Unveiling Swift Heavy Ion Track Morphology in Sr-Based High-Entropy Perovskites

The incorporation of multiple cations on a single lattice site in the high-entropy oxides is considered the key driving factor for modifying the known atomic-level response to energetic ion irradiation due to the presence of structural disorder; however, these effects are not well-understood yet. In this work, we present atomic-level insight into irradiation-induced nanoscale phase transformations in a perovskite-structured high-entropy oxide, Sr(Zr_0.2Sn_0.2Ti_0.2Hf_0.2Nb_0.2)O₃ (Sr(HE)O₃), subjected to 774 MeV swift Xe heavy ions, where damage is dominated by inelastic ion-lattice interactions. While these ions generally are known to create nanoscale disordered channels, "ion tracks", along the penetration direction in the material, this study shows the formation of discontinuous and partially recrystallized ion tracks in Sr(HE)O₃. Compared to SrTiO₃ irradiated under identical energy loss conditions, the ion tracks in Sr(HE)O₃ exhibit significantly reduced diameters and a markedly different interfacial structure. Notably, the crystalline-amorphous interface in Sr(HE)O₃ shows minimal lattice distortion, confined to approximately 2-3 monolayers, in contrast to the extended disordered shell commonly observed in SrTiO₃. Using in situ atomic-resolution electron microscopy, we further demonstrate that the amorphous/disordered regions within Sr(HE)O₃ ion tracks remain highly stable under electron irradiation, whereas tracks in SrTiO₃ readily recrystallize. This enhanced stability is attributed to the dominance of structural and chemical complexity arising from multiple B-site cations, which suppress defect migration and templated recrystallization driven by electronic excitations and local heating. Overall, this study highlights how high-entropy oxide chemistry fundamentally reshapes irradiation damage evolution, offering insights into defect formation and phase stability under extreme conditions.