Charge-coupled substituted garnets (Y3-xCa0.5xM0.5x)Fe5O12 (M = Ce, Th): Structure and stability as crystalline nuclear waste forms

Xiaofeng Guo, Ravi K. Kukkadapu, Antonio Lanzirotti, Matthew Newville, Mark H. Engelhard, Stephen R. Sutton, Alexandra Navrotsky

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21 Scopus citations

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, 2Y3+ = Ca2+ + M4+, where M4+ = Ce4+ or Th4+. 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 57Fe-Mössbauer spectroscopy indicated that M4+ and Ca2+ cations are restricted to the c site, and the local environments of both the tetrahedral and the octahedral Fe3+ 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.

Original languageEnglish (US)
Pages (from-to)4156-4166
Number of pages11
JournalInorganic chemistry
Volume54
Issue number8
DOIs
StatePublished - Apr 20 2015
Externally publishedYes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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