Multi-phonon (percolation) behavior and local clustering of Cd_xZn_1−xSe-cubic mixed crystals (x ≤ 0.3): A Raman–ab initio study

We present a polarization-dependent pure transverse-optic (TO) Raman study of high-quality Cd_xZn_1−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 Cd_xZn_1−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).