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

Abstract

A new triple tungstate Rb 9Àx Ag 3+x Sc 2 (WO 4 ) 9 (0 x 0.15) synthesized by solid state reactions and spontaneous crystallization from melts presents a new structure type related to those of Cs 7 Na 5 Yb 2 (MoO 4 ) 9 and Na 13 Sr 2 Ta 2 (PO 4 ) 9 . The title compound in centrosymmetric space group Cmcm contains dimers of two ScO 6 octahedra sharing corners with three bridging WO 4 tetrahedra. Three pairs of opposite terminal WO 4 tetrahedra are additionally linked by AgO 2 dumbbells to form {Ag 3 [Sc 2 (WO 4 ) 9 ]} 9À 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 Rb 9 Ag 3 Sc 2 (WO 4 ) 9 ceramics revealed a first-order superionic phase transition at 570 K with a sharp increase in the electrical conductivity. The conductivity i = 1.8 Â 10 À3 S cm À1 at 690 K is comparable with the value of 1.0 Â 10 À3 S cm À1 (500 K) observed earlier for rubidium-ion transport in pyrochlore-like ferroelectric RbNbWO 6 .