X-Ray Absorption Fine Structure Spectra and the Oxidation State of Nickel in Some of Its Oxycompounds

XANES spectra of nickel K-edge and x-ray diffraction patterns of several nickel oxycompounds were measured. The shift in energy of the Ni K-absorption edge toward higher values correlates well with the increase in the oxidation state of nickel from +2 to +4 in the compounds studied. The inflection point energy of the Ni pre-edge structure shifts approximately 1 eV per unit change in valency. Our results provide evidence which supports the assignment hat the valency of Ni in ~-NiOOH is +3; Ni302(OH)4 and commercial nickel peroxide (NiO2) likewise contain trivalent nickel. The edge shift for quadrivalent nickel in KNiiO6 is reported for the first time. The structure of the higher oxide forms of nickel (where Ni has a valency greater than +2) is interesting from the standpoint of devel-oping advanced nickel batteries for consumer applications and for electric vehicle propulsion. Despite he considerable research work that has been published, there is still much uncertainty and confu-sion as to the stoichiometry and structure of the various oxides (hydroxides) that are formed during the charging (and discharging) of the nickel electrode. 1 The oxidation states of Ni in these com-pounds are particularly interesting and need clarification. The diffi