Superconductivity has been observed in many insulating transition metal dichalcogenides (TMDCs) under pressure. However, the origin of superconductivity is still a puzzle, due to the lack of study on their structures at low temperature. Here, we report a high-T_c superconducting (SC) state (SC-I phase) coexisting with other SC states in a compressed 1T-HfS_2­ crystal up to ~160 GPa. In situ synchrotron X-ray diffraction results exclude the existence of decomposed sulfur, and confirm two structural phase transitions at room temperature and an extra transition at low temperature, which are responsible for the multiple SC states. The SC-Ⅰ phase exhibits an unsaturated T_c of 16.4 K at 158 GPa, and has the largest upper critical field among the bulk TMDCs, μ₀H_c2(0) ≈ 29.7 T for a T_c ~15.2 K at 147 GPa, exceeding the weak-coupling Pauli limit. These results reveal abundant SC properties together with sensitive structures in compressed HfS₂, and thereby extend our understanding on TMDCs’ superconductivity.
 

high pressure,superconductivity,Transition metal dichalcogenides,X-ray Diffraction