Accommodation of the misfit strain energy in the BaO(100)/MgO(100) heteroepitaxial ceramic interface using computer simulation techniques

Static atomistic simulation techniques have been employed to investigate the accommodation of the misfit strain energy in the BaO(100)/MgO(100) interface. The materials return to their natural (bulk) lattice parameters a few planes from the interface, while maintaining expanded or contracted lattice parameters at the interface to ensure charge matching of counter ions. BaO also forms three-dimensional islands when grown on MgO(100), in accordance with molecular beam epitaxy results. This behaviour is attributed to the instability of a monatomic BaO layer on MgO compared with a BaO bilayer

Charge disproportionation and Jahn-Teller distortion in LiNiO2 and NaNiO2: A density functional theory study

Density functional theory calculations have been performed on three potential ground-state configurations of LiNiO2 and NaNiO2. These calculations show that, whereas NaNiO2 shows the expected cooperative Jahn-Teller distortion (and therefore a crystal structure with C2/m symmetry), LiNiO2 shows at least two possible crystal structures very close in energy (within 3 meV/f.u.): P21/c and P2/c. Moreover, one of them (P2/c) shows charge disproportionation of the (expected) Ni3+ cations into Ni2+ and Ni4+. We discuss the implications of this complex ground state for the interpretation of the available electron and neutron structure data, its electronic and complex magnetic behaviour

Molecular dynamics studies of the bonding properties of amorphous silicon nitride coatings on crystalline silicon

In this paper we present molecular dynamics simulations of silicon nitride, both in bulk and as an interface to crystalline silicon. We investigate, in particular, the bonding structure of the silicon nitride and analyze the simulations to search for de- fective geometries which have been identified as potential charge carrier traps when silicon nitride forms an interface with silicon semiconductors. The simulations reveal how the bonding patterns in silicon nitride are dependent upon the stoichiometry of the system. Furthermore we demonstrate how having an “interphase”, where the nitrogen content in silicon gradually reduces towards pure silicon across a boundary region, as opposed to an interface where there is an abrupt drop in nitrogen con- centration at the boundary, can result in significantly different numbers of certain important carrier tra

Structural and electronic properties of silver/silicon interfaces and implications for solar cell performance

We present the results of an experimental and atomistic modelling investigation of the Sili- con/Silver (Si/Ag) interfaces found in industrial solar cells. We use small ab initio calculations to parameterize a new interatomic potential for the Si/Ag interaction. This interatomic potential is then validated against larger ab initio calculations as well as the results of previous experimental and theoretical studies of Si/Ag systems. The interatomic potential allows us to perform a large- scale search of the conformational space of Si/Ag interfaces identified from transmission electron microscopy (TEM) studies. The most favourable geometries thus identified are then used as the input for more accurate ab initio calculations. We demonstrate that the two interfaces which we identify experimentally have significantly different geometric and electronic structures. We also demonstrate how these different structures result in significantly different Schottky barriers at the interfaces

The role of extracellular DNA in uranium precipitation and biomineralisation.

Bacterial extra polymeric substances (EPS) have been associated with the extracellular precipitation of uranium. Here we report findings on the biomineralisation of uranium, with extracellular DNA (eDNA) used as a model biomolecule representative of EPS. The complexation and precipitation of eDNA with uranium were investigated as a function of pH, ionic strength and varying concentrations of reactants. The role of phosphate moieties in the biomineralisation mechanism was studied by enzymatically releasing phosphate (ePO4) from eDNA compared to abiotic phosphate (aPO4). The eDNA-uranium precipitates and uranium minerals obtained were characterised by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FT-IR) spectroscopy, Scanning Electron Microscopy-Energy Dispersive X-Ray analysis (SEM-EDX), X-Ray Powder Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS). ATR-FT-IR showed that at pH 5, the eDNA-uranium precipitation mechanism was predominantly mediated by interactions with phosphate moieties from eDNA. At pH 2, the uranium interactions with eDNA occur mainly through phosphate. The solubility equilibrium was dependent on pH with the formation of precipitate reduced as the pH increased. The XRD data confirmed the formation of a uranium phosphate precipitate when synthesised using ePO4. XPS and SEM-EDX studies showed the incorporation of carbon and nitrogen groups from the enzymatic orthophosphate hydrolysis on the obtained precipitated. These results suggested that the removal of uranium from solution occurs via two mechanisms: complexation by eDNA molecules and precipitation of a uranium phosphate mineral of the type (UO2HPO4)·xH2O by enzymatic orthophosphate hydrolysis. This demonstrated that eDNA from bacterial EPS is a key contributor to uranium biomineralisation

The analysis of impedance spectra for core–shell microstructures : why a multiformalism approach is essential

The impedance response of a core–shell microstructure with 80% core volume fraction has been simulated using finite‐element modeling and compared to two equivalent circuits for a wide range of shell permittivity and conductivity values. Different equivalent circuits, corresponding to different variants of the well‐known brick layer model, are applicable for different combinations of material properties in the microstructure. When the shell has a similar conductivity or permittivity to the core, adding a parallel pathway increases the accuracy of the fit by ≈±10%. When both the conductivity and permittivity values of the core and shell regions are different the series circuit is a better fit. This is confirmed by multiformalism impedance analysis, which reveals features in the data that are not apparent using a single formalism. Finally, the conductivity and permittivity values for both the shell and core are extracted from the simulated spectra using all formalisms and compared to the original input values. The accuracy of the extracted values often depends on the impedance formalism used. It is concluded that impedance spectroscopy data must be analyzed using multiple formalisms when considering core–shell microstructures

Amino acid and oligopeptide effects on calcium carbonate solutions

Biological organisms display sophisticated control of nucleation and crystallization of minerals. In order to mimic living systems, deciphering the mechanisms by which organic molecules control the formation of mineral phases from solution is a key step. We have used computer simulations to investigate the effects of the amino acids arginine, aspartic acid, and glycine on species that form in solutions of calcium carbonate (CaCO3) at lower and higher levels of supersaturation. This provides net positive, negative, and neutral additives. In addition, we have prepared simulations containing hexapeptides of the amino acids to consider the effect of additive size on the solution species. We find that additives have limited impact on the formation of extended, liquid-like CaCO3 networks in supersaturated solutions. Additives control the amount of (bi)carbonate in solution, but more importantly, they are able to stabilize these networks on the time scales of the simulations. This is achieved by coordinating the networks and assembled additive clusters in solutions. The association leads to subtle changes in the coordination of CaCO3 and reduced mobility of the cations. We find that the number of solute association sites and the size and topology of the additives are more important than their net charge. Our results help to understand why polymer additives are so effective at stabilizing dense liquid CaCO3 phases

The transformation of amorphous calcium carbonate to calcite and classical nucleation theory

The interfaces of calcite with water, dehydrated and hydrated amorphous calcium carbonate are studied with molecular dynamics simulations. The interfacial energies demonstrate that the calcite interface is most stable when in contact with water or low concentration solutions rather than amorphous calcium carbonate. These values are used to test the interplay between supersaturation and the interfacial energy for calcite. They demonstrate that a dissolution-reprecipitation process should always be energetically preferred to a solid state transformation of amorphous calcium carbonate to calcite

Using metadynamics to obtain the free energy landscape for cation diffusion in functional ceramics : dopant distribution control in rare earth-doped BaTiO3

Barium titanate is the dielectric material of choice in most multilayer ceramic capacitors (MLCCs) and thus in the production of ≈3 trillion devices every year, with an estimated global market of ≈$8330 million per year. Rare earth dopants are regularly used to reduce leakage currents and improve the MLCC lifetime. Simulations are used to investigate the ability of yttrium, dysprosium, and gadolinium to reduce leakage currents by trapping mobile oxygen defects. All the rare earths investigated trap oxygen vacancies, however, dopant pairs are more effective traps than isolated dopants. The number of trapping sites increases with the ion size of the dopant, suggesting that gadolinium should be more effective than dysprosium, which contradicts experimental data. Additional simulations on diffusion of rare earths through the lattice during sintering show that dysprosium diffuses significantly faster than the other rare earths considered. As a consequence, its greater ability to reduce oxygen migration is a combination of thermodynamics (a strong ability to trap oxygen vacancies) and kinetics (sufficient distribution of the rare earth in the lattice to intercept the migrating defects)

The Water–Amorphous Calcium Carbonate Interface and Its Interactions with Amino Acids

Amorphous calcium carbonate is often the first phase to precipitate during the mineralisation of calcium carbonate, before the formation of one of the crystalline polymorphs. In vivo, this phase is believed to be essential for the manufacture of minerals displaying non-equilibrium morphologies. The precipitation of this, usually transient, phase and its subsequent transformation into one of the crystalline polymorphs can be controlled by organic molecules. Here, we present a series of Molecular Dynamics simulations that explore the amorphous calcium carbonate – water interface, the attachment of amino acids onto both hydrous and anhydrous amorphous calcium carbonate and their effect on the surface. The results show that surface ions have a different coordination number distribution from bulk ions and can diffuse up to two orders of magnitude faster than their bulk counterparts, suggesting that crystallisation is much more likely to occur in this region. All the amino acids investigated bind to the amorphous calcium carbonate surfaces. However, acidic amino acids have a clear preference for the surface of amorphous CaCO3.H2O. The favoured mode of interaction of the amino acids is through amine and/or guanidine moieties. The important ramifications of the results for our understanding of protein-mineral interactions are discussed