Phase formation in the Li 2 MoO 4 –Rb 2 MoO 4 –Fe 2 (MoO 4 ) 3 system and crystal structure of a novel triple molybdate LiRb 2 Fe(MoO 4 ) 3

X-ray investigation of solid state interaction of the components in the Li2MoO4–Rb2MoO4–Fe2(MoO4)3 system was carried out, and a subsolidus phase diagram of the said system was constructed. The subsystem Rb2MoO4–LiRbMoO4–RbFe(MoO4)2 was shown to be non-quasiternary. Formation of a novel triple molybdate LiRb2Fe(MoO4)3 was established, conditions of solid state synthesis and crystallization of the compound were found. Its crystal structure (orthorhombic, space group Pnma, Z=4, a=24.3956(6), b=5.8306(1), c=8.4368(2)Å) represents a new structure type and includes infinite two-row ribbons {[Fe(MoO4)3]3−}∞ parallel to the b axis and composed of FeO6 octahedra, terminal Mo(3)O4 tetrahedra, and bridge Mo(1)O4 and Mo(2)O4 tetrahedra connecting two or three FeO6 octahedra. The ribbons are connected to form 3D framework via corner-sharing LiO4 tetrahedra. Rubidium cations are 11- and 13-coordinated and located in cavities of this heterogeneous polyhedral framework.