ADDITIONAL OXYGEN ORDERING IN LA2NIO4.25 (LA8NI4O17) .2. STRUCTURAL FEATURES

On the basis of data obtained from the refinement of neutron diffraction experiments, the structural features of La8Ni4O17 (“LaNiO4.25”) are discussed. It is shown that the additional oxygen atoms in the La2O2 layers form (O5-3) polyoxides including one delocalized hole. In the NiO2 planes, a periodic modulation of the equatorial Ni-O distances characterizes various electronic configurations of the nickel cations and a 2D charge density wave occurs, which is more pronounced at low temperature than at room temperature. A strong coupling between the (O5-3) polyoxides and the NiO2 planes results from charge transfer mechanisms, which also account for the stabilization of monovalent nickel (Ni+) in this compound. The formation of -Ni+-(O5-3)-Ni+-(O5-3)- chains along given <111>T directions is correlated to the interstitial oxygen ordering previously evidenced. © 1993 Academic Press, Inc.