MATERIALS SYNTHESIS VIA SOLID-STATE METATHESIS REACTIONS

The synthesis of solid-state materials generally requires high tempera tures and long reaction times in order to overcome diffusion barriers and achieve homogeneous products. Recently chemical routes using react ive precursors have allowed for greater control over product character istics including stoichiometry and crystallite size. Here we report on one such method which couples reactive solid metal halides with alkal i metal-main group compounds in very rapid, exothermic reactions leadi ng to crystalline products and alkali halide salt byproducts. These so lid-state metathesis (SSM) reactions provide synthetic control over pr oduct crystallinity, homogeneity, and phase. Important compounds as di verse as metal oxides, phosphides, sulfides, nitrides, and silicides h ave been synthesized using SSM synthesis. Through extensive examples, this review describes the synthetic versatility of this approach, incl uding routes to homogeneous mixed nonmetal solid solutions, product cr ystallite size reduction through the addition of an inert heat sink (e .g., NaCl), and the synthesis of metastable high temperature phases. T he initiation and propagation of these rapid reactions depends strongl y on the chemical properties and reactivity of the precursors. Initiat ion generally occurs when one precursor changes phase or decomposes, a llowing for increased surface contact and more reaction. Once an SSM r eaction is initiated it becomes rapidly self-sustaining and can reach high temperatures (>1000 degrees C) for very short periods (<2 sec). T he results from different systems provide indirect evidence for mechan isms involving either ionic or elemental intermediates in these reacti ons.