Pressure-induced structural transition, metallization, and topological superconductivity in PdSSe

Pressure not only provides a powerful way to tune the crystal structure of transition-metal dichalcogenides (TMDCs) but also promotes the discovery of exotic electronic states and intriguing phenomena. Structural transitions from the quasi-two-dimensional layered orthorhombic phase to three-dimensional cubic pyrite phase, metallization, and superconductivity under high pressure have been observed experimentally in TMDC materials ${\mathrm{PdS}}_{2}$ and ${\mathrm{PdSe}}_{2}$. We report a theoretical prediction of the pressure-induced evolutions of crystal structure and electronic structure of PdSSe, an isomorphous intermediate material of the orthorhombic ${\mathrm{PdS}}_{2}$ and ${\mathrm{PdSe}}_{2}$. A series of pressure-induced structural phase transitions from the layered orthorhombic structure into an intermediate phase and then to a cubic phase is revealed. The intermediate phase features the same structure symmetry as the ambient orthorhombic phase, except for drastic collapsed interlayer distances and striking changes of the coordination polyhedron. Furthermore, the structural phase transitions are accompanied by electronic structure variations from semiconductor to semimetal, which are attributed to bandwidth broadening and orbital-selective mechanisms. Specifically, the cubic phase PdSSe is distinct from the cubic ${\mathrm{PdS}}_{2}$ and ${\mathrm{PdSe}}_{2}$ materials by breaking inversion and mirror-plane symmetries but showing similar superconductivity under high pressure, which originates from strong electron-phonon coupling interactions concomitant with topologically nontrivial Weyl and high-fold fermions. The intricate interplay between lattice, charge, and orbital degrees of freedom as well as the topologically nontrivial states in these compounds will further stimulate wide interest to explore the exotic physics of the TMDC materials.