title: Charge-coupled substituted garnets (Y3-xCa0.5xM0.5x)Fe5O12 (M = Ce, Th): structure and stability as crystalline nuclear waste forms.
abstract: The garnet structure has been proposed as a potential crystalline nuclear waste form for accommodation of actinide elements, especially uranium (U). In this study, yttrium iron garnet (YIG) as a model garnet host was studied for the incorporation of U analogs, cerium (Ce) and thorium (Th), incorporated by a charge-coupled substitution with calcium (Ca) for yttrium (Y) in YIG, namely, 2Y(3+) = Ca(2+) + M(4+), where M(4+) = Ce(4+) or Th(4+). Single-phase garnets Y3-xCa0.5xM0.5xFe5O12 (x = 0.1-0.7) were synthesized by the citrate-nitrate combustion method. Ce was confirmed to be tetravalent by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. X-ray diffraction and (57)Fe-Mössbauer spectroscopy indicated that M(4+) and Ca(2+) cations are restricted to the c site, and the local environments of both the tetrahedral and the octahedral Fe(3+) are systematically affected by the extent of substitution. The charge-coupled substitution has advantages in incorporating Ce/Th and in stabilizing the substituted phases compared to a single substitution strategy. Enthalpies of formation of garnets were obtained by high temperature oxide melt solution calorimetry, and the enthalpies of substitution of Ce and Th were determined. The thermodynamic analysis demonstrates that the substituted garnets are entropically rather than energetically stabilized. This suggests that such garnets may form and persist in repositories at high temperature but might decompose near room temperature.
authors: ‡Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States.
authors: Xiaofeng
authors: X
authors: Guo
authors: ∥Environmental Molecular Sciences Laboratory, Pacific Northwest Laboratory, Richland, Washington 99354, United States.
authors: Ravi K
authors: RK
authors: Kukkadapu
authors: Antonio
authors: A
authors: Lanzirotti
authors: Matthew
authors: M
authors: Newville
authors: ∥Environmental Molecular Sciences Laboratory, Pacific Northwest Laboratory, Richland, Washington 99354, United States.
authors: Mark H
authors: MH
authors: Engelhard
authors: Stephen R
authors: SR
authors: Sutton
authors: Alexandra
authors: A
authors: Navrotsky