Pressure-Induced Metallization Accompanied by Elongated S–S Dimer in Charge Transfer Insulator NiS_2

The insulator-metal transition triggered by pressure in charge transfer insulator NiS_2 is investigated by combining high-pressure electrical transport, synchrotron x-ray diffraction and Raman spectroscopy measurements up to 40–50 GPa. Upon compression, we show that the metallization firstly appears in the low temperature region at \sim3.2 GPa and then extends to room temperature at \sim 8.0 GPa. During the insulator-metal transition, the bond length of S–S dimer extracted from the synchrotron x-ray diffraction increases with pressure, which is supported by the observation of abnormal red-shift of the Raman modes between 3.2 and 7.1 GPa. Considering the decreasing bonding-antibonding splitting due to the expansion of S–S dimer, the charge gap between the S-pp\pi^* band and the upper Hubbard band of Ni-3d e_\rm g state is remarkably decreased. These results consistently indicate that the elongated S–S dimer plays a predominant role in the insulator-metal transition under high pressure, even though the p-d hybridization is enhanced simultaneously, in accordance with a scenario of charge-gap-controlled type.