Structural phase transition of Nd 2NiO 4 + δ (0.106 ≤ δ ≤ 0.224)

Abstract

Crystal phases, crystal systems and lattice constants of Nd 2NiO 4 + δ at δ = 0.106–0.224 have been studied up to 970 K to examine the orthorhombic–tetragonal phase transition of Nd 2NiO 4 + δ . In the room temperature phase diagram of Nd 2NiO 4 + δ , two orthorhombic phases are found at δ = 0.13–0.17 and 0.205–0.224 and two mixed phases are found at δ = 0.106 and 0.17–0.205. The ( b– a) value of the orthorhombic phase at δ = 0.205–0.224 decreases with decreasing the δ value and is expected to become zero at δ = 0.13. A Rietveld analysis of the phase at δ = 0.106 shows that the phase consists of 90% of an orthorhombic phase and 10% of a tetragonal phase. The excess oxygen will be concerned for the orthorhombic–tetragonal phase transition of Nd 2NiO 4 + δ . However, between the orthorhombic phase at δ = 0.205–0.224 and the phase at δ = 0.106, another orthorhombic phase, which ( b– a) value increases with decreasing the δ value, exists at δ = 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 δ = 0.205–0.224 or the phase at δ = 0.13–0.17 is important. The phase transition temperature of Nd 2NiO 4 + δ is 880–890 K in air, and at 760–860 K the ( b– a) value increases with increasing the oxygen partial pressure, that is, with increasing the δ value. The orthorhombic phase at 760–860 K is not the phase at δ = 0.13–0.17 but is likely the phase at δ = 0.205–0.224. The phase transition of Nd 2NiO 4 + δ should be caused by the decrease of the δ value due to the increase of temperature.