Soft-Phonon-Induced Raman Scattering in IV-VI Compounds

At both the cubic-rhombohedral phase-transition temperature $T_c$ and the zero-gap temperature $T_b$, anomalous increases in Raman intensities are observed in ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ (for $x=0-1\mathrm{}$) single crystals. These are attributed to Raman scattering enhanced by the softening of the TO phonon near $T_c$ and also allowed by enhanced interband-mixing effects near $T_b$. These interband-mixing effects not only enhance the Raman intensity, but also increase $T_c$.