Non-nominal value of the dynamical effective charge in alkaline-earth oxides

We calculate ab initio the electronic states and the Born dynamical charge ${\mathrm{Z}}^{\mathrm{*}}$ of the alkaline-earth oxides in the local-density approximation. We investigate the trend of increasing ${\mathrm{Z}}^{\mathrm{*}}$ values through the series, using band-by-band decompositions and computational experiments performed on fake materials with artificially modified covalence. The deviations of ${\mathrm{Z}}^{\mathrm{*}}$ from the nominal value 2 are due to the increasing interaction between O 2p orbitals and unoccupied cation d states. We also explain the variations, along the series, of the individual contributions to ${\mathrm{Z}}^{\mathrm{*}}$ arising from the occupied band manifolds.