Phase Formation in the Rb_2MoO_4-Er_2(MoO_4)_3-Hf(MoO_4)_2 System and the Crystal Structure of New Triple Molybdate Rb_5ErHf(MoO_4)_6

摘要

The subsolidus region of the Rb_2MoO_4-Er_2(MoO_4)_3-Hf(MoO_4)_2 ternary salt system is studied using X-ray powder diffraction. A novel 5:1:2 triple molybdate, Rb_5ErHf(MoO_4)_6, is found to form in the system. Crystals of Rb_5ErHf(MoO_4)_6 are flux-grown under spontaneous nucleation conditions. The composition and crystal structure of Rb_5ErHf(MoO_4)_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) A, c = 38.6543(7) A, V= 3869.31(9) A~3, d_(calc) = 4.462 g/cm~3, Z- 6, space group R3 c. The mixed three-dimensional framework of the structure is formed of MoO_4 telrahedra, each sharing corners with two ErO_6 and HfO_6 octahedra. Two types of Rb atoms occupy large cavities of the framework. The distribution of the Er~(3+) and Hf~(4+) cation over two positions is refined in the course of structure solution.