AB-INITIO LATTICE-DYNAMICS AND CHARGE FLUCTUATIONS IN ALKALINE-EARTH OXIDES

We present an ab initio calculation of lattice-dynamical properties of the alkaline-earth oxides MgO, CaO, and SrO in the rocksalt structure . We employ the density-functional perturbation theory within the loca l-density approximation. We show phonon dispersion curves along severa l symmetry directions in the Brillouin zone. The calculated frequencie s agree well with the available experimental data. The investigation o f the phonon-induced charge-density fluctuations of MgO and CaO at the L point of the Brillouin zone partially supports the breathing-shell model of lattice dynamics and rules out the charge-transfer model for this class of materials. Moreover, our calculations show that the brea thinglike charge-density response is more pronounced for oxygen than f or the cations in these compounds.