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 pre pared by the deposition of metalorganic solutions followed by furnace annea ling at temperatures between 600 and 800 degreesC. Annealing temperatures a re decisive factors to control the electrical properties and electronic str ucture. X-ray photoelectron spectroscopy revealed that the specimens contai ned a mixture of Cu1+ and Cu2+ cations., and the annealing caused the chang e of the oxidation state from Cu1+ to Cu2+ which was accompanied by the red uction of manganese cations from Mn4+ to Mn3+. The Cu 2p core level from th e Cu1+ state along with Cu 3d levels showed unusually large negative bindin g energy shifts (2P(3/2) at 930.8 eV and 2p(1/2) at 950.6 eV). Extended x-r ay absorption fine structure showed that all manganese ions were located in octahedral sites of the spinel lattice, and both Cu1+ and Cu2+ cations occ upy the tetrahedral sites regardless of the annealing temperature. X-ray ab sorption near edge structure spectra of the Mn K edge confirmed the reducti on of manganese at high temperature. Cu K-edge spectra confirmed the presen ce of the cations with anomalous position in the 3d states, thus the negati ve shift of the Cu1+ core was attributed to the tetrahedral coordination of these cations in the spinel structure. (C) 2001 American Vacuum Society.