X-Ray Absorption Spectra and the Local Structure of Nickel in Some Oxycompounds and Fluorides

X-ray absorption spectra were measured for a number of nickel oxycompounds and fluorides which contain nickel with valencies in the range of 2 to 4. Information on the local structure and nature of bonding of nickel were derived using theoretical standards genmted with the FEFF code. The energies of the main Ni K-edge and the pre-edge peak were found to shift to higher values by about 1.5 and 0.6 eV, respectively, per unit increase in the valency of nickel. The average Ni-0 and Ni-F bond length within the octahedron decreased linearly with increase in nickel valency from 2 to 4. The local structure parameters for the higher valency nickel oxycompounds are consistent with a multi-phase model in which up to three phases corresponding to Ni2+, Ni3+, and Ni4+ could be present in different proportions depending on preparation procedures.