OXYGEN DEFECT STRUCTURE IN La₂MO_4+d (M = Cu, Ni, Co). W. Paulus^1,2, A. Cousson¹, G. Heger², A. Revcolevschi³, G. Dhalenne³, S. Hosoya⁴, V. Kvardakov⁵. ¹CE-Saclay, Lab. Léon Brillouin, F-91191 Gif-sur-Yvette; ²RWTH Aachen, Inst. of Crystallography, Jägerstr. 17-19 D-52056 Aachen, Germany; ³Lab. of Solid State Chemistry, Univ. Paris-Sud, F- 91405 Orsay, France; ⁴Univ. of Kofu, Institute of Inorganic Synthesis, Kofu 400, Japan; ⁵Kurchatov Institute Russian Science Centre, 123182 Moskau, Russia.

La₂MO₄ (M = Cu, Ni, Co) are isostructural (K₂NiF₄-type structure) and can incorporate oxygen on interstitial sites forming La₂MO_4+d. The amount of the extra oxygen is limited to d = 0.08, 0.16 and 0.25 for the Cu, Ni and Co homologous compounds respectively. With the formation of the oxygen-rich phases, strong changes in the physical properties are observed, e.g. the semiconductor La₂CuO_4.00 becomes metallic and superconducting with a T_c max. of 44K when oxygenated to d = 0.08. For the Ni and Co compounds no superconducting behaviour was observed for any oxygen stoichiometry. In all cases, however, a continuous vanishing of all a * a * c superstructure reflections goes along with the uptake of oxygen. These superstructure reflections are related to the tilting of the MO₆ octahedra observed for stoichiometric La₂MO_4.00 compounds in the orthorhombic phase at ambient temperature. The purpose of our studies is to understand the structural disorder induced by the uptake of extra oxygen in order to estimate the influence on phase transitions and electronical properties. We report here on our temperature-dependent neutron diffraction studies carried out on single crystals of La₂CoO_4+d and discuss the results in relation with those obtained for La₂NiO_4+d. and superconducting La₂CuO_4+d.