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

Polycrystalline samples of rare-earths molybdates and tungstates, i.e., CdRE₄Mo₃O₁₆ (RE = Eu, Gd, Y, Ho) and Pb_1–3x□_xEu_2xWO₄ (0 < x ≤ 0.1296 and □ denotes cationic vacancies) have been successfully prepared by high-temperature annealing of adequate CdMoO₄/RE₂MoO₆ and PbWO₄/Eu₂(WO₄)₃ mixtures, respectively. According to the X-ray diffraction analysis, the CdRE₄Mo₃O₁₆ compounds crystallize in a cubic, fluorite-related-type structure with space group \(Pn\bar{3}n\). In turn, new Pb_1–3x□_xEu_2xWO₄ phases crystallize in the scheelite-type, tetragonal symmetry, space group I4₁/a. Cadmium and rare-earth molybdates decompose in the solid state and the solid products of their decomposition are two RE molybdates, i.e., RE₂MoO₆ and RE₂(MoO₄)₃. Thermal stability of CdRE₄Mo₃O₁₆ decreases with decreasing of RE³⁺ radius. The melting point of each sample of Pb_1–3x□_xEu_2xWO₄ solid solution is lower than melting point of pure matrix, i.e., PbWO₄ (1116 °C), and it decreases with increasing in Eu content. Both CdRE₄Mo₃O₁₆ as well as Pb_1–3x□_xEu_2xWO₄ samples are insulators, and their optical band gap (E _g) is bigger than 3 eV.