Rb9-xAg3+xSc2(WO4)9: a new glaserite-related structure type, rubidium disorder, ionic conductivity.

A new triple tungstate Rb9-xAg3+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 σi = 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.

New triple molybdate Rb2AgIn(MoO4)3: synthesis, framework crystal structure and ion-transport behaviour.

A new triple molybdate, Rb2Ag1+3xIn1-x(MoO4)3 (0 ≤ x ≤ 0.02), was found in the course of a study of the system Rb2MoO4-Ag2MoO4-In2(MoO4)3 and was synthesized as both powders and single crystals by solid-state reactions and spontaneous crystallization from melts. The structure of Rb2Ag1+3xIn1-x(MoO4)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)O6 octahedra and distorted Ag1O6 trigonal prisms linked by common faces to form [Ag(In,Ag)O9] dimers connected to each other via MoO4 tetrahedra into an open three-dimensional (3D) framework. Between two adjacent [Ag(In,Ag)O9] dimers along the c axis, an extra Ag2O6 trigonal prism with about 1% occupancy was found. The Ag1O6 and Ag2O6 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 Rb2AgIn(MoO4)3 is a member of the series of rhombohedral 3D framework molybdate structure types with a ≉ 9-10 Šand 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. Rb2AgIn(MoO4)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.

Synthesis, crystal structures and properties of the new compounds K7-xAg1+x(XO4)4 (X = Mo, W).

Two new isostructural compounds, namely heptapotassium silver tetrakis(tetraoxomolybdate), K7-xAg1+x(MoO4)4 (0 ≤ x ≤ 0.4), and heptapotassium silver tetrakis(tetraoxotungstate), K7-xAg1+x(WO4)4 (0 ≤ x ≤ 0.4), have been synthesized and found to crystallize in the polar space group P63mc (Z = 2) with the unit-cell dimensions a = 12.4188 (2) and c = 7.4338 (2) Šfor K6.68Ag1.32(MoO4)4 (single-crystal data), and a = 12.4912 (5) and c = 7.4526 (3) Šfor K7Ag(WO4)4 (Rietveld analysis data). Both structures represent a new structure type, with characteristic [K1(XO4)6] `pinwheels' of K1O6 octahedra and six XO4 tetrahedra (X = Mo, W) connected by common opposite faces into columns along the c axes. The octahedral columns are linked to each other through Ag1O4 tetrahedra along with the K2 and K3/Ag2 polyhedra, forming the polar rods (...Ag1O4-X1O4-empty octahedron-Ag1O4...). Ag1 is located almost at the centre of the largest face of its coordination tetrahedron and seems to have some mobility. The new structure type is related to the Ba6Nd2Al4O15 and CaBaSiO4 types, and to other structures of the α-K2SO4-glaserite family. The differential scanning calorimetry (DSC) and second harmonic generation (SHG) results show that both compounds undergo first-order phase transformations to high-temperature centrosymmetric phases.