Evidence for percolation diffusion of cations and material recovery in disordered pyrochlore from accelerated molecular dynamics simulations

TL;DR

This study uses molecular dynamics simulations to reveal that in disordered pyrochlore, cation diffusion is facilitated by a percolation network of antisites, contrasting with oxygen diffusion behavior.

Contribution

It demonstrates that cation diffusion in disordered pyrochlore is enabled by a percolation network of antisites, a novel insight into disorder-related mass transport.

Findings

Cation diffusion is slow in low-disorder materials.

Higher disorder leads to faster cation diffusion and antisite annihilation.

Cation diffusivity decreases as the material reorders.

Abstract

We used classical and accelerated molecular dynamics simulations to characterize vacancy-mediated diffusion of cations in Gd$_2$Ti$_2$O$_7$ pyrochlore as a function of the disorder on the microsecond timescale. We find that cation vacancy diffusion is slow in materials with low levels of disorder. However, higher levels of disorder allow for fast cation diffusion, which is then also accompanied by fast antisite annihilation and ordering of the cations. The cation diffusivity is therefore not constant, but decreases as the material reorders. The results suggest that fast cation diffusion is triggered by the existence of a percolation network of antisites. This is in marked contrast with oxygen diffusion, which showed a smooth increase of the ionic diffusivity with increasing disorder in the same compound. The increase of the cation diffusivity with disorder is also contrary to observations from other complex oxides and disordered media models, suggesting a fundamentally different relation between disorder and mass transport. These results highlight the dynamic interplay between fast cation diffusion and the recovery of disorder and have important implications for understanding radiation damage evolution, sintering and aging, as well as diffusion in disorder oxides more generally.