*3.1. Impact on Structure and Chemistry*

DSC measurements on SHI irradiated CeO<sup>2</sup> by Shelyug et al. [53] revealed that only ~1% of the energy deposited by ions is subsequently stored in irradiation-induced defects within the structure. XRD measurements of irradiated samples display distinct changes in Bragg peak position, intensity, and width in CeO<sup>2</sup> after SHI irradiation, indicative of unit cell expansion and an increase in structural distortions around defects (microstrain) (Figure 3a) [14,36,38,39,41,45–47,52,54,56,59,60]. The original fluorite structure peaks are retained, and no diffuse scattering is apparent, indicating a very high resistance to irradiation-induced amorphization. The observed changes in the XRD patterns are consistent with the formation of point defects and defect clusters. HRTEM measurements suggest that oxygen Frenkel pairs are the primary defect produced following SHI irradiation [42], a finding supported by molecular dynamics simulations [63]. Dislocation loops have been observed in SHI irradiated CeO<sup>2</sup> above a threshold stopping power of 12 keV/nm [40]. retained, and no diffuse scattering is apparent, indicating a very high resistance to irradiation-induced amorphization. The observed changes in the XRD patterns are consistent with the formation of point defects and defect clusters. HRTEM measurements suggest that oxygen Frenkel pairs are the primary defect produced following SHI irradiation [42], a finding supported by molecular dynamics simulations [64]. Dislocation loops have been observed in SHI irradiated CeO2 above a threshold stopping power of 12 keV/nm [40].

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 6 of 24

within the structure. XRD measurements of irradiated samples display distinct changes in Bragg peak position, intensity, and width in CeO2 after SHI irradiation, indicative of unit cell expansion and an increase in structural distortions around defects (microstrain) (Figure 3a) [14,36,38,39,41,46–48,53,55,57,60,61]. The original fluorite structure peaks are

**Figure 3.** Structural and chemical changes in CeO2 after irradiation with 946 MeV 197Au ions, as adapted from Tracy et al. [47]. (**a**) Relative change in unit cell parameter (red) and heterogeneous microstrain (blue) based on X-ray diffraction measurements as a function of ion fluence and (**b**) corresponding X-ray absorption spectroscopy measurements of an unirradiated (blue) sample and after irradiation to a fluence of 5 × 1013 ions/cm2 (red). XAS and XPS measurements on SHI irradiated CeO2 reveal that structural changes are accompanied by partial reduction of nominally Ce4+ cations to Ce3+ [34,39,47,52,55,59]. **Figure 3.** Structural and chemical changes in CeO<sup>2</sup> after irradiation with 946 MeV <sup>197</sup>Au ions, as adapted from Tracy et al. [46]. (**a**) Relative change in unit cell parameter (red) and heterogeneous microstrain (blue) based on X-ray diffraction measurements as a function of ion fluence and (**b**) corresponding X-ray absorption spectroscopy measurements of an unirradiated (blue) sample and after irradiation to a fluence of 5 <sup>×</sup> <sup>10</sup><sup>13</sup> ions/cm<sup>2</sup> (red).

XAS spectra show a shift in the K-edge absorption energy of approximately −2 eV (Figure

3b), which suggests a partial reduction to the trivalent state rather than the transition of all cerium cations to the trivalent state, which corresponds to a shift of −7 eV [65]. This evidence of SHI irradiation-induced reduction is corroborated by magnetic measurements that display ferromagnetism in SHI irradiated CeO2, indicative of the magnetic moments produced by the 4f electrons in Ce3+ cations [39,57]. Iwase et al. [34] observed a saturation of this redox behavior at a Ce3+/Ce4+ ratio of ~12% at a maximum fluence of 6 × 1013 ions/cm2 using 200 MeV 132Xe ions. Coupled XRD and XAS measurements by Tracy et al. [47,60] revealed that the observed redox changes are directly linked with unit cell swelling and microstrain buildup, as the fluorite structure must distort to accommodate larger trivalent cations (1.14 Å versus 0.97 Å of initial tetravalent cations), as well as oxygen vacancies [4]. In certain cases, irradiation induced reduction occurs to a sufficient extent that the formation of a secondary phase is observed at high fluences [43,47,53]. This hypostoichiometric trigonal Ce11O20 phase consists of Ce4+ and Ce3+ cations along with ordered oxygen vacancies. While changes in unit cell parameter and microstrain of CeO2 (Figure 3a) follow a behavior that is consistent with a single impact model [31,47] (Equation (3)), it remains unclear whether or not the induced redox changes follow the same trend. XPS data from Iwase et al. [34] suggest a single impact mechanism for redox effects, based on the increase XAS and XPS measurements on SHI irradiated CeO<sup>2</sup> reveal that structural changes are accompanied by partial reduction of nominally Ce4+ cations to Ce3+ [34,39,46,51,54,58]. XAS spectra show a shift in the K-edge absorption energy of approximately −2 eV (Figure 3b), which suggests a partial reduction to the trivalent state rather than the transition of all cerium cations to the trivalent state, which corresponds to a shift of −7 eV [64]. This evidence of SHI irradiation-induced reduction is corroborated by magnetic measurements that display ferromagnetism in SHI irradiated CeO2, indicative of the magnetic moments produced by the 4f electrons in Ce3+ cations [39,56]. Iwase et al. [34] observed a saturation of this redox behavior at a Ce3+/Ce4+ ratio of ~12% at a maximum fluence of <sup>6</sup> <sup>×</sup> <sup>10</sup><sup>13</sup> ions/cm<sup>2</sup> using 200 MeV <sup>132</sup>Xe ions. Coupled XRD and XAS measurements by Tracy et al. [46,59] revealed that the observed redox changes are directly linked with unit cell swelling and microstrain buildup, as the fluorite structure must distort to accommodate larger trivalent cations (1.14 Å versus 0.97 Å of initial tetravalent cations), as well as oxygen vacancies [4]. In certain cases, irradiation induced reduction occurs to a sufficient extent that the formation of a secondary phase is observed at high fluences [43,46,52]. This hypostoichiometric trigonal Ce11O<sup>20</sup> phase consists of Ce4+ and Ce3+ cations along with ordered oxygen vacancies.

While changes in unit cell parameter and microstrain of CeO<sup>2</sup> (Figure 3a) follow a behavior that is consistent with a single impact model [31,46] (Equation (3)), it remains unclear whether or not the induced redox changes follow the same trend. XPS data from Iwase et al. [34] suggest a single impact mechanism for redox effects, based on the increase 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 (linked to chemical change).
