Simulation study of copper(I) and copper(II) species in ZSM-5 zeolite

Low energy configurations of CuI and CuII species in the ZSM-5 zeolite, probed by energy minimisation techniques, are found to be bound strongly to framework aluminium or copper species

Defect structures and migration mechanisms in oxide pyrochlores

Using computer simulation techniques the defect structure and oxygen ion migration mechanism of oxide pyrochlores (eg. Gd2Zr2O7) was investigated in order to explain the decreased activation enthalpy for oxygen ion conductivity as a function of order. Shell model potentials were found to be necessary in order to obtain sufficiently accurate physical properties for the pyrochlore compound. The oxygen Frenkel defect consisting of ‘a split 48f vacancy’ and 8b interstitial appeared to be the most stable instrinsic defect, but vacancies related to extended defect structures may play an important role in the diffusion mechanism too. The migration mechanism of oxygen ions is mainly based on 48f-48f jumps and involve 0.9 eV barrier energy, comparable with the experimental activation enthalpies of 70–85 kJ/mol

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

The conformation of apamin

AbstractEnergy minimisation techniques are used as a tool to distinguish between different proposed models for the structure of the bee venom polypeptide apamin. The influence of electrostatic interactions on the resultant energies is noted. The model of Hider and Ragnarsson [(1980) FEBS Lett. 111, 189-193] is found to be of consistently low energy

Intrinsic point defects and the n- and p-type dopability of the narrow gap semiconductors GaSb and InSb

The presence of defects in the narrow gap semiconductors GaSb and InSb affects their dopability and hence applicability for a range of optoelectronic applications. Here, we report hybrid density functional theory (DFT)-based calculations of the properties of intrinsic point defects in the two systems, including spin-orbit coupling effects, which influence strongly their band structures. With the hybrid DFT approach adopted, we obtain excellent agreement between our calculated band dispersions and structural, elastic, and vibrational properties and available measurements. We compute point defect formation energies in both systems, finding that antisite disorder tends to dominate, apart from in GaSb under certain conditions, where cation vacancies can form in significant concentrations. Calculated self-consistent Fermi energies and equilibrium carrier and defect concentrations confirm the intrinsic n- and p-type behavior of both materials under anion-rich and anion-poor conditions. Moreover, by computing the compensating defect concentrations due to the presence of ionized donors and acceptors, we explain the observed dopability of GaSb and InS

Quantum Mechanical/Molecular Mechanical (QM/MM) Approaches

Computational modeling techniques are now standard tools in solid‐state science. They are used routinely to model and predict structures, to investigate defect, transport, and spectroscopic properties of solids, to simulate sorption and diffusion, to develop models for nucleation and growth of solids, and increasingly to model and predict reaction mechanisms. They are applied to bulk solids, surfaces, and nanostructures, and successful applications are reported for all major classes of solid: metals, semiconductors, inorganic and ceramic materials, and molecular crystals. Modeling methods are now indeed tools that are used to guide, interpret, and predict experiment

Electron Counting in Solids: Oxidation States, Partial Charges, and Ionicity

The oxidation state of an element is a practically useful concept in chemistry. IUPAC defines it as “the charge an atom might be imagined to have when electrons are counted according to an agreed-upon set of rules”.(1) Once the composition of a compound is known, a trained chemist will immediately infer the oxidation states of its components, and in turn anticipate the structural, electronic, optical and magnetic properties of the material. This is a powerful heuristic tool

Oxidation states and ionicity

The concepts of oxidation state and atomic charge are entangled in modern materials science. We distinguish between these quantities and consider their fundamental limitations and utility for understanding material properties. We discuss the nature of bonding between atoms and the techniques that have been developed for partitioning electron density. While formal oxidation states help us count electrons (in ions, bonds, lone pairs), variously defined atomic charges are usefully employed in the description of physical processes including dielectric response and electronic spectroscopies. Such partial charges are introduced as quantitative measures in simple mechanistic models of a more complex reality, and therefore may not be comparable or transferable. In contrast, oxidation states are defined to be universal, with deviations constituting exciting challenges as evidenced in mixed-valence compounds, electrides and highly correlated systems. This Perspective covers how these concepts have evolved in recent years, our current understanding and their significance