Synthesis and thermal stability of rare-earths molybdates and tungstates with fluorite- and scheelite-type structure

摘要

AbstractPolycrystalline samples of rare-earths molybdates and tungstates, i.e., CdRE4Mo3O16(RE?=?Eu, Gd, Y, Ho) and Pb1–3x□xEu2xWO4(0?x?≤?0.1296 and □ denotes cationic vacancies) have been successfully prepared by high-temperature annealing of adequate CdMoO4/RE2MoO6and PbWO4/Eu2(WO4)3mixtures, respectively. According to the X-ray diffraction analysis, the CdRE4Mo3O16compounds crystallize in a cubic, fluorite-related-type structure with space group$$Pnar{3}n$$Pn3ˉn. In turn, new Pb1–3x□xEu2xWO4phases crystallize in the scheelite-type, tetragonal symmetry, space groupI41/a. Cadmium and rare-earth molybdates decompose in the solid state and the solid products of their decomposition are two RE molybdates, i.e., RE2MoO6and RE2(MoO4)3. Thermal stability of CdRE4Mo3O16decreases with decreasing of RE3+radius. The melting point of each sample of Pb1–3x□xEu2xWO4solid solution is lower than melting point of pure matrix, i.e., PbWO4(1116?°C), and it decreases with increasing in Eu content. Both CdRE4Mo3O16as well as Pb1–3x□xEu2xWO4samples are insulators, and their optical band gap (Eg) is bigger than 3?eV.]]>