ELECTROCHEMICAL PREPARATION AND STRUCTURAL CHARACTERIZATION OF LA2NIO4+DELTA PHASES (0 LESS-THAN-OR-EQUAL-TO DELTA LESS-THAN-OR-EQUAL-TO 0.25)

Electrochemical oxidation in alkaline solution has been used to control and to extend the oxygen overstoichiometry of La2NiO4+δ phases (0 ≤ δ ≤ 0.25). Chemical analysis has been carried out to determine the nickel (III) content and the related oxygen excess. Structural data (X-ray powder diffraction on La2NiO4+δ and neutron powder diffraction on La2NiO4.25) show that the interstitial oxygen atoms are introduced into the La2O2 layers. ESR spectra for the La2NiO4.25 compound recorded at 4.2 and 298 K characterize either low spin Ni3+(t62gd1x) in a flattened octahedral site or Ni+(t62gd2τd1x) in an elongated octahedron. The results show that the relaxation of the structure as a function of δ could arise either from a steric effect (i.e., formation of Ni3+) or from a disproportionation of Ni2+ into Ni+ and Ni3+. © 1993 Academic Press, Inc.