Strong Anharmonic and Quantum Effects in Pm-3n AlH3 Under High Pressure: A First-Principles Study.

Motivated by the absence of experimental superconductivity in the metallic Pm-3n phase of AlH3 despite the predictions, we reanalyze its vibrational and superconducting properties at pressures above 99 GPa making use of first-principles techniques. In our calculations based on the self-consistent harmonic approximation method that treats anharmonicity beyond perturbation theory, we predict a strong anharmonic correction to the phonon spectra and demonstrate that the superconducting critical temperatures predicted in previous calculations based on the harmonic approximation are strongly suppressed by anharmonicity. The electron-phonon coupling concentrates on the lowest-energy hydrogen-character optical modes at the X point of the Brillouin zone. As a consequence of the strong anharmonic enhancement of their frequency, the electron-phonon coupling is suppressed by at least a 30%. The suppression in {\lambda} makes Tc smaller than 4.2 K above 120 GPa, which is well consistent with the experimental evidence. Our results underline that metal hydrides with hydrogen atoms in interstitial sites are subject to huge anharmonic effects.