Insulator-to-Superconductor Transition in Quasi-One-Dimensional HfS₃ under Pressure

Various transition-metal trichalcogenides (TMTC) show unique electronic properties, such as metal-insulator transition, topological insulator, and even superconducting transition. Currently, almost all metallic TMTC compounds can show superconductivity either at ambient pressure or at high pressure. However, most TMTC compounds are semiconductors and even insulators. Does superconductivity exist in any non-metallic TMTC compound by artificial manipulation? In this work, the electronic behavior of highly insulating HfS₃ has been manipulated in terms of pressure. HfS₃ undergoes an insulator-to-semiconductor transition near 17 GPa with a band gap reduction of ∼1 eV. Optical absorption, Raman spectroscopy, and X-ray diffraction measurements provide consistent results, suggesting the structural origin of the electronic transition. Upon further compression, HfS₃ becomes a superconductor without further structural transition. The superconducting transition occurs as early as 50.6 GPa, and the T_c reaches 8.1 K at 121 GPa, which sets a new record for TMTCs. This work reveals that all TMTCs may be superconductors and opens a new avenue to explore the abundant emergent phenomena in the TMTC material family.