**6. Summary and Outlook**

Fluorite-structured CeO<sup>2</sup> is generally resistant to structural modification by SHI irradiation. Defect production accompanied by unit cell expansion and microstrain constitute the dominant observable radiation responses. Ion tracks with core-shell morphologies are observed in this material, a remnant of the induced atomic-scale structural and chemical changes. The highly transient conditions along the SHI path lead to oxygen movement in radial directions. This results in an oxygen-depleted core region, where Ce4+ cations are partially reduced to Ce3+. This core is surrounded by an oxygen rich shell that contains small peroxide-like oxygen defect clusters.

These redox driven processes under SHI irradiation are an important characteristic of CeO<sup>2</sup> and are very sensitive to alterations in sample microstructure and chemistry (grain size, stoichiometry, and dopants), as well as ion-beam parameters (ion mass, energy, fluence, flux, irradiation temperature, and pressure). This makes CeO<sup>2</sup> a suitable model material to study fundamental aspects of ion-matter interactions over a wide range of conditions. On the other hand, this complex behavior poses a challenge in terms of disentangling the individual contributions of each parameter and comparing results from different research groups and experiments.

Future ion irradiation studies should systematically examine the radiation response of CeO<sup>2</sup> under a wide range of experimental conditions, including coupled effects that are of relevance for nuclear applications. Such efforts should also cover the mostly unexplored irradiation regime between low energy ions (predominately nuclear d*E*/d*x*) and swift heavy ions (predominately electronic d*E*/d*x*) to gain insight into the material response under simultaneous contributions from both types of energy deposition.

**Author Contributions:** Conceptualization, W.F.C., C.L.T. and M.L.; Data curation, W.F.C., C.L.T. and M.L.; writing—original draft preparation, W.F.C., C.L.T. and M.L.; writing—review and editing, W.F.C., C.L.T. and M.L.; supervision, M.L.; project administration, M.L.; funding acquisition, M.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Department of Energy (DOE) Office of Nuclear Energy's Nuclear Energy University Program under US-DOE, contract DE-NE0008895. Synchrotron XRD measurements were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA's Office of Experimental Sciences. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. HPCAT beam time was provided by the Chicago/DOE Alliance Center. The research at ORNL's Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. W.F.C. was funded by an Integrated University Program Graduate Fellowship.

**Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article.

**Acknowledgments:** The authors gratefully acknowledge technical support by scientists at the different ion-beam, neutron, and X-ray user facilities: Christina Trautmann and Daniel Severin (GSI Helmholtz Center, Darmstadt, Germany), Maxim Zdorovets (Institute of Nuclear Physics, Astana, Kazakhstan), Vladimir Skuratov (Joint Institute for Nuclear Research, Dubna, Russia), Michelle Everett and Jörg Neuefeind (Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, USA) and Changyong Park (Advanced Photon Source, Argonne National Laboratory, Argonne, USA).

**Conflicts of Interest:** The authors declare no conflict of interest.
