Multi-scale structural analysis of swift heavy ion-irradiated ZrO₂ and HfO₂

The radiation-induced monoclinic-to-tetragonal phase transition in ZrO₂ and HfO₂ has been the subject of many investigations, but the transformation pathways and underlying structural mechanisms are still not well understood. In this study, microcrystalline powder samples of ZrO₂ and HfO₂ were irradiated with 946 MeV and 1470 MeV Au ions to a wide fluence range up to 3 × 10¹³ ions/cm². To characterize beam-induced structural modifications across all spatial length scales, complementary experimental techniques such as synchrotron X-ray diffraction and spallation neutron total scattering were used. The phase evolution of the tetragonal polymorph with increasing ion fluence is accurately described by a heterogeneous track-overlap model that incorporates both direct- and double-impact processes. These damage accumulation processes are an expression of a core-shell ion track morphology that depends on irradiation conditions and target material. Neutron pair distribution function analysis revealed that ion-beam-induced tetragonal ZrO₂ is merely a configurational average of short-range orthorhombic ( Pbcn ) domains stabilized by a dense network of domain walls. This knowledge is critical for a better understanding of how crystalline-to-crystalline phase transformations proceed at the atomic scale under extreme conditions.