Progress toward Solid State Synthesis by Design.

Ages of history are defined by the underlying materials that promoted human development: stone, bronze, and iron ages. Since the middle of the last century, humanity has lived in a silicon age, where the development of the transistor ushered in new technologies previously thought inconceivable. But as technology has advanced, so have the requirements for new materials to sustain increasing physical demands. The field of solid state chemistry is dedicated to the discovery of new materials and phenomena, and though most materials discoveries in history have been through serendipity rather than careful reaction design, the last few decades have seen an increase in the number of materials discovered through a consideration of chemical reaction kinetics and thermodynamics. Materials by design have changed the way solid state chemists approach the synthesis of possible materials with interesting and useful properties. Unlike other chemistry subfields such as organic chemistry and biochemistry, solid state chemistry does not currently benefit from a toolbox of reactions that can allow for the synthesis of any arbitrary material. The diversity and complexity of the solid state phase space likely inhibits chemists from ever having such a toolbox. However, a thorough understanding of the various synthetic techniques involved in the synthesis of stable and metastable solids may be realized through an understanding of the reaction kinetics and thermodynamics. In the Account, we review the common synthesis techniques involved in the formation of metastable materials and break down their underlying chemistry to the simplest reaction mechanisms involved. The synthesis reactions of most metastable materials can be understood through these three reaction driving parameters, which include the exploitation of Le Chatelier's principle, thermo-kinetic reaction coupling, and lowering the activation energy of formation of the metastable product, and we identify several materials whose syntheses are described either by one or a combination of these driving parameters. We identify what exists at the frontier of materials discovery by design, including novel applications of supercritical fluids for tuning between "gas" and "solvent"-like environments. While conventional solvation requires changes in either the temperature or composition of the system, supercritical fluid solvation requires only changes in the fluid density, which opens up the possibilities for the synthesis of new materials. Most importantly, however, we look toward the future of materials synthesis by design and see that it must be a collaborative one. At present, chemists design materials using knowledge about chemical structure and reactivity but often target specific materials with very specific properties. In contrast, computational chemists perform calculations on millions of different elemental combinations and find many candidates of possible materials with interesting properties, though most of these are not realizable synthetically due to limitations in reactivity, kinetics, or thermodynamics. Synthetic harmony can be achieved through active collaboration and communication between these two subfields of chemistry, such that new calculations can incorporate complete knowledge about reaction kinetics and thermodynamics, and new syntheses target computationally predicted materials derived from an understanding of mapped reaction landscapes.