Phase formation in the Rb2MoO4-Er2(MoO4)3-Hf(MoO4)2 system and the crystal structure of new triple molybdate Rb5ErHf(MoO4)6

The subsolidus region of the Rb2MoO4-Er-2(MoO4)(3)-Hf(MoO4)(2) ternary salt system is studied using X-ray powder diffraction. A novel 5: 1: 2 triple molybdate, Rb5ErHf(MoO4)(6), is found to form in the system. Crystals of Rb5ErHf(MoO4)(6) are flux-grown under spontaneous nucleation conditions. The composition and crystal structure of Rb5ErHf(MoO4)(6) are refined in a single-crystal X-ray diffraction experiment (X8 APEX diffractometer, MoK (alpha) radiation, 1753 reflections, R = 0.0183). The crystals are trigonal; a = 10.7511(1) angstrom, c = 38.6543(7) angstrom, V = 3869.31(9) angstrom(3), d (calc) = 4.462 g/cm(3), Z = 6, space group P1. The mixed three-dimensional framework of the structure is formed of MoO4 tetrahedra, each sharing corners with two ErO6 and HfO6 octahedra. Two types of Rb atoms occupy large cavities of the framework. The distribution of the Er3+ and Hf4+ cation over two positions is refined in the course of structure solution.