Shear-Induced Isostructural Phase Transition and Metallization of Layered Tungsten Disulfide under Nonhydrostatic Compression

Pressure-induced structural and electronic transformations of tungsten disulfide (WS2) have been studied to 60 GPa, in both hydrostatic and nonhydrostatic conditions, using four-probe electrical resistance measurements, micro-Raman spectroscopy, and synchrotron X-ray diffraction. The results show the evidence for an isostructural phase transition from hexagonal 2Elc phase to hexagonal 2Ha phase, which accompanies the metallization at similar to 37 GPa. This isostructural transition occurs displacively over a large pressure range between 15 and 45 GPa and is driven by the presence of strong shear stress developed in the layer structure of WS2 under nonhydrostatic compression. Interestingly, this transition is absent in hydrostatic conditions using He pressure medium, underscoring its strong dependence on the state of stress. We attribute the absence to the incorporation of He atoms between the layers, mitigating the development of shear stress. We also conjecture a possibility of magnetic ordering in WS2 that may occur at low temperature near the metallization.