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Title: Charge-coupled substituted garnets (Y 3–x Ca 0.5x M 0.5x )Fe₅O₁₂ (M = Ce, Th): Structure and stability as crystalline nuclear waste forms

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 formmore » and persist in repositories at high temperature but might decompose near room temperature.« less
Authors:
 [1] ;  [2] ;  [3] ;  [3] ;  [2] ;  [3] ;  [4]
  1. Univ. of California, Davis, CA (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  2. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
  3. Univ. of Chicago, Chicago, IL (United States)
  4. Univ. of California, Davis, CA (United States)
Publication Date:
OSTI Identifier:
1193657
Report Number(s):
LA-UR--15-22411
Journal ID: ISSN 0020-1669
Grant/Contract Number:
SC0001089; AC52-06NA25396
Type:
Accepted Manuscript
Journal Name:
Inorganic Chemistry
Additional Journal Information:
Journal Volume: 54; Journal Issue: 8; Journal ID: ISSN 0020-1669
Publisher:
American Chemical Society (ACS)
Research Org:
Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); USDOE National Nuclear Security Administration (NNSA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 36 MATERIALS SCIENCE