  The effect of pressure on Raman scattering (RS) in the bulk HfS$_2$ is
investigated under hydrostatic and non-hydrostatic conditions. The RS lineshape
does not change significantly in the hydrostatic regime, showing a systematic
blueshift of the spectral features. In a non-hydrostatic environment, seven
peaks emerge in the spectrum ($P$=7 GPa) dominating the lineshape up to
$P$=10.5 GPa. The change in the RS lineshape manifests a pressure-induced phase
transition in HfS$_2$. The simultaneous observation of both low-pressure (LP)
and high-pressure (HP) related RS peaks suggests the corresponding coexistence
of two different phases over a large pressure range. We found that the
HP-related phase is metastable, persisting during the decompression cycle down
to $P$=1.2 GPa with the LP-related features finally recovering at even lower
pressures. The angle-resolved polarized RS (ARPRS) performed under $P$=7.4 GPa
revealed a strong in-plane anisotropy of both the LP-related A$_{1g}$ mode and
the HP peaks. The anisotropy is related to the possible distortion of the
structure induced by the non-hydrostatic component of the pressure. We describe
the obtained results by the influence of the non-hydrostatic pressure on the
observed phase transition. We interpret our results in terms of a distorted
$Pnma$ phase as a possible HP induced structure of HfS$_2$.
