Quantum Crystal Structure in the 250 K Superconducting Lanthanum Hydride*

Hydrogen-rich materials at high pressures are at the verge of reaching room-temperature superconductivity. Electrical and x-ray diffraction measurements determined a weakly pressure-dependent Tc for LaH10 between 137 and 218 gigapascals in a structure with a fcc arrangement of La atoms. Here we show that quantum atomic fluctuations stabilize in this pressure range a high-symmetry Fm-3m crystal structure consistent with experiments. Even if ab initio classical calculations predict this structure to distort below 230 GPa, the inclusion of quantum effects evidences the Fm-3m as the true ground state. The agreement between the calculated and experimental Tc values further supports this phase as responsible for the 250 K superconductivity. The relevance of quantum fluctuations questions many of the crystal structure predictions made for hydrides within a classical approach that at the moment guide experiments. Furthermore, quantum effects are revealed to be crucial to stabilize solids with extraordinary electron-phonon coupling, reducing the pressures needed for their synthesis.