High-pressure Raman scattering spectrum of K$_{0.74}$Fe$_{1.67}$Se$_{1.6}$S$_{0.4}$

We perform high-pressure Raman scattering measurements on K$_{0.74}$Fe$_{1.67}$Se$_{1.6}$S$_{0.4}$ single crystal up to 20 GPa at room temperature. Anomalous changes of both the A$_{\mathrm{g}}$ and B$_{\mathrm{g}}$ phonon modes are found at a critical pressure around 5.0 GPa. Above this pressure, all Raman modes exhibit a hardening behavior at higher pressures. The superconducting transition temperature of this compound was observed to decrease with increasing pressure and vanish at the critical pressure. These results suggest that the lattice modification and the associated electronic structure are important to understand the superconductivity in the FeSe-based superconductors.