Structure and Stability\nof Charge-Coupled Lanthanide-Substituted\nCa<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>F<sub>2</sub> as a Potential\nFluoride Bearing Nuclear Waste Form

The structure and stability of charge-coupled lanthanide-substituted\nCa<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>F<sub>2</sub> as a potential\nfluoride-bearing nuclear waste form for the back-end fuel cycle of\nGen-IV molten salt reactor have been studied in detail. Here, calcium\nfluorapatite (CaFAp) as a model structure was taken for incorporation\nof trivalent lanthanides (Lns, La–Lu except Pm) in a charge-coupled\nfashion, i.e., 2Ca<sup>2+</sup> = Na<sup>+</sup> + Ln<sup>3+</sup>. In these fluorapatite phases, Na<sup>+</sup> is substituted exclusively\nat nine coordinated sites, Ca1, while Ln<sup>3+</sup> is preferentially\nsubstituted at seven coordinated sites, Ca2. These compositions are\nfurther characterized for the local structure by Fourier transform\ninfrared (FTIR) and Raman spectroscopy. Thermal expansion was measured\nby high-temperature X-ray diffraction (XRD) and the instantaneous\nthermal expansion coefficient correlates well with the unsubstituted\nCaFAp. The heat capacities of these solids were measured by differential\nscanning calorimetry and drop calorimetry, whereas enthalpies of formation\nwere obtained by high-temperature oxide melt solution calorimetry.\nThe thermodynamic analysis demonstrated that lanthanides having ionic\nradii closure to Ca<sup>2+</sup> (Sm<sup>3+</sup> and Gd<sup>3+</sup>) imparted higher thermodynamic stability to the substituted CaFAp\nas compared to that of other Ln<sup>3+</sup>. According to structural\nand thermodynamic investigations, entropy-stabilized fluorapatite\nwaste from NaPr<sub>0.125</sub>Nd<sub>0.125</sub>Sm<sub>0.125</sub>Eu<sub>0.125</sub>Gd<sub>0.125</sub>Tb<sub>0.125</sub>Dy<sub>0.125</sub>Ho<sub>0.125</sub>Ca<sub>8</sub>(PO<sub>4</sub>)<sub>6</sub>F<sub>2</sub> (WF-Ln) was successfully synthesized for the first time.\nFurthermore, electron beam irradiation studies probed by XRD, FTIR,\nRaman, and X-ray absorption (XAS) spectroscopy implied the radiation\nresistance nature of this substituted CaFAps up to 20 MGy.