Materials Synthesis Via Solid-State Metathesis Reactions

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Abstract
The synthesis of solid-state materials generally requires high temperatures and long reaction times in order to overcome diffusion barriers and achieve homogeneous products. Recently chemical routes using reactive precursors have allowed for greater control over product characteristics including stoichiometry and crystallite size. Here we report on one such method which couples reactive solid metal halides with alkali metal-main group compounds in very rapid, exothermic reactions leading to crystalline products and alkali halide salt byproducts. These solid-state metathesis (SSM) reactions provide synthetic control over product crystallinity, homogeneity, and phase. Important compounds as diverse as metal oxides, phosphides, sulfides, nitrides, and silicides have been synthesized using SSM synthesis. Through extensive examples, this review describes the synthetic versatility of this approach, including routes to homogeneous mixed nonmetal solid solutions, product crystallite size reduction through the addition of an inert heat sink (e.g., NaCl), and the synthesis of metastable high temperature phases. The initiation and propagation of these rapid reactions depends strongly on the chemical properties and reactivity of the precursors. Initiation generally occurs when one precursor changes phase or decomposes, allowing for increased surface contact and more reaction. Once an SSM reaction is initiated it becomes rapidly self-sustaining and can reach high temperatures (< 1000°C) for very short periods (>2 sec). The results from different systems provide indirect evidence for mechanisms involving either ionic or elemental intermediates in these reactions.