Publications

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Classical molecular dynamics (MD) simulations were performed to provide a conceptual understanding of the amorphous-crystalline interface for a candidate negative thermal expansion (NTE) material, ZrW2O8. Simulations of pressure-induced amorphization at 300 K indicate that an amorphous phase forms at pressures of 10 GPa and greater, and this phase persists when the pressure is subsequently decreased to 1 bar. However, the crystalline phase is recovered when the slightly distorted 5 GPa phase is relaxed to 1 bar. Simulations were also performed on a two-phase model consisting of the high-pressure amorphous phase in direct contact with the crystalline phase. Upon equilibration at 300 K and 1 bar, the crystalline phase remains unchanged beyond a thin layer of disrupted structure at the crystalline-amorphous interface. Differences in local atomic structure at the interface are quantified from the simulation trajectories.

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Here we report molecular level details regarding the adsorption of sarin (GB) gas in a prototypical zirconium-based metal-organic framework (MOF, UiO-66). By combining predictive modeling and experimental spectroscopic techniques, we unambiguously identify several unique bindings sites within the MOF, using the P=O stretch frequency of GB as a probe. Remarkable agreement between predicted and experimental IR spectrum is demonstrated. As previously hypothesized, the undercoordinated Lewis acid metal site is the most favorable binding site. Yet multiple sites participate in the adsorption process; specifically, the Zr-chelated hydroxyl groups form hydrogen bonds with the GB molecule, and GB weakly interacts with fully coordinated metals. Importantly, this work highlights that subtle orientational effects of bound GB are observable via shifts in characteristic vibrational modes; this finding has large implications for degradation rates and opens a new route for future materials design.

This report describes the potential of a novel class of materials--a-ZrW 2 0 8 , Zr 2 WP 2 0 12 , and related compounds that contract upon amorphization as possible radionuclide waste-forms. The proposed ceramic waste-forms would consist of zoned grains, or sintered ceramics with center- loaded radionuclides and barren shells. Radiation-induced amorphization would result in core shrinkage but would not fracture the shells or overgrowths, maintaining isolation of the radionuclide. In this report, we have described synthesis techniques to produce phase-pure forms of the materials, and how to fully densify those materials. Structural models for the materials were developed and validated using DFPT approaches, and radionuclide substitution was evaluated; U(IV), Pu(IV), Tc(IV) and Tc(VII) all readily substitute into the material structures. MD modeling indicated that strain associated with radiation-induced amorphization would not affect the integrity of surrounding crystalline materials, and these results were validated via ion beam experimental studies. Finally, we have evaluated the leach rates of the barren materials, as determined by batch and flow-through reactor experiments. ZrW 2 0 8 leaches rapidly, releasing tungstate while Zr is retained as a solid oxide or hydroxide. Tungsten release rates remain elevated over time and are highly sensitive to contact times, suggesting that this material will not be an effective waste-form. Conversely, tungsten releases rates from Zr2WP2012 rapidly drop, show little dependence on short-term changes in fluid contact time, and in over time, become tied to P release rates. The results presented here suggest that this material may be a viable waste-form for some hard-to-handle radionuclides such as Pu and Tc. ACKNOWLEDGEMENTS The authors acknowledge the contributions to this report from Sandia National Laboratories researchers Steven Meserole, Mark Rodriguez, Clay Payne, Tim Boyle, Nate Padilla, Khalid Hattar, Anthony Monterrosa, Trevor Clark, and Daniel Perry.

A molecular-scale understanding of the transition between hydration states in clay minerals remains a challenging problem because of the very fast stepwise swelling process observed from X-ray diffraction (XRD) experiments. XRD profile modeling assumes the coexistence of multiple hydration states in a clay sample to fit the experimental XRD pattern obtained under humid conditions. While XRD profile modeling provides a macroscopic understanding of the heterogeneous hydration structure of clay minerals, a microscopic model of the transition between hydration states is still missing. Here, for the first time, we use molecular dynamics simulation to investigate the transition states between a dry interlayer, one-layer hydrate, and two-layer hydrate. We find that the hydrogen bonds that form across the interlayer at the clay particle edge make an important contribution to the energy barrier to interlayer hydration, especially for initial hydration.

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The phonon, infrared, and Raman spectroscopic properties of zirconium tungsten phosphate, Zr2(WO4)(PO4)2 (space group Pbcn, IT No. 60; Z = 4), have been extensively investigated using density functional perturbation theory (DFPT) calculations with the Perdew, Burke, and Ernzerhof exchange-correlation functional revised for solids (PBEsol) and validated by experimental characterization of Zr2(WO4)(PO4)2 prepared by hydrothermal synthesis. Using DFPT-simulated infrared, Raman, and phonon density-of-state spectra combined with Fourier transform infrared and Raman measurements, new comprehensive and extensive assignments have been made for the spectra of Zr2(WO4)(PO4)2, resulting in the characterization of its 29 and 34 most intense IR- and Raman-active modes, respectively. DFPT results also reveal that ν1(PO4) symmetric stretching and ν3(PO4) antisymmetric stretching bands have been interchanged in previous Raman experimental assignments. Negative thermal expansion in Zr2(WO4)(PO4)2 appears to have very limited impact on the spectral properties of this compound. This work shows the high accuracy of the PBEsol exchange-correlation functional for studying the spectroscopic properties of crystalline materials using first-principles methods.

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Toward the development of classical force fields for the accurate modeling of clay mineral-water systems, we have extended the use of metal-O-H (M-O-H) angle bending terms to describe surface Si-O-H bending for hydrated kaolinite edge structures. Kaolinite, comprising linked octahedral Al and tetrahedral Si sheets, provides a rigorous test by combining aluminol and silanol groups with water molecules in hydrated edge structures. Periodic density functional theory and classical force fields were used with molecular dynamics to evaluate the structure, dynamics, hydrogen bonding, and power spectra for deriving optimum bending force constants and optimal equilibrium angles. Cleavage energies derived from density functional theory molecular dynamics calculations indicate the relative stabilities of both AC 1 and AC 2 edge terminations of kaolinite where Si-OH and Al-(OH 2 ) or Si-OH, Al-OH, and Al-(OH 2 ) groups exist, respectively. Although not examined in this study, the new Si-O-H angle bending parameter should allow for improved modeling of hydroxylated surfaces of silica minerals such as quartz and cristobalite, as well as amorphous silica-based surfaces and potentially those of other silicate and aluminosilicate phases.

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