Stoichiometry and topological features of triple molybdates AxByCz(MoO4)n with the heteropolyhedral open MT-frameworks: Synthesis, crystal structure of Rb5{Hf1.5Co0.5(MoO4)6}, and comparative crystal chemistry

Abstract

In the study of the Rb2MoO4–CoMoO4–Hf(MoO4)2 system a new compound, Rb5Hf1.5Сo0.5(MoO4)6, was obtained, which was synthesized by ceramic technology at 530°С for 80 h in air. Rb5Hf1.5Сo0.5(MoO4)6 single crystals were grown by spontaneous crystallization from a melt solution. Synthesized molybdate melt incongruently at 670 °C. The crystal structure has been refined using single-crystal X-ray diffraction data. Triple molybdate Rb5Hf1.5Сo0.5(MoO4)6 (a = 10.6169(4) Å, c = 38.1282(5) Å. V = 3722.02(2) Å3; space group R3‾c, Z = 6) is characterized by the heteropolyhedral open MT-framework formed by two types of MO6-octahedra and one type of MoO4-tetrahedron. Both MO6-octahedra are predominantly occupied by hafnium with the admixture of cobalt, but characterized by the different ratio between them. Large Rb + cations fill the large cavities and occupy A(1)O9- and A(2)O12-polyhedra. The topological features of the heteropolyhedral open MT-framework in related molybdates with the general formula AxByCz(MoO4)n are discussed.