LO multi-phonons cooperative phenomenon in ZnSe–BeSemixed crystals

Abstract

We extend to longitudinal optical (LO) phonons the percolation model set for the basic understanding of the atypical transverse optical (TO) 1-bond→2-mode behavior observed by Raman scattering in the Be–Se spectral range of the random Zn 1- x Be x Se alloy (0≤ x≤1), which opens the class of mixed crystals with contrast in the bond stiffness. While the TO signal clearly discriminates between Be–Se vibrations within the hard (h) Be-rich region and the soft (s) Zn-rich one, vibration coupling of the LO modes via the long range longitudinal polarization field generates a massive transfer of oscillator strength from the low-frequency LO - (h,s)-mixed mode to the high-frequency LO + one. This leaves the false impression of a traditional 1-bond→1-mode behavior in the LO symmetry. Moreover the contrasts between the Zn–Se and Be–Se bond lengths and stiffnesses are proposed to force a Verleur and Barker (VB)-like discrete multi-mode Raman response from each region. Accordingly LO - and LO + intra-mode transfers of oscillator strength account for the spectacular distortions of the LO + lineshape. The puzzling LO behavior can be regarded as the result of a cooperative phenomenon between two discrete assemblies of polar phonons. Also, the VB description accounts for subtle unexplained behaviors in the TO symmetry. More generally it provides an attractive area for the discussion of Raman lineshape asymmetries in random alloys, as a possible alternative to the much debated spatial correlation model or to internal/external strain effects.