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 multi-phase 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