The encapsulation selectivity for anionic fission products imparted by an electride

The nanoporous oxide 12CaO•7Al2O3 (C12A7) can capture large concentrations of extra-framework species inside its nanopores, while maintaining its thermodynamical stability. Here we use atomistic simulation to predict the efficacy of C12A7 to encapsulate volatile fission products, in its stoichiometric and much more effective electride forms. In the stoichiometric form, while Xe, Kr and Cs are not captured, Br, I and Te exhibit strong encapsulation energies while Rb is only weakly encapsulated from atoms. The high electronegativities of Br, I and Te stabilize their encapsulation as anions. The electride form of C12A7 shows a significant enhancement in the encapsulation of Br, I and Te with all three stable as anions from their atom and dimer reference states. Successive encapsulation of multiple Br, I and Te as single anions in adjacent cages is also energetically favourable. Conversely, Xe, Kr, Rb and Cs are unbound. Encapsulation of homonuclear dimers (Br2, I2 and Te2) and heteronuclear dimers (CsBr and CsI) in a single cage is also unfavourable. Thus, C12A7 offers the desirable prospect of species selectivity

Encapsulation of heavy metals by a nanoporous complex oxide 12CaO · 7Al₂ O₃

The nanoporous oxide 12CaO ⋅ 7Al2O3 (C12A7) offers the possibility of capturing large concentrations of environmentally damaging extra-framework species in its nanopores. Using density functional theory with a dispersion correction, we predict the structures and energetics of some heavy metals (Cr, Ni, Cu, Zn, Cd, Hg, and Pb) trapped by the stoichiometric and electride form of C12A7. In the stoichiometric form, while Zn, Cd, Hg, and Pb are encapsulated weakly, Cr, Ni, and Cu exhibit strong encapsulation energies. The electride form of C12A7 shows a significant enhancement in the encapsulation of Cr, Ni, Cu, and Pb. Successive encapsulation of multiple Cr, Ni, Cu, and Pb as single species in adjacent cages of C12A7 is also energetically favorable

Crystal structure, thermodynamics, magnetics and disorder properties of Be-Fe-Al intermetallics

The elastic and magnetic properties, thermodynamical stability, deviation from stoichiometry and order/disorder transformations of phases that are relevant to Be alloys were investigated using density functional theory simulations coupled with phonon density of states calculations to capture temperature effects. A novel structure and composition were identified for the Be-Fe binary {\epsilon} phase. In absence of Al, FeBe_5 is predicted to form at equilibrium above ~ 1250 K, while the {\epsilon} phase is stable only below ~ 1650 K, and FeBe_2 is stable at all temperatures below melting. Small additions of Al are found to stabilise FeBe_5 over FeBe_2 and {\epsilon}, while at high Al content, AlFeBe_4 is predicted to form. Deviations from stoichiometric compositions are also considered and found to be important in the case of FeBe_5 and {\epsilon}. The propensity for disordered vs ordered structures is also important for AlFeBe_4 (which exhibits complete Al-Fe disordered at all temperatures) and FeBe_5 (which exhibits an order-disorder transition at ~ 950 K).Comment: 14 pages, 10 figures, accepted for publication in J. Alloy Compd. on 14 March 201

Molecular dynamics study of oxygen diffusion in Pr₂NiO_4+δ

Oxygen transport in tetragonal Pr2NiO4+δ has been investigated using molecular dynamics simulations in conjunction with a set of Born model potentials. Oxygen diffusion in Pr2NiO4+δ is highly anisotropic, occurring almost entirely via an interstitialcy mechanism in the a-b plane. The calculated oxygen diffusivity has a weak dependence upon the concentration of oxygen interstitials, in agreement with experimental observations. In the temperature range 800-1500 K, the activation energy for migration varied between 0.49 and 0.64 eV depending upon the degree of hyperstoichiometry. The present results are compared to previous work on oxygen self-diffusion in related K2NiF4 structure materials

Anisotropic oxygen diffusion in tetragonal La₂NiO_4+δ: molecular dynamics calculations

Molecular dynamics simulations, used in conjunction with a set of Born model potentials, have been employed to study oxygen transport in tetragonal La2NiO4+δ. We predict an interstitialcy mechanism with an activation energy of migration of 0.51 eV in the temperature range 800-1100 K. The simulations are consistent with the most recent experiments. The prevalence of oxygen diffusion in the a-b plane accounts for the anisotropy observed in measurements of diffusivity in tetragonal La2NiO4+δ

Peroxide as a mechanism to accommodate excess oxygen

Atomic scale simulations are used to predict how excess oxygen is accommodated across three distinct groups of oxides: group II monoxides [1], fluorite di-oxides [2] and zirconate perovsites [3]. In addition to the crystallographic position and orientation of the peroxide molecule, transport of the species is also considered. For each group, three different cations are considered in order to determine how stability and structure change as a function of cation size. Peroxide molecular vibrational frequencies are also predicted to facilitate experimental investigation of the various structural models. For all simulations, the density functional code VASP was employed. Starting with the monoxides, in all cases, the preference is to form a peroxide ion centered at an oxygen site, rather than a single oxygen species, although the peroxide molecular orientation changes from \u3c100\u3e to \u3c110\u3e to \u3c111\u3e with increasing cation radius. The enthalpy for accommodation of excess oxygen in BaO is strongly negative, whereas in SrO it is only slightly negative and in CaO and MgO the energy is positive. Interestingly, the increase in material volume due to the accommodation of oxygen (the defect volume) does not vary greatly as a function of cation radius. Calculations of the BO2 dioxide structures have also been carried out. For these materials the oxygen vacancy formation energy is always positive (1.0–1.5 eV per oxygen removed) indicating that they exhibit a surprisingly small oxygen deficiency. For the di-oxides, accommodation of hyperstoichiometry is considered in CeO2, ThO2 and UO2. Calculations indicate a preference for the peroxide species over an isolated interstitial in CeO2 and ThO2 but not in UO2. Frenkel pair defects are investigated to understand if the interstitial component could assume a peroxide like configuration in the vicinity of the vacancy. While it is expected that this would not be the case for UO2 since peroxide was not stable, it is also not found to be the case for CeO2 and ThO2 with the peroxide disassociating into a lattice species and a separate interstitial ion. For the zirconate perovskites, again group II cations are the variable: BaZrO3, SrZrO3 and CaZrO3. While facilitated by peroxide, in contrast to the monoxide, the solution energy of O2 is predicted to be positive (though close to zero) for BaZrO3. Similar to the monoxides, the peroxide molecule is less favourably accommodated in SrZrO3 or CaZrO3. This trend is tested experimentally by exposing SrZrO3 and BaZrO3 to hydrogen peroxide solution and carrying out Raman spectroscopy measurements to look for a peak indicative of peroxide ions. A peak was observed at 1000 cm-1 in both compositions, suggesting the theoretically predicted peroxide ion is present. The transport of the excess oxygen through the perovskite lattice was predicted to proceed with activation energies of less than 1 eV in each of the systems. 1. Middleburgh S. C., Lagerlof K. P. D. & Grimes R. W. “Accommodation of Excess Oxygen in Group II Monoxides” J. Am. Ceram. Soc. 96, 308 (2013). 2. Middleburg S. C., Lumpkin G. R. & Grimes R. W. “Accommodation of excess oxygen in fluorite dioxides” Solid State Ionics, 253, 119 (2013). 3. Middleburgh S. C., Karatchevtseva I., Kennedy B. J., Burr P. A., Zhang Z., Reynolds E., Grimes R. W. & Lumpkin G. R. “Peroxide defect formation in zirconate perovskites” J. Mater. Chem. A, 2, 15883 (2014)

One-dimensional yttrium silicide electride (Y5Si3:e−) for encapsulation of volatile fission products

Better ways are needed to capture radioactive volatile fission products (Kr, Xe, Br, I, Te, Rb, and Cs) discharged during the reprocessing of spent nuclear fuel in order to reduce the volumes of produced waste and minimize environmental impact. Using density functional theory, we examine the efficacy of a one-dimensional yttrium silicide electride (Y5Si3:e−) as a host matrix to encapsulate these species. Endoergic encapsulation energies calculated for Kr, Xe, Rb, and Cs imply they are not captured by Y5Si3:e−. Encapsulation is exoergic for Br, I, and Te with respect to their atoms and dimers as reference states, meaning that they can be captured effectively due to their high electronegativities. This is further supported by the formation of anions due to charge transfer between Y5Si3:e− and Br (I and Te). The selectivity of this material for these volatile species makes it promising for use in nuclear filters

Interstitialcy diffusion of oxygen in tetragonal La₂CoO_4+δ

We report on the mechanism and energy barrier for oxygen diffusion in tetragonal La2CoO4+δ. The first principles-based calculations in the Density Functional Theory (DFT) formalism were performed to precisely describe the dominant migration paths for the interstitial oxygen atom in La2CoO4+δ. Atomistic simulations using molecular dynamics (MD) were performed to quantify the temperature dependent collective diffusivity, and to enable a comparison of the diffusion barriers found from the force field-based simulations to those obtained from the first principles-based calculations. Both techniques consistently predict that oxygen migrates dominantly via an interstitialcy mechanism. The single interstitialcy migration path involves the removal of an apical lattice oxygen atom out from the LaO-plane and placing it into the nearest available interstitial site, whilst the original interstitial replaces the displaced apical oxygen on the LaO-plane. The facile migration of the interstitial oxygen in this path is enabled by the cooperative titling-untilting of the CoO6 octahedron. DFT calculations indicate that this process has an activation energy significantly lower than that of the direct interstitial site exchange mechanism. For 800-1000 K, the MD diffusivities are consistent with the available experimental data within one order of magnitude. The DFT- and the MD-predictions suggest that the diffusion barrier for the interstitialcy mechanism is within 0.31-0.80 eV. The identified migration path, activation energies and diffusivities, and the associated uncertainties are discussed in the context of the previous experimental and theoretical results from the related Ruddlesden-Popper structures

Nitrogen-vacancy defects in germanium

While nitrogen doping has been investigated extensively in silicon, there is only limited information on its interaction with vacancies in germanium, despite most point defect processes in germanium being vacancy controlled. Thus, spin polarized density functional theory calculations are used to examine the association of nitrogen with lattice vacancies in germanium and for comparison in silicon. The results demonstrate significant charge transfer to nitrogen from nearest neighbour Ge and strong N-Ge bond formation. The presence of vacancies results in a change in nitrogen coordination (from tetrahedral to trigonal planar) though the total charge transfer to N is maintained. A variety of different nitrogen vacancy clusters are considered all of which demonstrated strong binding energies. Substitutional nitrogen remains an effective trap for vacancies even if it has already trapped one vacancy

Encapsulation of volatile fission products in a two-dimensional dicalcium nitride electride

The efficient capture of volatile fission products released during spent fuel reprocessing is a crucial concern for the nuclear community. Here, we apply the density functional theory to examine the efficacy of a two-dimensional dicalcium nitride electride (Ca2N:ē) to encapsulate volatile fission products. Encapsulation is endoergic for Kr, Xe, Rb, and Cs meaning that they are not encapsulated. Conversely, strong encapsulation is exhibited for Br, I, and Te with respect to their atoms and dimers as reference states. The preference for Br, I, and Te encapsulation is a consequence of charge transfer from Ca2N:ē to form encapsulated anions. This makes the electride a promising material for the selective trapping of volatile Br, I, and Te