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Solid state chemistry
Solid State Communications, Vol. 102, No. 2-3, pp. 79-85, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All fights reserved 0038-1098/97 $17.00+.00
Pergamon
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SOLID STATE CHEMISTRY F.J. DiSalvo Department of Chemistry, Cornell University, Ithaca, NY 14853, U.S.A. Even though non-molecular solids comprise only a small fraction of all known compounds, they have been technologically very important. These materials are the focus of Solid State Science. Once a new compound is synthesized, we have powerful tools of analysis to determine its structure and a host of physical properties and to connect these through increasingly sophisticated theoretical models. With perhaps the exception of high temperature superconductivity, the understanding of physical phenomena in solids is impressive. A similar understanding of the chemistry of solids is still, however, rather elusive in that we are quite unable to predict new crystal structures or the composition of compounds that are more than derivatives of known materials. In spite of this shortcoming, new discoveries through empirical searches and focused inquiries are accelerating. This article gives a short, admittedly biased and certainly incomplete, view of current progress and challenges in Solid State Chemistry and points out promising new developments in experimental and theoretical research. © 1997 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
o f " rules" that guide both the invention and synthesis of new compounds. This set of rules comprise the "rational synthetic method" of organic chemistry. No such rules are known for the design and synthesis of non-molecular (i.e. extended structure) solids. Unfortunately, it appears that organic systems cannot reproduce or improve upon some of the properties of extended structure compounds. The best conducting and magnetic materials by far, for example, are likely to remain such extended structure compounds. There are many other unique and useful properties of such solid state materials as well and many technologies are limited by the performance of those materials. Rather than recount these properties, we simply take it as generally known that there is a considerable need for new solid state materials that would deliver these improvements or display new and useful phenomena. While some predictive understanding of possible new structures has developed in a few restricted areas of solid state chemistry, such as in silicate and in zeolite compounds [1], prediction of the stoichiometry and structure of new compounds in the general case is presently impossible, at least on a useful time scale. This makes solid state chemistry a subject rich in possibilities for advancing the art of synthesis and rich in challenges for developing the necessary understanding
Increasingly, advances in condensed matter science as well as technology depend on novel materials. These may be known compositions of matter that are "processed" in new ways, such as All_xGaxAs multilayer systems, or completely new compounds with new structural arrangements of the constituent atoms, such as many of the recent superconducting copper oxide compounds. Solid State Chemists often focus on the synthesis of new compounds, not only in hopes of discovering new phenomena or enhanced properties, but also to understand the complexities of the myriad of possible bonding arrangements that can occur with the elements of the periodic table. Since the relationship between structure and properties has been a central theme of condensed matter science, it is clear that the discovery of new phenomena and of improved properties will depend in a large part on the discovery of new compounds and in modifying those compounds in deliberate ways. However, it is surprising to many that the discovery of new solid state compounds, especially those with novel structure types, is largely an empirical or "Edisonian" process. In contrast, the design and synthesis of organic molecules is a well developed science with a simple set 79
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to build a "rational synthesis" approach to the design and production of new compounds. In this article I focus on some of the recent achievements in solid state synthesis, in understanding structures and on the challenges ahead. I cannot mention all the exciting work in the field, but choose only a few examples for illustrative purposes. Nor do I discuss the molecular organic solid state [2], although the increasing interest in materials containing organic molecules that are part of an extended network will be briefly mentioned. I apologize not only to those whose contributions to the advancement of solid state chemistry have been important and/or many and yet are not included here, but also for being so lazy and parochial as to use some of the work done in my research group as examples of such advances. Further, my views of the challenges that lay ahead are my interpretation of what many others have observed. Any errors in fact or fancy are my own. 2. BACKGROUND While rather complex structures and compositions, such as Ag19Pb4568SbElZ18, have been reported, only binary compounds have been systematically studied [3]. Binary phase diagrams for any element combined with any other element in the periodic table have been well investigated for more than 80% of all possible combinations and some work has been done on most of the remainder. Of the approximately 25 000 known binary compounds, most adopt simple stoichiometries (the chemical formula contains small whole numbers, such as MX, MX2, MX3 or MaXs) and simple structures. Indeed only about 100 structure types are necessary to classify the vast majority of these binary compounds [2]. This huge data base has allowed empirically based "structure maps" to be developed that "predict" the structures with greater than 95% confidence (after the fact!) based on a few empirical atomic variables such as the electronegativity, atomic radii and number of valence electrons [3, 4]. In the few cases where binary phase diagrams are incompletely known, such structure maps may help in the prediction of the composition and structure of the stable phases. In synthetic solid state chemistry, then, the challenge lies in the synthesis and understanding of ternary or greater compositions. Of the approximately 100 000 possible ternary phase diagrams, only about 5% have been investigated [4]. Already there are more then 700 known ternary structure types; consequently, we might expect several thousand more. As is the case for binary compounds, most of the known ternaries adopt simple stoichiometries with small integer molar ratios. Except for some empirical predictions concerning the potential existence of at least one intermetallic ternary compound (at any corn-
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position) in a given phase diagram [5], there are no general schemes which can predict ternary compound composition or structure. Of course, chemical similarity of some elements and electron counting rules do allow one to suggest what chemical substitutions might be made in a known structure class so as to preserve the structure. For example, there are hundreds of compounds and their alloys that adopt the ThCr2Si2 structure type and new ones are still being discovered. In quaternary and quinary systems we have barely scratched the surface of what could possibly be synthesized. There are more than 50 million possible quinary phase diagrams. At present we do not know if there will be many quinary structures - that is, a structure type that is only adopted when 5 elements are present - or if the vast majority of such five component systems will prefer to form a mixture of binary and ternary compounds or their alloys, for example. In ternary systems containing one electronegative element (such as fluorine, oxygen, etc.), the free energy of formation of the ternary compound from parent binaries of that electronegative element is small when compared to the formation energy of the binaries themselves. This could lead to the expectation that the thermodynamic driving force for the formation of quinary structures, rather than a mixture of simpler structures, could become quite small. Thus the potential number of these quinary structures could be much smaller than might beexpected on the basis of what little we know about ternary phases. Extrapolation from the few known quaternary and quinary structures known is risky to say the least, but combining metals that prefer different coordination polyhedra may be one way to force the adoption of quaternary or quinary structures. Perhaps less risky is to conclude on the basis of the work on superconducting copper oxides that the structural and electronic degrees of freedom offered by complex quaternary and quinary structures allows at least enhanced if not novel behavior and properties. Because of our inability to predict the composition or structure of new compounds, advances in preparing new phases have been made largely by empirical methods based on prior art and invention as well as intuition based on experience. However, some research is beginning to focus on the specific challenges posed by the need for predictability and by handling the vast numbers of potential ternary, quaternary, etc. compounds that might be synthesized. We next discuss some of these advances and approaches and then look a little into the future. 3. RECENT ADVANCES IN SYNTHETIC METHODS The synthesis of extended structure compounds usually takes place at high temperatures (typically in
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the range of 500°C to 2500°C). At these temperatures the system typically reaches thermodynamic equilibrium in hours or days, which is a common solid state reaction time. Thermodynamic products are thus obtained, often with 100% yield. Again this situation is in considerable contrast to organic chemistry, where the products are determined primarily by kinetics rather than thermodynamics. The central issues in organic synthesis, then, are mechanisms of molecular change and separation of kineticaUy competing products. In solid state chemistry the synthetic issues are nucleation and diffusion (growth). Nucleation is the step in which atoms first organize themselves in the new structural arrangement of the product from that of reactants and is imagined to form a "seed" a few nanometers in dimension. While the principles of diffusion and growth are reasonably well understood, the nucleation step(s) is in most cases a mystery - but is sometimes implicated as the problem in a particular synthesis. 3.1. Nucleation David Johnson and coworkers are beginning to address nucleation issues by studying multilayer films using X-ray diffraction, TEM and DSC [6]. If the individual layers of a multilayer sandwich of different elements are very thin (less than 50 A), the first process observed upon heating is diffusion without nucleation to make an amorphous film. Further heating usually results in nucleation of a crystalline phase that grows by diffusion. Interestingly, the nucleated phase is sometimes metastable and/or different than that expected from the high temperature phase diagram. A significant future challenge is to develop experimental techniques to study nucleation phenomena in more detail. Since nucleation often occurs at composition gradients in the bulk or at interfaces between bulk solids, a central challenge is to design probes that can " s e e " what is happening in those regions inside the solid reaction mass. o
3.2. Synthesis methods More traditional methods of compound synthesis or crystal growth have been extended in many ways: to higher pressures, to lower temperatures and in different media (solvents and melts or fluxes). These trends will certainly continue to produce new compounds and structures that cannot be obtained by the historical method of "heat and beat". A few examples that interest me follow. 3.2.1. Low temperature. Low temperature synthesis is that accomplished below approximately 400°C, often even as low as room temperature. Such methods have been developed to produce a variety of novel compounds that cannot be prepared in any other way (at least so far!).
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This low temperature chemistry is sometimes called "chimie douce", a term coined by Jean Rouxel meaning "soft chemistry" or even "sweet chemistry". Typically a compound (the host), which is often prepared at higher temperatures by traditional methods, is modified by intercalation or deintercalation of a species (the guest) into or out of the host material. Such reactions are possible when the guest species in a given structure is mobile (i.e. has a high diffusion coefficient) and can be chemically or electrochemically removed or replaced in bulk of the material in a redox or exchange reaction [7, 8]. Using such methods, many more new phases that cannot be prepared by other methods because of their metastability will be obtained in the future. 3.2.2. Fluxes. Fluxes are substances that melt at elevated temperatures and are good solvents and crystal growth media for extended structure materials. While other methods of crystal growth are useful in specific cases, the flux technique is the most general. If the structure of a new material is not simple, it is often necessary to use small single crystals to determine that structure. Powder diffraction techniques for determining structure are rapidly improving, but the more complicated the structure the more likely that single crystals will be needed. When larger crystals are needed, especially for technological applications, the flux method is in less common use [9]. Metal [10], metal salts and oxide fluxes have been used to prepare a large number of compounds including the superconducting copper oxides [11]. More recently other fluxes have also been developed, such as hydroxides (12) or polychalocogenides [13, 141. When the flux is present in low volume fraction, it is usually called a mineralizer. The effect of diopside (CaMgSi206) mineralizer on the growth of oxide phases depends on its content: below 5 wt.%, it acts as a mineralizer for porcelain (an aluminosilicate); in the range from 10 to 15 wt.%, diopside favors crystallization of anorthite (CaAl2Si2Os) , reduces the quantity of muUite (A165i2013) formed and intensifies the dissolution of quartz [15]. Ceramics formed using this mineralizer are noted for their improved mechanical properties, white color and translucence. In the search for new compounds, the flux technique will continue to be invaluable. In some cases, new fluxes must be developed because traditional substances do not work. See for example the discussion of nitrides below. 3.2.3. Solvothermal methods. Hydrothermal crystal growth has been observed in nature and in the laboratory as an excellent preparative route to many crystalline materials, especially oxides. However, when water is either reactive or inert as a solvent, other molecular liquids (sometimes supercritical) are potentially useful.
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Several solvents have been exploited as media for crystal growth, including: ammonia [16, 17], ethylenediamine [17, 18] and methanol [19]. A wide range of compounds have been formed from the solvothermal technique, for example, chalcogenides, phosphates and carbonates [20, 21]. Given the recent success in using such solvents, it is likely that the technique will be extended to many other polar solvents in specific and difficult cases. 4. NOVEL MATERIALS Now I turn to a specific classes of compounds. Even for the relatively small field of solid state chemistry, there is a large variety in the materials of current interest. Oxides are the largest and most studied class of solid state compounds, in part because many minerals are oxides. I have chosen to mention a few other systems because they caught my eye or because I am involved in such research. By no means should this list be considered all inclusive or even representative of the field as a whole. 4.1. Cluster compounds Traditionally the chemistry of solids could be divided into intermetallic phases and normal valence compounds. In the latter case the compounds formally contain cations and anions, oxidation states can be assigned and the structures are always arranged so that the cations have anions as near neighbors and vice versa. Some thirty years ago, with the work of Franzen [22] and others, it became apparent that many compounds were intermediate in their nature; that is, they contain metalmetal bonds as well as metal-anion bonds [23]. One can describe the structure of such phases as the packing of molecular scale metal clusters that are surrounded by anions. Many related phases can similarly be described as resulting from the condensation of such clusters to form one-, two-, or even three-dimensional networks [24, 25]. The discovery of such phases continues unabated, especially in the early transition metals and rare earths. Some, such as the now famous Chevrel phases, have physical properties that have attracted the attention of many solid state scientists. Even more recently, chemists have begun to build exactly the same metal clusters via solution methods methods that are more typical of organic or inorganic chemistry [26, 27]. Further, in some cases it has proved possible to excise clusters from the solids mentioned in the previous paragraph and bring them into solution [28]. One focus of current research is the attempt to join such solvated clusters via organic linking groups into one-, two-, or three-dimensional networks or to trap the clusters in a polymer matrix [29]. Such a building-block approach to solids may prove to be a powerful technique for
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designing solids with specific structural features that combine both organic and solid state properties. 4.2. Coordination solids In recent years, multitopic organic ligands (those with several different binding sites for metals) and transition metals have been used to construct networks that have the same connectivity as well-known solid state structure types [30]. This might be thought of as an extension of Prussian Blue type compounds, which again are themselves the focus of recent research [31]. In this building-block approach [32, 33], solid state structural prototypes are chosen as targets and are constructed using organic ligands with the required geometry and a transition metal ion with the required coordination propensity. Structures that have been constructed by this approach include the analogs of diamond, AIB2, a-ThSi2, oL-Po, ruffle, PtS, SrSi2 and rare [3, 10] nets [34-36]. This synthetic methodology has also lead to the design and construction of materials with properties such as porosity and guest exchangeability generally associated with zeolites [37]. The use of organic ligands in these materials is very attractive since the size, shape and nature of the cavities can be suitably tailored. The ability to do catalysis within these cavities in the coordination solids has also been demonstrated. With more and more structural prototypes being constructed through this approach, much like organic chemistry, it should be possible to establish certain recurrent relationships between the molecular structure(s) and the final solid state structure. Networks with strong covalent bonds to transition metals, rather than weaker covalent bonds, have also been recently prepared [38]. Such relationships should hopefully pave the way for the rational design and construction of solid state structures with desired structural properties. This approach could also be combined with the cluster chemistry discussed above. It remains to be seen what range electronic or even magnetic properties can be expected from such materials. 4.3. Ternary nitrides While most of the binary nitrides which form have been fairly thoroughly investigated [39] and in a number of cases are of technological significance, the chemistry of ternary and higher nitrides has only recently attracted the broad interest of solid state chemists. This is a little surprising, since compounds made from neighboring elements (C, Si, P, S and especially O) have been much more widely investigated. Recent reviews summarize what is currently known about this young field [40-42]. Here we mention two features of the recent research.
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First, a number of novel structures are observed in ternary nitrides. Based on electronegativity differences, the bonding in nitrides might be expected to be less ionic than in oxides, but perhaps less covalent than carbides or sulfides. The variety of structures observed bear this out. For example, silicate and germinate chemistry is completely dominated by tetrahedral M+404 units that share only comers between two tetrahedra. In temary silicon and especially in germanium nitrides both corner sharing of M+aN4 units (between two or three tetrahedra) and edge sharing are common. In germanium nitrides a variety of Ge oxidation states can be stabilized: Ge +4, Ge+2 and even Ge - 2. Such behavior in oxides is unknown, while in sulfides only the first two oxidation states are known. Second, the preparation of single crystais of ternary nitrides has been a considerable challenge. Several new methods have met with some success: alkali metal [40] or alkali metal amide [12] fluxes and high pressure ammonia solvents [43]. Yet these methods are not sufficient to prepare all ternary phases, especially many of those containing early transition metals. There remains considerable opportunity for development of fluxes or other techniques to prepare these unusual compounds. In fact, it appears that the alkali metal flux method may be useful in the preparation of GaN crystals as substrates for blue lasers. Many oxynitrides [44] show promise as catalysts [45] and recently, framework nitrides with the semi-dense sodalite framework have been synthesized [46]. Routes to framework nitrides with large cavities are starting to be explored and these may compete favorably with current catalysts of commercial importance. 5. OTHER CHALLENGES FOR THE FUTURE As mentioned in the introduction, only a very small fraction of ternary or quaternary phase diagrams are known. So it is likely that there are a huge number of new compounds and structures yet to be discovered. If there are new or enhanced phenomena yet to be discovered in these new materials (only with considerable hubris can we claim that all interesting or useful phenomena in solids are now known), then the challenge is how to find these compounds and phenomena as efficiently as possible. There are perhaps two new approaches to this question: Combinatorial Synthesis and Direct Computation. We discuss each a little more below.
5.1. Combinatorial synthesis In a combinatorial methodology, from a given set of starting materials, a library of compounds is synthesized in a parallel fashion. Such parallel synthesis allows sampling of a large number of compounds whose syntheses may otherwise be considered impracticable.
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This methodology has already gained wide popularity in the area of drug design [46]. This initial approach is confined to the synthesis of libraries of organic compounds. Recently, Schultz and co-workers applied this approach to solid state synthesis of some inorganic materials [47]. Specifically, they synthesized arrays of multilayer films by sputtering. Each film in the array had a different composition that was determined by masks that were used during the film deposition. Superconducting copper oxide thin films derived from Cu, Bi203, CaO, PbO, SrCO3, Y203 and BaCO3 targets were prepared. From a library consisting of a 128-member array, using resistance measurements, the optimal superconducting phase as the composition corresponding to the previously known BiESr2Ca2Cu3010 was identified. The combinatorial methodology was also used to search for materials with large magnetoresistance [48] in perovskite (ABO3) and related structural families (AEBO4 or An+lBnO3n+l).The search indeed resulted in the discovery of a family of Co-containing compounds with compositions LnxMyCoO3_y, where M = Ba, Sr, Ca or Pb and Ln = La or Y, with colossal magnetoresistance (CMR) properties. These examples are just initial steps. The methodology used was to "engineer" a known class of materials to optimize a property by varying the composition or looking at related elements. This is what Materials Chemists already do quite well, but this approach may allow them to do it much faster. The ultimate challenge is to explore ternary and quaternary systems for new compounds, structures and even properties at a considerably enhanced rate. If one is only searching for a property that is easily detected by mass scanning, such as superconductivity, then the technology is close at hand. However, if a broad structural search is desired, the structure would have to be determined on thousands of samples, many multiphase, in a short period of time. It is just becoming possible to imagine that X-ray diffraction systems could be developed to obtain diffraction patterns in seconds from each sample in the library and that computers could determine or match structures in a similar time frame. It may be a decade or so before this is possible; it may take synchrotron X-ray intensities and bigger, faster workstations, as well as continued advances in the presently rapid improvements in algorithms for determining structures from powder or single crystal diffraction data. I do not want to minimize the many practical problems in implementing this approach, but I think these could be overcome in the next decade or even sooner. 5.2. Computation and structure prediction This issue of the journal contains an article by M. Parrinello on ab initio Molecular Dynamic calculations
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to predict the structures of solid state compounds. There are now perhaps a dozen groups world wide that are making some progress in this most challenging theoretical problem. These also include M. Teter at Cornell, S. Lee at Michigan, M. Cohen at Berkeley, J. Joannapolous at MIT and M. Jansen at Bonn. For such an approach to be useful to most solid state chemists, direct access to the programs using local workstations is necessary. To be useful to the synthetic chemist, the programs need to be fast and have moderate reliability in predicting characteristics of interest, such as structure. Such programs could be a useful guide in determining which phase diagrams deserve early experimental scrutiny. We are just at the beginning of this process, which will require faster computers with massive memories and further development of efficient codes and algorithms. Already some solid state chemists have found these programs useful in obtaining a starting point in difficult structural determinations [49]. Predicting exotic properties in new materials, such as high temperature superconductivity, will be even more challenging and is much less likely to happen in the next decade.
3.
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
6. SUMMARY
15.
Over the last decade Solid State Chemistry has been a growing field of research, primarily due to the huge excitement generated by the discovery of high temperature superconductors. However, there are major challenges to making a predictable synthetic science and in speeding up the process of discovery. Nonetheless, advances in understanding fundamental processes such as nucleation have recently appeared. There are many structurally interesting systems beside oxides and I have mentioned several. There are some recent developments that suggest experimental methods to speed up the synthesis and discovery of new phases and perhaps even theoretical methods to help in the search. The next decade of R&D in solid state chemistry is likely to be exciting and formative.
16.
Acknowledgements---I am indebted to my research group who helped with the writing of this manuscript, especially Simon Clarke, Glen Kowach, Sami Malik and Dhandapani Venkataraman. Errors and misconceptions, however, are my own. REFERENCES 1. Lewis, D.W., Willcock, D.J., Catlow, C.R.A., Thomas, J.M. and Hutchings, G.J., Nature, 382, 1996, 604. . For an overview of organic solid state materials see: (a) Crystal Engineering: The Design of Organic Solids (Edited by G.R. Desiraju), Elsevier,
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Amsterdam, 1989; and Supramolecular Chemistry (Edited by J.M. Lehn). VCH, Weinheim, 1995. For an overview see: Trends in Materials Data: Regularities and Predictions (Edited by J.R. Podgers and P. Villars), Feb 1993 Issue of the MRS Bulletin. Villars, P., J. Less-Common Met., 92, 1983, 215; 99, 1984, 33; 102, 1984, 199. Villars, P., J. Less-Common Metals, 119, 1986, 175. Noh, M., Johnson, C.D., Hornbostel, M.D., Theil, J. and Johnson, D.C., Chem. Mater., 8, 1996, 1625. Rouxel, J.,Advances in the Synthesis and Reactivity of Solids, 2, 1994, 27. Gopalakrishnan, J., Chem. Mater., 7, 1995, 1265. The Growth of Single Crystals (E,dited by R.A. Laudise). Prentice-Hall, New Jersey, 1970. Canfield, P.C. and Fisk, Z., Phil, Mag., B65, 1992, 1117. Schneemeyer, L.F., Process. Prop. High-To Supercond., 45 (Edited by Jin and Sungho). World Sci.; Singapore, 1993. Sandford, D., Marquez, L.N. and Stacy, A.M., Appl. Phys. Lett., 67, 1995, 422. Kanatzidis, M.G., Chem. Mater., 2, 1990, 353. Sunshine, S.A., Kang, D. and Ibers, J.A., J. Am. Chem. Soc., 109, 1987, 6202. Maslennikova, G.N., Platov, Yu.T. and Koneshova, T.I., Inorg. Mater., 31, 1995, 734. Jerome, J.E., Wood, P.T., Pennington, W.T. and Kolis, J.W., Inorg. Chem., 33, 1994, 1733. Dhingra, S.S. and Haushalter, R.C., Chem. Mater., 6, 1994, 2376. Fehlner, T.P., Inorg. Chem., 4, 1992, 315. Das, B.K. and Kanatzidis, M.G., Inorg. Chem., 34, 1995, 5721. Chippindale, A.M., Cowley, A.R. and Walton, R.J., J. Mater. Chem., 6, 1996, 611. Gutschke, S.O., Zlawin, A.M. and Wood, P.T.J. Chem. Soc. Chem. Commun., 1995, 2197. Franzen, H.F. and Graham, J., Inorg. and Nucl. Chem., 28, 1966, 377; Z. Kristallog., 123, 1966, 2. Simon, A., Angew. Chem. Int. Ed. Engl., 27, 1988, 159. Simon, A., J. Alloys and Comp., 229, 1995, 158. Corbett, J.D. and McCarley, R.E., Crystal Chemistry and Properties of Materials with Quasi-OneDimensional Structures, 179. D. Reidel, 1986. Zhang, X. and McCarley, R.E., Inorg. Chem, 34, 1995, 2678. Saito, T., Yamamoto, N., Nagase, T., Kobayashi, K., Yamagata, T., Imoto, H. and Unoura, K., lnorg. Chem., 29, 1990, 764. Tian, Y. and Hughbanks, T., lnorg. Chem, 34, 1995, 6250. Golden, J.H., Deng, H., DiSalvo, F.J., Frechet, J.MJ. and Thompson, P.M., Science, 268, 1995, 1463. Hoskins, B.F. and Robson, R., J. Am. Chem. Soc., 112, 1990, 1546. See, for example, Entley, W.R. and Girolami, G.S., Inorg. Chem., 33, 1994, 5165.
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32. Simard, M., Su, D. and Wuest, J.D., J. Am. Chem. Soc., 113, 1991, 4696. 33. Kasynski, P., Friedli, A.C. and Michl, J., J. Am. Chem. Soc., 114, 1992, 601. 34. Gardner, G.B., Venkataraman, D., Moore, J.S. and Lee, S., Nature, 374, 1995, 792. 35. Carlucci, L., Ciani, G., Proserpio, D.M. and Sironi, A., J. Chem. Soc., Chem. Commun., 1996, 1393. 36. Abrahams, B.F., Batten, S.R., Hmait, H., Hoskins, B.F. and Robson, R.,J. Chem. Soc., Chem. Commun., 1996, 1313. 37. Gardner, G.B., Kiang, Y.H., Lee, S., Asgaonkar, A. and Venkataraman, D., J. Am. Chem. Soc., 118, 1996, 6946. 38. Vaid, T.P. and Wolczanski, P.T. To be published. 39. Brese, N.E. and O'Keefe, M., Structure and Bonding, 79, 1992, 309. 40. DiSalvo, F.J. and Clarke, S.J., Current Opinion in Solid State and Materials Science, 1, 1996, 241. 41. zur Loye, H.C., Houmes, J.D. and Bem, D.S., in The Chemistry of Transition Metal Carbides and
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Nitrides (Edited by S.T. Oyama). Blackie Academic and Professional, Glasgow, UK, 1995. Niewa, R. and Jacobs, H., Chem. Rev., 96, 1996, 2053. Jacobs, H. and Schmidt, D., Current Topics in Materials Science, 8, 1982, 379. Marchand, R., Laurent, Y., Guyader, J., L'Haridon, P. and Verdier, P., J. Eur. Ceramic Soc., $, 1991, 197. Fripiat, N. and Grange, P., Chem. Commun., 1996, 1409. Schnick, W., Zeolites and Related Microporous Materials: State of the Art 1994; Studies in Surface Science and Catalysis, 84, 1994, 2221. An issue of Accounts of Chemical Research (March 1999, Vol. 29) was devoted to combinatorial chemistry. Xiang, X.D., Sun, X., Briceno, G., Lou, Y., Wang, K.A., Chang, H., Wallace-Freedman, W.G., Chen, S.W. and Schultz, P.G., Science, 268, 1995, 1738. Briceno, G., Chang, H., Sun, X., Schultz, P.G. and Xiang, X.D., Science, 270, 1995, 273. Dronskowski, R. To be published.
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SOLID STATE CHEMISTRY F.J. DiSalvo Department of Chemistry, Cornell University, Ithaca, NY 14853, U.S.A. Even though non-molecular solids comprise only a small fraction of all known compounds, they have been technologically very important. These materials are the focus of Solid State Science. Once a new compound is synthesized, we have powerful tools of analysis to determine its structure and a host of physical properties and to connect these through increasingly sophisticated theoretical models. With perhaps the exception of high temperature superconductivity, the understanding of physical phenomena in solids is impressive. A similar understanding of the chemistry of solids is still, however, rather elusive in that we are quite unable to predict new crystal structures or the composition of compounds that are more than derivatives of known materials. In spite of this shortcoming, new discoveries through empirical searches and focused inquiries are accelerating. This article gives a short, admittedly biased and certainly incomplete, view of current progress and challenges in Solid State Chemistry and points out promising new developments in experimental and theoretical research. © 1997 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
o f " rules" that guide both the invention and synthesis of new compounds. This set of rules comprise the "rational synthetic method" of organic chemistry. No such rules are known for the design and synthesis of non-molecular (i.e. extended structure) solids. Unfortunately, it appears that organic systems cannot reproduce or improve upon some of the properties of extended structure compounds. The best conducting and magnetic materials by far, for example, are likely to remain such extended structure compounds. There are many other unique and useful properties of such solid state materials as well and many technologies are limited by the performance of those materials. Rather than recount these properties, we simply take it as generally known that there is a considerable need for new solid state materials that would deliver these improvements or display new and useful phenomena. While some predictive understanding of possible new structures has developed in a few restricted areas of solid state chemistry, such as in silicate and in zeolite compounds [1], prediction of the stoichiometry and structure of new compounds in the general case is presently impossible, at least on a useful time scale. This makes solid state chemistry a subject rich in possibilities for advancing the art of synthesis and rich in challenges for developing the necessary understanding
Increasingly, advances in condensed matter science as well as technology depend on novel materials. These may be known compositions of matter that are "processed" in new ways, such as All_xGaxAs multilayer systems, or completely new compounds with new structural arrangements of the constituent atoms, such as many of the recent superconducting copper oxide compounds. Solid State Chemists often focus on the synthesis of new compounds, not only in hopes of discovering new phenomena or enhanced properties, but also to understand the complexities of the myriad of possible bonding arrangements that can occur with the elements of the periodic table. Since the relationship between structure and properties has been a central theme of condensed matter science, it is clear that the discovery of new phenomena and of improved properties will depend in a large part on the discovery of new compounds and in modifying those compounds in deliberate ways. However, it is surprising to many that the discovery of new solid state compounds, especially those with novel structure types, is largely an empirical or "Edisonian" process. In contrast, the design and synthesis of organic molecules is a well developed science with a simple set 79
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to build a "rational synthesis" approach to the design and production of new compounds. In this article I focus on some of the recent achievements in solid state synthesis, in understanding structures and on the challenges ahead. I cannot mention all the exciting work in the field, but choose only a few examples for illustrative purposes. Nor do I discuss the molecular organic solid state [2], although the increasing interest in materials containing organic molecules that are part of an extended network will be briefly mentioned. I apologize not only to those whose contributions to the advancement of solid state chemistry have been important and/or many and yet are not included here, but also for being so lazy and parochial as to use some of the work done in my research group as examples of such advances. Further, my views of the challenges that lay ahead are my interpretation of what many others have observed. Any errors in fact or fancy are my own. 2. BACKGROUND While rather complex structures and compositions, such as Ag19Pb4568SbElZ18, have been reported, only binary compounds have been systematically studied [3]. Binary phase diagrams for any element combined with any other element in the periodic table have been well investigated for more than 80% of all possible combinations and some work has been done on most of the remainder. Of the approximately 25 000 known binary compounds, most adopt simple stoichiometries (the chemical formula contains small whole numbers, such as MX, MX2, MX3 or MaXs) and simple structures. Indeed only about 100 structure types are necessary to classify the vast majority of these binary compounds [2]. This huge data base has allowed empirically based "structure maps" to be developed that "predict" the structures with greater than 95% confidence (after the fact!) based on a few empirical atomic variables such as the electronegativity, atomic radii and number of valence electrons [3, 4]. In the few cases where binary phase diagrams are incompletely known, such structure maps may help in the prediction of the composition and structure of the stable phases. In synthetic solid state chemistry, then, the challenge lies in the synthesis and understanding of ternary or greater compositions. Of the approximately 100 000 possible ternary phase diagrams, only about 5% have been investigated [4]. Already there are more then 700 known ternary structure types; consequently, we might expect several thousand more. As is the case for binary compounds, most of the known ternaries adopt simple stoichiometries with small integer molar ratios. Except for some empirical predictions concerning the potential existence of at least one intermetallic ternary compound (at any corn-
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position) in a given phase diagram [5], there are no general schemes which can predict ternary compound composition or structure. Of course, chemical similarity of some elements and electron counting rules do allow one to suggest what chemical substitutions might be made in a known structure class so as to preserve the structure. For example, there are hundreds of compounds and their alloys that adopt the ThCr2Si2 structure type and new ones are still being discovered. In quaternary and quinary systems we have barely scratched the surface of what could possibly be synthesized. There are more than 50 million possible quinary phase diagrams. At present we do not know if there will be many quinary structures - that is, a structure type that is only adopted when 5 elements are present - or if the vast majority of such five component systems will prefer to form a mixture of binary and ternary compounds or their alloys, for example. In ternary systems containing one electronegative element (such as fluorine, oxygen, etc.), the free energy of formation of the ternary compound from parent binaries of that electronegative element is small when compared to the formation energy of the binaries themselves. This could lead to the expectation that the thermodynamic driving force for the formation of quinary structures, rather than a mixture of simpler structures, could become quite small. Thus the potential number of these quinary structures could be much smaller than might beexpected on the basis of what little we know about ternary phases. Extrapolation from the few known quaternary and quinary structures known is risky to say the least, but combining metals that prefer different coordination polyhedra may be one way to force the adoption of quaternary or quinary structures. Perhaps less risky is to conclude on the basis of the work on superconducting copper oxides that the structural and electronic degrees of freedom offered by complex quaternary and quinary structures allows at least enhanced if not novel behavior and properties. Because of our inability to predict the composition or structure of new compounds, advances in preparing new phases have been made largely by empirical methods based on prior art and invention as well as intuition based on experience. However, some research is beginning to focus on the specific challenges posed by the need for predictability and by handling the vast numbers of potential ternary, quaternary, etc. compounds that might be synthesized. We next discuss some of these advances and approaches and then look a little into the future. 3. RECENT ADVANCES IN SYNTHETIC METHODS The synthesis of extended structure compounds usually takes place at high temperatures (typically in
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the range of 500°C to 2500°C). At these temperatures the system typically reaches thermodynamic equilibrium in hours or days, which is a common solid state reaction time. Thermodynamic products are thus obtained, often with 100% yield. Again this situation is in considerable contrast to organic chemistry, where the products are determined primarily by kinetics rather than thermodynamics. The central issues in organic synthesis, then, are mechanisms of molecular change and separation of kineticaUy competing products. In solid state chemistry the synthetic issues are nucleation and diffusion (growth). Nucleation is the step in which atoms first organize themselves in the new structural arrangement of the product from that of reactants and is imagined to form a "seed" a few nanometers in dimension. While the principles of diffusion and growth are reasonably well understood, the nucleation step(s) is in most cases a mystery - but is sometimes implicated as the problem in a particular synthesis. 3.1. Nucleation David Johnson and coworkers are beginning to address nucleation issues by studying multilayer films using X-ray diffraction, TEM and DSC [6]. If the individual layers of a multilayer sandwich of different elements are very thin (less than 50 A), the first process observed upon heating is diffusion without nucleation to make an amorphous film. Further heating usually results in nucleation of a crystalline phase that grows by diffusion. Interestingly, the nucleated phase is sometimes metastable and/or different than that expected from the high temperature phase diagram. A significant future challenge is to develop experimental techniques to study nucleation phenomena in more detail. Since nucleation often occurs at composition gradients in the bulk or at interfaces between bulk solids, a central challenge is to design probes that can " s e e " what is happening in those regions inside the solid reaction mass. o
3.2. Synthesis methods More traditional methods of compound synthesis or crystal growth have been extended in many ways: to higher pressures, to lower temperatures and in different media (solvents and melts or fluxes). These trends will certainly continue to produce new compounds and structures that cannot be obtained by the historical method of "heat and beat". A few examples that interest me follow. 3.2.1. Low temperature. Low temperature synthesis is that accomplished below approximately 400°C, often even as low as room temperature. Such methods have been developed to produce a variety of novel compounds that cannot be prepared in any other way (at least so far!).
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This low temperature chemistry is sometimes called "chimie douce", a term coined by Jean Rouxel meaning "soft chemistry" or even "sweet chemistry". Typically a compound (the host), which is often prepared at higher temperatures by traditional methods, is modified by intercalation or deintercalation of a species (the guest) into or out of the host material. Such reactions are possible when the guest species in a given structure is mobile (i.e. has a high diffusion coefficient) and can be chemically or electrochemically removed or replaced in bulk of the material in a redox or exchange reaction [7, 8]. Using such methods, many more new phases that cannot be prepared by other methods because of their metastability will be obtained in the future. 3.2.2. Fluxes. Fluxes are substances that melt at elevated temperatures and are good solvents and crystal growth media for extended structure materials. While other methods of crystal growth are useful in specific cases, the flux technique is the most general. If the structure of a new material is not simple, it is often necessary to use small single crystals to determine that structure. Powder diffraction techniques for determining structure are rapidly improving, but the more complicated the structure the more likely that single crystals will be needed. When larger crystals are needed, especially for technological applications, the flux method is in less common use [9]. Metal [10], metal salts and oxide fluxes have been used to prepare a large number of compounds including the superconducting copper oxides [11]. More recently other fluxes have also been developed, such as hydroxides (12) or polychalocogenides [13, 141. When the flux is present in low volume fraction, it is usually called a mineralizer. The effect of diopside (CaMgSi206) mineralizer on the growth of oxide phases depends on its content: below 5 wt.%, it acts as a mineralizer for porcelain (an aluminosilicate); in the range from 10 to 15 wt.%, diopside favors crystallization of anorthite (CaAl2Si2Os) , reduces the quantity of muUite (A165i2013) formed and intensifies the dissolution of quartz [15]. Ceramics formed using this mineralizer are noted for their improved mechanical properties, white color and translucence. In the search for new compounds, the flux technique will continue to be invaluable. In some cases, new fluxes must be developed because traditional substances do not work. See for example the discussion of nitrides below. 3.2.3. Solvothermal methods. Hydrothermal crystal growth has been observed in nature and in the laboratory as an excellent preparative route to many crystalline materials, especially oxides. However, when water is either reactive or inert as a solvent, other molecular liquids (sometimes supercritical) are potentially useful.
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Several solvents have been exploited as media for crystal growth, including: ammonia [16, 17], ethylenediamine [17, 18] and methanol [19]. A wide range of compounds have been formed from the solvothermal technique, for example, chalcogenides, phosphates and carbonates [20, 21]. Given the recent success in using such solvents, it is likely that the technique will be extended to many other polar solvents in specific and difficult cases. 4. NOVEL MATERIALS Now I turn to a specific classes of compounds. Even for the relatively small field of solid state chemistry, there is a large variety in the materials of current interest. Oxides are the largest and most studied class of solid state compounds, in part because many minerals are oxides. I have chosen to mention a few other systems because they caught my eye or because I am involved in such research. By no means should this list be considered all inclusive or even representative of the field as a whole. 4.1. Cluster compounds Traditionally the chemistry of solids could be divided into intermetallic phases and normal valence compounds. In the latter case the compounds formally contain cations and anions, oxidation states can be assigned and the structures are always arranged so that the cations have anions as near neighbors and vice versa. Some thirty years ago, with the work of Franzen [22] and others, it became apparent that many compounds were intermediate in their nature; that is, they contain metalmetal bonds as well as metal-anion bonds [23]. One can describe the structure of such phases as the packing of molecular scale metal clusters that are surrounded by anions. Many related phases can similarly be described as resulting from the condensation of such clusters to form one-, two-, or even three-dimensional networks [24, 25]. The discovery of such phases continues unabated, especially in the early transition metals and rare earths. Some, such as the now famous Chevrel phases, have physical properties that have attracted the attention of many solid state scientists. Even more recently, chemists have begun to build exactly the same metal clusters via solution methods methods that are more typical of organic or inorganic chemistry [26, 27]. Further, in some cases it has proved possible to excise clusters from the solids mentioned in the previous paragraph and bring them into solution [28]. One focus of current research is the attempt to join such solvated clusters via organic linking groups into one-, two-, or three-dimensional networks or to trap the clusters in a polymer matrix [29]. Such a building-block approach to solids may prove to be a powerful technique for
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designing solids with specific structural features that combine both organic and solid state properties. 4.2. Coordination solids In recent years, multitopic organic ligands (those with several different binding sites for metals) and transition metals have been used to construct networks that have the same connectivity as well-known solid state structure types [30]. This might be thought of as an extension of Prussian Blue type compounds, which again are themselves the focus of recent research [31]. In this building-block approach [32, 33], solid state structural prototypes are chosen as targets and are constructed using organic ligands with the required geometry and a transition metal ion with the required coordination propensity. Structures that have been constructed by this approach include the analogs of diamond, AIB2, a-ThSi2, oL-Po, ruffle, PtS, SrSi2 and rare [3, 10] nets [34-36]. This synthetic methodology has also lead to the design and construction of materials with properties such as porosity and guest exchangeability generally associated with zeolites [37]. The use of organic ligands in these materials is very attractive since the size, shape and nature of the cavities can be suitably tailored. The ability to do catalysis within these cavities in the coordination solids has also been demonstrated. With more and more structural prototypes being constructed through this approach, much like organic chemistry, it should be possible to establish certain recurrent relationships between the molecular structure(s) and the final solid state structure. Networks with strong covalent bonds to transition metals, rather than weaker covalent bonds, have also been recently prepared [38]. Such relationships should hopefully pave the way for the rational design and construction of solid state structures with desired structural properties. This approach could also be combined with the cluster chemistry discussed above. It remains to be seen what range electronic or even magnetic properties can be expected from such materials. 4.3. Ternary nitrides While most of the binary nitrides which form have been fairly thoroughly investigated [39] and in a number of cases are of technological significance, the chemistry of ternary and higher nitrides has only recently attracted the broad interest of solid state chemists. This is a little surprising, since compounds made from neighboring elements (C, Si, P, S and especially O) have been much more widely investigated. Recent reviews summarize what is currently known about this young field [40-42]. Here we mention two features of the recent research.
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First, a number of novel structures are observed in ternary nitrides. Based on electronegativity differences, the bonding in nitrides might be expected to be less ionic than in oxides, but perhaps less covalent than carbides or sulfides. The variety of structures observed bear this out. For example, silicate and germinate chemistry is completely dominated by tetrahedral M+404 units that share only comers between two tetrahedra. In temary silicon and especially in germanium nitrides both corner sharing of M+aN4 units (between two or three tetrahedra) and edge sharing are common. In germanium nitrides a variety of Ge oxidation states can be stabilized: Ge +4, Ge+2 and even Ge - 2. Such behavior in oxides is unknown, while in sulfides only the first two oxidation states are known. Second, the preparation of single crystais of ternary nitrides has been a considerable challenge. Several new methods have met with some success: alkali metal [40] or alkali metal amide [12] fluxes and high pressure ammonia solvents [43]. Yet these methods are not sufficient to prepare all ternary phases, especially many of those containing early transition metals. There remains considerable opportunity for development of fluxes or other techniques to prepare these unusual compounds. In fact, it appears that the alkali metal flux method may be useful in the preparation of GaN crystals as substrates for blue lasers. Many oxynitrides [44] show promise as catalysts [45] and recently, framework nitrides with the semi-dense sodalite framework have been synthesized [46]. Routes to framework nitrides with large cavities are starting to be explored and these may compete favorably with current catalysts of commercial importance. 5. OTHER CHALLENGES FOR THE FUTURE As mentioned in the introduction, only a very small fraction of ternary or quaternary phase diagrams are known. So it is likely that there are a huge number of new compounds and structures yet to be discovered. If there are new or enhanced phenomena yet to be discovered in these new materials (only with considerable hubris can we claim that all interesting or useful phenomena in solids are now known), then the challenge is how to find these compounds and phenomena as efficiently as possible. There are perhaps two new approaches to this question: Combinatorial Synthesis and Direct Computation. We discuss each a little more below.
5.1. Combinatorial synthesis In a combinatorial methodology, from a given set of starting materials, a library of compounds is synthesized in a parallel fashion. Such parallel synthesis allows sampling of a large number of compounds whose syntheses may otherwise be considered impracticable.
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This methodology has already gained wide popularity in the area of drug design [46]. This initial approach is confined to the synthesis of libraries of organic compounds. Recently, Schultz and co-workers applied this approach to solid state synthesis of some inorganic materials [47]. Specifically, they synthesized arrays of multilayer films by sputtering. Each film in the array had a different composition that was determined by masks that were used during the film deposition. Superconducting copper oxide thin films derived from Cu, Bi203, CaO, PbO, SrCO3, Y203 and BaCO3 targets were prepared. From a library consisting of a 128-member array, using resistance measurements, the optimal superconducting phase as the composition corresponding to the previously known BiESr2Ca2Cu3010 was identified. The combinatorial methodology was also used to search for materials with large magnetoresistance [48] in perovskite (ABO3) and related structural families (AEBO4 or An+lBnO3n+l).The search indeed resulted in the discovery of a family of Co-containing compounds with compositions LnxMyCoO3_y, where M = Ba, Sr, Ca or Pb and Ln = La or Y, with colossal magnetoresistance (CMR) properties. These examples are just initial steps. The methodology used was to "engineer" a known class of materials to optimize a property by varying the composition or looking at related elements. This is what Materials Chemists already do quite well, but this approach may allow them to do it much faster. The ultimate challenge is to explore ternary and quaternary systems for new compounds, structures and even properties at a considerably enhanced rate. If one is only searching for a property that is easily detected by mass scanning, such as superconductivity, then the technology is close at hand. However, if a broad structural search is desired, the structure would have to be determined on thousands of samples, many multiphase, in a short period of time. It is just becoming possible to imagine that X-ray diffraction systems could be developed to obtain diffraction patterns in seconds from each sample in the library and that computers could determine or match structures in a similar time frame. It may be a decade or so before this is possible; it may take synchrotron X-ray intensities and bigger, faster workstations, as well as continued advances in the presently rapid improvements in algorithms for determining structures from powder or single crystal diffraction data. I do not want to minimize the many practical problems in implementing this approach, but I think these could be overcome in the next decade or even sooner. 5.2. Computation and structure prediction This issue of the journal contains an article by M. Parrinello on ab initio Molecular Dynamic calculations
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to predict the structures of solid state compounds. There are now perhaps a dozen groups world wide that are making some progress in this most challenging theoretical problem. These also include M. Teter at Cornell, S. Lee at Michigan, M. Cohen at Berkeley, J. Joannapolous at MIT and M. Jansen at Bonn. For such an approach to be useful to most solid state chemists, direct access to the programs using local workstations is necessary. To be useful to the synthetic chemist, the programs need to be fast and have moderate reliability in predicting characteristics of interest, such as structure. Such programs could be a useful guide in determining which phase diagrams deserve early experimental scrutiny. We are just at the beginning of this process, which will require faster computers with massive memories and further development of efficient codes and algorithms. Already some solid state chemists have found these programs useful in obtaining a starting point in difficult structural determinations [49]. Predicting exotic properties in new materials, such as high temperature superconductivity, will be even more challenging and is much less likely to happen in the next decade.
3.
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
6. SUMMARY
15.
Over the last decade Solid State Chemistry has been a growing field of research, primarily due to the huge excitement generated by the discovery of high temperature superconductors. However, there are major challenges to making a predictable synthetic science and in speeding up the process of discovery. Nonetheless, advances in understanding fundamental processes such as nucleation have recently appeared. There are many structurally interesting systems beside oxides and I have mentioned several. There are some recent developments that suggest experimental methods to speed up the synthesis and discovery of new phases and perhaps even theoretical methods to help in the search. The next decade of R&D in solid state chemistry is likely to be exciting and formative.
16.
Acknowledgements---I am indebted to my research group who helped with the writing of this manuscript, especially Simon Clarke, Glen Kowach, Sami Malik and Dhandapani Venkataraman. Errors and misconceptions, however, are my own. REFERENCES 1. Lewis, D.W., Willcock, D.J., Catlow, C.R.A., Thomas, J.M. and Hutchings, G.J., Nature, 382, 1996, 604. . For an overview of organic solid state materials see: (a) Crystal Engineering: The Design of Organic Solids (Edited by G.R. Desiraju), Elsevier,
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