Pressure-Induced Structural Transformations and Superconductivity in Janus RhSeCl

Layered mixed-anion Janus compounds provide an attractive platform for coupling structural distortion with emergent electronic phenomena. However, the microscopic structural mechanism by which pressure reconstructs Janus frameworks and reshapes their electronic states remains largely unexplored. Here, we combine in situ high-pressure single-crystal and powder X-ray diffraction, Raman spectroscopy, electrical transport measurements, and first-principles calculations to investigate the mixed-anion Janus compound RhSeCl up to 160 GPa. Under compression, RhSeCl follows a progressive structural evolution from a hexagonal lattice toward a tetragonal phase through symmetry-lowering intermediates. This transformation proceeds through differential structural responses of the Se- and Cl-terminated sides of the Janus layers, accompanied by the increase in Rh coordination from quasi-octahedral RhSe3Cl3 to square-prismatic RhSe4Cl4 geometry. The structural evolution is accompanied by an experimentally observed semiconductor-to-metal transition, while electronic-structure calculations predict an intermediate nodal-ring semimetal state arising from crossings between Rh-d and Se/Cl-p bands. Superconductivity emerges above approximately 90 GPa as the structure evolves toward the tetragonal phase. Electron–phonon coupling calculations indicate that superconductivity originates from strong coupling between low-frequency Rh-dominated phonons and in-plane Rh- dx2-y2orbitals at the Fermi level, which enhances the electron hopping within the Rh square plane. These results link pressure-induced changes in the Janus structure to the predicted nodal-ring semimetal state and observed superconductivity in RhSeCl, suggesting mixed-anion Janus compounds as candidates for exploring new electronic phases under pressure.