New triple molybdate Rb₂AgIn(MoO₄)₃: synthesis, framework crystal structure and ion-transport behaviour

A new triple molybdate, Rb₂Ag_1+3xIn_1-x(MoO₄)₃ (0 ≤ x ≤ 0.02), was found in the course of a study of the system Rb₂MoO₄-Ag₂MoO₄-In₂(MoO₄)₃ and was synthesized as both powders and single crystals by solid-state reactions and spontaneous crystallization from melts. The structure of Rb₂Ag_1+3xIn_1-x(MoO₄)₃ (x ≉ 0.004) is of a new type crystallizing in the centrosymmetric space group R-3c [a = 10.3982 (9), c = 38.858 (4) Å, Z = 12 and R = 0.0225] and contains (In,Ag)O₆ octahedra and distorted Ag1O₆ trigonal prisms linked by common faces to form [Ag(In,Ag)O₉] dimers connected to each other via MoO₄ tetrahedra into an open three-dimensional (3D) framework. Between two adjacent [Ag(In,Ag)O₉] dimers along the c axis, an extra Ag2O₆ trigonal prism with about 1% occupancy was found. The Ag1O₆ and Ag2O₆ prisms are located at levels of z ≉ 1/12, 1/4, 5/12, 7/12, 3/4 and 11/12, and can facilitate two-dimensional ionic conductivity. The 12-coordinate Rb atoms are in the framework cavities. The structure of Rb₂AgIn(MoO₄)₃ is a member of the series of rhombohedral 3D framework molybdate structure types with a ≉ 9-10 Å and long c axes, which contain rods of face-shared filled and empty coordination polyhedra around threefold axes. Electrical conductivity of ceramics is measured by impedance spectroscopy. Rb₂AgIn(MoO₄)₃ undergoes a `blurred' first-order phase transition at 535 K with increasing electrical conductivity up to 1.1 × 10^-2 S cm^-1 at 720 K. Thus, the compound may be of interest for developing new materials with high ionic conductivity at elevated temperatures.