Synthesis, Structure, and Conductivity of Alluaudite-Related Phases in the Na2MoO4-Cs2MoO4-CoMoO4 System: Synthesis, Structure, and Conductivity of Alluaudite-Related Phases in the Na2MoO4-Cs2MoO4-CoMoO4 System

Phase formation study of the Na2MoO4-Cs2MoO4-CoMoO4 system resulted in new cesium-containing alluaudite-related phases. The solid solution Na4-2x-yCsyCo1+x(MoO4)(3) (0 <= x, y <= 0.30), based on the alluaudite-type Na4-2xCo1+x(MoO4)(3), and triple molybdate Na-10(Cs4-xNax)Co-5(MoO4)(12) (0 <= x <= 0.30) were found, and their structures were solved. In the structure of Na3.21Cs0.37Co1.21(MoO4)(3) (a = 13.0917(8) angstrom, b = 13.5443(8) angstrom, c = 7.1217(4) angstrom, space group C2/c, beta = 112.331(2), Z = 4), the cesium ions partially substitute the Na+ in the channels running along the c-axis. The structure of Na-10(Cs3.77Na0.23)Co-5(MoO4)(12) (a = 13.6572(3) angstrom, b = 11.5063(3) angstrom, c = 27.9898(5) angstrom, space group Pbca, Z = 4) was proved to be the aristotype for the pseudo orthorhombic Na25Cs8R5(MoO4)(24) (R = Fe, Sc, In). The compounds contain alluaudite-like layers of MoO4 tetrahedra and pairs of edge-shared (Co, Na)O-6 or (R, Na)O-6 and NaO6 octahedra, which are connected by bridging MoO4 tetrahedra to form 3D frameworks differing from the alluaudite type. The frameworks contain channels along the c-axis filled by Cs+ and Na+ ions. Bond valence sum (BVS) maps show that the alluaudite-related molybdates can have a 2D sodium-ion conductivity at elevated temperatures in contrast to the alluaudite-type cathode material Na2+2xFe2-x(SO4)(3) with a 1D conductivity. The measured ionic conductivity of Na4-2xCo1+x(MoO4)(3), Na4-2x-yCsyCo1+x(MoO4)(3), and Na10Cs4Co5(MoO4)(12) reaches 10(-3)-10(-2) S cm(-1) at 500 degrees C.