AN X-RAY-DIFFRACTION STUDY OF THE OXYGEN-CONTENT PHASE-DIAGRAM OF LA2NIO4+DELTA

The oxygen concentration phase diagram of La2NiO4+δ has been constructed from room temperature X-ray powder diffraction data on specimens spanning a wide range of oxygen stoichiometries. Contrary to some previous reports, we find multiple biphasic regions. The most stoichiometric phase (δ ≈ 0.00) is orthorhombic with Bmab (low temperature orthorhombic, LTO) symmetry in agreement with previous studies. A new distorted tetragonal phase with P 42/ncm (low temperature tetragonal, LTT) symmetry is observed for oxygen excess levels in the range 0.020 < δ < 0.030. Undistorted tetragonal (high temperature tetragonal, HTT) symmetry is observed for oxygen compositions from δ ≈ 0.055 to at least δ = 0.137. An orthorhombic phase appears for δ ≈ 0.168. The temperature dependence of the reduction boundary has been determined; however, in contrast with the Pr2NiO4+δ system, no evidence of the oxidation boundary has been observed. The room temperature LTT phase is consistent with lower temperature observations of weak ferromagnetism associated with spin canting. © 1993 Academic Press, Inc.