New triple molybdate Rb 2 AgIn(MoO 4 ) 3 : synthesis, framework crystal structure and ion-transport behaviour.

A new triple molybdate, Rb ₂ Ag _1+3x In _1-x (MoO ₄ ) ₃ (0 ≤ x ≤ 0.02), was found in the course of a study of the system Rb ₂ MoO ₄ -Ag ₂ MoO ₄ -In ₂ (MoO ₄ ) ₃ and was synthesized as both powders and single crystals by solid-state reactions and spontaneous crystallization from melts. The structure of Rb ₂ Ag _1+3x In _1-x (MoO ₄ ) 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)O ₆ octahedra and distorted Ag1O ₆ trigonal prisms linked by common faces to form [Ag(In,Ag)O ₉ ] dimers connected to each other via MoO ₄ tetrahedra into an open three-dimensional (3D) framework. Between two adjacent [Ag(In,Ag)O ₉ ] dimers along the c axis, an extra Ag2O ₆ trigonal prism with about 1% occupancy was found. The Ag1O ₆ and Ag2O ₆ 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 Rb ₂ AgIn(MoO ₄ ) 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. Rb ₂ AgIn(MoO ₄ ) 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.