Solid-state diffusion in amorphous zirconolite

TL;DR

This study uses molecular dynamics simulations to show that amorphization significantly increases solid-state diffusion in zirconolite, with implications for nuclear waste containment.

Contribution

It provides new microscopic insights into how amorphization and density affect diffusion in zirconolite, a key material for nuclear waste encapsulation.

Findings

Amorphization greatly enhances diffusion in zirconolite.

Decreasing density increases diffusion pre-factors but not activation energy.

Different atomic species respond uniquely to amorphization and density changes.

Abstract

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.