Synthesis, structure and lithium-ion conductivity of Li_2-2x Mg_2+x (MoO₄)₃ and Li₃ M(MoO₄)₃(M^III= Cr, Fe)

Abstract

Lithium magnesium molybdates of the general formula Li_2-2xMg_2+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 onedimensional 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 Li_2-2xMg_2+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.