Polymorphism in Li₂MSiO₄ ( M = Fe, Mn): a variable temperature diffraction study

Lithium transition metal silicates, Li2MSiO4 (M = Fe, Mn, Co), have attracted much interest as positive electrodes for Li-ion batteries due to their high theoretical capacities and their rich crystal chemistry. Low () and high () temperature forms of these tetrahedral structures differ in the ordering/distribution of cations within tetrahedral sites of an hcp based packing of oxygen. We have carried out VT powder neutron and X-ray diffraction on Li2MSiO4 (M = Fe, Mn) to characterize the rich polymorphism observed across the Li2MSiO4 (M = Fe, Mn) system as a function of temperature. For Li2FeSiO4 below 500 degrees C a (II) phase was observed with slight Li/Fe disorder. Above this temperature there was a sluggish transition to the (s) phase, which was observed up to ca. 820 degrees C, with the (II) polymorph seen at highest temperatures. Excellent agreement with phases obtained by quenching was observed, except that the phases exhibit a statistical distribution of Li/Fe, in contrast to the well-ordered structures of quenched phases. The Li2MnSiO4 polymorphism is less complex, the (II) phase is stable from room temperature to 750 degrees C, at which temperature it transforms to the structure with disordered Li/Mn.