Synthesis, structure and lithium-ion conductivity of Li 2−2x Mg 2+x (MoO 4 ) 3 and Li 3 M(MoO 4 ) 3 (M III = Cr, Fe)

Lithium magnesium molybdates of the general formula Li2−2xMg2+x(MoO4)3, for 0 ≤ x ≤ 0.3, have been synthesized and their structure and lithium ion conductivity investigated. Determination of crystal structure of one of the members, Li2Mg2(MoO4)3, has revealed a three-dimensional framework consisting of metal–oxygen octahedra and trigonal prisms (where Li and Mg reside) which are interconnected by MoO4 tetrahedra. Although the framework is three-dimensional, lithium-ion conductivity appears to be restricted to the one-dimensional channels formed by interconnected trigonal prisms. Isotypic molybdates, Li3M(MoO4)3 (M = Cr, Fe), where lithium ions occupy exclusively the trigonal prismatic channels, exhibit a higher lithium ion conductivity than Li2−2xMg2+x(MoO4)3, lending support to the idea that the conductivity is one-dimensional in these materials. 7Li NMR spectral data are consistent with this interpretation.