Determine whether hydrogen diffusion contributes to unit-cell expansion

Determine whether hydrogen diffusion into the synthesized Fm\bar{3}m-LaH$_{10\pm\delta}$ films contributes to the observed increase in unit-cell volume during long-term storage under high pressure, beyond the contribution from pressure decreases.

Background

The paper reports small increases in the refined unit-cell volumes of the Fm\bar{3}m phase over the monitoring period. The authors attribute these changes primarily to pressure decreases of approximately 5 GPa in the diamond anvil cells, but acknowledge that hydrogen diffusion could also contribute. Because the available diffraction measurements do not distinguish these effects, the contribution of hydrogen diffusion remains unresolved.

References

We cannot rule out a further contribution to the volume increase due to hydrogen diffusion into the sample.

— Synthesis and stability of high-$T_c$ LaH$_{10\pmδ}$ films at high pressures  (2608.18865 - Cross et al., 19 Aug 2026) in Section “Temporal stability of the crystal structure and superconductivity of LaH$_{10\pm\delta}$ films”