Comparison of experimental and theoretical valence charge densities of cubic ZnSe

Abstract

The valence charge density distribution (VCD) of cubic ZnSe was studied by means of theoretical and experimental methods. The theoretical VCDs were calculated in terms of the Hartree–Fock (HF), density functional (DF) and pseudopotential (PP) approach provided by the CRYSTAL95 program package. The experimental VCD was calculated via inverse Fourier transformation of X-ray structure factors. To avoid the extinction problem, we only measured a few low-indexed weak reflections ( h+ k+ l=4 n+2) in high precision, which are most sensitive for the chemical bond. The measured structure factors were analysed in terms of a bond charge model including charge transfer. The experimental data set was completed by reflections with h+ k+ l≠4 n+2 calculated in terms of the spherical atom model, but corrected by the parameters which received from the weak reflections. All structure factors were extrapolated to zero temperature using experimental temperature factors including anharmonicity. Despite the approximation made, the experimental VCD confirms the ab initio calculations qualitatively. The experimental and theoretical VCDs of ZnSe are characterised by almost spherical charge densities around the ions carrying an effective charge of about +0.7 electrons. A bond charge was found to be located inside the electron cloud around the Se nucleus. Comparing the VCDs of the isoelectronic series Ge, GaAs and ZnSe the bond charge decreases in amount and shifts closer to the anion, increasing the charge transfer between the ions.