Structural phase transition of Nd2NiO4+δ (0.106≤δ≤0.224)

Crystal phases, crystal systems and lattice constants of Nd2NiO4+delta at delta=0.106-0.224 have been studied up to 970 K to examine the orthorhombic-tetragonal phase transition of Nd2NiO4+delta. In the room temperature phase diagram of Nd2NiO4+delta, two orthorhombic phases are found at delta=0.13-0.17 and 0.205-0.224 and two mixed phases are found at delta=0.106 and 0.17-0.205. The (b-a) value of the orthorhombic phase at delta=0.205-0.224 decreases with decreasing the delta value and is expected to become zero at delta=0.13. A Rietveld analysis of the phase at delta=0.106 shows that the phase consists of 90% of all orthorhombic phase and 10% of a tetragonal phase. The excess oxygen will be concerned for the orthorhombic-tetragonal phase transition of Nd2NiO4+delta. However, between the orthorhombic phase at delta=0.205-0.224 and the phase at delta=0.106, another orthorhombic phase, which (b-a) value increases with decreasing the 3 value, exists at delta=0.13-0.17. The concernment of the excess oxygen is not clear. For considering the phase transition, the orthorhombic phase just below the phase transition temperature is the phase at delta=0.205-0.224 or the phase at delta=0.13-0.17 is important. The phase transition temperature of Nd2NiO4+delta is 880-890 K ill air, and at 760-860 K the (b-a) value increases with increasing the oxygen partial pressure. that is, with increasing the 5 value. The orthorhombic phase at 760-860 K is not the phase at delta=0.13-0.17 but is likely the phase at delta=0.205-0.224. The phase transition of Nd2NiO4+delta should be caused by the decrease of the 6 value due to the increase of temperature. (c) 2005 Elsevier B.V. All rights reserved.