Electronic Structure and Optical Properties of Inverse-spinel MnCo2O4 Thin Films

The electronic structure and the optical properties of MnCo2O4 thin films were investigated by using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and spectroscopic ellipsometry (SE) and were compared with those of Co3O4. Polycrystalline films of the spinel oxides were prepared on Al2O3(0001) substrates by using a sol-gel method. XPS investigation indicated that the Mn ions in MnCo2O4 had a valence of +3 and occupied the octahedral sites of the spinel lattice. XRD results revealed that the inverse-spinel MnCo2O4 sample exhibited distortion of the lattice to a tetragonal structure while the Co3O4 sample had a cubic structure. The lattice distortion implies a high-spin t(2g)(3)e(g)(1) d-electron configuration of the octahedral Mn3+ ions, which is susceptible to the Jahn-Teller effect. The dielectric function spectra of MnCo2O4 and Co3O4 measured by using SE in the visible-ultraviolet (1.5 -8.5 eV) range exhibited a number of optical absorption structures that could be explained in terms of electronic charge-transfer (CT) transitions between the ions located at different lattice sites. The valence-electronic states involved in the CT transitions were identified from the electronic structure predicted by the XPS analyses.