The Metastable Pressure-Induced Transformation of Raman Scattering Spectrum of Bulk HfS₂

The effect of hydrostatic pressure on the lattice dynamics of bulk hafnium disulphide (HfS₂) grown by chemical vapour transport method is investigated. The pattern of powder X-Ray diffraction of the investigated material was found to be in perfect agreement with that of the octahedral 1T phase of HfS₂.<br/>The Raman scattering has been studied at room temperature in diamond anvil cells in hydrostatic pressure up to 13.7 GaP.<br/>There are four Raman modes observed at ambient pressure, which is consistent with previous studies.<br/>There is an agreement in literature on the attribution of peaks at 266 cm^-1 and 344 cm^-1 to in-plane E_g and out-of-plane A_1g Raman-active modes, respectively. The peak at 325 cm^-1 is attributed to the infrared-active E_u (LO) or A_2u(LO) alike mode.<br/>Not clear is also the assignment of the weak feature at 141 cm^-1.Its attribution to a second-order difference processes was suggested, as such peaks can be observed in Raman scattering spectra of some transition metal dichalchogenides (TMDs) however the E_u (TO) was also suggested as the signature of the mode.<br/>The Raman scattering modes blueshift with increasing hydrostatic pressure with the pressure rates from 1.6 cm^-1/GPa to 4.4 cm^-1/GPa, which is similar to the values reported before.<br/>The lineshape of the Raman scattering spectrum does not significantly change with increasing hydrostatic pressure up to 5.7 GPa, however a substantial enhancement of the A_1g peak intensity can be noticed under the highest pressure.<br/>A change in the Raman scattering spectrum is observed in hydrostatic pressure between 6.2 GPa and 7.2 GPa.<br/>Seven vibrational modes can be observed in the spectrum above the transformation, as compared to four modes before the transformation.<br/>The frequencies of the Raman scattering modes observed change linearly with pressure coefficients up to 2.6 cm^-1/GPa.<br/>A negative pressure coefficinet (-0.13 cm^-1/GPa) of the Raman peak observed at the lowest enegy ( approx 95 cm^-1) can also be noticed.<br/>The overall lineshape of spectrum above the transformation does not change under pressure up to 13.7 GPa during compression.<br/>Surprisingly, the seven Raman modes can be observed under hydrostatic pressure down to 1.5 GPa during decompression.<br/>The apparent hysteresis of the Raman spectrum must reflect a metastable change in the sample structure.<br/>We analyse the effect in the context of iodine (I₂) molecules, which are found to be present in van der Waals gaps between the HfS₂layersin the investigated structure. Possible scenarios leading to the observed metastabilty are proposed.