Pressure-induced Mott insulator-metal transition and structural phase transitions in the triangular antiferromagnet NiGa₂S₄

Mott insulator NiGa2S4 has attracted much attention because of its elusive magnetic ground state due to the geometric frustration and layered structures. To our knowledge, however, additional studies of the turning properties through external parameters are rather scarce in this system. Here, we comprehensively performed high-pressure studies for the NiGa2S4 single crystal. With synchrotron x-ray diffraction and Raman-scattering results, we found the starting trigonal P (3) over bar m1 phase quickly transfers to the R3m phase at a lower pressure of 1.6 GPa due to the intralayer sliding. With further compressing, another two structural phase transitions were observed, NiGa2S4 transfers to tetragonal P (4) over bar 2m phase at 15.5 GPa and then enters into orthogonal Pna2(1) phase beyond 18.2 GPa. In addition, electrical transport measurements show a Mott insulator-metal transition close to 17 GPa, which can be related to the R3m -> P (4) over bar 2m transition where a similar to 17.5% collapse in the unit-cell volume and a sharp reduction in Raman intensity occur. The metallic NiGa2S4 shows no superconducting transition down to 1.8 K. This paper enriches our understanding of NiGa2S4 especially its structure information, the tuning between the Mott insulating state and metallic state simultaneously accompanied by a structural phase transition sheds light on further understanding of magnetism and Mott physics in NiGa2S4.