High-pressure superconductivity in hexagonal CaSH3 with a nonclathrate molecular hydrogen structure.

Understanding the interplay between pressure-driven Fermi surface electronic states and phonon hardening is essential for exploring high-temperature superconducting materials. In this study, using first-principles structure searches, we identify two thermodynamically stable ternary Ca-S-H phases at 200 GPa: P6̄m2 CaSH3 and C2/m Ca2SH6. Structural and bonding analyses reveal that both compounds consist of non-cage-like quasi-H2 units formed by H-H covalent bonds, together with weak ionic interactions between Ca and S, and exhibit metallic character. Among them, CaSH3 exhibits a superconducting transition temperature (Tc) of 82.35 K at 200 GPa. The electron-phonon coupling (EPC) is found to originate primarily from an acoustic E' mode at the L point, corresponding to out-of-plane shear vibrations of S atoms, with low- to medium-frequency phonon modes dominating the total EPC. Furthermore, pressure exerts a dual effect on Tc: on one hand, increasing pressure reduces the density of electronic states at the Fermi level, which suppresses the electron-phonon coupling; on the other hand, pressure induces phonon hardening (i.e., increased phonon frequencies), thereby enhancing Tc. This work reveals the competition between the electronic density of states and phonon hardening under high pressure, offering new insights into the superconducting mechanism of high-pressure hydrides.