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Near-forward Raman scattering combined with ab initio phonon and bond lengthcalculations is used to study the phonon-polariton transverse optical modes(with mixed electrical and mechanical character) of the II-VI ZnSeS mixedcrystal under pressure. The goal of the study is to determine the pressuredependence of the poorly resolved percolation-type Zn-S Raman doublet of thethree oscillator [1(Zn-Se),2(Zn-S)] ZnSe68S32 mixed crystal, which exhibits aphase transition at approximately the same pressure as its two end compounds(~14 GPa, zincblende-to-rocksalt), as determined by high-pressure x-raydiffraction. We find that the intensity of the lower Zn-S sub-mode ofZnSe68S32, due to Zn-S bonds vibrating in their own (S-like) environment,decreases under pressure (Raman scattering), whereas its frequencyprogressively converges onto that of the upper Zn- S sub-mode, due to Zn-Svibrations in the foreign (Se-like) environment (ab initio calculations).Ultimately, only the latter sub-mode survives. A similar phonon freezing wasearlier evidenced with the well-resolved percolation-type Be-Se doublet ofZnBeSe [Pradhan et al. Phys. Rev. B 81, 115207 (2010)], that exhibits a largecontrast in the pressure-induced structural transitions of its end compounds.We deduce that the above collapse and convergence process is intrinsic to thepercolation doublet of a short bond under pressure, at least in a ZnSe-basedmixed crystal, and not due to any pressure-induced structural transition.

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