Structural analysis of La2−xSrxNiO4 + δ by high temperature X-ray diffraction

High temperature X-ray diffraction measurements were made on oxygen nonstoichiometric La2 − xSrxNiO4 + δ in N2–O2 atmosphere at 873–1173 K. Crystal structure of La2−xSrxNiO4 + δ at high temperatures was analyzed using the tetragonal symmetry, I4/mmm. 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 decreases. As a consequence, the cell volume is almost constant regardless of the oxygen content variation. The lattice parameters essentially depend on temperature and δ. The relationship among the lattice parameters, temperature, and δ is expressed by the total differential form of the lattice parameters. The model with linear approximation can well explain the variation of the lattice parameters with δ and temperature. Apparent and true thermal expansion coefficients were calculated from the variation of the lattice parameters with temperature. Crystal structure of La2 − xSrxNiO4 + δ was estimated by the Rietveld analysis. It is elucidated that the space in the rock salt layer decreases as the acceptor concentration (x + 2δ) increases. The variation of the space in the rock salt layer is consistent with the oxygen nonstoichiometric behavior that the interstitial oxygen formation is suppressed as x and δ increase.