Lattice dynamics of Fe-doped CoO from first principles

The vibrational dynamics of substitutional Fe and its influence on the vibrational properties of the host CoO matrix have been investigated using ab initio calculated Hellmann-Feynman forces. Corrections for the strong on-site interaction have been taken into account via the Hubbard potential U and the local exchange interaction J. Calculations were performed with constant U on Co and variable U on Fe. It was found that Fe impurities exhibit higher values of effective force constant than the original Co. New localized modes are created in the host CoO matrix due to force constant defect between Fe and Co. Iron impurities affect the optical phonon vibrations, while the long wavelength acoustic phonons do not experience changes upon the doping. Mean-squared vibrational amplitudes of cations and anions in the ideal and Fe-doped CoO are compared to the available experimental data. The calculated mean-squared displacements of Fe remain in very good agreement with those measured by Mössbauer spectroscopy.