[en] Here, we investigate the electronic structure, energetics of cation ordering, and effect of biaxial strain on double perovskite CsRbCaZnCl₆ using first-principles calculations based on density functional theory. The two constituents (i.e., CsCaCl₃ and RbZnCl₃) forming the double perovskite exhibit a stark contrast. While CsCaCl₃ is known to exist in a cubic perovskite structure and does not show any epitaxial strain induced phase transitions within an experimentally accessible range of compressive strains, RbZnCl₃ is thermodynamically unstable in the perovskite phase and exhibits ultra-sensitive response at small epitaxial strains if constrained in the perovskite phase. We show that combining the two compositions in a double perovskite structure not only improves overall stability but also the strain-polarization coupling of the material. Our calculations predict a ground state with P4/nmm space group for the double perovskite, where A-site cations (i.e., Cs and Rb) are layer-ordered and B-site cations (i.e., Ca and Zn) prefer a rocksalt type ordering. The electronic structure and bandgap in this system are shown to be quite sensitive to the B-site cation ordering and is minimally affected by the ordering of A-site cations. We find that at experimentally accessible compressive strains CsRbCaZnCl₆ can be phase transformed from its paraelectric ground state to an antiferroelectric state, where Zn atoms contribute predominantly to the polarization. Furthermore, both energy difference and activation barrier for a transformation between this antiferroelectric state and the corresponding ferroelectric configuration are predicted to be small. The computational approach presented here opens a new pathway towards a rational design of novel double perovskites with improved strain response and functionalities

[en] Using first-principles calculations, we systematically predict the order-disorder energetics of series of zirconate (A₂Zr₂O₇), hafnate (A₂Hf₂O₇), titanate (A₂Ti₂O₇), and stannate (A₂Sn₂O₇) pyrochlores. The disordered defect-fluorite structure is modeled using an 88-atom two-sublattice special quasirandom structure (SQS) that closely reproduces the most relevant near-neighbor intrasublattice and intersublattice pair-correlation functions of the random mixture. The order-disorder transition temperatures of these pyrochlores estimated from our SQS calculations show overall good agreement with existing experiments. We confirm previous studies suggesting that the bonding in pyrochlores is not purely ionic and thus electronic effects also play a role in determining their disordering tendencies. Our results have important consequences for numerous applications, including nuclear waste forms and fast ion conductors

[en] 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.

[en] Collision cascade simulations were performed in the Er₂O₃ sesquioxide. The resulting point defects observed at the end of the ballistic phase of the collision cascade were analysed and their evaluation over longer time examined using temperature accelerated dynamics and the kinetic Monte Carlo method. The result shows that the large mass difference between the Er and O atoms results in cascades with different structures where an initially energetic O atom can channel over long distances, depositing energy in smaller sub-regions, whereas denser cascades with vacancy-rich cores develop from Er primary knock-on atoms. The most mobile defect that can form is the isolated O vacancy but when this occurs as part of a larger defect cluster it becomes trapped. The energy barriers for all other defects to move are very high.

[en] In this Brief Report, we use density functional theory to predict the existence of a heretofore unobserved crystalline compound, BaCl, and additionally predict it to be isostructural with NaCl (rocksalt). Due to the chemistry of Ba, which strongly prefers a 2+ charge state, compounds where Ba nominally exhibits a +1 charge (e.g., BaCl) are unlikely to be synthesized via conventional solid-state approaches. However, in considering the chemical evolution of ¹³⁷Cs to ¹³⁷Ba via β^- radioactive decay in a model nuclear waste form CsCl, we find that BaCl may be indeed relevant. The mechanical stability of this surprising structure is confirmed through examination of its elastic constants and phonon-dispersion relations. We have also analyzed the chemical bonding of rocksalt BaCl and found it to exhibit a complex mixture of ionic, metallic, and covalent characters. From our results, we demonstrate that the chemical evolution of crystalline structures due to radioactive decay may be a viable synthesis route for unforeseen materials with interesting properties.

[en] Using atomic-level calculations with empirical potentials, we have found that electrostatic dipoles can be created at grain boundaries formed from nonpolar surfaces of fluorite-structured materials. In particular, the Σ5(310)/[001] symmetric tilt grain boundary reconstructs to break the symmetry in the atomic structure at the boundary, forming the dipole. This dipole results in an abrupt change in electrostatic potential across the boundary. In multilayered ceramics composed of stacks of grain boundaries, the change in electrostatic potential at the boundary results in profound electrostatic effects within the crystalline layers, the nature of which depends on the electrical boundary conditions. For open-circuit boundary conditions, layers with either high or low electrostatic potential are formed. By contrast, for short-circuit boundary conditions, electric fields can be created within each layer, the strength of which then depend on the thickness of the layers. These electrostatic effects have important consequences for the behavior of defects and dopants within these materials.

[en] We report on density functional theory calculations that have been performed to systematically investigate the hydrogen-surface interaction as a function of surface orientation. The interactions that were analyzed include stable atomic adsorption sites, molecular hydrogen dissociation and absorption energies, migration pathways and barriers on tungsten surfaces, and the saturation coverage limits on the (1 1 1) surface. Stable hydrogen adsorption sites were found for all surfaces. For the reconstructed W(1 0 0), there are two primary adsorption sites: namely, the long-bridge and short-bridge sites. The threefold hollow site (3F) was found to be the most stable for W(1 1 0), while the bond-centered site between the first and second layer was found to be most stable for the W(1 1 1) surface. No bound adsorption sites for H₂ molecules were found for the W surfaces. Hydrogen (H) migration on both the (1 0 0) and (1 1 0) surfaces is found to have preferred pathways for 1D motion, whereas the smallest migration barrier for net migration of H on the W(1 1 1) surface leads to 2D migration. Although weaker H interactions are predicted for the W(1 1 1) surface compared to the (1 0 0) or (1 1 0) surfaces, we observe higher H surface concentrations of Θ  =  4.0 at zero K, possibly due to the corrugated surface structure. These results provide insight into H adsorption, surface saturation coverage and migration mechanisms necessary to describe the evolution from the dilute limit to concentrated coverages of H. (paper)

[en] Density functional theory was used to study the effects of charge localization on the structure and mobility of the highly mobile hexa-interstitial cluster in MgO. It was found that the relative stability of the configurations changed as charge was localized, with the higher energy intermediate configuration of the neutral cluster becoming the lowest energy configuration for the doubly charged cluster. The singly charged cluster was found to have the lowest migration barrier, with a barrier of 0.18 eV. The high mobility of the singly charged hexa-interstitial cluster could have a significant effect on microstructure evolution following radiation damage, while the detailed properties will be sensitive to the level of doping in the material.

[en] Building upon work in which we examined defect production and stability in spinels, we now turn to defect kinetics. Using temperature accelerated dynamics (TAD), we characterize the kinetics of defects in three spinel oxides: magnesium aluminate MgAl₂O₄, magnesium gallate MgGa₂O₄, and magnesium indate MgIn₂O₄. These materials have varying tendencies to disorder on the cation sublattices. In order to understand chemical composition effects, we first examine defect kinetics in perfectly ordered, or normal, spinels, focusing on point defects on each sublattice. We then examine the role that cation disorder has on defect mobility. Using TAD, we find that disorder creates local environments which strongly trap point defects, effectively reducing their mobility. We explore the consequences of this trapping via kinetic Monte Carlo (KMC) simulations on the oxygen vacancy (V_O) in MgGa₂O₄, finding that V_O mobility is directly related to the degree of inversion in the system

[en] This work, with an emphasis on helium irradiation rates appropriate for fusion-plasma conditions, advances the understanding of helium evolution at grain boundaries in W, an important consideration in the understanding of W as a plasma-facing component. Using accelerated molecular dynamics, helium bubble nucleation and growth at a symmetric Σ5[100](310) tilt grain-boundary in W is studied. The simulations reveal that the growth mode associated with bubble growth at the grain-boundary leads to a suppression of the helium supply to the bubble and hence to arrested growth. Such an unconventional bubble growth mode may dominate in materials with a high density of sinks.