ALLUAUDITE-LIKE PHASES IN THE Na2MoO4-Cs2MoO4-NiMoO4 SYSTEM

The phase formation in the Na2MoO4-Cs2MoO4-NiMoO4 system is studied using the solid-state synthesis and flux crystallization. The formation of a cesium-containing solid solution based on double molybdate Na4-2xNi1+x(MoO4)(3) of the alluaudite type and new triple molybdate of a related structure is established. Their structures contain layers of MoO4 tetrahedra and coupled (Ni, Na)O-6 octahedra bridged by MoO4 tetrahedra into three-dimensional open frameworks with tunnels along the c axis filled by sodium cations. By the example of isostructural Na3.39Ni1.31(MoO4)(3) (I) and Na3.18Cs0.29Ni1.26(MoO4)(3) (II), it is shown that in the alluaudite-type solid solution, cesium ions partially replace sodium cations in the framework tunnels, which provides the sodium ion conductivity. The structure of triple molybdate Na2.64Cs0.64Ni1.36(MoO4)(3) (III) is a new type of the alluaudite superstructure deformed and tripled in volume. Its formation is caused by a significant differentiation of cations at (Ni, Na) and (Na, Cs) positions. In terms of the degree of deformation, this structure is intermediate between alluaudite and (pseudo)orthorhombic Na10Cs4M5(MoO4)(12) (M = Mn, Co) and Na25Cs8R5(MoO4)(24) (R = Fe, Sc, In). Among the crystallization products in the Na2MoO4-Cs2MoO4-NiMoO4 system, crystals with a similar incommensurate modulation of the alluaudite structure along the c axis are found. Calculations of bond valence sum maps for sodium ions in the structures of I-III confirm the possibility of the one-dimensional sodium-ion conductivity for them at its lowest threshold for double molybdate for which the twodimensional transport is also probable at elevated temperatures.