Structural and electronic phase transition in the van der Waals crystal HfS2 under high pressure

The two-dimensional (2D) to three-dimensional (3D) structural evolution of transition metal dichalcogenides (TMDCs) under high pressure is a significant subject since the change of dimensionality could induce a drastic change in various physical properties. A more compact 3D structure of TMDCs could result in metallization and even superconductivity. In this work, we report an unusual 3D state of an $\mathrm{Hf}{\mathrm{S}}_{2}$ crystal that can still maintain its semiconducting nature over a wide pressure range. X-ray diffraction results show that the layered $\mathrm{Hf}{\mathrm{S}}_{2}$ undergoes a reversible transformation to a 3D $Immm$ structure at $\ensuremath{\sim}12$ GPa, accompanied by a semiconductor-to-semiconductor transition. The Raman and optical absorption measurements show that the phase transition may occur as early as $\ensuremath{\sim}9.2$ GPa. The semiconducting state is stable up to $\ensuremath{\sim}65$ GPa, and the metallization occurs at 68.6 GPa, the maximum pressure in this work. This work not only reports the 2D-to-3D structural phase transition in an $\mathrm{Hf}{\mathrm{S}}_{2}$ crystal driven by pressure, but also gives a clear physical picture that shows the intrinsic electrical transport properties and electronic behavior that enriches our understanding of the electronic behavior of TMDCs and sheds light on the future design of novel optoelectronic devices.