Phase equilibria in the systems Rb2MoO4-R2(MoO4)3-Hf(MoO4)2 (R = Al, In, Sc, Fe(III)) and the crystal structure of double molybdate RbFe(MoO4)2

The systems Rb2MoO4-R2(MoO4)3-Hf(MoO4)2 have been investigated in the subsolidus region by X-ray powder diffraction, DTA, and IR spectroscopy. Triple molybdates of the composition 5: 1: 2 are formed in the systems with R = Al, In, Sc, and Fe. Molybdates of composition 5: 1: 3 and 1: 1: 1 are found in the iron(III)-containing system in addition to the 5: 1: 2 molybdate. Single crystals of the double molybdate RbFe(MoO4)2, which is formed in the Rb2MoO4-Fe2(MoO4)3 system, have been grown. The structure of this double molybdate has been refined using X-ray diffraction data (X8 APEX automated diffractometer, MoKα radiation, 373 F(hkl), R = 0.0287). The trigonal unit cell parameters are the following: a = b = 5.6655(2) Å, c = 7.5061(4) Å, V = 208.65(1) Å3, Z = 1, ρcalc = 3.670 g/cm3, space group R3m1. The structure is formed by layers of FeO6 octahedra sharing corners with MoO4 tetrahedra and RbO12 icosahedra.