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Title: The role of surfaces, chemical interfaces, and disorder on plutonium incorporation in pyrochlores

Abstract

Pyrochlores, a class of complex oxides with formula A 2B 2O 7, are one of the candidates for nuclear waste encapsulation, due to the natural occurrence of actinide-bearing pyrochlore minerals and laboratory observations of high radiation tolerance. In this work, we use atomistic simulations to determine the role of surfaces, chemical interfaces, and cation disorder on the plutonium immobilization properties of pyrochlores as a function of pyrochlore chemistry. We find that both Pu 3+ and Pu 4+ segregate to the surface for the four low-index pyrochlore surfaces considered, and that the segregation energy varies with the chemistry of the compound. We also find that pyrochlore/pyrochlore bicrystals A 2B 2O 7/A 2'B 2'O 7 can be used to immobilize Pu 3+ and Pu 4+ either in the same or separate phases of the compound, depending on the chemistry of the material. Finally, we find that Pu 4+ segregates to the disordered phase of an order/disorder bicrystal, driven by the occurrence of local oxygen-rich environments. However, Pu 3+ is weakly sensitive to the oxygen environment, and therefore only slightly favors the disordered phase. This behavior suggests that, at some concentration, Pu incorporation can destabilize the pyrochlore structure. Together, these results provide newmore » insight into the ability of pyrochlore compounds to encapsulate Pu and suggest new considerations in the development of waste forms based on pyrochlores. Particularly, the phase structure of a multi-phase pyrochlore composite can be used to independently getter decay products based on their valence and size.« less

Authors:
ORCiD logo [1];  [1]; ORCiD logo [1]
  1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1346846
Report Number(s):
LA-UR-16-26968
Journal ID: ISSN 1463-9076; PPCPFQ; TRN: US1701759
Grant/Contract Number:
AC52-06NA25396
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Physical Chemistry Chemical Physics. PCCP (Print)
Additional Journal Information:
Journal Name: Physical Chemistry Chemical Physics. PCCP (Print); Journal Volume: 18; Journal Issue: 33; Journal ID: ISSN 1463-9076
Publisher:
Royal Society of Chemistry
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Perriot, Romain, Dholabhai, Pratik P., and Uberuaga, Blas P.. The role of surfaces, chemical interfaces, and disorder on plutonium incorporation in pyrochlores. United States: N. p., 2016. Web. doi:10.1039/C6CP03543D.
Perriot, Romain, Dholabhai, Pratik P., & Uberuaga, Blas P.. The role of surfaces, chemical interfaces, and disorder on plutonium incorporation in pyrochlores. United States. doi:10.1039/C6CP03543D.
Perriot, Romain, Dholabhai, Pratik P., and Uberuaga, Blas P.. 2016. "The role of surfaces, chemical interfaces, and disorder on plutonium incorporation in pyrochlores". United States. doi:10.1039/C6CP03543D. https://www.osti.gov/servlets/purl/1346846.
@article{osti_1346846,
title = {The role of surfaces, chemical interfaces, and disorder on plutonium incorporation in pyrochlores},
author = {Perriot, Romain and Dholabhai, Pratik P. and Uberuaga, Blas P.},
abstractNote = {Pyrochlores, a class of complex oxides with formula A2B2O7, are one of the candidates for nuclear waste encapsulation, due to the natural occurrence of actinide-bearing pyrochlore minerals and laboratory observations of high radiation tolerance. In this work, we use atomistic simulations to determine the role of surfaces, chemical interfaces, and cation disorder on the plutonium immobilization properties of pyrochlores as a function of pyrochlore chemistry. We find that both Pu3+ and Pu4+ segregate to the surface for the four low-index pyrochlore surfaces considered, and that the segregation energy varies with the chemistry of the compound. We also find that pyrochlore/pyrochlore bicrystals A2B2O7/A2'B2'O7 can be used to immobilize Pu3+ and Pu4+ either in the same or separate phases of the compound, depending on the chemistry of the material. Finally, we find that Pu4+ segregates to the disordered phase of an order/disorder bicrystal, driven by the occurrence of local oxygen-rich environments. However, Pu3+ is weakly sensitive to the oxygen environment, and therefore only slightly favors the disordered phase. This behavior suggests that, at some concentration, Pu incorporation can destabilize the pyrochlore structure. Together, these results provide new insight into the ability of pyrochlore compounds to encapsulate Pu and suggest new considerations in the development of waste forms based on pyrochlores. Particularly, the phase structure of a multi-phase pyrochlore composite can be used to independently getter decay products based on their valence and size.},
doi = {10.1039/C6CP03543D},
journal = {Physical Chemistry Chemical Physics. PCCP (Print)},
number = 33,
volume = 18,
place = {United States},
year = 2016,
month = 7
}

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  • The pyrochlores in the series A{sub 2}Sb{sub 2}O{sub 7} have been synthesized and characterized as exhibiting spin glass transitions at T{sub SG}=41, 4.5, and 2.6 K (for A=Mn{sup 2+}S=5/2 , Co{sup 2+}S=3/2 and Ni{sup 2+}S=1, respectively) despite the lack of chemical disorder. Since the Curie-Weiss temperature remains essentially constant for all members in the series ({theta}{approx}-40K), the frustration index for these materials increases significantly as the moment size is reduced from f=|theta|/T{sub SG}=1.1 (Mn{sup 2+}), to 9.3 (Co{sup 2+}) to 14.6 (Ni{sup 2+}). There is also a corresponding change in the spin dynamics measured by the shift in the ACmore » susceptibility signal as a function of frequency. These new materials provide an avenue to investigate the effect of quantum fluctuations on the Heisenberg pyrochlore lattice in the low spin limit, and show there is a dramatic change in the spin dynamics as the quantum regime is approached. - A comparison of the spin glass ordering temperature, T{sub SG}, and the frustration index f as a function of the spin for the pyrochlore series A{sub 2}Sb{sub 2}O{sub 7}. In the limit of low spin, the frustration index increases by an order of magnitude.« less
  • We investigate the role of static disorder on defect energetics on examples of interstitial oxygen atoms in amorphous (a)-SiO{sub 2}. We generate representative amorphous structures using molecular dynamics with empirical potentials and refine them using the periodic plane-wave density-functional method (DFT). We calculate the DFT distribution of incorporation energies for 96 peroxy-linkage (PL) configurations in a periodic model of a-SiO{sub 2}. The calculations show a big site-to-site variation of incorporation energies. We partition the oxygen atom incorporation energy into contributions from a small local cluster around the defect and from the rest of the amorphous network. The striking result ismore » that the incorporation of a defect can create as well as release the strain energy in the embedding network. The variation of the PL incorporation energy is dominated by the contribution from the surrounding amorphous network, with the distortion of the local geometry of the defect contributing only about one third of the total variation. The two contributions are statistically independent. Our results provide an analysis of the distribution of defect incorporation energies in a-SiO{sub 2} and emphasize the importance of disorder and statistical approaches, which cannot be achieved in crystalline and cluster models of amorphous structure. Additionally, since the defect energies can be so strongly dependent on the longer-range strain fields, amorphous samples prepared differently and hence having different distributions of strain may perform differently in applications.« less
  • A widely used assignment scheme for Si 2p core-level photoemission studies of silicon oxidation relies solely on the formal oxidation state of the silicon. The tacit assumption of this assignment methodology is that second-neighbor effects have no measurable effect on observed Si 2p binding energies. In this letter, new experiments are combined with literature precedents to make the case that the second-neighbor effects play an important role in determining binding energy shifts. {copyright} {ital 1998 American Institute of Physics.}
  • No abstract prepared.
  • The structure of Y{sub 2}(Zr{sub y}Ti{sub 1-y}){sub 2}O{sub 7} solid solutions progressively changes, with increasing y, from an ordered pyrochlore structure A{sub 2}B{sub 2}O{sub 7}{four_gradient}, space group Fd3m, to a deflect-fluorite structure (A,B){sub 4} (O{sub 0.875}{four_gradient}{sub 0.125}){sub 8}, space group Fm3m, at y = 0.90. The anion array consists of three independent sites O(1), O(2), and O(3), occupying positions 48f, 8a and 8b, respectively, of which 8b is unoccupied in a fully ordered pyrochlore. Rietveld powder-profile analysis of data collected with 1.5453-{angstrom} thermal neutrons was used to determine the structural state of four samples with increasing Zr content (y =more » 0.30, 0.45, 0.60, and 0.90). Refinements that employed only the pyrochlore superstructure intensity data provided weighted profile residuals that ranged 8.06 to 8.67% compared with expected values of 7.13 to 7.87% derived from counting statistics. The onset of disorder at y = 0.30 is marked by filling of the vacant 8b site with oxygen ions displaced from the nearest-neighbor anion shell - i.e., O(1) in 48f. Only for y > 0.45 does O(2) participate in the disorder. Mixing of the occupancy of cation sites A and B begins only with under occupancy of the O(2) site. The eight-coordinated A site, position 16c, is occupied solely by Y for y {le} 0.45 and is predominantly Y at y = 0.60. The substituted Zr{sup 4+} thus replaces Ti{sup 4+} in the six-coordinated B site for most of the solid-solution series. Complete mixing of all three cation species occurs abruptly over the narrow compositional range 0.60 < y {le} 0.90.« less