A New Model on Cation Distribution in Cation-Disordered Li1+xTM1-xO2 Cathodes

The search for new materials that could improve the energy density of Li-ion batteries (LIB) is one of today’s most challenging issues. Recently, cation-disordered lithium-excess metal oxides have emerged as a promising new class of cathode materials for LIB, due to their high reversible capacities and nice structural stability. However, a full structural model of the Li-transition metal (TM) sharing sublattice and the origin of short range ordering (SRO) of cation ions requires further investigation. In this work, we put forward a Monte Carlo strategy of building a cation-disordered rocksalt material supercell model. The cation ions of Li1.0Ti0.5Ni0.5O2 (LTNO) are placed at the FCC sublattice sites with the constraint of Pauling’s electroneutrality rule, instead of a random way. This constraint causes the Li-Ti and Ni-Ni clustering (the cation short range ordering). Based on this model, we discussed the relationship between the short range ordering, the local distorting, the theoretic capacity and the order-disorder strengths. A unified understanding of these factors in cation-disordered materials may enable a better design of disordered-electrode materials with high capacity and high energy density.

Influence of Oxygen Vacancy Density on the Polaronic Configuration in Rutile

Polaronic configurations that were introduced by oxygen vacancy in rutile TiO2 crystal have been studied by the DFT + U method. It is found that the building block of TiO6 will expand when extra electron is trapped in the central Ti atom as polaron. With manually adjusting the initial geometry of oxygen vacancy structure, a variety of polaronic configurations are obtained after variable-cell relaxation. By calculating different sizes of supercell model, it is found that the most stable configuration can be influenced by the density of oxygen vacancy. With increasing interaction between vacancies, the most stable polaronic configuration change from small polaronic configuration to mixed configuration.

Ab initio hybrid DFT calculations of BaTiO3 bulk and BaO-terminated (001) surface F-centers

Using a supercell model and a hybrid B3PW exchange-correlation functional, we have performed first principles calculations for the F-center in the BaTiO3 bulk and on the BaO-terminated (001) surface. We find that two Ti atoms nearest to the bulk F-center are repulsed, while nearest eight oxygen and four barium atoms relax toward the oxygen vacancy (by 1.06, 0.71 and 0.08% of the lattice constant [Formula: see text], respectively). The magnitudes of atomic displacements around the F-center located on the BaO-terminated (001) surface in most cases (except for Ti) are larger than those around the bulk F-center (0.1, 1.4 and 1.0% of [Formula: see text], respectively). Our calculated BaTiO3 bulk [Formula: see text]–[Formula: see text] bandgap of 3.55 eV is in an acceptable agreement with the respective experimental bandgap value of 3.2 eV. The pristine BaO-terminated (001) surface [Formula: see text]–[Formula: see text] bandgap (3.49 eV) is reduced with respect to the bulk bandgap value. The bulk and BaO-terminated (001) surface F-center bands in BaTiO3 matrix are located only at 0.23 eV and 0.07 eV under the conduction band (CB) bottom, indicating that the F-center is a shallow donor. The F-center in the BaTiO3 bulk contains charge of 1.103[Formula: see text], whereas slightly less charge, only 1.052[Formula: see text], are localized inside the F-center on the BaO-terminated (001) surface. Our calculations demonstrate considerable increase of the chemical bond covalency between the BaTiO3 bulk F-center and its two nearest Ti atoms equal to 0.320[Formula: see text], and even larger increase for BaO-terminated (001) surface F-center and its nearest Ti atom 0.480[Formula: see text], in comparison to the relevant Ti–O chemical bond covalency in the perfect BaTiO3 bulk 0.100[Formula: see text]. The difference between F-center formation energy in BaTiO3 bulk (10.3 eV) and on the BaO-terminated (001) surface (10.2 eV) trigger the segregation of the F-center from the bulk toward the BaO-terminated (001) surface.

Use of Wyckoff position splittings in the supercell model of crystals with point defects

The atomic sublattices occupying different Wyckoff positions in a host crystal are shown to be subdivided in the supercell model owing to the splitting of the occupied Wyckoff positions. The site symmetries of the split Wyckoff positions are in general different, which significantly increases the number of possible occupation schemes for impurities and defects in the supercell model. It is demonstrated that the use of the programs and retrieval tools of the Bilbao Crystallographic Server considerably simplifies the group-theoretical analysis of Wyckoff position splittings in the supercell model of a crystal with a point defect.Ab initiocalculations of electronic states of single defects in ZnO (Zn and O neutral vacancies) within the supercell model are performed to demonstrate the influence of Wyckoff position splittings on the results.

Ab initio study of graphene interaction with O2, O and O-

<p>A systematic <em>ab initio</em> (DFT-GGA) study of adsorption of various oxygen species on graphene has been performed in order to find out general trends and provide a good starting point to analyze the oxidation of more complex carbon materials. Particular attention was paid to finding an appropriate supercell model. According to our findings, atomic O is characterized by stable adsorption on graphene and very strong adsorption on defective graphene. On the other hand, O₂ does not adsorb on graphene and is allowed to diffuse freely to the defect, where it is expected to dissociate into two strongly adsorbed O atoms. The obtained results were compared with available theoretical data in the literature and good agreement was achieved.</p><p> </p>

Monte Carlo Simulation of Inclined Reactivity Control Rod and Boron Poisoning for the Canadian-SCWR Supercell

The Canadian-SCWR is a heavy-water moderated supercritical light-water-cooled pressure tube reactor. It is fueled with CANada deuterium uranium (CANDU)-type bundles (62 elements) containing a mixture of thorium and plutonium oxides. Because the pressure tubes are vertical, the upper region of the core is occupied by the inlet and outlet headers render it nearly impossible to insert vertical control rods in the core from the top. Insertion of solid control devices from the bottom of the core is possible, but this option was initially rejected because it was judged impractical. The option that is proposed here is to use inclined control rods that are inserted from the side of the reactor and benefit from the gravitational pull exerted on them. The objective of this paper is to evaluate the neutronic performance of the proposed inclined control rods. To achieve this goal, we first develop a three-dimensional (3D) supercell model to simulate an inclined rod located between four vertical fuel cells. Simulations are performed with the SERPENT Monte Carlo code at five axial positions in the reactor to evaluate the effect of coolant temperature and density, which varies substantially with core height, on the reactivity worth of the control rods. The effect of modifying the inclination and spatial position of the control rod inside the supercell is then analyzed. Finally, we evaluate how boron poisoning of the moderator affects their effectiveness.

Modelling pair distribution functions (PDFs) of organic compounds: describing both intra- and intermolecular correlation functions in calculated PDFs

The methods currently used to calculate atomic pair distribution functions (PDFs) from organic structural models do not distinguish between the intramolecular and intermolecular distances. Owing to the stiff bonding between atoms within a molecule, the PDF peaks arising from intramolecular atom–atom distances are much sharper than those of the intermolecular atom–atom distances. This work introduces a simple approach to calculate PDFs of molecular systems without building a supercell model by using two different isotropic displacement parameters to describe atomic motion: one parameter is used for the intramolecular, the other one for intermolecular atom–atom distances. Naphthalene, quinacridone and paracetamol were used as examples. Calculations were done with theDiffPy-CMIcomplex modelling infrastructure. The new modelling approach produced remarkably better fits to the experimental PDFs, confirming the higher accuracy of this method for organic materials.