Phase diagram and effect of excess oxygen atoms on the spin and charge ordering in La2NiO4+delta crystals

Heat capacities of La2NiO4+delta a crystals were measured in the temperatur e range 14-400 K and composition range 0.120 less than or equal to delta le ss than or equal to 0.154 with an adiabatic calorimeter. Heat-capacity anom alies were observed in the range 240-360 K due to positional order-disorder phase transitions of excess oxygen atoms. The phase relation showed phase separations of eutectoid and penitectoid types in the ranges 0.10<delta<0.1 4 and delta>0.14, respectively, and the presence of the low-temperature sta ble phase (designated as the zeta phase) with delta approximate to 0.14. In the low-temperature zeta phase, in which charge and spin orderings in NiO2 planes have been reported, a heat-capacity anomaly was observed at around 100 K due to the cooperative antiferromagnetic spin ordering, but no apprec iable heat-capacity anomaly indicating the cooperative charge (hole) orderi ng was observed. The arrangement of excess oxygen atoms in the zeta phase a nd the effect of excess oxygen atoms on the spin and charge orderings are d iscussed. [S0163-1829(99)03746-7].