New triple molybdate Rb 22 AgIn(MoO 44 ) 33 : synthesis, framework crystal structure and ion‐transport behaviour

摘要

A new triple molybdate, Rb 2 Ag 1+3 x 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+3 x In 1– x (MoO 4 ) 3 ( x ≈ 0.004) is of a new type crystallizing in the centrosymmetric space group R c [ a ?= 10.3982?(9), c = 38.858?(4)??, 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?? 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.