X-ray study of the spinel LiMn2O4 at low temperatures

Stoichiometric LiMn2O4 undergoes a structural phase transition close to roo m 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 superstru cture of the cubic spinel structure, induced by a charge ordering on the ma nganese sites. By use of synchrotron X-ray diffraction, it is shown that th e transition is first-order with the coexistence of two phases in a small r ange of temperatures and that the cell volume very slightly increases on co oling. Synchrotron X-ray diffraction confirms the symmetry of the low-tempe rature phase and reveals that the orthorhombic phase studied at 230 K progr essively shifts to a symmetry close to tetragonal when cooled to lower temp eratures. However, the symmetry of LiMn2O4 is never tetragonal, even down t o 1.5 K, and this tends to exclude the onset to a complete charge-ordered s tate.