SUCCESSIVE STRUCTURAL PHASE-TRANSITIONS IN STOICHIOMETRIC LA2NIO4 OBSERVED BY X-RAY-DIFFRACTION

Phase transition in La2NiO4+delta near stoichiometric oxygen content has been studied between 10 and 800 K by powder X-ray diffraction. In addition to a second-order transition from a high-temperature tetragonal (HTT) phase into a low-temperature orthorhombic (LTO1) phase at 700 K on cooling, a transition into another orthorhombic (LTO2) phase was observed at 75 K. The LTO2 phase developed into domains large enough for X-ray powder diffraction. We present evidence that the LTO2 phase is apparently an orthorhombic one and the second transition is of first order. The nature of phase transitions in La2MO4 (M=Ni, Cu) is also discussed in relation to the temperature dependence of the orthorhombicity.