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

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

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

The effect of cluster reconfiguration and non-stoichiometry on uranium vacancy migration in UO2

During reactor operation the release of fission gases from the fuel pellet is an important safety issue as it can lead to over-pressurization and failure of the fuel cladding. Uranium vacancy migration has been identified as the limiting step in the diffusion of fission gases through bulk UO2. The uranium vacancy migration energy is, therefore, an important parameter in this phenomenon, as well as other atomic scale processes, such as recovery from radiation damage. Chemical changes under taken by the fuel during irradiation lead to deviations from stoichiometric UO2 and the charge compensating defects that bind to the uranium vacancy also change. Therefore, we have examined the change in the migration energy for a uranium vacancy when bound to either two oxygen vacancies (Schottky defect) or to four U5+ cations (hole defects) representing UO2 and UO2+x respectively. By using empirical potentials within statics we were able to sample a large array of metastable cluster configurations to identify lower energy migration pathways that involve the reconfiguration of the cluster from the ground state configuration to metastable configurations (see UO2+x results in Figure 1). The work is published in ref [1]. Please click Additional Files below to see the full abstract