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

Abstract

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 °C. 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 (2p3/2 at 930.8 eV and 2p1/2 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.