*2.1. Irradiation Conditions*

The controlled production of SHIs requires large, dedicated user facilities, rather than the more prevalent and accessible laboratory-based tandem ion accelerators. SHI irradiation of CeO<sup>2</sup> materials has been performed at several large accelerator facilities worldwide [23]. However, these facilities cover different ranges of ion species and energies. The energy deposited in a material per unit length (known as energy loss, d*E*/d*x*) depends on ion mass and energy and is a key parameter in SHI irradiation experiments. For highly energetic ions, energy is deposited primarily to a material's electronic subsystem (electronic energy loss, d*E*/d*xe*), with only minor contributions from nuclear collisions (nuclear energy loss, d*E*/d*xn*).

The effects of SHI energy deposition parameters on the induced structural modifications in CeO<sup>2</sup> has been investigated over a broad d*E*/d*x* range between 15 and 45 keV/nm, as shown in Figure 2. Ions of specific energy ranging from 0.5–30 MeV/u (with u being the number of nucleons) and species ranging from <sup>127</sup>I–238U have been utilized in these experiments [32–56]. The strong dependence of the energy loss on the ion energy leads to continuous change in d*E*/d*x* along the ion's penetration path in the material (Figure 2), which must be considered for characterization in order to accurately relate irradiation effects to the specific d*E*/d*x* within the volume probed by a particular characterization technique [57].

**Figure 2.** Energy loss profiles as a function of penetration depth determined with SRIM 2013 [57] for the ions used in the majority of studies [32−56] on SHI irradiated CeO2, assuming 100% theoretical density. The color scale corresponds to the specific energy of ions and indicates the change in energy as ions penetrate into the material. The three ion species illustrate the range of masses and species used in these studies. **Figure 2.** Energy loss profiles as a function of penetration depth determined with SRIM 2013 [57] for the ions used in the majority of studies [32–56] on SHI irradiated CeO<sup>2</sup> , assuming 100% theoretical density. The color scale corresponds to the specific energy of ions and indicates the change in energy as ions penetrate into the material. The three ion species illustrate the range of masses and species used in these studies.

must be considered for characterization in order to accurately relate irradiation effects to the specific d*E*/d*x* within the volume probed by a particular characterization technique.

Other experimental parameters that are typically controlled in SHI irradiation studies include the ion flux, fluence, irradiation angle relative to the sample surface, and in situ environmental conditions (e.g., temperature and pressure). The many irradiation studies previously conducted on CeO2 differ also with respect to sample properties. For example, various microstructures have been employed, ranging from single crystals [59] to polycrystalline pellets [32] to loose powder compacts [14]. As this review will show, all of these parameters greatly influence the response of CeO2 to SHI irradiation. To gain further insight into radiation damage mechanisms, the impact of all ion-beam conditions and sample properties on the induced structural and chemical modifications must be understood at a holistic level. Other experimental parameters that are typically controlled in SHI irradiation studies include the ion flux, fluence, irradiation angle relative to the sample surface, and in situ environmental conditions (e.g., temperature and pressure). The many irradiation studies previously conducted on CeO<sup>2</sup> differ also with respect to sample properties. For example, various microstructures have been employed, ranging from single crystals [58] to polycrystalline pellets [32] to loose powder compacts [14]. As this review will show, all of these parameters greatly influence the response of CeO<sup>2</sup> to SHI irradiation. To gain further insight into radiation damage mechanisms, the impact of all ion-beam conditions and sample properties on the induced structural and chemical modifications must be understood at a holistic level.

#### *2.2. Characterization 2.2. Characterization*

A wide range of characterization techniques have been employed to investigate the complex structural and chemical effects induced in CeO2 by SHI irradiation. Such measurements are performed either ex situ (after ion irradiation) or in situ (directly at accelerator beamlines with various dedicated characterization infrastructures). Analytical methods used in prior work include various forms of scattering, spectroscopy, calorimetry, and electron microscopy. Some of these techniques provide direct insight into the damage structure within individual ion tracks (e.g., electron microscopy), while others are based on net damage accumulation and fluence-dependent measurements (e.g., X-ray diffraction). Characterization is commonly performed on materials after irradiation at ambient conditions to study damage formation, yet select works on irradiation at high temperatures and post-irradiation thermal annealing provide insight into damage recovery and defect dynamics. The following section gives a brief overview of the most prevalent characterization techniques used to investigate SHI irradiation effects in CeO2 and other A wide range of characterization techniques have been employed to investigate the complex structural and chemical effects induced in CeO<sup>2</sup> by SHI irradiation. Such measurements are performed either ex situ (after ion irradiation) or in situ (directly at accelerator beamlines with various dedicated characterization infrastructures). Analytical methods used in prior work include various forms of scattering, spectroscopy, calorimetry, and electron microscopy. Some of these techniques provide direct insight into the damage structure within individual ion tracks (e.g., electron microscopy), while others are based on net damage accumulation and fluence-dependent measurements (e.g., X-ray diffraction). Characterization is commonly performed on materials after irradiation at ambient conditions to study damage formation, yet select works on irradiation at high temperatures and post-irradiation thermal annealing provide insight into damage recovery and defect dynamics. The following section gives a brief overview of the most prevalent characterization techniques used to investigate SHI irradiation effects in CeO<sup>2</sup> and other fluorite-structured oxide materials.

fluorite-structured oxide materials. Scattering techniques provide information on the short-and long-range structural modifications to materials following SHI irradiation. Highly penetrating X-ray and neutron probes are commonly used for this purpose. X-ray scattering, which is sensitive to changes in the cation sublattice, is performed with either laboratory-based diffractometers or at large synchrotron facilities. Neutron probes are useful for investigating the structure of light (low-*Z*) atomic constituents in a material (e.g., O in CeO2) because, unlike X-rays, Scattering techniques provide information on the short-and long-range structural modifications to materials following SHI irradiation. Highly penetrating X-ray and neutron probes are commonly used for this purpose. X-ray scattering, which is sensitive to changes in the cation sublattice, is performed with either laboratory-based diffractometers or at large synchrotron facilities. Neutron probes are useful for investigating the structure of light (low-*Z*) atomic constituents in a material (e.g., O in CeO2) because, unlike X-rays, neutrons scatter efficiently on atoms of low atomic mass (*Z* of the atomic constituent).

neutrons scatter efficiently on atoms of low atomic mass (*Z* of the atomic constituent). Two scattering techniques have been most extensively used in the study of irradiated CeO2: diffraction and total scattering. Diffraction experiments allow for quantification of the long-range volumetric changes (unit cell swelling) caused by the production of defects [14,36,38,39,41,45–47,52,54,56,59,60], as well as heterogeneous microstrain and phase transformations, should they occur. Total scattering experiments are used to study the local defect structure using real-space analysis. Recently, intense spallation neutrons have become available for materials research at dedicated facilities, such as the Nanoscale Ordered Materials Diffractometer (NOMAD) at the Spallation Neutron Source, Oak Ridge National Laboratory (ORNL). High-resolution pair distribution function (PDF) analysis is utilized at NOMAD [61] to investigate short-range structural changes associated with irradiation-induced defects, which are inaccessible to conventional long-range diffraction methods [50,53].

Spectroscopic techniques provide insight into the local damage structure and changes to the chemistry of irradiated materials. X-ray absorption spectroscopy (XAS) has been used to probe cation oxidation state changes in irradiated CeO<sup>2</sup> and associated modifications to the local bonding environment [35,46,54,60]. X-ray photoelectron spectroscopy (XPS) provides insight into the electronic structure of cations by X-ray induced electron excitation and the consequent emission of characteristic photons during de-excitation [34,35,51,58]. Both XAS and XPS are valuable characterization tools for CeO<sup>2</sup> due to its tendency to chemically reduce under highly ionizing irradiation. Raman spectroscopy reveals the impact of defects on correlated atomic vibrations [38,49,55]. Structural modifications in fluorite-structured materials lead to a breakdown of selection rules and the appearance of so-called Raman forbidden modes in the spectra. Quantitative analysis of these forbidden modes provides information on defect concentrations. Raman spectroscopy further reveals the formation of Ce–*V<sup>O</sup>* defect complexes in irradiated CeO<sup>2</sup> [62]; thus, this technique is also useful for studying the material's redox response under SHI irradiation.

Electron microscopy (EM) is a powerful tool for the analysis of radiation effects in materials, as it provides direct imaging of the damage structure and its spatial distribution. High-resolution transmission electron microscopy (HRTEM), using state-of-the-art microscopes, has recently provided valuable insight into the atomic-scale nature of defects in irradiated CeO<sup>2</sup> [32,33,37,40,42,44,48]. This technique has provided new information on the size and damage morphology of individual SHI tracks. In modern microscopes, imaging capabilities are often coupled with other modes of operation such as electron diffraction and spectroscopy, which are utilized to determine complementary structural and electronic defect properties. Finally, heating stages in electron microscopes are useful to monitor changes in the damage structure and defect recovery as a function of increasing temperature.

Thermodynamic techniques like differential scanning calorimetry (DSC) provide information on the heat capacity of a material through comparison of its thermal behavior with that of an unirradiated reference sample under a precisely controlled high-temperature environment. With respect to irradiated samples, DSC is used to quantify the stored defect energy [53], providing insight into the nature of the initial defects and their kinetics during thermally induced recovery processes. When DSC is used in concert with complementary structural characterization techniques, the structure–energetics relationship of defects can be directly established. For example, in situ scattering measurements show how the defect structure recovers at high temperature, while DSC measurements yield the associated energetics of these processes [53].
