Abstract
Fluorite-structured oxides constitute an important class of materials for energy technologies. Despite their high level of structural symmetry and simplicity, these materials can accommodate atomic disorder without losing crystallinity, making them indispensable for uses in environments with high temperature, changing chemical compositions, or intense radiation fields. In this contribution, we present a set of simple rules that predict whether a compound may adopt a disordered fluorite structure. This approach is closely aligned with Pauling’s rules for ionic crystal structures and Goldschmidt’s rules for ionic substitution.
Highlights
Materials that are isostructural with the mineral fluorite (CaF2) have been studied for nearly 100 years and were instrumental to Pauling (1927) and Goldschmidt (1926) in development of the first sets of atomic radii
Disordered fluorite oxides exhibit a variety of useful physical properties such as high ion conduction, low thermal conductivity (Clarke and Phillpot, 2005), and excellent radiation tolerance (Sickafus et al, 2000) which permits their use as fuel cell electrolytes (Navrotsky, 2010), thermal barrier coatings (Xu et al, 2006), and nuclear fuels (Ewing et al, 2004)
Experimental data show that ternary hafnate oxides (A2Hf2O7) exhibit a disordered fluorite structure for A Dy-Lu and Y (Klee and Weitz, 1969; Stanek and Grimes, 2002; Ewing et al, 2004), while ordered pyrochlore forms for A La-Tb
Summary
Materials that are isostructural with the mineral fluorite (CaF2) have been studied for nearly 100 years and were instrumental to Pauling (1927) and Goldschmidt (1926) in development of the first sets of atomic radii. Based on the atomic disorder involved, these compounds have been referred to as disordered fluorites (Norberg et al, 2012; O’Quinn et al, 2020), anion-deficient fluorites (Sickafus et al, 2007; Tang et al, 2007), or defect fluorites (De Los Reyes et al, 2013). In YSZ, despite the hypo-stoichiometric anion sublattice, the remaining oxygen readily forms a simple cubic framework in which the two metal cations, Y3+ and Zr4+, distribute themselves randomly (Götsch et al, 2016). Another well-known example is the nuclear fuel uranium dioxide (UO2+x + fission products). Despite the prevalence and importance of this material class, there has been limited efforts to predict the structural stability field of disordered fluorite oxides
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