Pressure-induced phase transition on layered HgPSe3 revealed by optical, structural and vibrational studies.

The family of transition metal phosphorus trichalcogenides (MPX3, where M is a transition metal and X = S or Se) has recently attracted considerable attention due to their tunable band gaps and synergistic effects among electronic, magnetic, structural, and optical properties. We examine the influence of hydrostatic pressure on the optical, structural and vibrational properties of layered HgPSe3 through spectroscopic and diffraction techniques combined with first-principles calculations. Optical absorption measurements reveal a phase transition at 3.6 GPa, evidenced by a decrease in the band gap energy of approximately 200 meV, resulting in a change in sample color from bright to dark red. The same phase transition was confirmed by structural methods: powder X-ray diffraction and Raman spectroscopy. We propose that the compound undergoes a structural transition from its ambient pressure monoclinic phase to a high-pressure triclinic phase. Birch-Murnaghan fits give an experimental bulk modulus of B0 = 29.7 GPa, in very good agreement with the theoretical value of B0 = 28.1 GPa. Experimental pressure coefficients are obtained and compared with theoretical predictions. Our findings unveil a complex interplay between the structural and optical properties of HgPSe3 under pressure, which could be exploited to fabricate novel applications based on the tunable properties of van der Waals materials.