NON-NOMINAL VALUE OF THE DYNAMICAL EFFECTIVE CHARGE IN ALKALINE-EARTHOXIDES

We calculate nb initio the electronic states and the Born dynamical ch arge Z of the alkaline-earth oxides in the local-density approximatio n. We investigate the trend of increasing Z values through the series , using band-by-band decompositions and computational experiments perf ormed on fake materials with artificially modified covalence. The devi ations of Z from the nominal value 2 are due to the increasing intera ction between O 2p orbitals and unoccupied cation d states. We also ex plain the variations, along the series, of the individual contribution s to Z arising from the occupied band manifolds.