*4.4. Combined Pressure and Ion Irradiation*

Pressure is another parameter which can be adjusted during SHI irradiation. While irradiation is typically carried out in vacuum conditions, a limited number of investigations have focused on the combined effects of ion irradiation and high pressure. The use of SHIs, as opposed to ions of lower energies, is essential in such efforts, as energies on the order of 200 MeV/u are required to penetrate the mm-thick anvils of a conventional highpressure cell. The synergistic effects of pressure and ion irradiation often yield material modifications that cannot be obtained otherwise [73].

The high-pressure response of CeO2, in the absence of irradiation, is characterized by a sluggish phase transformation above ~30 GPa to the PbCl2-type cotunnite phase [74,75]. This transformation is typical of fluorite-structured materials, and in ceria it reaches completion at ~50 GPa. When irradiation is conducted at high pressure, this transformation is modified. Figure 8 illustrates the effects of separate and coupled pressure and irradiation, revealing that pressure significantly modifies the response of CeO<sup>2</sup> to SHI irradiation and vice versa. *Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 14 of 24

**Figure 8.** X-ray diffraction patterns of CeO2 exposed to 4 × 1012 946 MeV 197Au ions/cm2 at ambient conditions (blue), pressurized to 21.7 GPa in a diamond anvil cell (DAC) in the absence of irradiation (red), and exposed to a combination of pressure and irradiation, utilizing compression in a DAC to 21.8 GPa and irradiation with 7100 MeV 238U ions to a fluence of 4 × 1012 ions/cm2 (violet). **Figure 8.** X-ray diffraction patterns of CeO<sup>2</sup> exposed to 4 <sup>×</sup> <sup>10</sup><sup>12</sup> 946 MeV <sup>197</sup>Au ions/cm<sup>2</sup> at ambient conditions (blue), pressurized to 21.7 GPa in a diamond anvil cell (DAC) in the absence of irradiation (red), and exposed to a combination of pressure and irradiation, utilizing compression in a DAC to 21.8 GPa and irradiation with 7100 MeV <sup>238</sup>U ions to a fluence of 4 <sup>×</sup> <sup>10</sup><sup>12</sup> ions/cm<sup>2</sup> (violet).

Irradiation with 7100 MeV 238U ions to a fluence of 4 × 1012 ions/cm2 at a pressure of only 21.8 GPa (roughly 10 GPa below the typical transformation onset pressure) triggers a complete fluorite-to-cotunnite transformation. The highly localized and dense electronic excitations produced by SHIs provide a means of overcoming the energy barrier for cotunnite high-pressure phase formation. According to ab initio calculations, hyper- and Irradiation with 7100 MeV <sup>238</sup>U ions to a fluence of 4 <sup>×</sup> <sup>10</sup><sup>12</sup> ions/cm<sup>2</sup> at a pressure of only 21.8 GPa (roughly 10 GPa below the typical transformation onset pressure) triggers a complete fluorite-to-cotunnite transformation. The highly localized and dense electronic excitations produced by SHIs provide a means of overcoming the energy barrier for cotunnite high-pressure phase formation. According to ab initio calculations, hyper-

track cores might therefore explain the transformation to the cotunnite phase at a lowerthan-expected pressure. Moreover, the formation of the cotunnite phase is proposed to proceed through buckling of the (111) cation planes into adjacent anion planes in the fluorite structure, effectively increasing cation coordination [76]. Expelling anions from the track core during SHI irradiation at high pressure should reduce the resistance of cation-plane buckling and enhance the efficiency of cotunnite formation. An interplay of thermal and dynamic pressure effects during the highly transient ion irradiation process could also play a role in the observed transformation behavior. This was supported by compression experiments on pre-irradiated ceria samples that did not find any evidence of an accelerated fluorite-to-cotunnite transformation. More systematic research is needed to fully understand the effects of coupled extremes of irradiation, pressure, and tempera-

This review has so far focused on SHI irradiation effects in pure CeO2 having ideal or near-ideal stoichiometry. However, the study of related materials that deviate from this ideal composition can also provide valuable insight into its radiation response. First, swift heavy ion irradiation has been shown to induce local nonstoichiometry in CeO2, such that later ion impacts will interact not with ideal CeO2, but rather with a nonstoichiometric phase. Second, since CeO2 is used as a surrogate for nuclear fuel materials, doping with different atomic species (mimicking the accumulation of fission products) is an important aspect to consider in SHI irradiations. Finally, because the redox chemistry of Ce appears to play a key role in the radiation response of CeO2, a comparative study of structurallyrelated materials featuring cations with distinct redox behavior can help to isolate the ef-

fects of cation chemistry on the response of this material to SHI irradiation.

ture.

*5.* **Effects of Chemical Composition**

and hypo-stoichiometry increase and decrease, respectively, the critical phase transition pressure in fluorite-structured oxides [76,77]. The hypostoichiometry observed in CeO<sup>2</sup> ion track cores might therefore explain the transformation to the cotunnite phase at a lower-than-expected pressure. Moreover, the formation of the cotunnite phase is proposed to proceed through buckling of the (111) cation planes into adjacent anion planes in the fluorite structure, effectively increasing cation coordination [76]. Expelling anions from the track core during SHI irradiation at high pressure should reduce the resistance of cation-plane buckling and enhance the efficiency of cotunnite formation. An interplay of thermal and dynamic pressure effects during the highly transient ion irradiation process could also play a role in the observed transformation behavior. This was supported by compression experiments on pre-irradiated ceria samples that did not find any evidence of an accelerated fluorite-to-cotunnite transformation. More systematic research is needed to fully understand the effects of coupled extremes of irradiation, pressure, and temperature.
