

ABBREVIATIONS

REFERENCES

I. INTRODUCTION

A. Organic-Inorganic Hybrids

Complex structures, based on a molecular scale composite of inorganic and organic components, provide a substantial opportunity for tailoring new and functional materials for scientific exploration and technological applications. Inorganic materials, typically characterized by covalent and ionic interactions, offer the potential for high electrical mobility, a wide range of band gaps (enabling the design of insulators, semiconductors, metals, and superconductors), interesting magnetic interactions, a range of dielectric properties, substantial mechanical hardness, and thermal stability. Organic molecules, which generally interact through weaker interactions such as hydrogen bonding and van der Waals interactions, provide the possibility of

SYNTHESIS, STRUCTURE, AND PROPERTIES OF PEROVSKITES 3

Periostracum Outer Prismatic Layer Nacre Myostracum Nacre Inner Prismatic Layer

I I

Figure 1. Idealized cross section of a bivalve shell. The magnified views show the schematic structure of the nacre and prismatic layers of the shell. Within each view, the white sections of each block are the calcium carbonate crystals, whereas the black lines represent the organic material separating the crystals. Note the very different scales for the two insets. [Adapted from ( 1 ) and (4).1

structural diversity, highly efficient luminescence, a large degree of polarizability, plastic mechanical properties, and in some cases can even be made conducting or superconducting. Generally, organic-inorganic hybrid research focuses on employing the range of interactions found within organic and inorganic chemistry to create a composite with some enhanced property relative to that achievable with either organic or inorganic materials alone, or to combine useful properties of the two components within a single material. In some cases, the goal is to search for new phenomena that result from the interaction between the organic and inorganic subunits.

Nature has long made use of the beneficial properties arising from organic-inorganic hybrids. The nacre section of a mollusk shell (Fig. l), for example, consists of a highly organized laminated microstructure of aragonite CaCOl crystals (with a thickness of - 0.25 pm) separated by a thin (300-500 A) layer of proteinaceous organic matter. The resultant strength and fracture toughness of this “brick and mortar” microstructure is orders of magnitude higher than either of the constituents (1-5). The useful properties of this biological composite arise from the highly organized and appro- priately proportioned combination of a hard brittle inorganic phase with a soft plastic organic phase, with strong interfacial bonding between the two

4 DAVID B. MITZI

components. The weaker prismatic component of a seashell consists of long columnar crystals of aragonite or calcite, running normal to the surface of the shell, and a relatively thick (up to 5 pm) organic matrix around each crystal. The advantages of the prismatic layer are that it can be laid down more rapidly than nacre and that the modulus of elasticity can be controlled by changing the thickness of the protein matrix between the crystals (1).

In mammalian teeth, a high volume fraction of calcium phosphate, primarily in the form of Calo(P04)6(OH), (hydroxyapatite) rods, are bound together by protein to form an outer wear-resistant enamel shell covering the tougher dentine component (1, 4). Bone is a similar composite, consisting of organic fibers (mainly the protein collagen) and an inorganic crystalline phase (mainly hydroxyapatite or carbonated apatite), capable of holding up the body's weight while also withstanding sudden impacts of many times greater force (1, 4, 6 ) . The weight percentage of the calcium phosphate salt in bone is substantially lower than in mature tooth enamel (65 as opposed to 95 wt.%, on average). As a result of the composition and microstructure, bone can be bent to some degree without shattering, despite the fact that the inorganic component alone would be expected to be quite brittle. Remark- ably, bone is a "living" organic-inorganic hybrid in that it is continually growing and being remodeled.

Recently, there have been several synthetic composites that take advantage of the unique properties enabled by combining organic and inorganic constituents. Polyimides are used for microelectronics applications because of their heat resistance, chemical stability, and superior electrical properties. In an effort to reduce the coefficient of thermal expansion and the amount of moisture absorption, a small amount of a clay (montmorillonite) is dispersed on a molecular level in the polymer (Fig. 2). Only 2 wt.% addition of montmorillonite lowers the permeability coefficients for various gases to less than one-half of the values for pure polyimide, while at the same time also reducing the thermal expansivity (7). The substantial lowering of the permeability apparently arises from the sheetlike morphology of the clay particles which, as a result of the large surface area, increases the distance a diffus- ing molecule must travel to get around the inorganic sheets and through the material. These and other related molecularly dispersed polymer/layered silicate nanocomposites have been shown to achieve a higher degree of stiff- ness, strength, heat and flame resistance, and barrier properties, with far less inorganic content than comparable glass (or mineral) reinforced "filled" polymers, as a result of the molecular scale interaction between the two components (7-9). By enabling the formation of lighter materials with the same degree of toughness, many potential applications can be envisioned. Recently, for example, the Toyota Motor Company has successfully introduced an auto- motive timing belt cover made from a Nylon-layered silicate nanocomposite


Exfoliated Nanocomposite

Phase-Separated "Filled" Polymer

Figure 2. Schematic rcpresentations of the organic-inorganic composite structures obtained by mixing polymer with layered silicates. The gray bars represent the silicate layers. In stan- dard filled polymers (a), the ingredients are immiscible, resulting in a macrocomposite with chemically distinct phases. In the organic-inorganic nanocomposites (h), the two components arc mixed on a molecular scale, enabling better perforniing organic-inorganic composites with a smaller volume fraction of the more dense inorganic component. [Adapted from Giannelis (81.1

(9). Other proposed applications include airplane interiors, fuel tanks, under- the-hood (automobile) structural parts, brakes, and tires (8, 9).

Organic-inorganic superlattices, created using sequential evaporation of the organic and inorganic components, are another area of active interest. Takada et al. (10) created a multilayer structure consisting of alternating amorphous layers of copper phthalocyanine (CuPc) and TiO,, with an arti- ficial period of greater than 40 A. The organic CuPc layers have an energy gap of approximately 2.0 eV and absorb light in the visible region to generate electrons and holes. The titanium oxide layers, on the other hand, have a larger energy gap and therefore exhibit no photoconductive sensitivity over most of the visible spectrum. They do, however, have a larger electron affinity and a substantially larger mobility. Consequently, the electrons generated in the CuPc layer are expected to transfer to the TiO., layer. By physically separating the carriers, the probability of recombination is reduced and the photoconductivity should be enhanced over the values observed for pure CuPc thin films. While this description is very informal, especially given

6 DAVID R. MITZI

the amorphous nature of the two components of the structure, the observed 40 times enhancement of the photoconductive response of the composite, compared to a CuPc thin film, demonstrates the substantial improvement in photoconductive properties that can be achieved by combining an efficient organic photoconductive material with an inorganic material having larger mobility and electron affinity (10). A similar superlattice structure has also recently been prepared with alternating layers of 8-hydroxyquinoline alu- minum (Alq) and MgF2 (11). By confining the organic fluorescent material between the inorganic layers in a superlattice structure, the exciton energy can be shifted to higher values with decreasing Alq layer thickness (for thicknesses below - 50 A). These two examples suggest the possibility of tailoring sequentially deposited organic-inorganic superlattice structures for specific optical and optoelectronic device applications.

Another interesting recent advance is the self-organization of monodis- persed inorganic nanocrystallites into three-dimensional (3D) quantum dot superlattices, through the derivatization of the surface of the nanocrystals with a monolayer of coordinating organic ligands (1 2). One example of this can be seen in the plan-view transmission electron microscope (TEM) image (Fig. 3) of a superlattice consisting of 48 A diameter CdSe nanocrystals, each surrounded by an 11 A thick monolayer of trioctylphosphine oxide. Formation of bulk colloidal crystals of the organic-inorganic nanocrystallites occurs by gentle evaporation of the quantum dot dispersion using established two-solvent recrystallization methods. The size, shape, and functionalization of the organic coating on the particles controls the degree to which the inorganic particles (which typically range in size from 15 to 150 A) communi- cate with each other, influences the packing of the particles in the 3D lattice, and also plays an important role in the original synthesis of the nanocrystallites ( 1 2). By rational selection of semiconducting, insulating, or magnetic inorganic particles, and by controlling the separation between the particles (through the appropriate choice of the organic capping layer), quantum dot superlattices can be created that maintain ordering over many microns and provide the potential for application in the areas of optical and electronic devices, as well as in magnetic data storage.

While the above examples are only a small sampling of the exciting and potentially useful organic-inorganic hybrids that are being developed (1 3), they also generally represent systems that do not have full long-range structural ordering, often consisting of either amorphous organic or inorganic layers. This chapter will focus on a particular class of crystalline organic-inorganic hybrids, which have an extended inorganic framework interacting with stereoregular organic side groups. Crystalline materials have the advantage that they can readily be structurally characterized using techniques such as X-ray and neutron diffraction, making it possible to correlate

SYNTHESIS, STRUCTURE, AND PROPERTIES OF PEROVSKITES I

Figure 3. (u ) High resolytion TEM image of the (101) projection through a face centered cubic superlattice of 48 A surface-derivatized CdSe nanocrystallites. ( h ) Higher magnifica- tion view of the inorganic nanocrystallites (dark regions) and trioctylphosphine oxide coating (white regions). (c) Small angle electron diffraction pattern of a 2 pm diameter portion of the superlattice, demonstrating the periodicity of the superlattice over this length scale. [Courtesy of C. €3. Murray.]

structural features with specific material properties. Concentrating on systems with extended inorganic frameworks neglects most of organometallic chemistry. But for electronic and optical applications, the extended inorganic framework provides the advantage of a potentially high mobility pathway for electrical carriers to pass. It also enables the possibility of stronger extended magnetic interactions along one, two, or three dimensions. In addition, the covalent bonding holding the inorganic framework together tends to provide mechanical robustness for these materials, a desirable feature for many applications.

Generally speaking, extended organic-inorganic materials can be loosely categorized according to the strength of the interaction between the two com-

8 DAVID B. MITZI

van der Waals ionic covalent Figure 4. Schematic representation of extended organic-inorganic materials. The gray regions represent the extended inorganic framework, which can be either ID, 2D, or 3D, while the white ovals represent organic niolecules, which interact with the inorganic component either through van der Waals, ionic, or covalent interactions. In real systems, the situation is often intermediate to those shown above, with some combination of interaction types between the organic and inorganic components.

ponents, ranging from weak van der Waals type interactions, to hydrogen bonded, ionic, and finally covalently bonded compounds (Fig. 4). Com- pounds within the first category are typically formed by reversibly interca- lating organic species (guest molecules) into the interlayer space of a pre- existing inorganic layered structure (host). They have largely been observed and studied in layered silicates and in transition metal oxide, chalcogenide, and phosphate systems (14-17). The structure of the inorganic framework is little affected by the guest molecule, highlighting the small degree of interaction between the two components. For neutral organic molecules and inorganic layers the relevant bonding distances between the two species are fairly long, being governed by van der Waals interactions, leading to a situation largely analogous to physisorption. Depending on the degree of charge transfer between the intercalated species and the host material, intercalated compounds may be closely related to the next grouping of organic-inorganic materials, which are characterized by ionic interactions.

In ionic compounds, the organic component is an intimate part of the overall structure and in fact the structure depends on the organic ion for overall charge neutrality. In contrast to the previous group, these compounds have specific stoichiometries, with the organic ions occupying well-defined sites in the crystal lattice. Furthermore, the ionic bond length is substan-


tially smaller than typical van der Wads distances, providing the potential for a stronger interaction between the organic and inorganic components. These compounds are mainly formed in divalent transition, main group, and rare earth metal halides with one (1D)-, two (2D)-, or three-dimensional (3D) perovskite and related structures [e.g. (18-27)J. In the family (C4HyNH3)2(CH3NH3), - I Snn13,+ 1 , for example, conducting “n”-layer thick perovskite-based anion slabs alternate with much wider band gap butylammonium cation bilayers, leading to multiple quantum well structures with controllable well width ( 19). Other similar examples of ionic organic-inorganic systems include the oxides, CntHzm + I NH3TiNb05 (28) and (CmH2m+ lNH3)2Ti409 (29), which are formed by an intercalative reaction involving the hydrous titanates (HTiNbOs or HzTi409 . nHz0) and alkyl- amine species in solution. By an analogous reaction, the layered perovskites, (C,H2m+1NH3)Ca2Na,, 3Nbn03n+l (3 I n I 7) have aIso been prepared (30).

Although there are numerous reported compounds with covalent bonding between organic and inorganic units, most feature isolated molecules or clusters. A few examples, however, including Zr(HPO4)(C6Hs0PO3) .2H20

CHjRe03 (33). (CH3)zSnFz (34), and C,Hz,+ lBiI2 (35), contain extended inorganic frameworks. The first two compounds consist of Zr phosphate/ phosphite layers with long-chain organic groups covalently bonded to them through P-0 bonds. The proposed structure of CH3Re03 features two- dimensional Re03 layers, while (CH3)2SnF2 consists of an infinite 2D network of octahedrally coordinated tin atoms with bridging fluorine atoms and apical methyl groups above and below each inorganic sheet. The CnHz,l + I BiI? compound consists of extended BiI2 chains with the alkyl groups covalently bonded to one side of each chain. Closely related to these covalent systems are compounds in which organic ligands are directly coordinated to metal ions within an inorganic layer or chain, as in M03(CsHsN)

While the influence of the organic component on the geometry and bonding of the inorganic framework becomes progressively more important in the van der Waals and ionically bonded systems, the interaction is especially significant in the covalently bonded systems. In C,H2, + 1 BiI2, for example, the carbon bonding to the bismuth atom has a fundamental effect on the bonding geometry about this atom. Whereas in BiI3 the bismuth is surrounded by an octahedral coordination of iodine atoms, indicating that the bismuth lone pair orbital is stereochemically inactive, in CfIHzn + I BiI2 bismuth adopts a square pyramidal coordination, with four iodines in the basal plane of the pyramid and the carbon occupying the apical site. In this system the bismuth lone pair is stereochemically active and is trans to the carbon atom from the alkyl side-

Zr(HPO3)I.2(03P-R-P02)0.4 LR -- (CH3)?H2C6C6H2(CH3)21 (32),

(36) or {[Rh(CH3CN)4l(BF4)1.5}, (37).

10 DAVID B. MITZI

group. The square pyramidal geometry arises from the system minimizing the amount of carbon antibonding character in the highest occupied molecular orbital (HOMO) (35), highlighting the importance of the carbon-metal bond on the structure of the extended inorganic framework.

Each of the loosely defined classes of extended organic-inorganic hybrids are of substantial scientific and technological interest. However, the degree of interaction between the organic and inorganic species in van der Waals interacting systems is relatively small, and the class of extended organometallic and other covalently interacting compounds is somewhat limited at present, generally containing simple organic fragments (typically alkyl groups). This chapter will focus on the class of ionically interacting organic-inorganic systems based on the perovskite framework. In addition to the increasing number of new structures that have been recently reported in this family, many potentially useful physical properties have been observed, including enhanced exciton binding energies due to a dielectric confinement effect (with a resulting intense photoluminescence peak at room temperature) (38, 39), nonlinear optical properties with the potential for third harmonic gen- eration (26, 40), and electroluminescence (4 1). The recent demonstration of intense green electroluminescence at liquid nitrogen temperature from a device based on (CgH&*H4NH3)2PbL (42) has led to the speculation that these materials might be useful for luminescent display applications.

B. Inorganic Perovskites

The basic building component of the organic-inorganic perovskite family is the ABX3 perovskite structure (Fig. 5). This simple structure consists of a 3D network of corner-sharing BX6 octahedra, where the B atom is typically a metal cation and X is an anion (02-, Cl-., Br , I-, or in a few instances S2-), with the appropriate charge to balance the A and B cations. The A cations fill the large 12-fold coordinated holes between the octahedra. Some examples of undistorted cubic perovskite structures include SrTiO3 (43), CsSnBr3 (44), and B%6&.4Bi03 (45). More generally, the structure is distorted as a result of cation displacements, as in BaTi03 (46), or by tilting of the octahedra, as in CaTiO3 (47). The cation displacements give rise to the useful properties of ferroelectricity and antiferroelectricity in many of the perovskite systems.

In addition to the 3D perovskites, layered perovskites can also be formed by taking an n-layer-thick cut from the 3D perovskite structure along some crystallographic direction, and alternating these layers with some other type of modulation layer. Among the possible terminating planes, (100) and (110) are the most common (Fig. 6). The (1 10) oriented perovskites generally adopt the formula AnMnX3n+2, where A is a cation or a mixture of cations, M is a metal cation, and X is an anion (generally, 02-, F-, or C1 ). Several

SYNTHESIS, STRUC'I'IIRE. AND PROPERTIES 01: PEROVSKITES 11

o w - l

ABX3

::::- C

L a Figure 5. Basic ABX3 perovskite structure (a) , and a schematic representation of how the structure extends in three dimensions (h ) . The dashed square in this representation corresponds to the unit cell shown on the left.

examples include CaCrFS (n - 1), Ba2M2F8 with M = Zn, Mn, Fe, Co, or Ni (n - Z), and CajNb,O,, (n = 4) (48-51). The inorganic framework for the n - 1 structure is reduced to chains of comer-sharing MXs octahedra extending down one axis. These can therefore be considered 1D perovskite structures and provide an interesting comparison to the layered (or 2D), and the 3D (large n) perovskite systems. A number of these compounds exhibit ferroelectricity, pyroelectricity, and magnetic-ordering transitions.

The literature of ( 1 00) oriented layered perovskites is considerably more extensive than that for the (1 10) oriented counterparts. The Ruddlesden- Popper series (52), A,, + I BnX7,z+ I, which consists of n layers of corner-sharing BXs octahedra separated by AX rock salt layers (Fig. 6), forms the framework for one major branch of this family. A typical example (with n = 2) is the system (Sr?-,Lnl+.)Mn2O7 (0 I x I 0.5, Ln = trivalent lanthanide), which exhibits interesting giant magnetoresistance effects (53). The Aurivillius compounds (mostly oxides) (54), with the general formula (Bi?02)An - B,03, + 1 , are a second extensive branch of the (100) oriented family and consist of alternating layers of fluorite-like (Bi?O#+ and n layer thick (A,, 1B,03,+ perovskite sheets. Members of this group often pos- sess interesting ferroelectric properties with potential for use in nonvolatile

12 DAVID B. MITZI

<1 1 O> Oriented Cubic Perovskite <loo> Oriented

X-

A

M < I CI

c

/ :Pi

<loo> cut -

i A”MflX,fl+2 ABX, Afl+lBflX3fl+l ( Bi24)An-,Bf14n+,

(n=2) (n=2) (n=2)

Figure 6. Different possible terminations for the layered perovskite structures. The cuhic perovskite (center) can hc cut along either the (110) or (100) crystallographic directions to produce the ( 1 10) oriented and (100) oriented families of layered perovskites. The thickness of the perovskite sheets can vary depending on how many layers of the original perovskite structure are taken in the cut (denoted by n) . Both the Ruddlesden-Popper, A,, + 1 BnX3,,+ 1 ,

and the Aurivillius. (Ri202)An . I B,Og,,+ 1 , series are shown for the (100) oriented family.

memory devices (55). Commercial nonvolatile RAMS are currently being produced using BizSrTaIOg and Bi?Sr(NbTa)Oy as the ferroelectric cotnpo- nent (56).

The most famous (100) oriented layered perovskites include the rapidly expanding family of high-temperature superconductors (YBa2Cu307 - 6 ,

LazCuO4, BizSr2CaCu208 6 , etc.). These structures, which include several copper oxide-based Ruddlesden-Popper and Aurivillius-like phases as well as many more complex structures, consist of “active” CuOz based perovskite layers (where the superconductivity occurs) separated by electrically and mechanically “soft” modulation layers (Biz02, CuO chain layer, etc. ). The modulation layers can be used to tune the properties of the perovskite sheets without introducing substantial disorder into these active layers. They are mechanically “soft” in the sense that they contain atoms that are less tightly bound (oxygen can generally be pulled in and out of the structure from these layers), and electronically “soft” in the sense that the layers generally contain ions that can adopt several different valence states or can be substituted with ions in a different valence state. These two features enable the modulation layers to act as a charge reservoir for the Cu02 sheets, a feature that is presumably important for the observation of high-temperature superconductivity.


In the above strictly inorganic families, it is natural to attempt replacing the inorganic modulation layer with an organic layer, since organic chemistry offers considerably more flexibility in controlling molecular parameters (e.g., molecule length, width, degree of saturation, polymerization, polarizability, and luminescent character). The impetus of this modification is to generate a greater range of structures and properties within the perovskite family, and to have greater flexibility in designing materials to suit a particular application.

C. Scope

The area of organic-inorganic perovskites is a broad and growing field, and therefore this chapter will attempt to provide an overview, with a number of specific (perhaps biased) examples chosen primarily from among recent (1980-1998) developments. In Section 11, the crystallography of the organic-inorganic perovslute family will be discussed. After briefly con- sidering the 3D organic-inorganic perovskite systems, several examples of simple ( I 00) oriented single-layered perovskites will be presented for structures based on divalent transition metal, Group 14 (IVA) metal, and rare earth metal halides, in combination with simple aliphatic or aromatic organic cations. In addition to these basic 2D systems several other recent developments will be covered, including the incorporation of more complex organic molecules and polymer layers, the multilayer perovskite structures, the ability to control the orientation (( 100) vs. ( I 10)) and dimensionality of the perovskite sheets, and the structural transitions as a function of temperature.

In Section 111, a variety of synthetic techniques for the organic-inorganic perovskites will be examined, with a particular focus on recent progress that has enabled the synthesis and crystal growth of many of the new compounds discussed in Section 11. A very useful review of solution crystalliza- tion techniques for the layered perovskites known up to 1978 has been published by Arend et al. ( 1 8). The discussion in the present chapter is meant to complement and update this earlier work. In addition, several techniques for thin-film deposition will be discussed. The thin-film techniques are particu- larly relevant for the potential use of these materials in electronic or optical devices.

Finally, in Section IV there will be a discussion of selected properties of the perovskite family, with emphasis on the magnetic, luminescent, and conducting properties. Prior reviews on properties include the recent work by Ishihara (57), dealing with the optical properties of the lead(I1)-based layered perovskites, and a 1974 review of the magnetic properties of the first-row transition metal perovskites, which is included in a more general article on lower dimensional magnetism by de Jongh and Miedema (22).

14 DAVID B. MITZI

11. ORGANIC-INORGANIC PEROVSKITE STRUCTURES

A. Three-Dimensional Systems

In the basic cubic perovskite structure (Fig. 5) , the inorganic A cation can be replaced by a suitable organic cation. Since the organic cation must fit into a rigid and relatively small (from the point of view of organic molecules) cuboctahedral hole formed by the 12 nearest-neighbor X atoms, the selection of molecules for the A site is expected to be limited. In fact, a tolerance factor ( t ) can be defined, based on the geometric constraints imposed by a rigid sphere model for the perovskite structure, to establish how large the A cation can be given the radii for the B and X ions. For a perfectly packed cubic perovskite structure, the condition for the A, B, and X ions to be in contact is (RA + R,) - t& (RB + R,), where RA, RB, and R , are the corresponding ionic radii and the tolerance factor, t = 1. Empirically it is found that 0.8 I t 5 0.9 for most cubic perovskite structures, although the exact end-points of this range depend somewhat on which tables are used to obtain the ionic radii values. There is also a slightly expanded range for distorted structures (58). Taking, for example, a system with one of the largest possible values for R, + R,, B y Pb and X = I [R,, = 1.19 A and R, - 2.20 A (5?)], and using t = 1, we find that R , should not exceed approximately 2.6 A. Given that C-C or C-N bond lengths are of order 1.4 A, only the smallest organic molecules-those consisting of two or three atoms (exclud- ing hydrogens)-should fit into the structure.

Based on these considerations, the methylammonium cation is expected to be an appropriate choice for the 3D perovskite structure. In fact, the compounds C H ~ N H ~ M X J , with M - Sn and Pb, and X = C1, Br, and I, have all been synthesized and structurally characterized (60-69). Each of these systems adopt (as the highest temperature phase) the cubic perovskite structure. For the lead(I1) compounds, the cubic lattice constants vary from a = 5.657(2) A (X = C1) and a = 5.901(1) A (X = Br), to a = 6.3285(4) A (X - I) (66, 67). The corresponding tin(I1) compounds have the similar lattice constants, a = 5.89 A (X - Br) and a = 6.240(1) A (X = I) (60, 63).

Since the symmetry of the free methylammonium cation does not agree with the Oh site symmetry for the A cation of the cubic perovskite structure, the organic cation must be orientationally disordered in the high-temperature phase. In fact, for cubic CHJNH3PbX3, with X = C1, Br, or I, nuclear magnetic resonance (NMR) and nuclear quadruple resonance (NQR) spec- troscopies demonstrate that the methylammonium cation undergoes rapid isotropic reorientation (64, 65, 67). Upon cooling, the structures distort to lower symmetry (Table I) as the motion of the methylammonium cation becomes more restricted (64-67). In contrast to the isotropic reorientation at

SYNTHESIS, STRUCTURE. AND PROPERTIES OF PEROVSKITES 15

TABLE I Structural Data for CHxNH3PbXx (X 7 C1, Br, Iia

X ( K) Crystal System Space Group (1 (A) 0 (A) c (A) Z

CI >178.8 Cubic Pm3m 5.675 1 172.9-178.8 Tetragonal P4/mmni 5.656 5.630 1

<172.9 Orthorhombic P222 I 5.673 5.628 11.182 2

155.1-236.9 Tetragonal I4,/mcm 8.322(2) 11.832(7) 4 149.5-155.1 Tetragonal P4/mmm 5.894(2) 5.861(2) 1

~ 1 4 4 . 5 Orthorhombic Pna2l 7.979(1) 8.580(2) 11.849(2) 4

162.2-327.4 Tetragonal I4/mcm 8.855(6) 12.659(8) 4 <162.2 Orthorhombic Pna2 1 8.861(2) 8.581(2) 12.620(3) 4

Temperature

Br >236.9 Cubic Pm3m 5.901 (1 ) 1

I >327.4 Cubic Pm3m 6.3285(4) 1

“After (66).

high temperature, the lowest temperature phases for each of these systems are characterized by the organic cation being restricted to rotations about the C-N axis. All of these structural transitions are reported to be first order (64). The tin(I1)-based systems with X - Br and I undergo similar structural transitions upon cooling (60, 68, 69).

The compound C H ~ N H ~ E U I ~ has also recently been prepared and at room temperature is isostructural to the tetragonal (room temperature) CH3NH3Pb13 structure, with the lattice parameters, a = 8.917(2) A and c = 12.860(4) A (70). As of yet no low-temperature studies have been per- formed on the europium(I1) material, but the succession of phase transitions is likely to be similar to those observed in the lead(I1) and tin(I1) systems.

In addition, the methylammonium cation has been replaced by the larger formamidinium cation in the tin(I1)-based system, NH2CH=NH2SnI3, yielding a room temperature cubic perovskite structure with the lattice constant, a = 6.316(1) A, approximately 1.2% larger than that for the methylammonium analogue (71). While this system is cubic at room temperature, indicating a disordered organic cation, a sharp feature in the electrical resistivity as a function of temperature at approximately 75 K (see Section 1V.C) sug- gests that a structural transition occurs as the organic cation orders at lower temperature. The mixed-cation system (CH3NH 3 ) l ,(NH2CH=NH&SnI3 has also been prepared with x - 0.5 and has the room temperature lattice constant, a = 6.278( 1) A, half-way between the parameters observed for the x = 0 and x - 1 compounds (71). This mixed system demonstrates the possibility of forming a solid solution between the two organic cations and enables a study of the physical properties as a function of the cubic lattice constant. Note that the formamidinium cation consists of three non-hydrogen atoms

16 DAVID €3. MITZI

and two sites for potential hydrogen bonding, whereas the methylammonium cation has only two non-hydrogen atoms and one site for hydrogen bonding. An issue of current interest is how these extra impediments to disordering for the larger organic cation, and how mixing of the two organic cations, affect the low-temperature structural transitions.

B. Layered (100) Oriented Perovskites

Among the layered systems, the simplest examples include the compounds (R-NH3)2M&, where R-"Hi is an aliphatic or single ring aromatic ammonium cation, M is a divalent metal that can adopt an octahedral coordination, and X is a halogen. These systems consist of single layers of (100) oriented perovskite sheets separated by bilayers of organic ammonium cations, and are analogous to the inorganic n = 1 Ruddlesden-Popper compound K2NiF4 (or LazCu04), with the KlF2 (La202) rock salt layer replaced by the organic bilayer (Fig. 7). In contrast to the 3D perovskite structure, AMX3, where the organic cation (A') must fit into a rigidly defined hole, in the layered systems the distance between the perovskite sheets can vary, and therefore larger more complex organic cations (R-NH;) can be incorporated.

(R-N H3)2MX4 K2NiF, (0 rgan i c-i norgan ic) (i no rgan ic)

Figure 7. Basic single-layered organic-inorganic perovskite structure (a). Note the analogy with the inorganic n :- 1 Ruddlesden-Popper structure (6). The dashed rectangles in each structure highlight the fact that the two compounds are very similar with. however, the replacement of the inorganic rock salt layer (e.g., K2F2) with an organic bilayer. Van der Waals interaction between the organic tails of the organic cations hold the perovskite layers together in the organic-inorganic structure.

SYNTHESIS, STRIJCTURE, AND PROPERTIES OF PEROVSKITES 17

A variety of interactions are responsible for stabilizing the layered structures. The extended ionic-covalent 2D anions (MXj-) are sheathed on both sides by the organic cations, with the ammonium heads of the cations hydrogen-ionic bonding to the halogens in the inorganic sheets and the organic tails extending into the space between the layers. The resulting neutral organic-inorganic+rganic (R-NH3)zMX4 sandwiches are then stacked to form the full 3D structure, with van der Wads interactions between the tails of the organic cations holding the structure together. The degree to which the organic tails from successive layers interdigitate depends on the molecular conformation of the tails, as well as the spacing between nearest- neighbor organic cations within a layer (controlled by the MX; - framework). Whereas little interdigitation between the layers generally occurs in the transition metal chloride and bromide based systems, for the relatively spacious PbI; framework and long-chain alkylammonium cations, substantial over- lap between the alkyl chains in adjacent layers has been reported (72).

In addition to the monoammonium organic cation compounds, diammonium cations also stabilize the layered perovskite structure (Fig. S ) , yield-

(N H,-R-N H,)MX, “Eclipsed”

(b) Figure 8. Comparison of the n - 1 (100) oriented layered perovskites with organic monoammonium cations (a ) and diammonium cations (6). Note that for the idealized diammonium system, the metal atoms from one layer are directly over metal atoms in the adjacent layen (i.e., an “eclipsed” arrangement). In contrast, for the idealized monoammonium system. the metal atoms are shifted between adjacent layers (i.e., a “staggered‘ arrangement).

18 DAVID B. MITZI

ing the general formula (H3N-R-NH3)MK (18). In these systems, the organic cations hydrogen bond to the inorganic sheets at both ends rather than only at one end, thereby removing the van der Waals gap between the layers. As seen in the idealized structure (Fig. S), the metal atoms, M, are aligned from layer to layer, resulting in an “eclipsed” arrangement of layers, and providing for the possibility of enhanced interaction between the layers through interlayer terminal X - . - X contacts (for short organic R groups). In contrast, for the ideal monoammonium cation compounds, there is a shift of each successive perovskite sheet resulting in a “staggered” configuration for the layers. In actual structures, the detailed manner in which the organic cations hydrogen bond to the inorganic sheets influences the alignment of adjacent layers, thereby making the distinction between the staggered and eclipsed structures somewhat less clear-cut among the monoammonium and diammonium compounds.

The length of the organic cations clearly influences the perovskite sheet spacing and consequently the effective dimensionality of the compound with respect to physical properties. Note that, at least at room temperature, the organic cations are rarely perfectly aligned along a direction perpendicular to the perovskite sheets. The tilt angle adopted by the molecules is a function of the configuration of hydrogen bonding, as well as by space-filling considerations for the organic cations. While most of the cations considered in this section are relatively simple (generally either alkylammonium or single ring aromatic ammonium cations), the structural constraints for larger, more complex organic cations will be addressed in Section 1I.D.

Hydrogen bonding between the organic cation NH; group (or groups) and the perovskite layers is another important issue in the structural chemistry of the organic-inorganic layered perovskites. It influences not only the alignment and spacing of nearest-neighbor perovskite sheets, but also the degree of tilting and rotation of the comer-sharing MXg octahedra within the perovskite layers and the progression of structural transitions as a function of temperature. In principal, the ammonium head(s) of the R-NH; or ‘NH3-R-NH; cations can hydrogen bond to any of the eight halogens (four bridging and four terminal) within the holes formed by the corner- sharing MXg octahedra. In practice, due to the geometric constraints of the ammonium unit, the N-H. . .X interactions are generally either formed to two bridging halogens and one terminal halogen (bridging halogen configuration) or to two terminal halogens and one bridging halogen (terminal halogen configuration) (Fig. 9). The two types of bonding are also referred to as the “orthorhombic” and “monoclinic” configurations, respectively, because of the structures that result from these hydrogen-bonding schemes in the methylammonium cadmium and manganese halides and related systems (73). The former terminology is adopted in this chapter because, in general, the


C H,N

Bridging Halogen Terminal Halogen Con f i g u rat ion Configuration

Figure 9. Two hydrogen-bonding arrangements observed in the (100) oriented organic- inorganic layered perovskites. For simplicity, a methylammonium cation is displayed as the organic cation and the hydrogens on the carhon atom have been rernoved. Longer alkylam- moniuni cations and many aromatic arnnionium cations also adopt these same two bonding schemes (73).

symmetry of the structure depends on a number of other factors in addition to the hydrogen-bonding scheme.

The electronic structure and size of the metal atoms in the inorganic sheets also have an important impact on the local coordination and overall crystal structure. Early studies on the (R-NH3)2MX4 and (H3N-R-NH3)MG families generally considered systems with divalent first-row transition metals in the “M” site of the structure, as a result of the characteristically interesting lower dimensional magnetic properties found in these systems (22). The Jahn-Teller theorem implies that distortion of the octahedral MX6 coordination will occur whenever the resulting splitting of degenerate energy lev- els yields additional stabilization. The transition metal cations, Cu2+, high- spin C?+, and low-spin Co2+, with the corresponding electronic configura-

20 DAVID B. MITLI

tions, 3d9 (t2g6e,3), 3d4 (t2,3e,'), and 3d7 (t2,6eg1), are therefore expected to experience the most substantial Jahn-Teller distortion. The octahedra either elongate or contract along one axis, thereby potentially distorting the perovskite sheets containing these ions. The size of the metal cations is also often important. In Zn2+ and Cd2+ compounds, for example, the d shells are completely filled, with consequently no ligand field stabilization effects. The coordination chemistry for these atoms is primarily determined by the cation size and electrostatic/covalent bonding forces-thereby making Cd2+ more likely to adopt an octahedral coordination and the smaller Zn2+ ion more likely to be found with a lower coordination number (74). In fact, a large number of organic-inorganic perovskites have been synthesized containing Cd2+ in the metal site, while most similar examples containing Zn2+ form a different structure, typically with isolated ZnXi tetrahedra (75).

More recently, the Group 14 (IVA) metals (Ge, Sn, or Pb) (19, 21, 26, 57, 72,76), as well as the rare earth metals (27), have also been examined in the layered organic-inorganic perovskite structure. The Group 14 (IVA) divalent metal cations have the valence electronic configuration, ns2, formally leading to a lone pair of nonbonding electrons. The stereoactivity of these lone pairs for Ge2+, Sn2+, and Pb2+ plays an important role in determining the structure of the inorganic sheets in perovskites based on these cations.

As a result of the significant impact of the M2+ electronic structure and coordination chemistry on the crystallographic structure of the perovskite sheets, the detailed discussion of various members of the single-layer (100) oriented perovskite family will be categorized according to the chemical grouping of the metal atom. A summary of the room temperature crystal structure parameters for a number of these compounds can be found in Section 1I.J.

1 . Transition Metd Halides

In Section ILA, the methylammonium cation was found to fit into the 3D perovskite structure, CH3NH3MX3, where M is a divalent Group 14 (IVA) or rare earth metal cation. For many of the divalent transition metals, the methylammonium cation also stabilizes the single-layer perovskite structure. Several examples include (CH3NH3)?MCI4, where M = Cd (73), Mn (77), Fe (78), and Cu (79). Each of these compounds exhibit a series of structural transitions as a function of temperature, resulting from changes in the motion and hydrogen bonding of the methylammonium cation (see Section 1I.C). Among the transition metal halides, the copper(I1) chloride and bromide analogues of the layered perovskite structures have been examined in great detail as a result of the relative ease with which crystals can be prepared, as well as the interest in Jahn-Teller distortions and magnetic interactions in these systems.

edited by...the transition metal coordination chemistry of hemilabile ligands 233 caroline s. sidne....

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