OXYGEN INTERCALATION STAGE ORDERING, AND PHASE-SEPARATION IN N-SIMILAR-TO-DELTA-LESS-THAN-SIMILAR-TO-0.11(DELTA WITH 0.05)

We report neutron diffraction studies on a series of La2NiO4+delta sin gle crystals with 0.05 less than or similar to delta less than or simi lar to 0.11. At 300 K, all of the crystals have an average tetragonal structure (space group I4/mmm). On cooling below 290 K, one or more or thorhombic phases appear, characterized by incommensurate superlattice peaks at (0, k, l +/- DELTA) with k odd, l even, and 1/4 < DELTA less -than-or-equal-to 1/2. The positions, widths, and intensities of the s uperlattice peaks are quite sensitive to the cooling rate. We show tha t the incommensurate peaks are evidence of intercalated layers of oxyg en spaced periodically along the c axis, with a one-dimensional orderi ng similar to the staging of intercalates in graphite. The structure o f the La2NiO4 lattice between the interstitial layers is of the Bmab-t ype; the superlattice peaks result from the ordered autiphase domain b oundaries induced by the interstitial oxygens, which sit at (1/4, 1/4, 1/4)-type position.. Observed orderings involve interstitial layers s eparated by two to four Ni-O layers. Peak shapes and positions are mod elled quantitatively using the formulas of Hendricks and Teller for on e-dimensional disorder in a layer lattice. Besides the one-dimensional ordering of the intercalant layers, temperature-dependent phase separ ations are observed. Because of the slow ordering kinetics, phase sepa ration can be suppressed by rapid cooling.