High-pressure structural phase transitions and metallization in layered HfS2 under different hydrostatic environments up to 42.1 GPa

Abstract

A series of structural, vibrational and electrical transport behaviors for HfS₂ were systematically investigated by Raman spectroscopy, electrical conductivity, high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) in conjunction with first-principles theoretical calculations in the processes of compression and decompression under different hydrostatic environments. High-pressure Raman scattering and electrical conductivity results revealed that HfS₂ underwent two structural phase transitions and metallization at 8.0, 15.2 and 20.5 GPa under non-hydrostatic conditions. The first structural transition was characterized by the appearance of the M1 and M2 Raman peaks and the inflection point in the pressure-dependent electrical conductivity. The second structural transition was identified from the disappearance of the E_g, M1 and M2 Raman peaks, the emergence of the M3–M6 Raman peaks, and the discontinuities in the pressure coefficient of the A_1g mode and electrical conductivity. As the pressure increased to 20.5 GPa, HfS₂ underwent a metallization transition, which was attributed to the closure of the bandgap energy obtained from first-principles theoretical calculations. Under hydrostatic conditions, two structural transformations and metallization of HfS₂ occurred at relatively high pressures of 8.2, 17.2 and 23.1 GPa due to different deviatoric stress. Upon decompression, the phase transition was revealed to be reversible under different hydrostatic environments, which was verified by the HRTEM and AFM results. Our acquired results of the crystalline and electronic structures of HfS₂ facilitate the understanding of the high-pressure characterization of other HfX₂ compounds (X = Se and Te) under different hydrostatic environments.