Synthesis, crystal structure and lithium ion conductivity of LiMgFSO4

Abstract

Lithium magnesium fluoride sulfate, LiMgFSO₄, has been prepared by reaction of MgSO₄·H₂O, Li₂CO₃ and NH₄F at 450–500 °C. It crystallizes in space group P of the triclinic system, with Z = 2 and a = 5.1623(7), b = 5.388(1), c = 7.073(1) Å, α = 106.68(1), β = 107.40(1) and γ = 97.50(1)°. The structure is built up from single chains of corner-sharing MgO₄F₂ octahedra running parallel to the c axis, cross linked via vertex-sharing SO₄ tetrahedra, so that each tetrahedron links three different chains. Li resides within the cavities defined by the framework, wherein it is disordered between two half-occupied sites. This structure is isotypic with that of all members of the amblygonite (LiAlFPO₄)–montebrasite (LiAlOHPO₄) series, and that of tavorite (LiFeOHPO₄). It is topologically identical to the titanite (CaTiOSiO₄) and kieserite (MgSO₄·H₂O) structures. The framework structure of LiMgFSO₄ facilitates lithium ion conduction [σ (520 °C) = 1.5 × 10^-3 S cm⁻¹; E_a = 0.94 eV].