Order-disorder phase transitions of excess oxygen atoms and their appearance as eutectoid and peritectoid reactions in La2NiO4+delta (delta=0.047-0.116) crystals

Heat capacities of La2NiO4 + delta crystals were measured in the temperatur e range 14-400 K and composition range 0.047 less than or equal to delta le ss than or equal to 0.116 with an adiabatic calorimeter. Heat capacity anom alies were observed in the range 230-300 K due to positional order-disorder phase transitions of excess oxygen atoms. The phase relation, showing eute ctoid or peritectoid reactions, indicated the presence of low-temperature s table phases with delta = 0.07 and 0.10, and the transition entropies of 1. 35 and 1.77 J K-1 mol(-1) found in the delta= 0.073 and 0.098 crystals, res pectively, were interpreted as essentially consistent with formation of sta ge-3 and stage-2 structures for the respective low-temperature phases. Spon taneous heat evolution and absorption effects were observed in the range 16 0-240 K due to freezing in or out of rearrangement motion of excess oxygen atoms.