Synthesis, crystal structures, and properties of new acentric glaserite-related compounds Rb7Ag5-3xSc2+x(XO4)(9) (X = Mo, W)

DOI 10.1016/j.jssc.2021.122638

The subsolidus phase equilibria in the system Ag2MoO4-Rb2MoO4-Sc-2(MoO4)(3) were studied and two new triple molybdates, Rb9Ag3Sc2(MoO4)(9) and Rb7Ag5Sc2(MoO4)(9), were found. The structures of Rb7Ag5Sc2(MoO4)(9) and isostructural Rb7Ag5Sc2(WO4)(9) of the Cs7Na5Yb2(MoO4)(9) type (the space group 32) were determined. The found composition of the triple tungstate crystal, Rb7Ag4.61Sc2.13(WO4)(9), indicates a non-stoichiometric compound formula, Rb7Ag5-3xSc2+x(WO4)(9). Both structures have one incompletely occupied Ag site, and structure Rb7Ag4.61Sc2.13(WO4)(9) also contains two positions with mixed Ag and Sc. Both compounds contain 'lanterns' [M-2(XO4)(9)] (M = (Sc, Ag), Sc; X = Mo, W), which are strengthened by three AgO2 dumbbells to give isolated building blocks [Ag3M2(XO4)(9)] forming two-story hexagonal layers resembling the structure of glaserite K3Na(SO4)(2). Similar layers of [Ag3Sc2(WO4)(9)](9-) building blocks were also found by us in the structure of Rb9-xAg3+xSc2(WO4)(9), which is close to that of Rb9Ag3Sc2(MoO4)(9). Similar layers of the [M-2(TO4)(9)] units were also observed in Cs7Na5Yb2(MoO4)(9) and Na13Sr2Ta2(PO4)(9). The title compounds belong to the series of rhombohedral triple molybdates and tungstates with a approximate to 9-10 angstrom and large c-periods (more than 20 angstrom), which have layered or open 3D framework structures. Like many compounds of this series, Rb7Ag5Sc2(XO4)(9) (. = W, Mo) at elevated temperatures have significant ionic conductivity reaching values 6.1.10(-3) S cm(-1) at 703 K (X = Mo) and 1.4.10(-3) S cm(-1) at 733 K (X = W) with E-a = 0.7 eV and 0.6 eV, respectively.