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

The influence of oxygen vacancy and Ce3+ ion positions on the properties of small gold clusters supported on CeO2-x(111)

We studied the influence of oxygen vacancies on small Au clusters supported on CeO2 using dispersion-corrected density functional theory (DFT-D). Our results show that the effect of oxygen vacancies on Au clusters is highly dependent on the cluster size and the relative position of the cluster to the vacancy. We found that the Au particles are only affected by the vacancies if they are located directly within the cluster perimeter. Using Crystal Orbital Hamilton Population (COHP) analysis, we show that the oxygen vacancy can lead to the formation of Au–Ce bonds under destabilisation of the bonds to the Au atom at the vacancy site and subsequent distortion of the cluster structure. However, we found that such Au–Ce bond formation only occurs when the interactions between the Au atom at the vacancy site and the surrounding Au atoms are not critical for the overall cluster stability as, for example, in the case of the central atom in a planar Au7 cluster. The formation of an oxygen vacancy can change the charge of the supported gold cluster from positive (on stoichiometric CeO2) to neutral or negative on defective CeO2−x. Interestingly, the additional electron density is located only at the Au atom at the vacancy site and is not redistributed throughout the cluster. Investigation of the electrostatic potential of the cluster surface did not show any significant changes compared to the stoichiometric surface, other than those caused by structural changes of the Au cluster

Initiatives to increase colonoscopy capacity - is there an impact on polyp detection? A UK National Endoscopy Database analysis

\ua9 2023 Georg Thieme Verlag. All rights reserved.Background Mismatch between routine endoscopy capacity and demand means centres often implement initiatives to increase capacity, such as weekend working or using locums/agency staff (insourcing). There are concerns about whether increasing workload to meet demand could negatively impact quality. We investigated polyp detection, a key quality metric, in weekend vs weekday and insourced vs standard procedures using data from the UK National Endoscopy Database (NED). Methods We conducted a national retrospective cross-sectional study of diagnostic colonoscopies undertaken 01/01-04/04/2019. The primary outcome was mean number of polyps (MNP) and the secondary, polyp detection rate (PDR). Multi-level mixed-effect regression, fitting endoscopist as a random effect, was used to examine associations between procedure day (weekend/weekday) and type (insourced/standard) and these outcomes, adjusting for patient age, sex and indication. Results 92,879 colonoscopies (weekends: 19,977 (21.5%); insourced: 9,909 (10.7%)) were performed by 2,496 endoscopists. For weekend colonoscopies, patients were more often female and less often having screening-related procedures; for insourced colonoscopies, patients were younger and less often attending for screening-related procedures (all p<0.05). Case-mix adjusted MNP was significantly lower for weekend vs weekday (IRR=0.86, (95%CI 0.83-0.89)) and for insourced vs standard procedures (IRR=0.91, (95%CI 0.87-0.95)). MNP was highest for weekday standard procedures and lowest for weekend insourced procedures, but there was no interaction between procedure day and type. Similar associations were found for PDR. Conclusions Strategies to increase colonoscopy capacity may have adverse effects on polyp detection. Routine quality monitoring should be undertaken following such initiatives. Meantime, reasons for this unwarranted variation require investigation

Density Functional Theory Study of the Partial Oxidation of Methane to Methanol on Au and Pd Surfaces

The partial oxidation of methane to methanol has been a goal of heterogeneous catalysis for many years. Recent experimental investigations have shown how AuPd nanoparticle catalysts can give good selectivity to methanol with only limited total oxidation of CH4 using hydrogen peroxide as an oxidant in aqueous media. Interestingly, the use of colloidal nanoparticles alone, without a support material, leads to efficient use of the oxidant and the possibility of introducing oxygen from O2(g) into the CH3O2H primary product. This observation indicates that a radical mechanism is being initiated by H2O2 but then the oxygen addition step, catalyzed by these nanoparticles, can incorporate O2(ads). In this contribution, we use density functional theory (DFT) to study the elementary steps in the partial oxidation of methane to methanol using H2O2 as a radical initiator and molecular oxygen as an oxidant over the low index surfaces of Pd and Au. We are able to show that pure Pd nanoparticles are prone to oxidation by O2(g), whereas the competitive adsorption of water on Au surfaces limits the availability of O2(ads). Calculations with Au added to Pd or vice versa show that both effects can be alleviated by using mixed metal surfaces. This provides a rationalization of the need to use alloy nanoparticles experimentally, and the insights from these results will aid future catalyst development

A comparative analysis of the mechanisms of ammonia synthesis on various catalysts using density functional theory

In this review, we present the recent progress in ammonia synthesis research using density functional theory (DFT) calculations on various industrial catalysts, metal nitrides and nano-cluster-supported catalysts. The mechanism of ammonia synthesis on the industrial Fe catalyst is generally accepted to be a dissociative mechanism. We have recently found, using DFT techniques, that on Co₃Mo₃N (111) surfaces, an associative mechanism in the synthesis of ammonia can offer a new low-energy pathway that was previously unknown. In particular, we have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies which can activate dinitrogen, thereby promoting the associative mechanism. These fundamental studies suggest that a promising route to the discovery of low-temperature ammonia synthesis catalysts will be to identify systems that proceed via the associative mechanism, which is closer to the nitrogen-fixation mechanism occurring in nitrogenases

Bulk electronic, elastic, structural, and dielectric properties of the Weyl semimetal TaAs

We present results of electronic structure calculations of the bulk properties of the Weyl semimetal TaAs. The emergence of Weyl (massless) fermions in TaAs, due to its electronic band structure, is indicative of a new state of matter in the condensed phase that is of great interest for fundamental physics and possibly new applications. Many of the physical properties of the material, however, are unknown. We have calculated the structural parameters, dielectric function, elastic constants, phonon dispersion, electronic band structure, and Born effective charges using density functional theory within the generalized gradient approximation, including spin-orbit coupling where necessary. Our results provide essential information on the material; and our calculations agree well with the relatively small number of experimental data available. Moreover, we have determined the relative stability of the ground state body-centered tetragonal phase with respect to other common binary phases as a function of pressure at the athermal limit, predicting a transition to the CsCl cubic structure at 23.3 GPa. Finally, we have determined the band structure using an unbiased hybrid density functional that includes 25% exact exchange, in order to refine the previously determined positions in k space of the Weyl points

Screening Divalent Metals for A- and B-Site Dopants in LaFeO3

Doping LaFeO3, a mixed ionic electronic conductor, can serve to increase its ionic and electronic conductivity, as observed in La1–xSrxCo1–yFeyO3−δ (LSCF), a promising intermediate temperature solid oxide fuel cell (IT-SOFC) cathode material. In this study, ab initio methods have been employed to assess the viability of a range of divalent A- and B-site dopants for promoting ionic and electronic conductivity, through calculating solution energies and binding energies to charge compensating species. For the A-site, we find that all alkali earth metals considered promote increased conductivity properties, but strontium and calcium have the lowest solution energies and therefore will be suitable dopants, in full agreement with experiment. Surprisingly, we find manganese, which has typically been assumed to dope exclusively on the B-site, to have significant probability, on the basis of energetic considerations, to occupy the A-site and be equally as energetically favorable as the traditional strontium dopant under certain conditions. For the B-site, cobalt and nickel were found to be suitable dopants, promoting ionic and electronic conductivity, due to the variable oxidation state of transition metals. Magnesium also increases conductivity as a B-site dopant in contrast with the other alkali earth dopants studied, which favor the A-site. By considering two compensation mechanisms, O2– vacancy and hole compensation, we show both oxygen vacancies and holes will be promoted in the doped system, in agreement with the experimentally observed mixed ionic electronic conducting properties of doped systems, including LSCF

Defects and Oxide Ion Migration in the Solid Oxide Fuel Cell Cathode Material LaFeO3

LaFeO3, a mixed ionic electronic conductor, is a promising cathode material for intermediate temperature solid oxide fuel cells (IT-SOFC). Key to understanding the electronic and ion conducting properties is the role of defects. In this study ab initio and static lattice methods have been employed to calculate formation energies of the full range of intrinsic defects—vacancies, interstitials, and antisite defects—under oxygen rich and oxygen poor conditions, to establish which, if any, are likely to occur and the effect these will have on the properties of the material. Under oxygen rich conditions, we find that the defect chemistry favors p-type conductivity, in excellent agreement with experiment, but contrary to previous studies, we find that cation vacancies play a crucial role. In oxygen poor conditions O2– vacancies dominate, leading to n-type conductivity. Finally, static lattice methods and density functional theory were used to calculate activation energies of oxide ion migration through this material. Three pathways were investigated between the two inequivalent oxygen sites, O1 and O2; O2–O2, O1–O2, and O1–O1, with O2–O2 giving the lowest activation energy of 0.58 eV, agreeing well with experimental results and previous computational studies