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

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)

Solubility and partitioning of impurities in Be alloys

The most energetically favourable accommodation processes for common impurities and alloying elements in Be metal and Be-Fe-Al intermetallics were investigated using atomic scale simulations. Fe additions, combined with suitable heat treatments, may scavange Al and Si through their incorporation into the FeBe₅ intermetallic. In the absence of Fe, Al and Si will not be associated with Be metal. Li and Mg are also not soluble, but may react with other impurities if present (such as Al or H). Mg may also form the MgBe₁₃ intermetallic phase under certain conditions. He and H exhibit negligible solubility in all phases investigated and whilst He will tend to form bubbles, H can precipitate as BeH₂. Similarly, C additions will form the stable compound Be₂C. Finally, oxygen exhibits a strong affinity to Be, exhibiting both some degree of solubility in all phases considered here (though especially metallic Be) and a highly favourable energy of formation for BeO

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