Low temperature methods have been applied to the synthesis of many advanced materials. Non-hydrolytic sol-gel (NHSG) processes offer an elegant route to stable and metastable phases at low temperatures. Excellent atomic level homogeneity gives access to polymorphs that are difficult or impossible to obtain by other methods. The NHSG approach is most commonly applied to the preparation of metal oxides, but can be easily extended to metal sulfides. Exploration of experimental variables allows control over product stoichiometry and crystal structure. This paper reviews the application of NHSG chemistry to the synthesis of negative thermal expansion oxides and selected metal sulfides.
Low-temperature synthetic pathways can result in crystallization of metastable materials. These methods have been widely explored for the preparation of metal oxides. Adaptation of nonhydrolytic sol-gel chemistry to non-oxide systems offers an elegant route to transition-metal sulfides. The method can be exploited for the facile and reproducible synthesis of iron sulfide crystallizing in the troilite structure. This phase is only found in meteorites and planets and has previously been obtained by high-temperature or high-energy ball-milling methods. "Nonhydrolytic" sol-gel processing results in direct crystallization of troilite with no need for further calcination.
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