Structural phase transitions of a successive type and the two-dimensional nature of the antiferromagnetic ordering in La2NiO4 single crystal

The heat capacities of a single crystal of the stoichiometric compound La2NiO4 were measured over the temperature range 14-500 K with an adiabatic calorimeter. Three heat capacity anomalies were observed. The two anomalies taking place at around 80 K and 150 K were interpreted as being due to first-order and second-order structural phase transitions, respectively, potentially of the locked-in-commensurate-incommensurate-normal-commensurate type. The third anomaly at 330 K was indicated to be a three-dimensional antiferromagnetic phase transition found commonly in two-dimensional antiferromagnets of the K2NiF4 type. The two-dimensional antiferromagnetic short-range order is discussed on the basis of the magnetic contribution to the heat capacity curve on the high-temperature side of the transition temperature.