Rb_9-xAg_3+xSc₂(WO₄)₉: a new glaserite-related structure type, rubidium disorder, ionic conductivity

A new triple tungstate Rb_9-xAg_3+xSc₂(WO₄)₉ (0 ≤ x ≤ 0.15) synthesized by solid state reactions and spontaneous crystallization from melts presents a new structure type related to those of Cs₇Na₅Yb₂(MoO₄)₉ and Na₁₃Sr₂Ta₂(PO₄)₉. The title compound in centrosymmetric space group Cmcm contains dimers of two ScO₆ octahedra sharing corners with three bridging WO₄ tetrahedra. Three pairs of opposite terminal WO₄ tetrahedra are additionally linked by AgO₂ dumbbells to form {Ag₃[Sc₂(WO₄)₉]}^9- groups, which together with some rubidium ions are packed in pseudohexagonal glaserite-like layers parallel to (001), but stacking of the layers is different in these three structures. In the title structure, the layers stack with a shift along the b axis and their interlayer space contains disordered Rb⁺ cations partially substituted by Ag⁺ ions. Almost linear chains of incompletely filled close Rb3a-Rb3d positions (the shortest distances Rb-Rb are 0.46 to 0.64 Å) are found to locate approximately along the b axis. This positional disorder and the presence of wide common quadrangular faces of Rb2 and Rb3a-Rb3d coordination polyhedra favor two-dimensional ionic conductivity in the (001) plane with Rb⁺ and Ag⁺ carriers, which was confirmed with bond valence sum (BVS) maps. Electrical conductivity measurements on Rb₉Ag₃Sc₂(WO₄)₉ ceramics revealed a first-order superionic phase transition at 570 K with a sharp increase in the electrical conductivity. The conductivity σ_i = 1.8 × 10^-3 S cm^-1 at 690 K is comparable with the value of 1.0 × 10^-3 S cm^-1 (500 K) observed earlier for rubidium-ion transport in pyrochlore-like ferroelectric RbNbWO₆.