The Thursday Afternoon Lecture
The Chemistry of Solids: Retrospect and Prospect C. N. R. Rao Director, Solid State and Structural Chemistry Unit Indian Institute of Science, Bangalore, India
Abstract solid state chemistry has emerged as one of the frontier areas of investigation in recentyears. chemists hy virture of their innate abilities in structure, dvnamics, and synthesis haw nor only c o r ~ t r i h t r d1nua.h tc, u l r undcrct.mdi& of iht! nature and vruperties iriwlid materials hut ha\.e 31s) created new chemistry: While synthesizing novel, tailor-made materials of desired properties has always remained one of the prime objectives-of d i d state chemkts, they are showing increasing interest in areas like organic solid state and surface sciencerTherma1 and photochemical transformations of organic solids which are determined by crystal chemistry yield novel products offering interesting possibilities including asymmetric synthesis. Some of the other important aspects of solid state chemistry are ultramicrostructures of solids, chemistry of defect solids, phase transitions, magnetic and electronic materials. and amornhous materials. Success in ;Irtws like exploitatim uiaol;~rrnvrh? or dewl~q~in:: n1.w c .11n l w s cruciallv.(lt~ncnds . un our knowledre ~,t'.wlid;.In spite of the rapid progress made in this important area of chemistry, ~~~
100
Journal of Chemical Education
chemistry curricula in most institutions do not seem to incorporate solid state chemistry at undergraduate or postgraduate levels. In this presentation highlights of solid state chemistry will he reviewed. It will be shown how various aspects of solid state chemistry can be introduced in different types of chemistry courses much to our advantage.
Selected Paragraphs The unusual skill with which structures of compounds are related to their properties and new, tailor-made substances are synthesized by chemists makes their contributions to the study of solids unique. Metal Oxides. Metal oxides exhibit a wide range of electrical and magnetic properties. For example, in the case of transition metal monoxides of NaCl structure, we have a t one extreme, T i 0 which is
metallic and Pauli naramaenetic and at the other. MnO which u is an antiferromagnetic insulator. A number of transition metal oxides with hieh metallic conductivities are known. typical mr.; lwing RdJ.,, H u O , L a , i,Sr, c,, C O O ,m d CrO,. 'l'here is also a n exciting class oimeral crxides which exhihit transitions from insulathg to metallic state; typical examples of such oxides are VnO?, VO?, . Ti902 . .and NbO?. Synthetic ~ e t h o d s -several . new synthetic strategies have been developed hv chemists in preparing materials and in growing crystals. Some of the preparative methods depend on chemical reactions, typical ones are the chemical vapor deposition (CVD) technique and techniques employed to prepare microcrystalline substances (as distinct from polycrystalline or amorphous substances). Electrochemical methods of preparation are also employed extensively. Another development has been the use of molten metals as solvents for the synthesis of new intermetallic compounds. Major developments in the study of ionic systems in recent years are solid state electrochemistry and superionic conductors, both beine related to enerev -" research. Besides being able t o precisel; measure ionic and electronic compounds of conductivitv in solids. solid state electrochemical techniques are used tdprecisely control the stoichiometry in svstems like silver chalcoeenides. Organic Solids. hereh have been significant efforts in the study of the organic solid state particularly thermal and
photochemical transformations of organic molecules in the solid state including polymerization reactions. Efforts have also been made to-understand theoretically the packing, binding, and defects in organic crystals by employing model potentials. Ionics and Solid State Electrochemistry. High ionic conductivity with very low electronic conductivity has been observed in several materials. In superionic conductors, the ionic conductivitv is lareer than 5 X 10W n-' cm-I. and the electronic condu&ivityis a t least two orders of magnitude lower. Some of the well-known suoeriouic conductors are AeI ~ ~ p-alumina, I ~ related materials such as R ~ A and (Naz0,l1Al2O3).In recent years, various types of superionic conductors where the mobile ions are protons, Li+ and Fhave been discovered. Solid state ionics mav make a ereat technological impact through their use in advanced baitery systems. The sodium-sulfur battery has alreadv received much publicity. Surface Science. Modern surface science deals with the
surface while electron spectroscopy mainly provides ways of studying electron states.
Volume 59
Number 2
February 1982
101
