Comparison of experimental and theoretical valence charge densities of cubic ZnSe

The valence charge density distribution (VCD) of cubic ZnSe was studied by means of theoretical and experimental methods. The theoretical VCDs were ca lculated in terms of the Hartree-Fock (HF), density functional (DF) and pse udopotential (PP) approach provided by the CRYSTAL95 program package. The e xperimental 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 + I = 4n + 2) in high precision, which are most sensitive for the chemical bond. The measured structure factors we re analysed in terms of a bond charge model including charge transfer. The experimental data set was completed by reflections with h + k + l not equal 4n + 2 calculated in terms of the spherical atom model, but corrected by t he parameters which received from the weak reflections. All structure facto rs were extrapolated to zero temperature using experimental temperature fac tors including anharmonicity. Despite the approximation made, the experimen tal 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 electr ons. 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 an d ZnSe the bond charge decreases in amount and shifts closer to the anion, increasing the charge transfer between the ions. (C) 2001 Elsevier Science Ltd. All rights reserved.