Synthesis and Characterization of Advanced Materials (1984)

Basic research in the synthesis and characterization of advanced materials (SACAM) has been the principal source of new materials, structural information, bonding models and theories, thermodynamic data, and synthesis techniques and, as such, has provided the materials community with the scientific base on which materials technology has flourished. This area has been actively supported in universities, especially in Germany, the Scandinavian countries, the Netherlands, France, and Austria, during much of this century, and interest is growing in the Soviet Union and Japan.

For a variety of reasons, basic research in advanced materials in U.S. universities has not kept pace with other areas of basic research. Some of the obstacles that are frequently identified by members of the materials community are the following:

1. The prevalent structure of academic departments along the lines of individual research groups staffed by graduate students and postdoctoral researchers of short tenure;

2. The tradition of evaluating principal scientists primarily in terms of their individual contributions;

3. The emphasis on molecular science in undergraduate curricula, especially in chemistry, which is a key discipline in advanced materials synthesis and characterization;

4. The lack of a sense of identity and coherence among the members of the synthesis and characterization research community.

These obstacles have the following effects:

1. The short tenure of postdoctoral personnel and graduate students results in difficulties in maintaining continuity, which is particularly serious in those research areas of materials synthesis that require skills and techniques that are generally developed only with considerable experience.

2. The evaluation of scientists primarily on the basis of their individual contributions makes it difficult to establish a research program based on collaboration, yet collaborative efforts are crucial to research in materials.

3. The emphasis on molecular science in undergraduate education makes it significantly more difficult to attract graduate students, and, concomitantly, future faculty, into research groups doing solid-state preparation and characterization.

4. The lack of a feeling of community among the materials chemists, ceramists, metallurgists, and other scientists involved in synthesis and characterization makes it difficult to develop the interaction and cross-fertilization so important to the development of this field.

Our approach to these problems is to consider the status of a variety of materials research areas with the aim of providing an overview of the field and a synopsis of current and near-future research problems. This overview and synopsis, together with the scientific importance of the research problems discussed, provide the background and basis for the panel’s recommendations and for a number of those in the Summary Report (Part I).

It should be recognized that studies of novel materials should not be limited to the synthesis of materials in the sense that a chemist ordinarily uses the term “synthesis.” The preparation and study of materials that are of interest for their mechanical properties, especially durability, present a broad range of problems in basic research. Materials such as fiber composites, ultrahard boron compounds, high-temperature alloys, and spinodal compositions deserve basic study by metallurgists, ceramists, and chemists. In addition, the study of low-energy-cost materials processing is of obvious importance.

It should also be noted that there are fundamental problems in the chemical bondings and mass-transport processes that are operative in growing interfaces and

passivating surfaces. For example, the understanding and chemical control of surface oxidation, surface-bonding defects, transport of atoms along grain boundaries, and impurity diffusion to surfaces are of growing importance for electronic devices, magnetic materials, corrosion, fracture, and radiation-hardened coatings. These areas, which have direct bearing on many technological needs, offer substantial challenges and opportunities for basic research in the synthesis and characterization of solids.

We consider the current status of basic research in advanced materials synthesis and characterization using the following breakdown: electronic structure of solids, inorganic solids: new classes of compounds and their impact on solid-state concepts, highly conducting molecular solids, ceramics and amorphous solids, and predicting the lifetimes of materials. We then present our conclusions and recommendations.

In the electronic structure of solids, the interaction between those concerned with theory and with the development of synthesis and the resultant impact on characterization of materials are increasingly important. This interaction is largely a result of the availability of computers and relatively large-scale experimental facilities. Modern research efforts in materials frequently take the following form:

1. A structural or physical effect of potential interest is observed.

2. The effect is studied in detail by any one of a number of modern techniques [e.g., nuclear magnetic resonance (NMR), x-ray absorption field spectroscopy (XAFS), Auger spectroscopy, photoelectron emission (XPS)].

3. A rigorous quantitative theory of the effect is developed.

The requirements for collaboration are obvious. Among the benefits is that theory generally provides insight, guidance, and incentive for additional synthesis efforts.

Modern theoretical approaches include empirical and semiempirical approaches, such as the Brewer-Engel correlation of gaseous atom electronic states and alloy structure, Phillips and Van Vechten dielectric electro-

negativities, and the Mediema model of intermetallic solids, as well as ab initio calculations, such as augmented plane wave (APW) and linear augmented plane wave (LAPW), which are becoming increasingly available to support the work of synthesis scientists. The theoretical framework is being extended to more complex materials; these extensions are essential for the systematic development of advanced materials. This theoretical development is, in turn, being accelerated by the synthesis of completely new classes of compounds that are expanding our awareness of possibilities for chemical interactions and of the limited nature, of the concepts with which we have circumscribed our thinking.

Descriptions of electronic behavior in solids are based primarily on two extreme models: the localized model applies to electrons about discrete atomic centers, and the intinerant model applies to electrons distributed over the entire solid. Many materials exhibit intermediate behavior, thus efforts are under way to establish a theoretical bridge between the extremes of localized and itinerant electron behavior. The general problem is known as the “narrow-band” problem; its solution requires the introduction in zero order of electron-lattice interactions and electron-electron correlations. Successful treatment of the narrow-band problem is essential to our understanding of the relationship between phase instabilities, high-temperature superconductivity, and the appearance of spontaneous magnetization. Narrow-band effects are responsible for the disproportionation reaction, 2Fe⁺⁴=Fe⁺³+Fe⁺⁵ in CaFeO₃ (not found in isostructural SrFeO₃), for the semiconductor-metal transition in VO₂ (which forms V-V pairs at low temperatures), and for the unusual phase transitions found in NiS.

Other areas of theoretical development that will have an impact on synthesis and characterization are the effects of disorder, that is, the conceptual passage from the ordered solid to the amorphous state, and the electronic structure of surfaces and interfaces. The former is relevant to the study of defect formation and migration, and the latter is necessary in the investigation of the numerous surface properties of materials, of which catalysis and corrosion are two of the most important. The interplay of theory and synthesis benefits both areas. Systematic chemistry delineates the boundaries of applicability for the simplifying assumptions, either implicit or explicit, in the phenomenological models. Furthermore, inadequacies of models that emerge in the

attempt to predict synthesis behavior or properties of materials in new configurations frequently lead to the application of more rigorous or ab initio methods; these then provide a basis for the evaluation and refinement of the empirical theories. This interplay leads to feedback between theory and experiment and results in a rapidly developing area of scientific understanding. It merits strong support and encouragement.

In the area of synthesis per se, two extreme situations can be identified. One is the preparation of known and relatively well-characterized materials in higher purity, special form, or with specific doping or atomic distributions. The other, the subject of this section, is the creation of totally new classes of compounds exhibiting novel structures and bonding. The latter situation exemplifies the need to reconsider many of our traditional ways of thinking about chemistry, structure, and bonding, particularly in expanding our horizons about what is possible and what must be taken into account. Thus, the concepts associated with isotropic bonding, structure, and electronic properties, which have been so well studied for traditional metals, are now being tested for one- and two-dimensional metallic arrays. Some of these new types of compounds are expected to display new phenomena. Examples, most of which have been known for less than 10 years, make it clear that new and exciting types of compounds are to be found even in binary metal-halogen and metal-chalcogen compounds; ternary systems are much less well explored, although the remarkable Chevrel phases and their relatives that are now being reported suggest that such studies will be productive.

Nearly all of what may be classified as structurally “remarkable” and “novel” phases have the highly anisotropic metal-metal bonding associated with the unusual low-oxidation states of many metals. The reduction in the formal nonmetal-to-metal ratio is usually accompanied by some degree of metal-metal bonding, apparently to accommodate orbital-electron requirements already recognized for the metals and their more normal compounds. Metal-metal bonding often appears to control the resulting structure rather than occurring as a secondary, weak effect. Some examples are as follows:

1. Alkali metal-rich oxides have recently been found, Cs₇O and Cs₁₁O₃, for example, in which structured metal-oxygen clusters appear in a sea of metal.

2. Isolated metal tetrahedral or octahedral clusters occur in halides and chalcides, for example, the Chevrel phases Pb_xMo₆S₈, Mo₄S₄Br₄, Nb₇P₄, and CsNb₆I₁₁.

3. Infinite chains of metals are found in the form of (a) metal octahedra sharing adjoining edges in Nb₂Se, (b) metal octahedra sharing opposite edges in M₂Cl₃ (M=Y, Gd, Tb), Sc₇Cl₁₀, and Gd₅Br₈, (c) metal dodecahedra in Ta₆S, and (d) metal octahedra sharing verticles in Ti₅Te₄-type phases. Indeed, the last structure occurs with so many transition metal-posttransition nonmetal combinations that one wonders what remarkably flexible electronic requirements pertain.

4. Infinite double metal sheet structures in the monohalides of Zr, Hf, Sc, Y, and many rare earth elements and in Hf₂S, PtTe, Ag2F, and Ba₂N provide the first really two-dimensional metals for study, for example, in the areas of band theory, properties of interstitial impurities such as hydrogen, conduction, and catalytic behavior.

Nontransition elements also share in the structural and bonding significance:

5. Complex chains of tellurium occur in Te₃Cl₂ and Te₂Br and of phosphorus in many polyphosphides.

6. Metal clusters, chains, and ribbons are even found in a remarkable class of intermetallic phases involving metals of widely different electronegativities, for example, square Bi₄ units in Ca₁₁Bi₁₀, angular Sn₃ in ribbons in NaHg, and tin tetrahedra in KSn.

Obviously, these phases provide new horizons for what is possible, if not immediately explicable, in chemistry. The metal-metal bonding and low-dimensional electronic conductivity provide strong challenges to both the theorist and the experimentalist for explanation, interpretation, and characterization. Not only do these compounds imply the occurrence of new phenomena, but they also promise direct use as catalytic substrates, media for hydrogen storage, new electronic environments for nonmetals or for metal ions in electronic and magnetic applications, and as intermediates potentially important in corrosion of active and refractory metals and significant for their mechanical properties (e.g., stress corrosion cracking of zirconium by iodine).

There are some special conditions and circumstances that allow for such discoveries in what have sometimes been thought to be “sterile” systems. Surface blockage and kinetic limitations, even at 800°C, have made vapor-phase transport reactions invaluable and higher temperatures, melt-solvent reactions, and longer reaction periods increasingly necessary. Tantalum and similar container materials, high-quality (space-age) drybox facilities, and metal fabrication and welding equipment are often necessities. Characterizational tools such as microprobes, Guinier powder and single-crystal x-ray diffraction facilities, and photoelectron emission (XPS) equipment with high-quality drybox entrance capability are essential. Synthesis using molecular-beam epitaxial methods is promising. Most, if not all, academic institutions find it difficult to provide these sophisticated facilities without considerable assistance, yet these capabilities should not be limited to national laboratories and industrial firms.

During the past 10 years or so, a new area of materials synthesis and characterization emerged. It deals with the preparation and measurement of properties of molecular solids that are good electronic conductors. This area has undergone rapid development as a result of strong interactions among synthesis chemists, materials scientists, and solid-state physicists. The motivation for synthesizing and characterizing these molecular solids stems in part from the desire to obtain materials with unusual or unprecedented properties, some of which might be of technological importance.

The new phenomena already discovered have stimulated the advance of theoretical concepts of electronic conduction in solids. We expect that other new phenomena will be found that will further stimulate advances in this area. The results of work in this field have played a crucial role in developing concepts related to one-dimensional band theory, charge density waves, Peierls instability effects, Coulomb interactions, incommensurate lattice effects, and effects of defects.

Electronically conducting molecular solids fall into two broad categories: highly conducting substances (conductivity >1 ohm⁻¹ cm⁻¹ at 25°C) and semiconducting molecular solids, including some photoconductors and

photoresists. The first category, which includes the new and potentially important molecular metals, is one of vigorous basic research activity. We give it special attention.

The first example of a covalent polymer, which contains no metal atoms, is highly conducting (σ ^~ 5×10³ ohm⁻¹ cm⁻¹ at room temperature) and is a superconductor (T_c= 0.3 K), is polymeric sulfur nitride, (SN)_x. Potential future research includes synthesis efforts to produce substances isoelectronic with (SN)_x, for example, [(CH₃PN]_x, [S(CH)]_x. Another example is polyacetylene, (CH)_x. This material can be prepared in the form of large thin, free-standing, and flexible silvery films. Polyacetylene can be p- or n-doped to yield a series of semiconductors and the first examples of organic polymeric metals. In the semiconducting forms it has been used to fabricate p/n junctions and Schottky barriers, which act as rectifying diodes. In certain systems these exhibit a photovoltaic effect and, accordingly, have a potential for application as solar-cell materials. The use of different dopants and the replacement of hydrogen by organic or inorganic groups could result in new molecular metals and semiconductors.

Another class of materials in this category is the intercalation compounds of graphite. Through intercalation with a variety of electron-donating or electron-attracting species, the conductivity of graphite can be spectacularly modified. By doping with AsF₅, conductivities in excess of that of copper have been obtained. It seems likely that further studies will result in the discovery of interesting and useful electronic behavior.

The materials exhibiting marked anisotropy of electronic conduction and significant one-dimensional effects should also be mentioned. In this category fall the TTF derivatives (e.g., (TTF) (TCNQ) and (TTF) Br_0.6, the platinum chain compounds (KCP), the mercury chain compounds (Hg_2.84AsF₆), and the charge-transfer complexes, of which (TTF) (TCNQ) is an example. These compounds have already resulted in new concepts of electronic conduction in solids, which will undoubtedly continue as new materials are prepared and studied. The discovery in Hg_2.84AsF₆ of a sublattice that is incommensurate with the main crystal lattice has evoked considerable theoretical interest. New types of charge-transfer complexes, such as the iodine complexes of certain phthalocyanin and porphyrin derivatives, with

single-crystal, room-temperature conductivities up to 500 ohm⁻¹ cm⁻¹, have been reported.

Research on highly conducting molecular solids is advancing at an extraordinary rate. The synthesis of new chemical systems is essential to the continued growth of this field. The potential for increasing scientific knowledge through the synthesis of new compounds, the better characterization of known materials, and the theoretical analysis of new phenomena is great. The field presents a challenge and an opportunity for collaborative interaction among chemists, physicists, and materials scientists. Present knowledge suggests that in the next few years many new materials with unusual properties will be discovered, if the appropriate scientific climate can be fostered.

Ceramics and specially designed amorphous solids are attractive for use in advanced energy, electronic, magnetic, and optical systems. Examples of opportunities for significant new developments in ceramic materials include metallic oxides for use as electrode materials in electrochemical cells, for example, oxygen electrodes in fuel/electrolysis cells and solid-solution electrodes in high-specific-energy batteries; catalytic substrates and mixed ceramic-metal catalyst systems; high-temperature structural materials for heat engines and turbines; radiation-hardened and corrosion-resistant materials for nuclear fusion containment; photosensitized ceramics for photography and photoelectrolysis of water; controlled surface-reactive glasses and glass ceramics for replacement of bones or teeth; and tailor-made crystal chemicals for long-term encapsulation of nuclear wastes.

Because of the multicomponent and polyphase nature of these materials, characterization and synthesis are often highly complex and difficult. Consequently, research in this class of materials has been unduly influenced by applications, mechanical performance criteria, and consumer products, all of which have tended to limit innovation and the development of new processing and materials concepts. Several kinds of long-term basic research required to advance the fundamental science of these materials are the following.

Structural and physical characterization at the intermediate range of order (10–100 Å) and a physical

theory including such order are needed for ceramic and amorphous materials. Information is needed to provide insight into the statistical distribution of molecular species, bond types, bond angles, and relaxation times. Such research has been made possible by the recent development of a variety of new experimental approaches, including Fourier transform NMR, laser Raman spectroscopy, and XAFS. Such studies would lead to a better understanding of a variety of materials: amorphous solids with solutions of second and third components, for example, hydrogenated amorphous silicon and selenium hybrid crystalline materials with amorphous sublattices, such as β-alumina with an ordered A10 lattice and disordered Na⁺; stabilized unusual oxidation states in invert glasses; meta stable solubility gap materials; and microphase separated oxide and chalcogenide glasses.

A basic theory relating environmental sensitivity of mechanical properties to structural flaws and kinetics of reactions at interfaces is required for improving and developing new structural ceramics and glasses. High-temperature creep of Si₃N₄ and SiC materials, fatigue of single and polycrystalline oxides, and surface deterioration of glass optical fibers are examples of areas where such basic research is needed.

Quantitative characterization of the structure, composition, and phase state of grain boundaries and surfaces is essential to understanding of the properties of many advanced ceramic materials. Amorphous grain-boundary phases of 10–50 Å thin films cannot generally be analyzed by present instrumental methods, which is a basic limitation to progress in this class of materials. For example, electron-beam techniques result in structural alteration of grain boundaries and surface films.

Control of particulates used in processing of many types of ceramics requires improved theories of agglomeration, mixedness, liquid-particulate interactions, specific surface-adsorption and electrochemical effects, composition and defect gradients, and organic-inorganic interactions. A general theory of ceramic particulate systems needs to be related to the rheology, forming, drying, and subsequent firing steps in achieving tailored microstructures required for new energy systems, biomaterials, and electronic applications.

Several novel techniques are now being used for fabricating metastable alloys and structures with unique chemical, physical, and mechanical properties. Amorphous metal sheets, ribbons, fibers, and powders may be

produced by rapid cooling and subsequently consolidated into bulk structures. The chemical homogeneity and lack of the normal (crystallographic) deformation modes in these solids provide unique properties that have not been fully identified and exploited. In addition, amorphous, ultrafine, crystalline, or epitaxial surface layers on bulk metals are being formed by laser-quenching, electron-beam quenching, and ion implantation. These processes are of significant utility in coping with a variety of surface-related phenomena such as corrosion, friction, wear, fatigue, and catalysis. Moreover, ion implantation permits the formation of alloys that are unobtainable by other techniques and makes possible studies of diffusion, oxidation, and decomposition involving atom species that were not previously accessible. A new generation of instrumental analysis methods of dramatically greater resolution and sensitivity was developed during the last decade. Opportunities are many for the use of this new instrumentation for the characterization of surfaces and interphase boundaries, intermediate-range order, and the environment-structure interactions of ceramics and amorphous solids. The complexities that result from the polyphase and multicomponent nature of ceramic materials often make the new characterization techniques difficult to apply; however, the understanding of dynamic mechanisms in structural and atomic detail probably will yield new ceramic materials and improvements in properties and processing of existing materials.

Prediction of the structural reliability of materials exposed to high-performance conditions for long periods of time is a scientific problem of great practical importance. Nuclear waste encapsulants will be required to isolate radioactive constituents from the ecosphere for thousands of years. Such materials must withstand harsh combinations of thermal, chemical, mechanical, and radiation stresses. Basic research on the synergistic interaction of such complex factors during surface chemical reactions and microstructure evolution is needed to extrapolate from laboratory-time-scale experiments to geologic-time-scale needs.

The effort to ensure the reliability of isolated defense and energy-generating and -conversion facilities

requires prediction of the long-term performance of electronic and optical components exposed to combinations of severe thermal, chemical, mechanical, and radiation environments. Fracture mechanics theories that lead to lifetime predictions are currently based on empirically derived relationships involving environment-sensitive factors. Basic research to develop a molecular theory underlying the origin and magnitude of the environment-sensitive parameters in these fracture mechanics theories is urgently needed to assure accuracy of lifetime predictions. Synthesis of new structural materials with improved lifetimes and development of protective coatings cannot proceed on other than a trial-and-error basis without an improved physical understanding of materials degradation.

The problems of lifetime predictions in severe environments require a synthesis of theories of molecular structure, mechanical behavior, and surface chemical reactivities coupled with accurate characterization of flow distributions, heterogeneous surface states, microstructural parameters, and composition gradients.

Based on accomplishment and research potential, we conclude that SACAM is a healthy field of endeavor. A wide variety of materials is being synthesized in response to a diversity of scientific motivations; the materials are being characterized with a growing set of powerful experimental techniques; and the results of the synthesis and characterization have potential applications to a broad range of pressing national problems. Accordingly, there are many opportunities for the development of substantial and meaningful research programs. However, there are some aspects of materials synthesis and characterization that lead to special problems. Our perception of the problems and our recommendations follow.

1. The techniques required for synthesis research are generally sufficiently demanding and specialized that efforts to develop a synthesis program frequently preclude the development in the same group of the expertise required for the incisive investigation of properties or front-line theoretical analysis. Therefore, we recommend that collaborative efforts be encouraged, not only between different departments in the same institution but

also between different institutions. This objective can be accomplished by increased funding for domestic travel and for intermediate-term exchange appointments, especially across the industrial-academic interface.

2. There is concern about the attractiveness of solid-state science to students. We recommend that an effort be made to attract first-rate minds into the area by emphasizing the intellectually stimulating character of the problems and by better disseminating news of the field and the continuing substantial accomplishments and applications of solid-state research.

3. There is widespread agreement that research in synthesis and characterization requires more continuity than is generally possible in university-based basic research groups under current funding arrangements. We recommend that continuing efforts be made to ensure that research groups remain large enough so that all the diverse aspects of SACAM research are represented in these groups. We also recommend increased support for senior research technicians and postdoctoral research personnel. The creation of postdoctoral fellowships in solid-state science, to be awarded for from 3 to 5 years and in a research group of the fellow’s choosing, is one possible way of accomplishing some of these goals.