Pressure-induced phonon freezing in the ZnSeS II–VI mixed crystal: phonon–polaritons and ab initio calculations

Abstract

Near-forward Raman scattering combined with ab initio phonon and bond length calculations is used to study the ‘phonon–polariton’ transverse optical modes (with mixed electrical–mechanical character) of the II–VI ZnSe1−xSx mixed crystal under pressure. The goal of the study is to determine the pressure dependence of the poorly-resolved percolation-type Zn–S Raman doublet of the three oscillator [1 × (Zn–Se), 2 × (Zn–S)] ZnSe0.68S0.32 mixed crystal, which exhibits a phase transition at approximately the same pressure as its two end compounds (~14 GPa, zincblende → rocksalt), as determined by high-pressure x-ray diffraction. We find that the intensity of the lower Zn–S sub-mode of ZnSe0.68S0.32, due to Zn–S bonds vibrating in their own (S-like) environment, decreases under pressure (Raman scattering), whereas its frequency progressively converges onto that of the upper Zn–S sub-mode, due to Zn–S vibrations in the foreign (Se-like) environment (ab initio calculations). Ultimately, only the latter sub-mode survives. A similar ‘phonon freezing’ was earlier evidenced with the well-resolved percolation-type Be–Se doublet of Zn1−xBexSe (Pradhan et al 2010 Phys. Rev. B 81 115207), that exhibits a large contrast in the pressure-induced structural transitions of its end compounds. We deduce that the above collapse/convergence process is intrinsic to the percolation doublet of a short bond under pressure, at least in a ZnSe-based mixed crystal, and not due to any pressure-induced structural transition.