Lattice dynamics of the model percolation-type (Zn,Be)Se alloy: Inelastic neutron scattering, ab initio study, and shell-model calculations
Résumé
The random Zn1-xBexSe zincblende alloy is known to exhibit a peculiar three-mode [1×(Zn-Se),2×(Be-Se)]vibration pattern near the Brillouin zone (BZ) center, of the so-called percolation type, apparent in its Ramanspectra. This is due to an unusually large contrast between the physical properties (length, ionicity) of theconstituting bonds. In the present work, the inelastic neutron scattering is applied to study the dispersion ofmodes away from the BZ center, with special attention to the q dependence of the BeSe-like transverse opticdoublet. The discussion is supported by calculations of lattice dynamics done both ab initio (using the SIESTAcode) and within the shell model. The BeSe-like doublet is found to survive nearly unchanged throughout the BZup to the zone edge, indicating that its origin is at the ultimate bond scale. Themicroscopicmechanism of splittingis clarified by ab initio calculations. Namely, the local lattice relaxation needed to accommodate the contrast inphysical properties of the Zn-Se and Be-Se bonds splits the stretching and bending modes of connected, i.e.,percolativelike, (Be-Se) bonds.