  We discuss how structural disorder and amorphization affects solid-state
diffusion, and consider zirconolite as a currently important case study. By
performing extensive molecular dynamics simulations, we disentangle the effects
of amorphization and density, and show that a profound increase of solid-state
diffusion takes place as a result of amorphization. Importantly, this can take
place at the same density as in the crystal, representing an interesting
general insight regarding solid-state diffusion. We find that decreasing the
density in the amorphous system increases pre-factors of diffusion constants,
but not decreasing the activation energy. We also find that atomic species in
zirconolite are affected differently by amorphization and density change. Our
microscopic insights are relevant for understanding how solid-state diffusion
changes due to disorder and for building predictive models of operation of
materials to be used to encapsulate nuclear waste.
