Fractional subdiffusion of molecular hydrogen in irradiated portlandite: connecting atomistic insights and macroscopic release

During the storage of radioactive waste, the irradiation of interstellar ices, or the production of natural hydrogen in deep geological rocks, radiolysis generates H₂ in hydrogen-bearing materials. Its subsequent release may involve several processes, among which diffusion can play an important role. Establishing a direct link between atomistic migration mechanisms and macroscopic release kinetics remains a major challenge. Here, using irradiated portlandite as a model system, we show that H₂ transport within the crystal is subdiffusive and follows a time-fractional diffusion model rather than classical Fickian diffusion. The fitted anomalous exponent (α = 0.43) is consistent with the subdiffusive behavior reported in molecular dynamics simulations, although the generalized diffusion coefficients differ by two to four orders of magnitude and the apparent activation energies also differ. The model describes the measured release kinetics and permits extrapolations to other particle sizes.