Hydrogen Vacancy Induced Superconductivity Collapse in A15 Lanthanum Hydride.

Hydrogen-rich lanthanum compounds show the highest known superconducting transition temperatures 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 LaH_{5.75-x} hosts high-T_{c} superconductivity (T_{c}=98 K at 94 GPa) when available interstitial sites are fully occupied with hydrogen. Upon decompression, a site-selective hydrogen depopulation drives a superconductor-to-insulator transition with a threshold composition of LaH_{5}. Strikingly, the A15 framework is experimentally retained from 120 to 4 GPa, with reversible pressure-dependent hydrogen content changes from LaH_{5.75} down to LaH_{3.25}. 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.