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

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Abstract
The structure and stability of charge-coupled lanthanide-substituted Ca-10(PO4)(6)F-2 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., 2Ca(2+) = Na+ + Ln(3+). In these fluorapatite phases, Na+ is substituted exclusively at nine coordinated sites, Ca1, while Ln(3+) 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 Ln(3+). According to structural and thermodynamic investigations, entropy-stabilized fluorapatite waste from NaPr0.125Nd0.125Sm0.125Eu0.125Gd0.125Tb0.125Dy0.125Ho0.125Ca8(PO4)(6)F-2 (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.
Funding Information
  • Bhabha Atomic Research Centre

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