High-Pressure Hydrogen Sulfide from First Principles: A Strongly Anharmonic Phonon-Mediated Superconductor

We use first-principles calculations to study structural, vibrational, and superconducting properties of H₂S at pressures P ≥200 GPa . The inclusion of zero-point energy leads to two different possible dissociations of H₂S , namely 3 H₂S →2 H₃S +S and 5 H₂S →3 H₃S +HS₂ , where both H₃S and HS₂ are metallic. For H₃S , we perform nonperturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction (λ ≈2.64 at 200 GPa) and T_c. Anharmonicity hardens H-S bond-stretching modes and softens H-S bond-bending modes. As a result, the electron-phonon coupling is suppressed by 30% (λ ≈1.84 at 200 GPa). Moreover, while at the harmonic level T_c decreases with increasing pressure, the inclusion of anharmonicity leads to a T_c that is almost independent of pressure. High-pressure hydrogen sulfide is a strongly anharmonic superconductor.