Lithium magnesium fluoride sulfate, LiMgFSO 4 , has been prepared by reaction of MgSO 4 ?H 2 O, Li 2 CO 3 and NH 4 F at 450-500 uC. It crystallizes in space group P1 of the triclinic system, with Z~2 and a~5.1623(7), b~5.388(1), c~7.073(1) Å , a~106.68(1), b~107.40(1) and c~97.50(1)u. The structure is built up from single chains of corner-sharing MgO 4 F 2 octahedra running parallel to the c axis, cross linked via vertex-sharing SO 4 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 (LiAlFPO 4 )-montebrasite (LiAlOHPO 4 ) series, and that of tavorite (LiFeOHPO 4 ). It is topologically identical to the titanite (CaTiOSiO 4 ) and kieserite (MgSO 4 ?H 2 O) structures. The framework structure of LiMgFSO 4 facilitates lithium ion conduction [s (520 uC)~1.5610 -3 S cm 21 ; E a~0 .94 eV]. 374

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Metal oxides containing mobile lithium ions are technologically important materials in the context of design and development of electrolytes and electrodes for solid-state lithium batteries. Mobility of lithium in a solid manifests itself in the following measureable ways: ionic conductivity/diffusion, redox insertion/deinsertion and ion exchange. While ionic conductivity and redox insertion/deinsertion determine the practical use of a material as an electrolyte and electrodes, respectively, ion exchange involving lithium in aqueous/molten salt media under mild conditions not only provides a convenient probe for the investigation of lithium mobility in solids, but also enables synthesis of new metastable phases. In this article, we present a chemical (rather than electrochemical) perspective of lithium ion mobility in inorganic oxide materials, in an attempt to bring out the relationships between structure and properties associated with lithium ion mobility. The survey shows that considerable lithium ion mobility occurs both in closepacked (rocksalt and its relatives, spinel, LiNbO 3 , rutile and perovskite) as well as open-framework (e.g. NASICON) oxide structures. LiCoO 2 (a-NaFeO 2 ), LiMn 2 O 4 (spinel), LiNbO 3 /LiTaO 3 (structure based on HCP array of anions), LiNbWO 6 (trirutile) and (Li,La)TiO 3 (perovskite) are some of the oxide materials (structure type indicated in parentheses) where high lithium mobility has been well established by various experimental studies. An investigation of the factors that control lithium ion conductivity in the (Li,La)TiO 3 perovskite has enabled us to design new perovskite oxides in the Li-Sr-B-B'-O (B ~Ti, Zr; B' ~Nb, Ta) systems that exhibit high lithium ion mobility/conductivity. Among the framework materials, NASICON (e.g. Na 3 Zr 2 PSi 2 O 12 ) turns out to be a versatile structure that supports high lithium mobility under ion-exchange, ionic conductivity and redox insertion/deinsertion conditions. {Based on a lecture delivered at the international symposium ''Materials for Energy: Batteries and Fuel Cells '', November 2002,

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