Evolution of Structural and Electronic Properties of TiSe under High Pressure.

A pressure-induced structural phase transition and its intimate link with the superconducting transition was studied for the first time in TiSe up to 40 GPa at room temperature using X-ray diffraction, transport measurement, and first-principles calculations. We demonstrate the occurrence of a first-order structural phase transition at 4 GPa from the standard trigonal structure (S.G.3̅1) to another trigonal structure (S-G-3̅1). Additionally, at 16 GPa, the 3̅1 phase spontaneously transforms into a monoclinic 2/ phase, and above 24 GPa, the 2/ phase returns to the initial 3̅1 phase. Electrical transport results show that metallization occurs above 6 GPa. The charge density wave observed at ambient pressure is suppressed upon compression up to 2 GPa with the emergence of superconductivity at 2.5 GPa, with a critical temperature () of 2 K. A structural transition accompanies the emergence of superconductivity that persists up to 4 GPa. The results demonstrate that the pressure-induced phase transitions explored by the experiments along with the theoretical predictions may open the door to a new path for searching and controlling the phase diagrams of transition metal dichalcogenides.