Synthesis, structure and lithium-ion conductivity of Li2-2xMg2+x(MoO4)(3) and Li3M(MoO4)(3) (M-III = Cr, Fe)

Lithium magnesium molybdates of the general formula Li2-2xMg2+x(MoO4)(3), for 0 less than or equal to x less than or equal to 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. Li-7 NMR spectral data are consistent with this interpretation.