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The birth and development of useful technologies often hinge on the availability of solid-state materials with appropriate physical and chemical properties. Forty years ago, the preparation of pure semiconductors as single crystals led to a complete transformation of the electronics industry. Today, new high-temperature ceramic superconductors, nonlinear optical solids and thin films, supermagnetic alloys, and "nanophase" catalytic materials also promise to initiate sweeping changes in relevant technologies, including communications, computing, transportation, and chemical manufacturing, to name the most obvious.
Although many of these key solid-state materials have been discovered the old-fashioned way (by accident), rational synthesis is now playing an increasingly important role. The interplay between structural chemistry, physical measurement, and theory is today sufficiently refined that it is often possible to predict a priori the structure and composition of compounds that are expected to have a certain desirable property. The problem simply lies in how to make them. For example, one family of thallium cuprate superconductors may be represented by Tl sub 2 Ba sub 2 Ca sub n Cu sub n+1 O sub x , where n + 1 is the number of contiguous copper oxide planes between thallium-barium oxide layers. As n + 1 increases from 1 to 2 to 3, the superconducting transition temperature T sub c goes from approximately 80 to 110 to 125 K (1). Several theories point to the importance of interplane coupling in the superconductivity of these materials (2), and one might reasonably ask whether the highest T sub c values should be anticipated for the compound with n = x, if only it could be made. Unfortunately, attempts to prepare pure phases of this stoichiometry in the desired structure have to date been unsuccessful.
The problem with trying to make such materials is that the products of solid-state reactions today are predictable only in a very limited sense. Thus, although it is often possible to calculate properties on the basis of known or theoretical structures (3), predicting the most stable structure for a given composition of matter is still more of an art than a science. Through a massive inventory of known compounds and reactions, a framework of structural families and heuristic composition-structure relations has emerged; nevertheless, even seemingly innocent isomorphous substitutions in known compounds can...