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Backward/near-forward Raman scattering and ab initio Raman/phonon calculations are combined, together with x-ray diffraction and ellipsometry measurements to further inform the debate on the compact phonon behavior of the II–VI Cd1−xZnxTe alloy. The compacity favors the coupling of polar optic modes in both the transverse and longitudinal symmetries via the related (E L , T ) long-wave electric fields. The E L -coupling achieves maximum in the Zn-dilute limit, which enhances the (upper) ZnTe-like (impurity) mode at the expense of the (lower) CdTe-like (matrix-like) one, leaving the impression of a unique {Cd-Te,Zn−Te}-mixed longitudinal optic (LO) phonon across most of the composition domain. However, the purely mechanical (non-polar) transverse optic (PM-TO) phonons, that hardly couple, reveal an underlying three-mode {1 × (Cd-Te),2 × (Zn-Te)} fine structure that distinguishes between Zn–Te vibrations in Zn- and Cd-like environments up to second neighbors. Further refinement arises by exploring the phonon–polariton (i.e., polar-TO) regime at large Zn content. On reducing the scattering angle, the E T -coupling develops into a sequential softening of phonon–polaritons from ZnTe- down to CdTe-like ones, which transiently unveils a bimodal pattern behind the Cd–Te signal. Altogether, this results in a (rare) canonical four-mode {2 × (Cd-Te),2 × (Zn-Te)} percolation pattern for Cd1−xZnxTe, i.e., a close II–VI replica of the twin III−V In1−xGaxAs one—yet differing by two apparent LO modes and a sensitivity of bond vibrations limited to first-neighbors. Retrospectively, the difference in sensitivity of bond vibrations to the local environment between In1−xGaxAs (limited to first neighbors) and Cd1−xZnxTe (extending up to second neighbors) emerges as a rule throughout common (covalent) III–V and (ionic) II–VI semiconductor alloys. [ABSTRACT FROM AUTHOR]

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]

We present a polarization-dependent pure transverse-optic (TO) Raman study of high-quality CdxZn1−xSe single crystals with zincblende (cubic) structures (x ≤ 0.3) covering both the phonon and phonon-polariton variants of the TO modes, using suitable backward and near-forward scattering geometries, respectively. Insight into the native phonon regime of the phonon-polaritons is obtained at intermediate composition of the random crystal (x = 0.5 , 0.3) and at the (Cd,Zn)-dilute limits (x ∼ 0.1 ; using prototype impurity motifs) by applying ab initio codes to large supercells (64–216 atoms), with special attention to both the Raman intensities and the phonon frequencies. The experimental (Raman) and theoretical (ab initio) results converge onto a percolation-type three-phonon [ 1 × (Cd - Se) , 2 × (Zn - Se) ] pattern for CdxZn1−xSe. On the practical side, the interplay between the oscillator strengths of the two Zn-Se Raman modes is used to diagnose a pronounced trend toward local clustering in the studied crystals, presumably an early sign of the composition-induced zincblende ⇿ wurtzite structural transition (x ∼ 0.3). The deviation from the ideal Zn ⇿ Cd random substitution is estimated by working out a zincblende-version of the percolation model equipped with a relevant order parameter κ. The model is based on a sensitivity of the Zn-Se vibration to its local environment at the second-neighbor scale, independently supported by ab initio calculation of the Raman spectra in their dependence on κ (adjusted by simulated annealing). [ABSTRACT FROM AUTHOR]

We test a presumed ability behind the phenomenological percolation scheme used for the basic description of the multi-mode Raman spectra of mixed crystals at one dimension along the linear chain approximation, to determine, via the Raman intensities, the nature of the atom substitution, as to whether this is random or due to local clustering/anticlustering. For doing so, we focus on the model percolation-type GeySi1-y system characterized by six oscillators {1×(Ge-Ge),3×(Ge-Si),2×(Si-Si)} and place the study around the critical compositions y ~ (0.16, 0.71, and 0.84) corresponding to nearly matching of intensities between the like Raman modes from a given multiplet (Ge-Si triplet or Si-Si doublet). The interplay between the GeySi1-y Raman intensities predicted by the percolation scheme depending on a suitable order parameter k of local clustering/anticlustering is found to be consistent with ab initio calculations of the GeySi1-y Raman spectra done with the Ab Initio Modeling PROgram code using large (64-, 216-, and 512-atoms) disordered cubic supercells matching the required (y,k) values. The actual "percolation vs. ab initio" comparative insight at moderate/dilute-(Ge,Si) limits, with an emphasis on the k-induced intra-bond transfer of oscillator strength, extends a pioneering one earlier achieved at an intermediate composition (y ~ 0.50) by using small (32-atom) supercells [O. Pagès et al., J. Appl. Phys. 114, 033513 (2013)], mainly concerned with the inter-bond transfer of oscillator strength, providing altogether a complete picture. [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]