Asymmetric interfacial strain governing anomalous fluorine diffusion in ultrathin heterostructures
Phys. Rev. B 114, 094112 – Published 19 August, 2026
DOI: https://doi.org/10.1103/sz89-51zk
Abstract
The anomalously high fluorine ionic diffusivity reported in multilayer ultrathin heterostructures at around 600 K more than two decades ago [N. Sata et al., Nature (London) 408, 946 (2000)] has remained unresolved due to the complex distribution and diffusion behavior of vacancies and interstitials at the interface. Here, we address this long-standing issue by employing a cluster expansion model combined with machine-learning molecular dynamics, enabling atomic-level analysis of fluorine ion thermodynamics and kinetics. We find that fluorine vacancies preferentially accumulate at the -side interface, whereas fluorine interstitials appear only on the side. At 600 K, diffusion is dominated by interstitial-mediated fluorine diffusion in the region, whereas at 900 K vacancy-mediated fluorine diffusion on the side becomes active, and the coexistence of both mechanisms leads to a synergistic enhancement of the overall diffusivity. The spatial separation of defect distributions and diffusion pathways arises from the coupling between electronic stabilization and asymmetric interfacial strain. Electron pairing between trapped electrons in adjacent stabilizes clustering at the -side interface, whereas tensile strain on the side enlarges octahedral interstitial sites and promotes interstitial-mediated fluorine hopping; conversely, compressive strain on the side suppresses interstitial occupation. These findings provide microscopic insight into the origin of the anomalous diffusion and clarify how electronic stabilization of vacancy clustering and local strain modulation of interstitial formation jointly regulate defect distributions and diffusion mechanisms in fluorite-structured heterostructures.
