Vibrational spectroscopic study of the site occupancy distribution of cations in nickel cobalt oxides

Mid-infrared, far-infrared, and Raman spectra for Ni1-xCoxOy indicated the systematic variation from a spinel-type (x=1.0) to an inverse spinel-type (x=0.67) lattice, through a multiphase transition region, to an NaCl-type (x<0.25) structure. Further analysis reveals that a decrease in x in this system is accompanied by the formation of octahedral/Ni2+, which, in the case of spinel compositions Ni1-xCoxO4/3 (x=0.67 to 1.0), results in substitution of Co3+ into tetrahedral sites and/or the redistribution of charge around some of the oxygen ions. The proposed transition region explains the observed break in continuity, at around x=0.5, in the previously reported resistivity data for nickel cobalt oxide films. (C) 2004 American Institute of Physics.