Abstract
This thesis introduces ten new rare-earth and uranium silicates which were synthesized by high-temperature, high-pressure hydrothermal or flux-growth methods. Four rare-earth gallosilicates adopt the same structural type and uranium silicates are classified into two series based on their valences. All structures were characterized by single crystal X-ray diffraction (SXRD). Pure or major phase of products can be obtained by adjusting experiment conditions. Their purities were checked by powder X-ray diffraction. The variable temperature powder diffraction patterns were used to understand the thermal stability of the compounds. Solid state NMR, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy can give us structural information as compared with that from SXRD. Characterizations including photoluminescence, emission life time and second-harmonic generation were measured to investigate interesting properties of these compounds. The A series share the same rare earth gallosilicate structure, Rb2REGaSi4O12 (RE = Y, Eu, Gd and Tb), constructed by 2-D [GaSi4O12] double layers and 1-D infinite chains formed by edge-sharing REO7. The RE sites on Rb2YGaSi4O12 can be partially substituted by Eu3+ or Tb3+ respectively or simultaneously. These photoluminescent compounds display great potential in the application of phosphor materials. Different combination of fluxes were used to prepare four new uranyl silicates in B series. B1 and B2 are salt-inclusion structures containing MF (M = alkali ions) octahedraon in their 12-ring channels. B3 is constructed by unusual Si8O2212- oligosilicate anions and discrete Urφ4 tetragonal bipyramids. In series C, two mixed-valence uranium silicates were synthesized by high temperature, high pressure hydrothermal method with the addition of two alkali cations and fluoride ions. The existence of fluoride ions in synthesis can stabilize the U5+ species (UO2+) and the entry of second alkali metal may induce the mixed-valence uranium in C1 structure. There are three unique uranium sites in C2 and each uranium adopts different valence state. The XPS and XANES data reveal that there are two valences (4+ and 5+) in C1 and three (4+, 5+ and 6+) in C2.