X-ray diffraction studies on the nature of the phase transition in the stoichiometric LiMn2O4

Abstract

Crystal structure of the stoichiometric lithium–manganese oxide, LiMn2O4, below the temperature of phase transition from cubic (Fd3m) to orthorhombic (Fddd) structure has been investigated, using the high-resolution synchrotron X-ray powder diffraction. Phase analysis of low-temperature diffraction patterns makes it clear that, contrary to numerous earlier reports, only one structural transformation occurs in the stoichiometric LiMn2O4: it is the first-order transition from cubic to orthorhombic phase at about 285K, with the coexistence of both phases down to about 260K. At 250K the unit cell is face-centred orthorhombic, Fddd, with a = 24.7498(6)Å, b = 24.8655(6)Å and c = 8.1932(2)Å (b/a = 1.0047). With decreasing temperature, the orthorhombic phase shifts to a pseudo-tetragonal symmetry, and at a temperature of 10K, the unit-cell parameters are a = 24.7611(12)Å, b = 24.8003(12)Å and c = 8.1934(4)Å (b/a = 1.0016). The high-resolution synchrotron X-ray diffraction techniques enable to show unequivocally that the structure does not change to tetragonal.