Evolution of amorphous structure under irradiation: zircon case study

Abstract

The nature of the amorphous state has been notably difficult to ascertain at the microscopic level. In addition to the fundamental importance of understanding the amorphous state, potential changes to amorphous structures as a result of radiation damage have direct implications for the pressing problem of nuclear waste encapsulation. Here, we develop new methods to identify and quantify the damage produced by high-energy collision cascades that are applicable to amorphous structures and perform large-scale molecular dynamics simulations of high-energy collision cascades in a model zircon system. We find that, whereas the averaged probes of order such as pair distribution function do not indicate structural changes, local coordination analysis shows that the amorphous structure substantially evolves due to radiation damage. Our analysis shows a correlation between the local structural changes and enthalpy. Important implications for the long-term storage of nuclear waste follow from our detection of significant local density inhomogeneities. Although we do not reach the point of convergence where the changes of the amorphous structure saturate, our results imply that the nature of this new converged amorphous state will be of substantial interest in future experimental and modeling work.

Radiation damage effects on helium diffusion in zircon

“…second and third peaks) where no discernible peaks are seen in the fully amorphous structure. This is in agreement with our previous work [10] in which it was shown that even after 6 collision cascades further structural damage and amorphization was still possible. As well as continual change to the local PDFs a continued increase in the number of coordination defects was also observed in the overlapped damaged region.…”

“…In our previous work [10] we showed that the pair distribution function (PDF) of the most highly overlapped damaged region defined by a regular shape of a box containing 6000 atoms was very close to the PDF of the fully amorphous structure. In order to study He diffusion in the damaged structure, we have used a larger region damaged by overlapping collision cascades.…”

“…The interatomic potentials used in this work to model zircon have been described in previous papers [7,10]. The Zr-O and O-O pairwise potentials are in the form of a Buckingham potential, with the Si-O pairwise potential taking the form of a Morse potential.…”

“…This work uses the radiation damaged structures produced in our previous paper [10]. A short range repulsive ZBL (Ziegler Biersack Littmark) [36] stopping potential was added to the pair wise potentials during the cascade simulations, and the structures were then relaxed using the non-modfied Buckingham potentials afterwards.…”

“…These experiments observed that irradiation results in the amorphization of zircon, which is accompanied by a large volume expansion of approximately 18-20% [6]. In more recent years Molecular Dynamics (MD) simulations have been carried out to aid the understanding of radiation damage in zircon, predicting the formation of Si-O bridges, the percolation of damage, and density inhomogenities formed as a direct result of simulated collision cascades [7][8][9][10].…”