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A new material, Ga 2 Mo 3 O 12 , was prepared for the first time using non-hydrolytic sol-gel chemistry. Rietveld analysis of variable temperature X-ray diffraction data showed that this compound crystallizes in the monoclinic Sc 2 W 3 O 12 structure, and that it displays positive thermal expansion up to its decomposition temperature of 650 uC. No phase transitions were observed in this temperature range. The high-pressure behavior was investigated with an in situ synchrotron diffraction study. Reversible phase transitions to new monoclinic structures occurred at approximately 3.2 and 4.1 GPa. In addition, irreversible amorphization was observed above 8 GPa.

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.

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