Phonon-based partition of (ZnSe-like) semiconductor mixed crystals on approach to their pressure-induced structural...

The generic 1-bond → 2-mode "percolation-type" Raman signal inherent to the short bond of common A_1-xB_xC semiconductor mixed crystals with zincblende (cubic) structure is exploited as a sensitive "mesoscope" to explore how various ZnSe-based systems engage their pressure-induced structural transition (to rock-salt) at the sub-macroscopic scale-with a focus on Zn_1-xCd_xSe. The Raman doublet, that distinguishes between the AC- and BC-like environments of the short bond, is reactive to pressure: either it closes (Zn_1-xBe_xSe, ZnSe_1-xS_x) or it opens (Zn_1-xCd_xSe), depending on the hardening rates of the two environments under pressure. A partition of II-VI and III-V mixed crystals is accordingly outlined. Of special interest is the "closure" case, in which the system resonantly stabilizes ante transition at its "exceptional point" corresponding to a virtual decoupling, by overdamping, of the two oscillators forming the Raman doublet. At this limit, the chain-connected bonds of the short species (taken as the minor one) freeze along the chain into a rigid backbone. This reveals a capacity behind alloying to reduce the thermal conductivity as well as the thermalization rate of photo-generated electrons.