  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 and mechanical character) of the II-VI ZnSeS 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)] ZnSe68S32 mixed crystal, which exhibits a
phase transition at approximately the same pressure as its two end compounds
(~14 GPa, zincblende-to-rocksalt), as determined by high-pressure x-ray
diffraction. We find that the intensity of the lower Zn-S sub-mode of
ZnSe68S32, 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
ZnBeSe [Pradhan et al. Phys. Rev. B 81, 115207 (2010)], that exhibits a large
contrast in the pressure-induced structural transitions of its end compounds.
We deduce that the above collapse and 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.
