Pressure-Induced Insulator-to-Metal Transition in van der Waals compound CoPS$_3$

We have studied the insulator-to-metal transition and crystal structure evolution under high pressure in the van der Waals compound CoPS$_3$ through $\textit{in-situ}$ electrical resistance, Hall resistance, magnetoresistance, X-ray diffraction, and Raman scattering measurements. CoPS$_3$ exhibits a $C2/m$ $\rightarrow$ $P\overline{3}$ structural transformation at 7 GPa accompanied by a 2.9$\%$ reduction in the volume per formula unit. Concomitantly, the electrical resistance decreases significantly, and CoPS$_3$ becomes metallic. This metallic CoPS$_3$ is a hole-dominant conductor with multiple conduction bands. The linear magnetoresistance and the small volume collapse at the metallization suggest the incomplete high-spin $\rightarrow$ low-spin transition in the metallic phase. Thus, the metallic CoPS$_3$ possibly possesses an inhomogeneous magnetic moment distribution and short-range magnetic ordering. This report summarizes the comprehensive phase diagram of $M$PS$_3$ ($M$ = V, Mn, Fe, Co, Ni, and Cd) that metalize under pressures.