Structure and Stability of Charge-Coupled Lanthanide-Substituted Ca₁₀(PO₄)₆F₂ as a Potential Fluoride Bearing Nuclear...

The structure and stability of charge-coupled lanthanide-substituted Ca₁₀(PO₄)₆F₂ as a potential fluoride-bearing nuclear waste form for the back-end fuel cycle of Gen-IV molten salt reactor have been studied in detail. Here, calcium fluorapatite (CaFAp) as a model structure was taken for incorporation of trivalent lanthanides (Lns, La-Lu except Pm) in a charge-coupled fashion, i.e., 2Ca²⁺ = Na⁺ + Ln³⁺. In these fluorapatite phases, Na⁺ is substituted exclusively at nine coordinated sites, Ca1, while Ln³⁺ is preferentially substituted at seven coordinated sites, Ca2. These compositions are further characterized for the local structure by Fourier transform infrared (FTIR) and Raman spectroscopy. Thermal expansion was measured by high-temperature X-ray diffraction (XRD) and the instantaneous thermal expansion coefficient correlates well with the unsubstituted CaFAp. The heat capacities of these solids were measured by differential scanning calorimetry and drop calorimetry, whereas enthalpies of formation were obtained by high-temperature oxide melt solution calorimetry. The thermodynamic analysis demonstrated that lanthanides having ionic radii closure to Ca²⁺ (Sm³⁺ and Gd³⁺) imparted higher thermodynamic stability to the substituted CaFAp as compared to that of other Ln³⁺. According to structural and thermodynamic investigations, entropy-stabilized fluorapatite waste from NaPr_0.125Nd_0.125Sm_0.125Eu_0.125Gd_0.125Tb_0.125Dy_0.125Ho_0.125Ca₈(PO₄)₆F₂ (WF-Ln) was successfully synthesized for the first time. Furthermore, electron beam irradiation studies probed by XRD, FTIR, Raman, and X-ray absorption (XAS) spectroscopy implied the radiation resistance nature of this substituted CaFAps up to 20 MGy.