Controlled coordination and oxidation states of copper and manganese cations in complex nickel-copper-cobalt-manganese oxide thin films

Thin thermistor films of Ni0.48Co0.24CuxMn2.28-xO4 (x=0.6 and 1.2) were prepared by the deposition of metalorganic solutions followed by furnace annealing at temperatures between 600 and 800 degreesC. Annealing temperatures are decisive factors to control the electrical properties and electronic structure. X-ray photoelectron spectroscopy revealed that the specimens contained a mixture of Cu1+ and Cu2+ cations., and the annealing caused the change of the oxidation state from Cu1+ to Cu2+ which was accompanied by the reduction of manganese cations from Mn4+ to Mn3+. The Cu 2p core level from the Cu1+ state along with Cu 3d levels showed unusually large negative binding energy shifts (2P(3/2) at 930.8 eV and 2p(1/2) at 950.6 eV). Extended x-ray absorption fine structure showed that all manganese ions were located in octahedral sites of the spinel lattice, and both Cu1+ and Cu2+ cations occupy the tetrahedral sites regardless of the annealing temperature. X-ray absorption near edge structure spectra of the Mn K edge confirmed the reduction of manganese at high temperature. Cu K-edge spectra confirmed the presence of the cations with anomalous position in the 3d states, thus the negative shift of the Cu1+ core was attributed to the tetrahedral coordination of these cations in the spinel structure. (C) 2001 American Vacuum Society.