PHASE-SEPARATION, OXYGEN COMPOSITION, AND A GLASS-TRANSITION DUE TO FREEZING-IN OF THE OXYGEN REARRANGEMENT IN LA2NIO4.094 SINGLE-CRYSTALS

Heat capacities of La2NiO4.094 single crystal were measured in 15-400 K with an adiabatic calorimeter. Four anomalies were found, as due to a phase separation concerning the excess oxygen composition at around 285 K, a glass transition over a wide temperature range of 150-300 K, a first-order structural transition from tetragonal to orthorhombic ph ases at 240 K, and an antiferromagnetic phase transition at around 55 K. The glass transition appeared as giving rise to spontaneous, exothe rmic at low temperatures and endothermic at high temperatures, enthalp y relaxations due to irreversible equilibration processes with respect to the compositions and configurations of excess oxygen atoms. The pr operties of the phase separation are discussed on the basis of the rel axation rates and the heat capacity anomaly around the separation temp erature.