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The emerging CdTe–BeTe semiconductor alloy that exhibits a dramatic mismatch in bond covalency and bond stiffness clarifying its vibrational-mechanical properties is used as a benchmark to test the limits of the percolation model (PM) worked out to explain the complex Raman spectra of the related but less contrasted Zn1−xBex-chalcogenides. The test is done by way of experiment ( x ≤ 0.11 ), combining Raman scattering with X-ray diffraction at high pressure, and ab initio calculations (x ~ 0–0.5; x ~1). The (macroscopic) bulk modulus B 0 drops below the CdTe value on minor Be incorporation, at variance with a linear B 0 versus x increase predicted ab initio, thus hinting at large anharmonic effects in the real crystal. Yet, no anomaly occurs at the (microscopic) bond scale as the regular bimodal PM-type Raman signal predicted ab initio for Be–Te in minority (x ~0, 0.5) is barely detected experimentally. At large Be content (x ~1), the same bimodal signal relaxes all the way down to inversion, an unprecedented case. However, specific pressure dependencies of the regular (x ~0, 0.5) and inverted (x ~1) Be–Te Raman doublets are in line with the predictions of the PM. Hence, the PM applies as such to Cd1−xBexTe without further refinement, albeit in a "relaxed" form. This enhances the model's validity as a generic descriptor of phonons in alloys. [ABSTRACT FROM AUTHOR]

Raman scattering and ab initio Raman/phonon calculations, supported by X-ray diffraction, are combined to study the vibrational properties of Zn1−xBexTe under pressure. The dependence of the Be–Te (distinct) and Zn–Te (compact) Raman doublets that distinguish between Be- and Zn-like environments is examined within the percolation model with special attention to x ~ (0,1). The Be-like environment hardens faster than the Zn-like one under pressure, resulting in the two sub-modes per doublet getting closer and mechanically coupled. When a bond is so dominant that it forms a matrix-like continuum, its two submodes freely couple on crossing at the resonance, with an effective transfer of oscillator strength. Post resonance the two submodes stabilize into an inverted doublet shifted in block under pressure. When a bond achieves lower content and merely self-connects via (finite/infinite) treelike chains, the coupling is undermined by overdamping of the in-chain stretching until a «phonon exceptional point» is reached at the resonance. Only the out-of-chain vibrations «survive» the resonance, the in-chain ones are «killed». This picture is not bond-related, and hence presumably generic to mixed crystals of the closing-type under pressure (dominant over the opening-type), indicating a key role of the mesostructure in the pressure dependence of phonons in mixed crystals. [ABSTRACT FROM AUTHOR]

We investigate by near-forward Raman scattering a presumed reinforcement of the (A-C,B-C)- mixed phonon-polariton of a A1-xBxC zincblende alloy when entering its longitudinal optical-like (LO-like) regime near the Brillouin zone centre Γ, as predicted within the formalism of the linear dielectric response. A choice system to address such issue is ZnSe0.68S0.32 due to the moderate dispersion of its refractive index in the visible range, a sine qua non condition to bring the phononpolariton insight near Γ. The LO-like reinforcement regime is actually accessed by using the 633.0 nm laser excitation, testified by the strong emergence of the (Zn-Se,Zn-S)-mixed phononpolariton at ultimately small scattering angles. [ABSTRACT FROM AUTHOR]

The apparently universal 1-bond → 2-mode percolation behavior in the Raman spectra of zincblende semiconductor alloys is generally observed for the short bond only, and not for the long one. In this work we perform a combined high-pressure-backward/near-forward Raman study of the leading percolation-type (Zn,Be)Se alloy (~50 at.% Be), which exhibits a distinct percolation doublet in the spectral range of its short Be-Se bond, in search of a Zn-Se analogue. The high-pressure-backward insight is not conclusive per se, but clarifies the perspective behind the near-forward Raman study. The latter reveals an unique Zn-Se phonon-polariton. Its fair contour modeling depending on the scattering angle is achieved within the linear dielectric approach, based on ellipsometry measurement of the ZnBeSe refractive index. Somewhat surprisingly this reveals that the phonon-polariton in question is a 'fractional' one in that it carries only half of the available Zn-Se oscillator strength, as ideally expected in case of a BeSe-like bimodal Raman behavior of the long Zn-Se bond. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The random Zn1-xBexSe zincblende alloy is known to exhibit a peculiar three-mode [1×Zn-Se),2×Be-Se)] vibration pattern near the Brillouin zone (BZ) center, of the so-called percolation type, apparent in its Raman spectra. This is due to an unusually large contrast between the physical properties (length, ionicity) of the constituting bonds. In the present work, the inelastic neutron scattering is applied to study the dispersion of modes away from the BZ center, with special attention to the ... dependence of the BeSe-like transverse optic doublet. The discussion is supported by calculations of lattice dynamics done both ab initio (using the siesta code) and within the shell model. The BeSe-like doublet is found to survive nearly unchanged throughout the BZ up to the zone edge, indicating that its origin is at the ultimate bond scale. The microscopic mechanism of splitting is clarified by ab initio calculations. Namely, the local lattice relaxation needed to accommodate the contrast in physical properties of the Zn-Se and Be-Se bonds splits the stretching and bending modes of connected, i.e., percolativelike, (Be-Se) bonds. [ABSTRACT FROM AUTHOR]

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-mechanical character) of the II-VI ZnSe1-x S x 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)] ZnSe0.68S0.32 mixed crystal, which exhibits a phase transition at approximately the same pressure as its two end compounds (~14 GPa, zincblende  →  rocksalt), as determined by high-pressure x-ray diffraction. We find that the intensity of the lower Zn-S sub-mode of ZnSe0.68S0.32, 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 Zn1-x Be x Se (Pradhan et al 2010 Phys. Rev. B 81 115207), that exhibits a large contrast in the pressure-induced structural transitions of its end compounds. We deduce that the above collapse/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.