Thermally-induced and chemically-induced structural changes in layered perovskite-type oxides Nd2 − xSrxNiO4 + δ (x = 0, 0.2, 0.4)

High temperature X-ray diffraction measurements were carried out on K2NiF4-type Nd2 − xSrxNiO4 + δ (x = 0, 0.2, 0.4) in N2–O2 atmosphere at 873–1173 K. Diffraction patterns of Nd2 − xSrxNiO4 + δ were indexed by a tetragonal symmetry except for that of Nd2NiO4 + δ measured at 873 K in 1 bar O2. Tetragonal-orthorhombic phase transition was observed in Nd2NiO4 + δ. The phase transition temperature decreases as P(O2) decreases, e.g., about 883 K in P(O2) = 1 bar and about 758 K in P(O2) = 10− 4 bar. Temperature vs. oxygen content phase diagram for Nd2NiO4 + δ is illustrated from the relationship between phase transition temperature and oxygen content. Lattice parameters and atomic arrangement were estimated by the Rietveld analysis. As the amount of excess oxygen increases, the lattice parameter perpendicular to the perovskite and the rock salt layers increases and that parallel to the layers slightly decreases. As a consequence, the cell volume is almost independent of δ. The lattice parameters essentially depend on temperature and the amount of excess oxygen. Apparent thermal expansion coefficient was calculated from temperature dependence of lattice parameters in a constant atmosphere, while true thermal expansion coefficient was calculated from the temperature dependence of lattice parameters at the same oxygen content. Isothermal chemical expansion coefficient was calculated from the variation of the lattice constants with oxygen content. Thermal and chemical expansion coefficients are compared with expansion coefficients of other nonstoichiometric oxides. To make clear the correlation between oxygen nonstoichiometry and structural parametersʹ variation, space in the rock salt layer where interstitial oxygen is located is calculated from the structural information. Space in the rock salt layer decreases as the calculated acceptor concentration, x + 2δ, increases. This means that the interstitial oxygen formation is suppressed as the acceptor concentration increases. Similar tendency has been confirmed in oxygen nonstoichiometric behavior of Ni-based K2NiF4-type oxides.