Pressure-Raman study of optical phonon anharmonicity and metastable phase in $^{68}$Zn$^{76}$Se

The effects of hydrostatic pressure on the one- and two-phonon Raman spectra of isotopic purity $^{68}$Zn$^{76}$Se are studied to 15GPa at 300K. With increasing pressure the TO-TA(X,K) difference-mode shifts rapidly to higher energy, moving above the 2TA overtone band at 5.8GPa with no significant 4 -phonon mixing. Above 10GPa, the one-phonon TO($\Gamma )$ peak broadens rapidly, reaching $\sim $ 60cm$^{-1}$ FWHM and overlapping both TO-TA(X,K) and LO($\Gamma )$. After the sample undergoes the forward and reverse high-pressure transitions, the sphalerite-structure Raman features return (including the strongly broadened TO($\Gamma )$ peak) and a new sharp line attributed to a metastable ZnSe phase appears. The TO($\Gamma )$ broadening in ZnSe is much stronger than that due to pressure-tuning of the anharmonic decay TO($\Gamma )=>$ TA+LA(X,W,K) in GaP and ZnS.[1] Our results suggest that the resonant anharmonic interactions in ZnSe may be strongly enhanced by spatial confinement and disorder in the domains of nucleating phases. [1] J. Serrano et. al., Phys. Rev. B\underline {69}, 014301(2004).