Vibrational-mechanical properties of the highly-mismatched Cd1−xBexTe semiconductor alloy: experiment and ab initio calculations

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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 Zn 1−x Be x -chalcogenides. The test is done by way of experiment ( x≤ 0.11 x ≤ 0.11 ), combining Raman scattering with X-ray diffraction at high pressure, and ab initio calculations ( x x ~ 0–0.5; x x ~1). The (macroscopic) bulk modulus B₀ B 0 drops below the CdTe value on minor Be incorporation, at variance with a linear B₀ B 0 versus x 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 x ~0, 0.5) is barely detected experimentally. At large Be content ( x x ~1), the same bimodal signal relaxes all the way down to inversion, an unprecedented case. However, specific pressure dependencies of the regular ( x x ~0, 0.5) and inverted ( x x ~1) Be–Te Raman doublets are in line with the predictions of the PM. Hence, the PM applies as such to Cd 1−x Be x Te without further refinement, albeit in a “relaxed” form. This enhances the model’s validity as a generic descriptor of phonons in alloys.