Pressure-induced phase transitions in GeS under high pressures*

We have studied the pressure-induced structural and electronic phase transitions of layered GeS (\textit{Pnma}) to 30 GPa, using micro-Raman spectroscopy and electrical resistivity measurements in diamond anvil cells. The result shows a steady decrease in resistivity to that of metal at around 18 GPa. The visual appearance of GeS supports the insulator-metal transition: initially black GeS becomes opaque and eventually reflective with increasing pressure. The Raman result indicates that the metallization is preceded by a structural phase transition, presumably to the previously predicted \textit{Cmcm} structure.