Coexistence of superconductivity and electride states in Ca2H with an antifluorite-type motif under compression

Abstract

Hydrogen-abundant compounds, as highly potential candidates of near- or room-temperature superconductors, have recently attracted substantial attention. It is noted that metal-rich hydrides may exhibit unique electride feature, together with potential superconductivity, which indicates that they could be regarded as electride superconductor (ESC) hydrides. Herein, we performed the first-principles structure searches of calcium-rich hydrides for designing ESC hydrides. Strikingly, unprecedented calcium-based hydride Ca₂H with robust stability and excellent metallicity is identified under compression, where it possesses the zero-dimensional distribution feature of interstitial electrons serving as interstitial quasiatoms (ISQs), mainly originating from the donation of calcium atoms. Interestingly, cubic Fmm Ca₂H, with antifluorite-type configuration, consists of a host Ca sublattice and guests H and ISQs confined in the center of Ca₈ cubes. Furthermore, Ca₂H is predicted to have a superconducting critical temperature T_c of 6 K at 100 GPa and exhibit significantly enhanced superconductivity up to 11 K when dynamically stabilized to 30 GPa, which is predominantly attributed to the remarkable softening of low-frequency acoustic phonon modes involving Ca-dominated vibrations, accompanied by the strong coupling with Ca 3d states near the Fermi energy. Additionally, the yielded electride Na₂H via substituting Ca with Na in Ca₂H is found to have a notably high T_c of 46 K at 30 GPa. Our current work improved the understanding of structures and properties of electrides, as well as paving the way for exploring emergent ESCs in hydride systems.