  The effect of hydrostatic pressure up to 27 GPa on the Raman scattering (RS)
in bulk HfS$_2$ is investigated. There are two transformations of RS spectra,
which take place during compression at 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 structural change in the material of yet unknown
nature. The frequencies of the RS modes observed above the transformation
change linearly with pressure and corresponding pressure coefficients have been
determined. The other transition manifests itself as a change in the RS
lineshape. While a series of well-defined RS modes are observed under 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 permanent nature
of the high-pressure transition.
