X-ray absorption spectra and the structure of the higher oxide forms of nickel

X-ray absorption spectra have been measured for beta-NiOOH, Ni3O2(OH)(4), Ni2O3. 6H(2)O, chemically and electrochemically prepared gamma-NiOOH as well as commercially obtained NiO2. Information on the local structure and nature of bonding of nickel in these compounds have been derived using theoretical standards generated with the FEFF code. Spectra of Ni, NiO, and KNiIO6 were used as standard-reference for Ni-0, Ni2+, and Ni4+, respectively. Based on the x-ray absorption edge energies and local structure parameters, we conclude that the various nickel oxycompounds consist of a mixture of phases. One phase consists of Ni2+ in the form of alpha-Ni(OH)(2) and/or beta-Ni(OH)(2). The other phase contains Ni4+ and is likely to be NiO2, a material which can be obtained in pure form, e.g., by electrochemical oxidation of LiNiO2. The fraction of each phase depends on preparation conditions. A chemically prepared gamma-NiOOH sample showed the highest fraction of NiO2 (i.e., Ni4+).