RbAgSc(WO): a new glaserite-related structure type, rubidium disorder, ionic conductivity.

A new triple tungstate RbAgSc(WO) (0 ≤ x ≤ 0.15) synthesized by solid state reactions and spontaneous crystallization from melts presents a new structure type related to those of CsNaYb(MoO) and NaSrTa(PO). The title compound in centrosymmetric space group Cmcm contains dimers of two ScO octahedra sharing corners with three bridging WO tetrahedra. Three pairs of opposite terminal WO tetrahedra are additionally linked by AgO dumbbells to form {Ag[Sc(WO)]} 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 RbAgSc(WO) ceramics revealed a first-order superionic phase transition at 570 K with a sharp increase in the electrical conductivity. The conductivity σ = 1.8 × 10 S cm at 690 K is comparable with the value of 1.0 × 10 S cm (500 K) observed earlier for rubidium-ion transport in pyrochlore-like ferroelectric RbNbWO.