Simulation of thermodynamic properties of borosilicate melts containing alkaline-earth metal oxides

The possibility for simulation of thermodynamic parameters of MO-B₂O₃-SiO₂ melts (M = Ca, Sr, Ba) in terms of the UNIQUAC associated solution theory was demonstrated using the experimental data obtained previously by high-temperature mass spectrometry (1650–1800 K). The calculated numbers of different bonds formed in the examined melts were found to correlate with the deviations of their thermodynamic parameters from ideality.     

Synthesis of first representatives of alkaline-earth metal diselenophosphinates

A three-component reaction between secondary phosphines, elemental selenium, and calcium or barium hydroxides in a molar ratio of 1: 2: 0.5 proceeds under mild conditions (70?C75 °C, 10 min, aqueous ethanol) to give previously unknown Ca and Ba diselenophosphinates (78?C89%).     

Cohesive properties of CeN and LaN from first principles

The effect of electron-correlation on the ground-state properties of CeN and LaN is studied by ab initio quantum-chemical methods. The approach which is used combines two separate steps: (1) the ground-state Hartree-Fock calculations for the crystal; (2) application of the method of increments to the studied system, which allows an expansion of bulk properties using the information from quantum-chemical calculations performed for finite clusters. As can be expected, for CeN correlation plays a significant role: with Hartree-Fock method only 49% of the experimental cohesive energy has been recovered, whereas after correlation corrections (coupled-cluster approach) the ground-state properties were found to be in good agreement with the experimental data found in literature. Thus, we obtained about 90% of the expected cohesive energy; the computed lattice constants and bulk moduli also agree well with the experimental values. For comparison, the equivalent treatment has been performed for LaN, where no f orbital is occupied. There the HF contribution to the ground-state properties is larger and hence the correlation effects weaker.     

The properties of methylene- and amine-substituted zeolites from first principles

All-silica zeolite frameworks doped with methylene and amine groups are studied using density functional theory-based electron structure calculations. Strain energies are calculated in a novel way, by comparing zeolite energies with appropriate polymer reference systems. The modified zeolites are found to be mechanically stable structures with surprisingly little strain. Distortions due to impurities result in broadened Si-O-Si angle distributions in the lattice surrounding defects. Our results suggest that zeolites can accommodate both methylene and amine groups at high concentrations with minimal strain. The amine-doped zeolites are strong Lewis bases suggesting novel applications in base catalysis.     

Electronic and vibrational properties of nickel sulfides from first principles

We report the results of first-principles calculations (generalized gradient approximation-Perdew Wang 1991) on the electronic and vibrational properties of several nickel sulfides that are observed on Ni-based anodes in solid oxide fuel cells (SOFCs) upon exposure to H2S contaminated fuels: heazlewoodite Ni3S2, millerite NiS, polydymite Ni3S4, and pyrite NiS2. The optimized lattice parameters of these sulfides are within 1% of the values determined from x-ray diffraction. The electronic structure analysis indicates that all Ni-S bonds are strongly covalent. Furthermore, it is found that the nickel d orbitals shift downward in energy, whereas the sulfur p orbitals shift upward with increasing sulfur content; this is consistent with the decrease in conductivity and catalytic activity of sulfur-contaminated Ni-based electrodes (or degradation in SOFC performance). In addition, we systematically analyze the classifications of the vibrational modes at the point from the crystal symmetry and calculate the corresponding vibrational frequencies from the optimized lattice constants. This information is vital to the identification with in situ vibrational spectroscopy of the nickel sulfides formed on Ni-based electrodes under the conditions for SOFC operation. Finally, the effect of thermal expansion on frequency calculations for the Ni3S2 system is also briefly examined.     

Structural specific features and properties of alkaline-earth and rare-earth metal borates

The review summarizes literature data on the synthesis, crystal growth conditions, structures, and physicochemical properties of alkaline-earth and rare-earth element borates. The compounds under consideration were shown to be promising for the use as materials for electronics, laser and luminescent matrices.     

Thermochemical properties of anhydrous and hydrated alkaline-earth metal and magnesium perchlorates

A low-temperature method of obtaining anhydrous magnesium, calcium, strontium, and barium perchlorates and their hydrates was developed. Hexa-Mg(ClO₄)₂, Sr· (ClO₄)₂·H₂O, and Ba(ClO₄)₂·H₂O were prepared for the first time. The enthalpies of dissolution of anhydrous magnesium and strontium perchlorates and Mg, Ca, Sr, and Ba perchlorate hydrates in water were measured; the standard enthalpies of their formation were determined. The enthalpies of dehydration of M(ClO₄)₂· nH₂O in stages were determined.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 1978–1983, September, 1989.     

Removal of heavy metal ions from aqueous solutions by alkaline-earth metal phosphates

The possibility of utilization of calcium or magnesium phosphates of various composition for heavy and non-ferrous metal extraction from aqueous solutions has been studied. The efficiency of the phosphates in removal of Pb(II), Cr(III) and Fe(III) ions has been shown to decrease in the following sequence: Mg₃(PO₄)₂>MgNH₄PO₄>Ca₃(PO₄)₂>CaHPO₄>Ca₁₀(PO₄)₆(OH)₂ which is inverse to their hydrolytic stability series. It has been established that phosphates of non-apatite structure are capable of binding up to 12 mmol g⁻¹ of the named heavy metals by a chemical interaction. Hydroxyapatite interacts with the polyvalent metal ions via either the above mentioned or ion-exchange mechanism, depending on preparation method used for the apatite and the nature of metal.     

Electronic and mechanical properties of 5d transition metal mononitrides via first principles

The electronic and mechanical properties of 5d transition metal mononitrides from LaN to AuN are systematically investigated by use of the density-functional theory. For each nitride, six structures are considered, i.e., rocksalt, zinc blende, CsCl, wurtzite, NiAs and WC structures. Among the considered structures, rocksalt structure is the most stable for LaN, HfN and AuN, WC structure for TaN, NiAs structure for WN, wurtzite structure for ReN, OsN, IrN and PtN. The most stable structure for each nitride is mechanically stable. The formation enthalpy increases from LaN to AuN. For LaN, HfN and TaN, the formation enthalpy is negative for all the considered structures, while from WN to AuN, except wurtzite structure in ReN, the formation enthalpy is positive. The calculated density of states shows that they are all metallic. ReN in NiAs structure has the largest bulk modulus, 418 GPa. The largest shear modulus 261 GPa is from TaN in WC structure. Trends are discussed.     

Elastic, electronic and thermal properties of YSZ from first principles

First principles calculations were performed to investigate the elastic, electronic and thermal properties of 14% cubic yttria-stabilized zirconia (YSZ) using the pseudo potential plane-wave method within the gradient generalized approximation (GGA) for the exchange and correlation potential. Computed lattice constant parameters are in good agreement with the available experimental results. The three independent elastic constants were computed by means of the stress-strain method, indicating that 14% cubic YSZ is a mechanically stable structure. From the knowledge of the elastic constants, a set of related properties, namely bulk, shear modulus, Young’s modulus, sound velocity, Debye temperature, thermal capacity and minimum thermal conductivity are numerically estimated in the frame work of the Voigt-Reuss-Hill approximation for YSZ polycrystalline. The calculated bulk modulus, shear modulus, Young’s modulus, sound velocity, Debye temperature, thermal capacity and minimum thermal conductivity are in reasonable agreement with the available experimental and theory data. Density of states, charge density and Mulliken population analysis show that the 14% cubic YSZ is covalent and possess ionic character.     

Optical properties of silicon clusters in the presence of water: a first principles theoretical analysis

We investigate the impact of water on the optical absorption of prototypical silicon clusters. Our clusters contain 5 silicon atoms, tetrahedrally coordinated and passivated with either hydrogen or oxygen. We approach this complex problem by assessing the contributions of three factors: chemical reactivity, thermal equilibration, and dielectric screening. We find that the silanone (Si=O) functional group is not chemically stable in the presence of water and exclude this as a source of significant red shift in absorption in aqueous environments. We perform first principles molecular dynamics simulations of the solvation of a chemically stable, oxygenated silicon cluster with explicit water molecules at 300 K. We find a systematic 0.7 eV red shift in the absorption gap of this cluster, which we attribute to thermally induced fluctuations in the molecular structure. Surprisingly, we find no observable screening impact of the solvent, in contrast with consistent blue shifts observed for similarly sized organic molecules in polar solvents. The predicted red shift is expected to be significantly smaller for larger Si quantum dots produced experimentally, guaranteeing that their vacuum optical properties are preserved even in aqueous environments.     

Beta-sheet preferences from first principles

The natural amino acids have different preferences of occurring in specific types of secondary protein structure. Simulations are performed on periodic model beta-sheets of 14 different amino acids, at the level of density functional theory, employing the generalized gradient approximation. We find that the statistically observed beta-sheet propensities correlate very well with the calculated binding energies. Analysis of the calculations shows that the beta-sheet propensities are determined by the local flexibility of the individual polypeptide strands.     

Electrochemical potentials from first principles

The electrochemical potential is the fundamental parameter in the theory of electrochemistry. Not only does it determine the position of electrochemical equilibria but also it acts as the driving force for electron transfer reactions, diffusion-migration phenomena, and phase transformations of all kinds. In the present work, the electrochemical potential is defined as the total work done in transferring a single particle of a substance from a universal reference state to a specified location, at constant temperature and pressure. It is the sum of two scalar fields: the electrostatic potential energy and the chemical potential energy. The electrochemical potential is widely underutilized within the fields of solid-state science and electrochemical engineering. For historical reasons, many authors prefer to analyze driving forces in terms of electrode potentials, concentration gradients, or Gibbs free energies. In this paper, the author provides a short introduction to the electrochemical potential and then shows how some of the major branches of electrochemistry can benefit from using it. Topics examined include the Volta potential difference, the membrane potential difference, the scanning Kelvin probe microscope, the electromotive force, the proton motive force, and the activation of electron transfer.

     

Tafel slopes from first principles

Tafel slopes for multistep electrochemical reactions are derived from first principles. The derivation takes place in two stages. First, Dirac’s perturbation theory is used to solve the Schrödinger equation. Second, current–voltage curves are obtained by integrating the single-state results over the full density of states in electrolyte solutions. Thermal equilibrium is assumed throughout. Somewhat surprisingly, it is found that the symmetry factor that appears in the Butler–Volmer equation is different from the symmetry factor that appears in electron transfer theory, and a conversion formula is given. Finally, the Tafel slopes are compiled in a convenient look-up table.     

Electronic structure and peculiar bonding properties of NdNiMg5 from first principles

The newly found ternary compound NdNiMg₅ has been studied within DFT based methodologies. Results of cohesive energy, charge transfers, elastic constants and electron localized function mapping as well as electronic structure and bonding properties have been compared with those of isostructural binary NdNi. The calculation results have shown that Mg substructures interlayering NdNi – like slabs exhibit different magnitudes of charge transfers all within range of metallic behavior and the different Mg substructures selectively bind with Nd and Ni substructures. As a consequence an enhanced cohesion with respect to binary intermetallic NdNi is identified. The whole set of elastic constants and their combinations in orthorhombic symmetry confirm the mechanical stability of NdNiMg₅ with larger compressibility and less ductility (more brittleness) with respect substructures to NdNi. While in an intermetallic compound such as NdNi the bonding is ensured mainly by Nd–Ni interaction, in NdNiMg₅ Nd–Ni, Nd–Mg, Ni–Mg as well as Mg–Mg participate to the bonding and the extra electrons brought by Mg are found within bonding states thus illustrating furthermore the enhanced cohesion of the ternary versus the binary systems.     

Stability of intermediates in the glycerol hydrogenolysis on transition metal catalysts from first principles

The hydrogenolysis reaction catalyzed by a transition metal solid catalyst is a potential way to transform glycerol to 1,2-propylene glycol or 1,3-propylene glycol, two important chemicals. We explore the thermodynamic profile of this reaction from first principle simulation, comparing Ni, Rh and Pd catalysts modeled by (111) surfaces. The stability of adsorbed reactants, dehydrated intermediates, and hydrogenated propylene glycol is compared, with a special focus on the factors controlling the selectivity of the reaction. From a global thermodynamic view point, the formation of 1,2-propylene glycol is favored, and in addition the most stable intermediates in the gas phase (acetol and 1,2-aldol) lead to the formation of this product. The metal catalyst has three roles. First it stabilizes the dehydrated intermediates and renders the dehydration more exothermic. Second, the adsorption on the surface modifies the relative stability of the dehydrated intermediates, with implications on the reaction selectivity. Third it catalyses the hydrogenation step, leading to propylene glycol.