Strong Effects of Cation Vacancies on the Electronic and Dynamical Properties of FeO

We report pronounced modifications of electronic and vibrational properties induced in FeO by cation vacancies, obtained within density functional theory incorporating strong local Coulomb interactions at Fe atoms. The insulating gap of FeO is reduced by about 50% due to unoccupied electronic bands introduced by trivalent Fe ions stabilized by cation vacancies. The changes in the electronic structure along with atomic displacements induced by cation vacancies affect strongly phonon dispersions via modified force constants, including those at atoms beyond nearest neighbors of defects. We demonstrate that theoretical phonon dispersions and their densities of states reproduce the results of inelastic neutron and nuclear resonant x-ray scattering experiments \emph{only} when Fe vacancies and Coulomb interaction $U$ are both included explicitly in \emph{ab initio} simulations, which also suggests that the electron-phonon coupling in FeO is strong.