Electrochemical preparation and structural characterization of La[sub 2]NiO[sub 4+[delta]] phases (0 [le] [delta] [le] 0. 25)

Electrochemical oxidation in alkaline solution has been used to control and to extend the oxygen overstoichiometry of La[sub 2]NiO[sub 4+[delta]] phases (0 [le] [delta] [le] 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 La[sub 2]NiO[sub 4+[delta]] and neutron powder diffraction on La[sub 2]NiO[sub 4.25]) show that the interstitial oxygen atoms are introduced into the La[sub 2]O[sub 2] layers. ESR spectra for the La[sub 2]NiO[sub 4.25] compound recorded at 4.2 and 298 K characterize either low spin Ni[sup 3+] (t[sup 6][sub 2g]d[sup 1][sub x][sup 2][sub [minus]y][sup 2]) in a flattened octahedral site or Ni[sup +] (t[sup 6][sub 2g]d[sup 2][sub x][sup 2]d[sup 1][sub x][sup 2][sub [minus]y][sup 2]) in an elongated octahedron. The results show that the relaxation of the structure as a function of [delta] could arise either from a steric effect (i.e., formation of Ni[sup 3+]) or from a disproportionation of Ni[sup 2+] into Ni[sup +].