X-ray study of the spinel LiMn2O4 at low temperatures

Stoichiometric LiMn2O4 undergoes a structural phase transition close to room temperature from cubic to orthorhombic symmetry. The crystal structure at 230 K (space group Fddd and a = 24.743(1), b = 24.840(1.), and c = 8.199(1) Angstrom) corresponds to an orthorhombic similar to 3a x 3a x a superstructure of the cubic spinel structure, induced by a charge ordering on the manganese sites. By use of synchrotron X-ray diffraction, it is shown that the transition is first-order with the coexistence of two phases in a small range of temperatures and that the cell volume very slightly increases on cooling. Synchrotron X-ray diffraction confirms the symmetry of the low-temperature phase and reveals that the orthorhombic phase studied at 230 K progressively shifts to a symmetry close to tetragonal when cooled to lower temperatures. However, the symmetry of LiMn2O4 is never tetragonal, even down to 1.5 K, and this tends to exclude the onset to a complete charge-ordered state.