Pressure dependence of static and dynamic ionicity of SiC polytypes

Abstract

The ionicity of SiC polytypes and its pressure dependence is investigated by employing ab initio calculations using density-functional theory within the local-density approximation. Two approaches are used to quantify ionicity: a static one introduced by Garcia and Cohen involving a decomposition of the valence-charge density into symmetric and antisymmetric contributions with respect to the bond center and a dynamic one connected with Born effective charges. Both approaches yield increasing values of the ionicity which is in accordance with existing experimental data for 3C and 6H SiC for low hydrostatic pressure ( = 0.2 Mbar). A decrease of ionicity at ultrahigh pressures (up to 1.2 Mbar) as quoted for 6H SiC cannot be confirmed by our calculations. We discuss possible explanations of such a discrepancy.