Lattice dynamics of the model percolation-type (Zn,Be)Se alloy: Inelastic neutron scattering,ab initiostudy, and shell-model calculations

Abstract

The random Zn${}_{1\text{\ensuremath{-}}x}$Be${}_{x}$Se zincblende alloy is known to exhibit a peculiar three-mode [$1\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}$(Zn-Se),$2\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}$(Be-Se)] vibration pattern near the Brillouin zone (BZ) center, of the so-called percolation type, apparent in its Raman spectra. This is due to an unusually large contrast between the physical properties (length, ionicity) of the constituting bonds. In the present work, the inelastic neutron scattering is applied to study the dispersion of modes away from the BZ center, with special attention to the $\stackrel{P\vec}{q}$ dependence of the BeSe-like transverse optic doublet. The discussion is supported by calculations of lattice dynamics done both ab initio (using the siesta code) and within the shell model. The BeSe-like doublet is found to survive nearly unchanged throughout the BZ up to the zone edge, indicating that its origin is at the ultimate bond scale. The microscopic mechanism of splitting is clarified by ab initio calculations. Namely, the local lattice relaxation needed to accommodate the contrast in physical properties of the Zn-Se and Be-Se bonds splits the stretching and bending modes of connected, i.e., percolativelike, (Be-Se) bonds.