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., CdREMoO (RE = Eu, Gd, Y, Ho) and Pb□EuWO (0 < x ≤ 0.1296 and □ denotes cationic vacancies) have been successfully prepared by high-temperature annealing of adequate CdMoO/REMoO and PbWO/Eu(WO) mixtures, respectively. According to the X-ray diffraction analysis, the CdREMoO compounds crystallize in a cubic, fluorite-related-type structure with space group $$Pn\bar{3}n$$ . In turn, new Pb□EuWO phases crystallize in the scheelite-type, tetragonal symmetry, space group I4/ a. Cadmium and rare-earth molybdates decompose in the solid state and the solid products of their decomposition are two RE molybdates, i.e., REMoO and RE(MoO). Thermal stability of CdREMoO decreases with decreasing of RE radius. The melting point of each sample of Pb□EuWO solid solution is lower than melting point of pure matrix, i.e., PbWO (1116 °C), and it decreases with increasing in Eu content. Both CdREMoO as well as Pb□EuWO samples are insulators, and their optical band gap ( E ) is bigger than 3 eV. [ABSTRACT FROM AUTHOR]