Christopher R. Stanek

Mechanisms of nonstoichiometry in Y3Al5O12

Currently, Y2O3–Al2O3 phase diagrams do not show the technologically significant yttrium aluminum garnet (Y3Al5O12,YAG) phase as deviating from the stoichiometric ratio, i.e., YAG is always expressed as a line compound. In this paper, we not only report the synthesis of nonstoichiometric YAG, but also the use of atomistic simulation to predict the defect structure associated with the deviation. By comparing the experimental variation in the lattice parameter as a function of deviation from stoichiometry with the defect volume changes predicted by atomistic simulation, we predict that nonstoichiometry in YAG proceeds via cation antisite defects.

Thermodynamics of fission products in UO2 ± x

The stabilities of selected fission products-Xe, Cs, and Sr-are investigated as a function of non-stoichiometry x in UO(2 ± x). In particular, density functional theory (DFT) is used to calculate the incorporation and solution energies of these fission products at the anion and cation vacancy sites, at the divacancy, and at the bound Schottky defect. In order to reproduce the correct insulating state of UO(2), the DFT calculations are performed using spin polarization and with the Hubbard U term. In general, higher charge defects are more soluble in the fuel matrix and the solubility of fission products increases as the hyperstoichiometry increases. The solubility of fission product oxides is also explored. Cs(2)O is observed as a second stable phase and SrO is found to be soluble in the UO(2) matrix for all stoichiometries. These observations mirror experimentally observed phenomena.

Predicted pyrochlore to fluorite disorder temperature for A2Zr2O7 compositions

In a previous publication the order–disorder pyrochlore to fluorite transformation temperatures for a series of A 2 Hf 2 O 7 pyrochlores were predicted [C.R. Stanek and R.W. Grimes: Prediction of rare-earth A 2 Hf 2 O 7 pyrochlore phases. J. Am. Ceram. Soc. 2002, 85, p. 2139]. This was facilitated by establishing a relationship between these temperatures and the energy required to introduce a specific defect structure into the perfect pyrochlore lattice. Here an equivalent relationship for A 2 Zr 2 O 7 pyrochlores was generated, and from this the disorder temperatures for a number of compositions including Eu 2 Zr 2 O 7 were predicted.

Defect behavior in rare earth REAlO3 scintillators

The aluminate perovskites YAlO3 (YAP) and LuAlO3 (LuAP) have been identified as potential scintillator materials due to their high light output and short decay time. However, the performance of these materials is significantly reduced by point defects. In this paper, atomistic simulations provide insight into the types of point defects that are expected under various conditions in YAP and LuAP, as well as other REAlO3 compounds (where RE denotes a rare earth ion ranging from Lu to La or Y). For example, we predict that cation antisites are the dominate intrinsic defect for smaller REAlO3 compounds, with the concentration of Schottky-type defects increasing for compounds with larger RE ions. We also predict that cation vacancies will be present in association with the oxidation of the Ce activator. From these results, we show how defects affect different aspects of the scintillation process. Our aim is to provide information that can be used to aid in the intelligent optimization of this family of scintill...

Mechanism for transient migration of xenon in UO2

In this letter, we report recent work on atomistic modeling of diffusion migration events of the fission gas product xenon in UO2 nuclear fuel. Under nonequilibrium conditions, Xe atoms can occupy the octahedral interstitial site, in contrast to the thermodynamically most stable uranium substitutional site. A transient migration mechanism involving Xe and two oxygen atoms is identified using basin constrained molecular dynamics employing a Buckingham type interatomic potential. This mechanism is then validated using density functional theory calculations using the nudged elastic band method. An overall reduction in the migration barrier of 1.6–2.7 eV is obtained compared to vacancy-mediated diffusion on the uranium sublattice.

Anisotropic thermal conductivity in uranium dioxide

The thermal conductivity of uranium dioxide has been studied for over half a century, as uranium dioxide is the fuel used in a majority of operating nuclear reactors and thermal conductivity controls the conversion of heat produced by fission events to electricity. Because uranium dioxide is a cubic compound and thermal conductivity is a second-rank tensor, it has always been assumed to be isotropic. We report thermal conductivity measurements on oriented uranium dioxide single crystals that show anisotropy from 4 K to above 300 K. Our results indicate that phonon-spin scattering is important for understanding the general thermal conductivity behaviour, and also explains the anisotropy by coupling to the applied temperature gradient and breaking cubic symmetry.

Atomistic simulation of CeO2 surface hydroxylation: implications for glass polishing

Atomistic simulation techniques have been used to investigate the dissociative adsorption of water on the (110), (111), and (100) low index surfaces of CeO2, as well as a so-called “trench” surface configuration. Several different coverages of water have been considered to better understand how the hydroxylation process progresses. Hydroxylation energies and surface energies of CeO2 calculated via atomistic simulations are compared to similar results for other fluorite oxides. Finally, the modification of CeO2 crystallite morphology in the presence of water is predicted from the changes in surface energy and the implications of these morphological changes for glass polishing are discussed.

Computational study of the energetics of charge and cation mixing in U1-xCexO₂

The formalism of electronic density-functional theory (DFT), with Hubbard-U corrections (DFT+U), is employed in a computational study of the energetics of fluorite-structured U1-xCexO₂ mixtures. The computational approach makes use of a procedure which facilitates convergence of the calculations to multiple self-consistent DFT+U solutions for a given cation arrangement, corresponding to different charge states for the U and Ce ions in several prototypical cation arrangements. Results indicate a significant dependence of the structural and energetic properties on the nature of both charge and cation ordering. With the effective Hubbard-U parameters that reproduce well the measured oxidation-reduction energies for urania and ceria, we find that charge transfer between U⁴⁺ and Ce⁴⁺ ions, leading to the formation of U⁵⁺ and Ce³⁺, gives rise to an increase in the mixing energy in the range of 4–14 kJ/mol of the formula unit, depending on the nature of the cation ordering. The results suggest that although charge transfer between uranium and cerium ions is disfavored energetically, it is likely to be entropically stabilized at the high temperatures relevant to the processing and service of urania-based solid solutions.

The role of charge and ionic radius on fission product segregation to a model UO2 grain boundary

The segregation energies of a range of fission products in UO2 to a Σ5 symmetric tilt grain boundary have been calculated using empirical potentials and their dependency on site, charge, and ionic radius has been determined. Density functional theory calculations provide information about the detailed bonding environment around the segregates. While most of the fission products prefer to reside in sites with large free volume, there are some that form strong bonds with neighboring oxygen ions, and thus prefer sites with high oxygen coordination. This result provides insight into nuclear fuel design to enhance control of fission product retention.

Radioparagenesis: The formation of novel compounds and crystalline structures via radioactive decay

When a crystalline material is made with radioactive isotopes, the structure of that material will change as the radioisotope decays. Using density functional theory, we explore the potential structures formed from this decay, a process we term radioparagenesis. Using three systems as examples – CsCl, SrO, and Lu2O3 – we describe how in each case a here-to-fore unobserved crystalline phase of BaCl, ZrO, and Hf2O3 can be formed, resulting in novel crystalline materials. We examine how the formation of these phases depends on the parent structure and the pathways available to the system upon the decay of the radioisotope. We discuss the implications of this phenomenon for the formation of new materials.