A COMPARISON OF DEFECT ENERGIES IN MGO USING MOTT-LITTLETON AND QUANTUM-MECHANICAL PROCEDURES

The authors compare the predictions of Mott-Littleton calculations, based on empirical interatomic potentials, with predictions based on self-consistent solutions of the Schrodinger equation for embedded clusters. Simple vacancy and substitutional defects in MgO are modelled using both the classical Mott-Littleton and quantum mechanical methods. Particular attention is paid to the size of the quantum mechanical cluster, the different ways that polarisation is taken into account and the choice of basis set. Results are presented for closed-shell systems only, namely V"Mg and Vo vacancies and for Li'Mg, Na'Mg, AlMg, Fo and Clo substitutional impurities. They find a respectable level of agreement between the quite distinct approaches. This both validates the classical calculations and indicates useful generalisations combining the two approache

Microscopic origin of the optical processes in blue sapphire

Al2O3 changes from transparent to a range of intense colours depending on the chemical impurities present. In blue sapphire, Fe and Ti are incorporated; however, the chemical process that gives rise to the colour has long been debated. Atomistic modelling identifies charge transfer from Ti(III) to Fe(III) as being responsible for the characteristic blue appearance

GEOMETRY AND CHARGE-DISTRIBUTION OF H-CENTERS IN THE FLUORITE STRUCTURE

The analysis of experimental optical and spin-resonance data for the H centre gives a consistent picture of the local geometry and one-electron wavefunctions. One of the two ions in the F2- molecular ion remains very close to the perfect lattice site the other is at a distance close to that found in other F2- centres. This analysis is confirmed by atomistic calculations using the HADES code. The results are used to give a preliminary analysis of the self-trapped exciton data

Molecular dynamics study of liquid silica under high pressure

Structural changes of liquid silica are investigated under high pressure by molecular dynamics simulation. It is well known that high-silica liquids display anomalous pressure-dependent behavior in their diffusivities. The potential model, the so-called ‘soft potential’, is used, as it is expected to simulate the structural changes of silica at high temperature well. With increasing pressure, above the glass transition temperature, the simulated silica melt shows the so-called diffusivity maximum under a pressure of 20 GPa, as already shown by the previous studies. However, it is also found that this diffusivity maximum disappears above 2800 K. The analysis of Si coordination number suggests that the competition between the increase of five-fold and that of three-fold controls the extent of the anomaly. Secondly, the analysis of ‘local oxygen packing number (LOPN)’, that had been developed to investigate geometrical features in amorphous structures, is applied. In a complementary manner to the analysis of the Si coordination number, the local structure in the silica melt shows the gradual structural transformation from a low-density to high density packing on compression. Finally, a model explaining the two types of change of diffusivity in silica melt was proposed in combination with the LOPN analysis and the structon analysis that had been developed to investigate the thermal change of local structures

Efficient and accurate approach to modeling the microstructure and defect properties of LaCoO3

Complex perovskite oxides are promising materials for cathode layers in solid oxide fuel cells. Such materials have intricate electronic, magnetic, and crystalline structures that prove challenging to model accurately. We analyze a wide range of standard density functional theory approaches to modeling a highly promising system, the perovskite LaCoO3, focusing on optimizing the Hubbard U parameter to treat the self-interaction of the B-site cation's d states, in order to determine the most appropriate method to study defect formation and the effect of spin on local structure. By calculating structural and electronic properties for different magnetic states we determine that U=4 eV for Co in LaCoO3 agrees best with available experiments. We demonstrate that the generalized gradient approximation (PBEsol+U) is most appropriate for studying structure versus spin state, while the local density approximation (LDA+U) is most appropriate for determining accurate energetics for defect properties

Nonstoichiometry and Weyl fermionic behavior in TaAs

The band structure of TaAs provides the necessary conditions for the emergence of Weyl fermions. Measurements verifying this fact are remarkably robust, given the reported levels of nonstoichiometry in typical single crystals. Here we demonstrate the surprising fact that a small degree of nonstoichiometry is essential for such observations in a wide range of temperatures. From first principles, we compute how crystal defects influence the position of the Fermi level relative to the so-called Weyl points, a key factor in allowing the detection of these particles. We show that observations of Weyl fermions depend crucially on nonstoichiometry and only occur within narrow ranges of elemental composition and temperature, indicating a considerable degree of fortuity in their discovery. Our approach suggests that in some cases the drive to produce ultra-pure crystals for measurements of exotic emergent phenomena may be misplaced

DFT-D3 study of H-2 and N-2 chemisorption over cobalt promoted Ta3N5-(100),(010) and (001) surfaces

The reactants for ammonia synthesis have been studied, employing density functional theory (DFT), with respect to their adsorption on tantalum nitride surfaces. The adsorption of nitrogen was found to be mostly molecular and non-activated with side-on, end-on and tilt configurations. At bridging nitrogen sites (Ta–N–Ta) it results in an azide functional group formation with a formation energy of 205 kJ mol−1. H2 was found also to chemisorb molecularly with an adsorption energy in the range −81 to −91 kJ mol−1. At bridging nitrogen sites it adsorbs dissociatively forming >NH groups with an exothermic formation energy of −175 kJ mol−1 per H2. The nitrogen vacancy formation energies were relatively high compared to other metal nitrides found to be 2.89 eV, 2.32 eV and 1.95 eV for plain, surface co-adsorbed cobalt and sub-surface co-adsorbed cobalt Ta3N5-(010). Co-adsorption of cobalt was found to occur mostly at nitrogen rich sites of the surface with an adsorption energy that ranged between −200 to −400 kJ mol−1. The co-adsorption of cobalt was found to enhance the dissociation of molecular hydrogen on the surface of Ta3N5. The studies offer significant new insight with respect to the chemistry of N2 and H2 with tantalum nitride surfaces in the presence of cobalt promoters

The potential of manganese nitride based materials as nitrogen transfer reagents for nitrogen chemical looping

A systematic study was carried out to investigate the potential of manganese nitride related materials for ammonia production. A-Mn-N (A = Fe, Co, K, Li) materials were synthesised by nitriding their oxide counterparts at low temperature using NaNH2 as a source of reactive nitrogen. The reactivity of lattice nitrogen was assessed using ammonia synthesis as a model reaction. In the case of Mn3N2, limited reactivity was observed and only 3.1% of the available lattice nitrogen was found to be reactive towards hydrogen to yield ammonia while most of the lattice nitrogen was lost as N2. However, the presence of a co-metal played a key role in shaping the nitrogen transfer properties of manganese nitride and impacted strongly upon its reactivity. In particular, doping manganese nitride with low levels of lithium resulted in enhanced reactivity at low temperature. In the case of the Li-Mn-N system, the fraction of ammonia formed at 400 °C corresponded to the reaction of 15% of the total available lattice nitrogen towards hydrogen. Li-Mn-N presented high thermochemical stability after reduction with hydrogen which limited the regeneration step using N2 from the gas phase. However, the results presented herein demonstrate the Li-Mn-N system to be worthy of further attention