*5.4. Implications for CeO<sup>2</sup>*

Comparison of the SHI irradiation response of CeO<sup>2</sup> to those of closely-related fluorite and fluorite-derivative materials suggests that deviation from the ideal fluorite chemistry and crystallography tends to reduce the radiation tolerance of cerium oxides. Both Cebearing, bixbyite-structured materials (which can be considered anion-deficient fluorite derivatives) and Ce-bearing, pyrochlore-structured materials (which can be considered heavily doped, anion-deficient fluorite derivatives) are susceptible to irradiation-induced amorphization, a radiation response that has not previously been observed in pristine CeO2. Likewise, doping with relatively low levels of lanthanide cations has been shown to reduce the radiation tolerance of CeO2.

Substantial alterations in chemistry and crystallography are likely to occur in many of the harsh nuclear environments in which the use of CeO<sup>2</sup> has been proposed. For example, use of this material as a nuclear fuel matrix will necessarily entail the introduction of new cation species, including a diverse array of fission fragment elements. The accompanying exposure to highly ionizing radiation of fission fragments will induce redox changes and modification from ideal stoichiometry. As discussed in Section 5.2, substitution of Ce with Th or U greatly alters the redox response under SHI irradiation, and therefore impacts the induced structural changes. Similarly, doping with cation species that modify the energetics of redox processes [82] may yield a radiation response that is strongly dependent on the electronic structure of the dopant element.
