*3.2. Defect Formation and Ion Track Morphology*

Irradiation induced unit cell expansion and the accumulation of heterogeneous microstrain are typically studied with XRD experiments designed to track net damage accumulation in a series of samples irradiated to increasing ion fluences. This is useful for the

determination of ion track cross sections and effective track diameters, but provides no direct insight into the morphology of individual ion tracks. Electron microscopy, on the other hand, allows for direct imaging of tracks. determination of ion track cross sections and effective track diameters, but provides no direct insight into the morphology of individual ion tracks. Electron microscopy, on the other hand, allows for direct imaging of tracks.

Irradiation induced unit cell expansion and the accumulation of heterogeneous microstrain are typically studied with XRD experiments designed to track net damage accumulation in a series of samples irradiated to increasing ion fluences. This is useful for the

in Ce3+ cations as a function of ion fluence. Still, additional research is needed to accurately monitor the structural and chemical changes over a range of irradiation conditions, ideally using coupled XRD and XAS measurements to better understand the formation and accumulation of Frenkel-type defects (linked to structural changes) and redox-type defects

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 7 of 24

(linked to chemical change).

*3.2. Defect Formation and Ion Track Morphology* 

A recent HRTEM investigation by Takaki et al. [42] provided detailed insight into the size and damage morphology of 200 MeV <sup>132</sup>Xe ion tracks in CeO2. This provides the basis for more fundamental understanding of the defect mechanisms leading to the formation of SHI tracks. A core-shell track morphology was observed, wherein the interior of the track is oxygen deficient and the annular shell oxygen rich (Figure 4a). This suggests that SHI traversal and associated energy deposition causes the radial expulsion of oxygen from the ion path region. Since cerium's electronic structure is flexible, the oxygen vacancies within a core region are stabilized by charge compensation from partial cation reduction (Ce4+ to Ce3+). These measurements also showed that oxygen anions are displaced up to 17 nm from the center of the ion track [42]. Neutron total scattering measurements of CeO<sup>2</sup> irradiated with 2000 MeV <sup>197</sup>Au ions showed that a fraction of the displaced oxygen atoms form peroxide-like defect clusters (Figure 4b) [50]. These defect clusters may act as a structural and chemical compensation mechanism to balance the oxygen interstitials with their counterpart oxygen vacancies stabilized through cation reduction. A recent HRTEM investigation by Takaki et al. [42] provided detailed insight into the size and damage morphology of 200 MeV 132Xe ion tracks in CeO2. This provides the basis for more fundamental understanding of the defect mechanisms leading to the formation of SHI tracks. A core-shell track morphology was observed, wherein the interior of the track is oxygen deficient and the annular shell oxygen rich (Figure 4a). This suggests that SHI traversal and associated energy deposition causes the radial expulsion of oxygen from the ion path region. Since cerium's electronic structure is flexible, the oxygen vacancies within a core region are stabilized by charge compensation from partial cation reduction (Ce4+ to Ce3+). These measurements also showed that oxygen anions are displaced up to 17 nm from the center of the ion track [42]. Neutron total scattering measurements of CeO2 irradiated with 2000 MeV 197Au ions showed that a fraction of the displaced oxygen atoms form peroxide-like defect clusters (Figure 4b) [51]. These defect clusters may act as a structural and chemical compensation mechanism to balance the oxygen interstitials with their counterpart oxygen vacancies stabilized through cation reduction.

**Figure 4.** (**a**) HRTEM image of a single 200 MeV 132Xe ion track in CeO2 with a core-shell damage morphology, consisting of an oxygen depleted core (red) and oxygen interstitial rich shell (blue). (**b**) Neutron pair distribution functions (PDFs) of CeO2 before and after irradiation with 2000 MeV 197Au ions up to 5 × 1012 ions/cm2. A loss of structural order is indicated by the symmetric peak broadening and the decrease in the intensity of correlation peaks. A structural feature at ~1.45 Å is observed after irradiation (inset) indicative of the formation of peroxide-like defects. Adapted from (**a**) Takaki et al. [42] and (**b**) Palomares et al. [50]. **Figure 4.** (**a**) HRTEM image of a single 200 MeV <sup>132</sup>Xe ion track in CeO<sup>2</sup> with a core-shell damage morphology, consisting of an oxygen depleted core (red) and oxygen interstitial rich shell (blue). (**b**) Neutron pair distribution functions (PDFs) of CeO<sup>2</sup> before and after irradiation with 2000 MeV <sup>197</sup>Au ions up to 5 <sup>×</sup> <sup>10</sup><sup>12</sup> ions/cm<sup>2</sup> . A loss of structural order is indicated by the symmetric peak broadening and the decrease in the intensity of correlation peaks. A structural feature at ~1.45 Å is observed after irradiation (inset) indicative of the formation of peroxide-like defects. Adapted from (**a**) Takaki et al. [42] and (**b**) Palomares et al. [50].

A combination of the results from XRD and EM measurements provides more comprehensive information on SHI tracks in CeO2, including size, morphology, and internal damage structure. The areal extent of changes in unit cell parameters and microstrain within a single ion track can be deduced from the fitting of fluence-dependent XRD data (single-impact model, Equation (3)). The comparison of effective track diameters reveals A combination of the results from XRD and EM measurements provides more comprehensive information on SHI tracks in CeO2, including size, morphology, and internal damage structure. The areal extent of changes in unit cell parameters and microstrain within a single ion track can be deduced from the fitting of fluence-dependent XRD data (single-impact model, Equation (3)). The comparison of effective track diameters reveals a systematic discrepancy between those determined from unit cell expansion (3.9–5.8 nm) and microstrain (4.6–10.5 nm) over a range of irradiation conditions (132Xe and <sup>197</sup>Au ions of 167 and 946 MeV energies) [46,52]. Relating these XRD-based results with diameters obtained by HRTEM investigation of SHI tracks (core diameter: ~4 nm and core + shell: ~17 nm) suggests that most of the swelling induced in CeO<sup>2</sup> occurs in the core region, since this matches well with the track diameter determined from analysis of unit cell expansion data. Thus, the unit cell expansion can be attributed to defects within the track core, which are predominantly oxygen vacancies and reduced Ce3+ cations. Microstrain can instead be attributed to distortions arising from all defects within the core and shell, such that the

effective diameter associated with the region of increased microstrain represents the total ion track size, including both the core and shell periphery.

Besides complex ion tracks, SHIs have been shown to also induce interesting surface damage morphologies in CeO2, as described by Ishikawa et al. [44]. For ions incident in oblique directions relative to sample surfaces, the formation of hillocks was observed. These hillocks are spherical in shape and crystalline, with an ideal fluorite structure of similar atomic spacing to that of the unirradiated matrix. The spherical hillocks are located above ion tracks and have a mean diameter of 10.6 ± 1.3 nm for irradiation with 200 MeV <sup>197</sup>Au ions. Currently, CeO<sup>2</sup> and Gd2Zr2O<sup>7</sup> are the only materials in which SHI irradiation-induced hillocks have been shown to exhibit a fully crystalline structure with no amorphous component [44,65]. This is consistent with the exceptionally high radiation tolerance of CeO2.

The spherical, droplet-like shapes of these hillocks imply the influence of surface tension in a liquid phase, such that the observation of hillocks on the surface of SHI irradiated CeO<sup>2</sup> supports the conclusion that a thermal spike and localized melting occur within ion tracks. In this scenario, all atoms are displaced from their original sites over picosecond time frames, with oxygen anions moving further away from the location of the original ion path than cerium cations. Rapid quenching restores the initial crystal structure, but some defects and defect clusters remain. The core-shell damage morphology is therefore a remnant of these highly transient processes, and the separation of oxygen anions from the track core and incomplete recovery result in the observed oxygen defect clusters and cation oxidation state reduction. This is supported by the molecular dynamics modeling of Devanathan et al. [63], who showed that the rapid increase in temperature within ion tracks in CeO<sup>2</sup> and the subsequent quenching process do not result in complete restoration of the initial atomic arrangement.
