First-principles lattice dynamics of almost-unstable zinc-blende structures.

Abstract

Using a full-potential linear-muffin-tin-orbital method (LMTO-FP), ab initio calculations of the \ensuremath{\Gamma}-point phonon spectrum and elastic properties of the copper halides CuF, CuCl, CuBr, and CuI (and for purpose of comparison of Si, ZnSe, and ZnTe) were performed. Our results for the TO(\ensuremath{\Gamma})-phonon frequencies, the bulk modules, and the piezoelectrically stiffened shear modules agree very well with the experimental values, establishing thus the accuracy of the LMTO-FP method. The experimental determination of the optical mode Gr\uneisen parameters is quite difficult, as there is a strong dependence of the structure of the Raman spectrum on the applied hydrostatic pressure. These difficulties can be totally avoided by our method, so we obtain reliable optical mode Gr\uneisen and internal-strain parameters. Moreover, the LMTO-FP method has been used to test the proposed Cu off-center model for CuCl, which could not be confirmed by our ab initio calculation.