High-temperature, solid-state synthesis of targeted transition-metal silicates and germanates in molten-salt media

This dissertation research primarily focused on the exploratory synthesis of transition metal silicates and germanates in molten alkali and alkaline-earth halide media in the temperature range of 650--1000°C. These halide salts, having relatively low melting points, not only provide reaction media for crystal formation and growth, but also participate in reactions resulting in a novel class of framework solids. For the latter, a major part of the endeavor was to study the structure correlation between the frameworks and that of the incorporated salts.; A novel copper germanate Na2Cu3(GeO3) 4 and its silicate-substituted analogues, Na2Cu3(Ge 1-xSixO3)4 (x = 0.216(5), 0.432(4)) were isolated using the eutectic CsCl/NaCl flux. These compounds possess geometrically frustrated (GF) magnetic lattices, providing new additions for the fundamental study of spin exchange interactions in magnetic materials.; This study revealed that alkaline-earth metal halides (AIIX 2), e.g., BaCl2, tend to be incorporated in the transition metal silicate and germinate frameworks in a unique way compared to alkali metal halides (AIX) studied previously. The resulting novel salt incorporated solids include (i) layered compounds adopting the general formula (BaX)2·M(M 'O3)2 (X = Cl, Br; M = Mn, Co, Ni, Cu; M' = Si, Ge) and (BaCl)2FeO0.43(1)(SiO 3)2, whose structures are built by alternating layers of transition metal silicates and salts; (ii) polar non-centrosymmetric compounds including the silicate-based Ba2MnSi2O 7Cl adopting fresonite structure and two isostructural phases of Ba 6Mn4Si12O34Cl3 and Ba 6Fe4+xSi12-xO34Cl3 (x = 1) adopting a new structure type; (iii) porous compound Ba 10Mn4(SiO3)8Cl12 possessing muirite type framework. The salt lattices found in these compounds suggest the template effects of molten salts in the formation of various frameworks. It is important to note that the bonding at the interface of two chemically dissimilar lattices does not consist of M-X, which was previously observed in the AIX-incorporated, salt-inclusion solids, but merely AII-O bonds.; In addition, a new non-centrosymmetric compound BaMn(SiO3) 2 was synthesized in the BaCl2/NaCl eutectic flux in this study, demonstrating once again the utility of flux in the growth of crystals. The magnetic studies of BaMn(SiO3)2 show an antiferromagnetic-ferromagnetic transition at 16 K, which might be caused by weak spin-canting.; The success in synthesizing novel transition metal containing silicates and germanates via salt-inclusion methods further promises the utilities of molten salt for the continued endeavor in the synthesis of solid-state materials by design. The challenge remains in understanding the correlation between the salts and the final structures until more salt-incorporated solids are isolated and studied.