Hydrogen Vacancy Induced Superconductivity Collapse in A15 Lanthanum Hydride

Hydrogen-rich lanthanum compounds show the highest known superconducting transition temper- atures at high pressure. Despite the pivotal role of hydrogen within these systems, there has been no systematic exploration of how the composition and superconducting properties are intertwined. Our experimental and computational studies demonstrate that A15-type LaH5.75−x hosts high-Tc super- conductivity (Tc = 98K at 94GPa) when available interstitial sites are maximally occupied with hydrogen. Upon decompression, a site-selective hydrogen depopulation drives a superconductor- to-insulator transition with a threshold composition of LaH5. Strikingly, the A15 framework is experimentally retained from 120 GPa to 4 GPa, with reversible pressure-dependent hydrogen con- tent changes from LaH5.75 down to LaH3.25. Our results demonstrate the exceptional stability and tunability of the A15 framework, offering a unique platform to probe the interplay between composition, structure, and superconductivity in hydrogen-rich materials.