OXYGEN-DEFECT GEOMETRY IN OXYGEN-RICH LA2COXCU1-XO4+DELTA LAYERED OXIDES

The structural properties of the La2CoxCu1-xO4+δ (x = 0.25, 0.75) series have been studied by powder neutron diffraction. Excess oxygen is accommodated as an O2- ion in the interstitial space defined by the lanthanide bilayers and leads to the displacement of some of the neighboring apical oxygens of the (Cu, Co)O6 octahedra from their normal lattice sites. The rigid tilt of the metal-oxygen octahedral units is, however, suppressed in the Co-rich system and the Fmmm space group is adopted. The Cu-rich compound is found to remain isostructural with the parent La2CuO4 system (space group Bmab). The defect occupies a unique site in the x = 0.25 system but is equally shared between two symmetry-equivalent positions for x = 0.75. © 1994 Academic Press, Inc.