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

Crystal structure of the stoichiometric lithium-manganese oxide, LiMn 2O4, 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. © 2004 Elsevier B.V. All rights reserved.