Thirteen new phases, Li 2 MX0 4 , have been synthesised. Nine of these, labelled a, with: Μ = Mg, Μη, Fe, Co, Zn, have the tetragonal α-LiFe0 2 , rock salt-related structure and are isostructural with previously reported Li 2 MgZr0 4 . In their structures, Μ and X ions are disordered over the octahedral Fe sites of a-LiFe0 2 . Three phases, labelled β, with: MX = NiHf, CuZr, CuHf, have closely related structures but with a lower symmetry space group and significantly different c/a ratios for their tetragonal unit cells. A structural model involving ordering of Μ and X ions is proposed in which the driving force for ordering is a Jahn-Teller distortion associated with Ni and Cu. Two phases: X = NiHf, NiZr, form other high temperature polymorphs which are also related structurally to rock salt but with varying degrees of cation disorder.

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