Abstract
Optical properties of normal spinel MxCo3−xO4(M=CrandCu) films grown by sol-gel method have been investigated by the spectroscopic ellipsometry (SE) in the 1.5–4eV region. For x<1.0, the cubic lattice constant of CrxCo3−xO4 is found to increase linearly with x while that of CuxCo3−xO4 decreases slightly. By comparing the optical constants of the ternary oxides measured by SE with those of Co3O4, Mott-Hubbard, charge-transfer (CT) and crystal-field (CF) transitions are found to coexist in the same energy region. The changes in the optical absorption spectrum by Cr and Cu alloying into Co3O4 are explained in terms of the changes in the electronic structure of Co3O4 through the substitution of the octahedral Co3+ and the tetrahedral Co2+ sites of the spinel structure by Cr3+ and Cu2+ ions, respectively. The CT transitions are explained in terms of d states of the Co3+ and Co2+ ions and p states of O2− ion. The CF transitions are interpreted as originating from the CF multiplets of the octahedral Co3+ and Cr3+ ions.
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