Polymorphism and structural defects in Li2FeSiO4Electronic supplementary information (ESI) available: Atomic distances and angles in Li2FeSiO4 synthesised at 900°C. See DOI: 10.1039/c002815k

Li 2 FeSiO 4 , an interesting material with potential applications as the positive electrode in lithium batteries, shows complex crystal chemistry due to the versatility of cation ordering (Li + , Fe 2+ , Si 4+ ) within tetrahedral sites of buckled hexagonal close packed layers of oxygen atoms. This study, conducted through X-ray and electron diffraction experiments, focuses on three samples of Li 2 FeSiO 4 (obtained from ceramic synthesis at 700 °C, 800 °C and 900 °C) which may contain significant amounts of structural defects. Two polymorphs of Li 2 FeSiO 4 were isolated and investigated through X-ray diffraction and electron microscopy. A new form of Li 2 FeSiO 4 (space group Pmnb with a = 6.2853(5), b = 10.6592(8) Å and c = 5.0367(4) Å or alternatively P 2 1 / n with a = 6.2819(1) Å, b = 10.6575(2) Å, c = 5.0371(1) Å, β = 90.032(7)°) prepared at 900 °C, shows cooperative small displacements of lithium cations from one tetrahedral site (up) to another (down). Attempts to prepare the second, low-temperature, polymorph (space group P 2 1 / n , a = 8.2253(5) Å, b = 5.0220(1) Å, c = 8.2381(4) Å, β = 99.230(2)°) previously reported by Nishimura et al. , lead to crystals exempt of structural defects (at 700 °C) or built up by an intergrowth between the low temperature polymorph and a residue of the high temperature one. Under specific preparation conditions, Li 2 FeSiO 4 crystals may contain intergrowths of two different polymorphs with different relative orientations of LiO 4 , FeO 4 , and SiO 4 , tetrahedral along specific directions.