Structural properties and phase diagram of La2Mo4+δ (M = Ni, Cu)

The phase diagram of La2MO4+δ (M = Ni, Cu) and the temperature dependence of orthorhombicity for several orthorhombic phases were investigated by in situ X-ray powder diffraction. A successive phase transition, tetragonal I4/mmm-[second-order] → orthorhombic Bmab-[first-order] → orthorhombicc Pccn, was observed in La2NiO4.00. The orthorhombicity of the Pccn phase decreases on cooling in contrast to that of the Bmab phase. The orthorhombic La2NiO4.15 (Fmmm) shows a weak temperature dependence of orthorhombicity and exhibits a first-order phase transition to the tetragonal I4/mmm phase on heating. As judged from the temperature dependence of orthorhombicity, superconducting La2CuO4.07, which results from the phase separation in the high-pressure oxygenated sample, is Bmab, while La2CuO4.07 prepared by electrochemical oxidation at room temperature is Fmmm. 8 refs., 2 figs