Cubic⇆orthorhombic transition in the stoichiometric spinel LiMn2O4

The stoichiometric spinel LiMn2O4 undergoes a transition from cubic to orthorhombic symmetry close to room temperature. Differential scanning calorimetry and conductivity experiments show that the transition takes place at 290(3) K upon cooling with a hysteresis of similar to 13 K during heating. This transition makes LiMn2O4 adopt either the cubic or orthorhombic form at room temperature, depending on its thermal history. Rietveld refinements of neutron diffraction data recorded at 350 K confirm the cubic symmetry (Fd (3) over bar m, a = 8.2495(2) Angstrom) and an oxygen stoichiometry of 3.972(26) in our sample, a value which was fully confirmed by redox titration. At 230 K, the symmetry is orthorhombic and superstructure reflections are clearly observed, leading to a 3a x 3b x 3c supercell [Fddd, a' = 24.7435(5) Angstrom, b' = 24.8402(5) Angstrom, and c' = 8.1989(1) Angstrom]. (C) 1999 The Electrochemical Society. S1099-0062(98)08-078-X. All rights reserved.