Vibration properties of Zn1−xBexSe, a mixed II-VI semiconductor characterized by a high contrast in elastic properties of its pure constituents, ZnSe and BeSe, are simulated by first-principles calculations of electronic structure, lattice relaxation and frozen phonons. The calculations within the local density approximation has been done with the Siesta method, using norm-conserving pseudopotentials and localized basis functions; the benchmark calculations for pure endsystems were moreover done also by all-electron WIEN2k code. An immediate motivation for the study was to analyze, at the microscopic level, the appearance of anomalous phonon modes early detected in Raman spectra in the intermediate region (20 to 80%) of ZnBe concentration. This was early discussed on the basis of a percolation phenomenon, i.e., the result of the formation of wall-to-wall -Be-Se-chains throughout the crystal. The presence of such chains was explicitly allowed in our simulation and indeed brought about a softening and splitting off of particular modes, in accordance with experimental observation, due to a relative elongation of Be-Se bonds along the chain as compared to those involving isolated Be atoms. The variation of force constants with interatomic distances shows common trends in relative independence on the short-range order.

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We extend to longitudinal-optical (LO) phonons the percolation model set for the basic understanding of the atypical transverse-optical (TO) one-bond→ two-mode behavior observed by Raman scattering in the Be-Se spectral range of the random Zn 1−x Be x Se alloy ͑0 ഛ x ഛ 1͒, which opens the class of mixed crystals with contrast in the bond stiffness. The study is supported by contour modeling of the TO and LO Raman line shapes. This is achieved via application of the Hon and Faust treatment to a version of the modified-randomelement-isodisplacement model generalized to multioscillators. While the TO signal clearly discriminates between Be-Se vibrations within the hard Be-rich region and the soft Zn-rich one, complexity arises in the LO symmetry due to vibration coupling via the 1ong-range longitudinal polarization field. In particular this generates a massive transfer of oscillator strength from the low-frequency ͑LO − ͒ (hard, soft)-mixed mode to the high-frequency ͑LO + ͒ one, which results in an apparent LO + single-mode behavior. Moreover the contrasts between the Zn-Se and Be-Se bond lengths and bond stiffness are proposed to force a Verleur and Barker-like (VB) discrete multimode Raman response from each region. Accordingly LO − and LO + intramode transfers of oscillator strength superimpose to the LO − → LO + intermode one. This accounts for the spectacular distortions of the LO + line shape. On the whole, the puzzling LO behavior can be regarded as the result of a cooperative phenomenon between two discrete assemblies of polar LO phonons, driven by the long-range longitudinal polarization field. Also, the Verleur and Barker description accounts for subtle unexplained behaviors in the TO symmetry. More generally it appears to provide a much attractive area for the discussion of the asymmetries of the TO and LO Raman line shapes in random alloys, as a possible alternative to the much debated spatial correlation model or to internal/external strain effects.

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We demonstrate how to overcome serious problems in understanding and classification of vibration spectra in semiconductor alloys, following from traditional use of the virtual crystal approximation (VCA). We show that such different systems as InGaAs (1-bond→1-mode behavior), InGaP (modified 2-mode) and ZnTeSe (2-bond→1-mode) obey in fact the same phonon mode behavior -hence probably a universal one -of a percolation-type (1-bond→2-mode). The change of paradigm from the 'VCA insight' (an averaged microscopic one) to the 'percolation insight' (a mesoscopic one) offers a promising link towards the understanding of alloy disorder. The discussion is supported by ab initio simulation of the phonon density of states at the zone-center of representative supercells at intermediary composition (ZnTeSe) and at the impurity-dilute limits (all systems). In particular, we propose a simple ab initio 'protocol' to estimate the basic input parameters of our semi-empirical 'percolation' model for the calculation of the 1-bond→2-mode vibration spectra of zincblende alloys. With this, the model turns self-sufficient.

We use the 1-bond→2-phonon percolation doublet of zincblende alloys as a 'mesoscope' for an unusual insight into their phonon behavior under pressure. We focus on (Zn,Be)Se and show by Raman scattering that the original Be-Se doublet at ambient pressure, of the stretching-bending type, turns into a pure-bending singlet at the approach of the high-pressure ZnSe-like rocksalt phase, an unnatural one for the Be-Se bonds. The 'freezing' of the Be-Se stretching mode is discussed within the scope of the percolation model (mesoscopic scale), with ab initio calculations in support (microscopic scale).

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