Insights into Defect Kinetics, Mass Transport and Electronic Structure in Ion-Irradiated Bi₂O₃

Solid-state ion conductors like Bi₂O₃ are gaining attention for advanced energy storage and generation applications. In this study, we explore the ion irradiation response of the monoclinic α-phase of Bi₂O₃, which is easily synthesized and stable in diverse and extreme environments. Ion irradiation is known to produce disorder, phase transformations, and strain in materials, all of which influence mass transport and ionic conductivity. Here, we present the first study investigating α-Bi₂O₃ under noble gas ion irradiation, using helium (He), neon (Ne), and argon (Ar) to assess the effects on its structure, electronic properties, and mass transport. Our findings demonstrate a clear dependence of amorphization and mass transport on both ion species and fluence. Through transmission electron microscopy (TEM), 4D-STEM, and grazing-incidence X-ray diffraction (GIXRD), we reveal an increased propensity for amorphization with larger ion species (Ne and Ar) and at higher fluences, while He irradiation leads primarily to swelling. We observe a depth-dependent effect due to the combination of electronic stopping power, which dominates near the surface, and nuclear stopping power, which becomes more significant deeper into the bulk. Structural changes associated with irradiation are correlated with mass transport processes, evidenced by bubble nucleation and coarsening in the material. These bubbles, formed in response to ion bombardment, provide insight into the behavior of defects and gas mobility in Bi₂O₃. Additionally, we observe significant changes in the electronic structure of Bi₂O₃, including a widening of the band gap from 2.6 eV to 3.5 eV as the material undergoes amorphization. By correlating the kinetics of bubble formation with alterations in structural and electronic features, we provide new insights into the mechanisms governing ionic conductivity.

This study highlights the complex interplay between irradiation conditions, phase transformations, and mass transport in Bi₂O₃. Our results suggest that amorphization enhances ionic mobility, as indicated by the coarsening of bubbles and increased mass transport in irradiated regions. These findings provide new insights into the mechanisms governing ionic conductivity in disordered systems, contributing to the development of more resilient materials for extreme environments. The insights gained from this work also have broader implications for the use of Bi₂O₃ in next-generation solid-state ionics, particularly in applications such as solid oxide fuel cells, batteries, and nuclear reactors, where materials are exposed to high radiation doses.