Anomalous metal-semiconductor transition and superconductivity in van der Waals TiS2 under pressure

Van der Waals transition-metal dichalcogenides (TMDCs) form a vast family that has garnered significant attention in various fields. Among these, ${\mathrm{TiS}}_{2}$ has complex and controversial electronic properties at ambient and high pressures that need clarification, such as its semiconductor/metal nature, ambiguous high pressure structures, disputed electronic behaviors under pressure, etc. In this work, we confirm the anomalous metal-semiconductor-metal transition and reveal the emergence of superconductivity in ${\mathrm{TiS}}_{2}$. This metal-semiconductor transition near 15 GPa is clarified as an intrinsic property of single-crystal ${\mathrm{TiS}}_{2}$ due to the self-Ti-doping and an isostructural transition. Re-metallization occurs at 31 GPa, and superconductivity emerges above 88 GPa. The critical temperature ${T}_{c}$ increases monotonically with pressure, reaching 4.1 K at 136.6 GPa, and is still not saturated. As the lightest TMDC and the first superconducting layered fourth-period transition-metal disulfide, ${\mathrm{TiS}}_{2}$ offers valuable insights into its unique electronic properties. This research not only reveals the anomalous metal-semiconductor transition and superconductivity but also inspires further investigation into the evolution of electronic properties in other TMDCs under pressure.