W0238

Phase Separations and Charge Ordering in Layered Nickelates. D.J. Buttrey¹, V. Sachan² and J.M. Tranquada³, ¹Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 USA, ²Rodel Inc., 451 Bellevue Road, Newark DE 19713 and ³Department of Physics, Brookhaven National Laboratory, Upton, NY 11973 USA

The oxygen content phase diagrams of La₂NiO_4+d , and La_2-xSr_xNiO_4±d have been extensively studied using x-ray and neutron diffraction. A wide range of oxygen stoichiometries have been prepared by annealing under controlled atmospheres. The oxygen resulting from these anneals was determined from a combination of parallel thermogravimetry and iodometric titration. Using synchrotron x-ray powder diffraction and single crystal neutron diffraction, these systems were found to exhibit a complex array of phase separations and phase transitions, as well as ordering of charge, spin, and oxygen defects. Comparisons with the cuprate analogs will also be made.