Polymorphism and structural defects in Li2FeSiO4

Li2FeSiO4, 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+, Fe2+, Si4+) 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 Li2FeSiO4 (obtained from ceramic synthesis at 700 degrees C, 800 degrees C and 900 degrees C) which may contain significant amounts of structural defects. Two polymorphs of Li2FeSiO4 were isolated and investigated through X-ray diffraction and electron microscopy. A new form of Li2FeSiO4 (space group Pmnb with a = 6.2853(5), b = 10.6592(8) angstrom and c = 5.0367(4) angstrom or alternatively P2(1)/n with a = 6.2819(1) angstrom, b = 10.6575(2) angstrom, c = 5.0371(1) angstrom, beta = 90.032(7)degrees) prepared at 900 degrees 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 P2(1)/n, a = 8.2253(5) angstrom, b = 5.0220(1) angstrom, c = 8.2381(4) angstrom, beta = 99.230(2)degrees) previously reported by Nishimura et al., lead to crystals exempt of structural defects (at 700 degrees C) or built up by an intergrowth between the low temperature polymorph and a residue of the high temperature one.