Cubic <-> orthorhombic transition in the stoichiometric spinel LiMn2O4

The stoichiometric spinel LiMn2O4 undergoes a transition from cubic to orth orhombic symmetry close to room temperature. Differential scanning calorime try and conductivity experiments show that the transition takes place at 29 0(3) K upon cooling with a hysteresis of similar to 13 K during heating. Th is 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 3.97 2(26) in our sample, a value which was fully confirmed by redox titration. At 230 K, the symmetry is orthorhombic and superstructure reflections are c learly 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.