Pressure-induced optical anisotropy of HfS₂

The effect of pressure on Raman scattering (RS) in the bulk HfS₂ 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 HfS₂. 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 A1g 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 HfS₂.