Pressure-Driven Phase Transitions in Bulk HfS2

The effect of hydrostatic pressure up to 27 GPa on Raman scattering in bulk HfS2 is investigated. There are two transformations of Raman scattering spectra which take place during compression at a pressure between 5.7 GPa and 9.8 GPa as well as between 12.8 GPa and 15.2 GPa. Seven vibrational modes can be observed after the transformation, as compared to four modes before the transformation. The observed change suggests a structural change in the material of yet unknown nature. The frequencies of the Raman scattering modes observed above the transformation change linearly with pressure and the corresponding pressure coefficients have been determined. The other transition manifests itself as a change in the Raman scattering lineshape. While a series of well-defined Raman scattering modes are observed under the pressure below the transition, broad spectral bands can be seen at higher pressure. The overall lineshape of the spectra resembles that of disordered materials. The lineshape does not change during decompression, which suggests the permanent nature of the high-pressure transition.