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.
Mixed transition-metal oxide spinels exhibit high electrical conductivity and enhanced infrared transmissivity resulting from the presence of small polarons in the NiCo 2 O 4 lattice. Polarons are formed as a result of judicious choice of component metal cations and attendant resident cation charge states. Substitution of lithium for cobalt (Ni 1+x Co 2−x−z Li z O 4 ) while maintaining the spinel stoichiometry and controlled post-deposition annealing was found to influence measured conductivity in both solutionand sputter-deposited thin films. For lithium concentrations < 10% an improvement in conductivity was observed, depending upon whether the oxide film was deposited from solution (large increase) or sputter deposited (small increase). However, higher lithium concentrations degraded conductivity. Both XPS and SIMS analyses of films with high lithium concentrations (z > 0.3) revealed that lithium was concentrated near the film surface and the formation of carbonate was detected by XPS and Fourier transform infrared data. Results indicate that additions of lithium to transition-metal spinel oxides can lead to increased conductivity by increased polaron formation. However, in high concentration, lithium is an interstitial that diffuses to the surface to form compounds that degrade electrical conductivity.
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