*5.1. Effects of Doping*

In addition to trivalent cerium cations, CeO<sup>2</sup> can incorporate a number of dopant species while maintaining the fluorite structure [78]. To date, only a few studies have focused on the effects of doping on the response of CeO<sup>2</sup> to ion irradiation. These studies investigated CeO<sup>2</sup> doped with trivalent lanthanide cations, given their similarity to the lanthanide Ce. Doping with the lanthanide sesquioxides Gd2O<sup>3</sup> and Er2O<sup>3</sup> has been shown to enhance irradiation-induced swelling and structural disorder in CeO<sup>2</sup> irradiated with 200 MeV <sup>132</sup>Xe ions, compared with an undoped reference sample [60,79–81]. This was explained in terms of defect stabilization by localized strain fields around the dopant cations, but changes to the redox behavior of the cerium cations by chemical doping were not considered.

*Ab initio* modeling by Lucid et al. showed that incorporation of dopants alters the energetics of cerium cation redox processes in CeO2, with certain dopants inhibiting reduction (e.g., Sm) while others (e.g., Eu) facilitate it [82]. Since redox behavior plays a crucial role in SHI induced material modifications (particularly in nanocrystalline ceria), doping with certain lanthanide sesquioxides may provide a means of tuning ion-matter interactions and radiation stability. This should be addressed in future studies by comparing, for example, SHI induced unit cell changes in CeO<sup>2</sup> containing a range of dopant species (e.g., varying dopant sizes, masses, charge states, and redox energetics).
