The Phase Transition in HfS₂ Induced by The Pressure Non-Hydrostaticity

Layered van der Waals crystals are an exciting class of materials of properties, which strongly depend on their thickness. Their structure makes them very sensitive to the interlayer spacing, which can be modulated by temperature or strain.This motivates the recent interest in the strain engineering as an effective way to modify their mechanical, optical, and electronic properties.<br/>We have addressed the effect of pressure on hafnium disulphide (HfS2), material of promising electrical properties. We study Raman scattering (RS) in bulk HfS₂ using λ=632.8 nm (1.96 eV) and λ=561 nm (2.21 eV) at room temperature under pressure up to 10 GPa. A methanol-ethanol mixture or crystallographic oil were used in our diamond anvil cell (DAC) as the pressure transmitting medium (PTM).<br/>HfS₂ under ambient conditions adopts the 1T octahedral structure with the RS spectrum dominated by the out-of-plane A_1g mode.<br/>A systematic pressure-induced blue-shift of the RS features in the spectrum was observed when hydrostatic conditions of pressure were supported by a methanol-ethanol mixture as PTM, confirming that no phase transition occurred in HfS₂ up to 10 GPa.<br/>On the contrary, seven new modes appeared in the RS spectrum around P=7 GPa when crystallographic oil was used as PTM. The non-hydrostatic component of pressure in that case, which emerged around P=4 GPa was deduced from the characteristic splitting of ruby-related photoluminescence lines. Both ”low-pressure” and ”high pressure”-related peaks coexisted in the RS spectrum up to P=10 GPa with the former features systematically decreasing their intensity. The ”high-pressure” peaks were observed under decompression down to P=1.2 GPa pointing out to a metastability of the observed transition.<br/>Polarization angle-resolved measurements were performed at 7.4 GPa to study properties of all the observed RS peaks. A strong anisotropy of the RS for both the mode, evolving from the A_1gsymmetry vibrations and the ”high pressure” modes was observed. Moreover, the effect of excitation energy on the anisotropy was clearly appreciated. In particular for the A_1g-related mode, the two-fold (four-fold) polar symmetry of the RS intensity for co- and cross-polarization conditions was observed for l=561 nm (l=632.8 nm) excitation.<br/>We conclude that the observed evolution of the RS spectra under pressure corresponds to the partial phase transition in HfS₂. The transition was triggered around P=7 GPa by the non-hydrostatic component of pressure. Both phases coexist up to P=10 GPa and upon decompression down to P=1.2 GPa. The anisotropy of the RS polar dependence at P=7.4 GaP points out to the distortion of crystal lattice in both co-existing phases. The effect of excitation energy on the A_1g-related mode was explained in terms of the non-resonant (λ=632.8 nm) and resonant λ=561 nm) excitation conditions. Our results confirm the critical effect of the PTM on the pressure-induced phase transition in HfS₂, as revealed by the RS spectroscopy.