Emergence of Diffusional Hydrogen Escape in High-$T_c$ Superconducting Calcium Superhydride at Megabar Pressures

High-pressure metal superhydrides have attracted intense scientific interest due to their remarkable superconducting properties. While superconductivity is known to be sensitive to material composition, compositional variability is often overlooked in metal superhydrides at megabar pressures. Using ab initio path-integral simulations, we find that up to 7 % of the hydrogen atoms escape from the 215-kelvin superconducting CaH$_6$ upon decompression from 165 to 123 GPa. This loss of hydrogen leads to an elastic instability at low pressures, an "abnormal" positive pressure dependence of the superconducting $T_c$ and a quantum phase transition from a mixed superconducting-diffusive state to a pure superconducting phase. In this mixed phase, proton diffusivity reaches $10^{-8}$ $\mathrm{cm^2/s}$ at low temperatures and promotes to $10^{-7}$ $\mathrm{cm^2/s}$ at room temperature, which elucidates the escape of hydrogen at megabar pressures. Our results are consistent with many "anomalous" experimental observations and highlight the significance of composition effects in high-$T_c$ metal superhydrides.