Structural, electronic and magnetic properties of greigite Fe3S4by GGA and GGA+Uversus SCAN meta-GGA density functionals

The performance of exchange-correlation functional of density functional theory represented in generalized gradient approximation (GGA) and in the strongly constrained and appropriately normed (SCAN) meta-GGA scheme to study structural, electronic, and magnetic properties of greigite (Fe3S4) was investigated. The effects of inclusion of strong electron correlations represented by on-site Hubbard correctionU, and nonlocality of the long-range van der Waals (vdW) interactions were also considered. Geometry optimization yielded the inverse spinel structure and lattice parameter of greigite in good agreement with experimental data. Calculated electronic structure revealed a half-metallic nature of the greigite bands for the applied functionals except for GGA, which predicts metallic behavior. Antiferromagnetic coupling of iron ions in tetrahedral and octahedral coordinations makes the overall crystal structure ferrimagnetic. In general the GGA+Uand SCAN show comparable performance in prediction physical properties of greigite. Inclusion of the vdW correction does not change the character of the bands.

Superconductivity in high-entropy alloy system containing Th

Th-containing superconducting high entropy system with the nominal composition (NbTa)[Formula: see text](MoWTh)[Formula: see text] was synthesized. Its structural and physical properties were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, specific heat, resistivity and magnetic measurements. Two main phases of alloy were observed: major bcc structure and minor fcc. The experimental results were supported by numerical simulation by the DFT Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA).

Enhanced Superconducting Critical Parameters in a New High-Entropy Alloy Nb0.34Ti0.33Zr0.14Ta0.11Hf0.08

The structural and physical properties of the new titanium- and niobium-rich type-A high-entropy alloy (HEA) superconductor Nb0.34Ti0.33Zr0.14Ta0.11Hf0.08 (in at.%) were studied by X-ray powder diffraction, energy dispersive X-ray spectroscopy, magnetization, electrical resistivity, and specific heat measurements. In addition, electronic structure calculations were performed using two complementary methods: the Korringa-Kohn-Rostoker Coherent Potential Approximation (KKR-CPA) and the Projector Augmented Wave (PAW) within Density Functional Theory (DFT). The results obtained indicate that the alloy exhibits type II superconductivity with a critical temperature close to 7.5 K, an intermediate electron-phonon coupling, and an upper critical field of 12.2(1) T. This finding indicates that Nb0.34Ti0.33Zr0.14Ta0.11Hf0.08 has one of the highest upper critical fields among all known HEA superconductors.

Structure and energetics of FeO/Fe(001) interfaces

We report results of density functional theory calculations of structure and properties of 1-5 monolayer thin FeO(001) films and their interactions with the Fe(001) surface. It is found that deposition of an iron-oxide film affects weakly geometry of the Fe(001) support, causing small<2% expansion of the first interplanar distance compared to clean iron surface. Analysis of the electronic structure of the FeO/Fe system shows that after interface formation, the oxide layer remains semiconducting and the substrate metallic. Electronic structure of the FeO(001) layer in direct contact with the Fe(001) surface exhibits metallic character. Magnetism of the metal/semiconductor interface is only slightly disturbed compared to that of isolated components. The FeO adlayers preserve antiferromagnetic (AFM) nature of the oxide and the sharp boundary between higher AFM phase of FeO and lower ferromagnetic phase of Fe is observed at the interface.

Gold nanostructures on iron oxide surfaces and their interaction with CO

We review results of density functional theory calculations of the adsorption of single gold atoms and formation of sub-nanometer Aunstructures (n= 2 to 5) on most stable iron oxide surfaces: hematite (0001), and magnetite (111) and (001). Structural, energetic, and electronic properties of Aunstructures on both Fe- and O-rich oxide terminations are discussed. Different chemical character of the two oxide terminations is reflected in distinctly stronger binding of gold at the oxygen- than at the iron-terminated surface, and in different changes of the adsorption binding energy with the size of the Auncluster. On the iron-terminated oxide surface the binding energy increases whereas on the oxygen-rich termination it decreases with the number of Au atoms in the structure. Upon CO adsorption on magnetite surface all Aunstructures have a net positive charge and CO binds to the most cationic Au atom of a cluster. Interactions of Aunand CO with magnetite (111) show many similarities with those on hematite (0001) surface. The influence of the substrate relaxation effects on adsorption energy is also discussed.

Electrochemical determination of nitroaromatic explosives at boron-doped diamond/graphene nanowall electrodes: 2,4,6-trinitrotoluene and 2,4,6-trinitroanisole in liquid effluents

The study is devoted to the electrochemical detection of trace explosives on boron-doped diamond/graphene nanowall electrodes (B:DGNW). The electrodes were fabricated in a one-step growth process using chemical vapour deposition without any additional modifications. The electrochemical investigations were focused on the determination of the important nitroaromatic explosive compounds, 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitroanisole (TNA). The distinct reduction peaks of both studied compounds were observed regardless of the pH value of the solution. The reduction peak currents were linearly related to the concentration of TNT and TNA in the range from 0.05-15 ppm. Nevertheless, two various linear trends were observed, attributed respectively to the adsorption processes at low concentrations up to the diffusional character of detection for larger contamination levels. The limit of detection of TNT and TNA is equal to 73 ppb and 270 ppb, respectively. Moreover, the proposed detection strategy has been applied under real conditions with a significant concentration of interfering compounds - in landfill leachates. The proposed bare B:DGNW electrodes were revealed to have a high electroactive area towards the voltammetric determination of various nitroaromatic compounds with a high rate of repeatability, thus appearing to be an attractive nanocarbon surface for further applications.

Adsorption of gold subnano-structures on a magnetite(111) surface and their interaction with CO

Gold deposited on iron oxide surfaces can catalyze the oxidation of carbon monoxide.

Structure, physicochemical and biological properties of new complex salt of aqua-(nitrilotriacetato-N,O,O',O")-oxidovanadium(IV) anion with 1,10-phenanthrolinium cation

The crystal structure of new 1,10-phenathrolin-1-ium aqua-(nitrilotriacetato-N,O,O',O")-oxidovanadium(IV) semihydrate of molecular formula (phenH)[VO(NTA)(H2O)]∙1/2H2O was determined. This is the first example of structurally characterized compound that comprises a distinctly separated, monomeric [VO(NTA)(H2O)](-) coordination entity. The crystallographic measurements have subsequently been complemented by the IR spectroscopic characterization and thermal analysis. Furthermore, the electrochemical (cyclic voltammetry) as well as spectrophotometric (UV-vis) studies revealed that the compound is capable of scavenging the superoxide free radicals (O2(-)) as well as stable organic radicals i.e. 2,2'-azinobis(3-ethylbenzothiazoline-6 sulfonic acid) cation radical (ABTS(+)) and 2,2-diphenyl-1-picrylhydrazyl radical (DPPH), but its reactivity towards radicals is lower than that of VOSO4. Finally, biological properties of the complex in the range of 1-100 μM were investigated in relation to its cytoprotective activity against the oxidative damage generated exogenously by using hydrogen peroxide in the hippocampal neuronal HT22 cell line (the MTT and LDH tests). It has been established that in contrast to VOSO4 the title compound protects the HT22 from the oxidative damage. The paper presents a new perspective for oxidovanadium(IV) complexes as candidates for novel, low-molecular mass cytoprotective agents.

Effect of iron additions on intergranular cohesion in chromium

The structural, cohesive, and magnetic properties of (111) and (210) tilt grain boundaries (GBs) in pure Cr, and in Cr with Fe additions, are studied from first principles. Different concentration and position of solute atoms are considered. Our calculations show that Fe atoms placed in the GB interstice enhance cohesion, whereas Fe substituted for one of the Cr layer atoms, in most cases, has very small (or negligible) effect on the cohesion at GBs in Cr. We have found that Fe additions show a tendency to enrich the boundaries in Cr. In the presence of Fe additions the magnetic moments on the GB host atoms are substantially modified and those on Fe impurities are reduced. In most cases the moments on Fe additions remain much higher than the local moments on the Cr atoms.

Crystal structure of three diaza-crowns-18

The crystal structures of three diaza crowns-18, namely 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (crown 1), 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diacetonitrile (crown 2) and N,N′-(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyldi-2,1-ethanediyl)bis-[4-methyl-benzenesulfonamide] (crown 3) have the following space groups and unit cell parameters: crown 1(C

Molecular modeling of singlet-oxygen binding to anthraquinones in relation to the peroxidating activity of antitumor anthraquinone drugs

Anthraquinone derivatives are important anti-cancer drugs possessing, however, undesirable peroxidating and, in consequence, cardiotoxic properties. This results from the mediation by these compounds of the one-electron reduction processes of the oxygen molecule, which produces the highly toxic superoxide anion radical and other active oxygen species. This article summarizes the results of our studies on the molecular aspects of the mechanism of anthraquinone-mediated peroxidation which were carried out using enzymatic-assay, electrochemical, and quantum-mechanical methods.

Enthalpy of oxygen addition to anthraquinone derivatives determines their ability to mediate NADH oxidation

Anthraquinone derivatives are important anti-cancer drugs possessing undesirable cardiotoxic properties related to their peroxidating activity. Previous studies have suggested that this activity can be caused by the binding of a singlet oxygen molecule to an anthraquinone, followed by the one-electron reduction of the complex formed, and its further dissociation into anthraquinone and the superoxide anion radical. In this study, we have carried out semi-empirical PM3 calculations of the energetics of the formation of peroxides and hydroperoxides from hydroxy, amino and imino derivatives of 9,10-anthracenedione. These calculations were supplemented with ab initio calculations, using STO-3G, 4-31G and 6-31G basis sets, on the energetics of oxygen binding to 1,4-dihydroxy and 1,4-diaminobenzene. It was found that for anthraquinones possessing hydroxyl groups, the formation of hydroperoxides is significantly favored energetically compared with the formation of peroxides. In the case of anthraquinones containing only amino groups, the formation of hydroperoxides is less favorable, owing to a greater enthalpy of amino group deprotonation compared with that of hydroxyl group. The effect of electrostatic solvation on the energetics of oxygen addition was also investigated using the Conductor-like Screening Model (COSMO) approach. The effect of solvation on peroxide formation was found to be small, while in the case of hydroperoxides solvation was found to lower the enthalpy of this reaction by approximately 10 kcal/mol for epsilon = 78 (simulating an aqueous environment). Significant stabilization of hydroperoxides was estimated in weakly polar media (epsilon = 4) which can simulate the quinone-reducing center of the mitochondrial NADH dehydrogenase. The enthalpies obtained for oxygen addition to anthraquinones involving the formation of the most stable of the peroxide and hydroperoxide species are in good correlation with the rates of NADPH oxidation stimulated by these compounds and, in turn, with their peroxidating properties. This correlation can be directly implemented in the design of non-peroxidating anthraquinone-derived anti-cancer drugs.

An alternative concept for the molecular nature of the peroxidating ability of anthracycline anti-tumor antibiotics and anthracenodiones

Quantum chemical calculations of model anthraquinone molecules using the CNDO/2 method have revealed that superoxide anion radical formation following the single electron transfer mediated by anthraquinone anti-tumor antibiotics may occur in aerobic conditions as a result of the direct addition of an electron to the anthraquinone-oxygen low energy charge transfer complex that is formed with singlet oxygen. Cyclovoltammetric measurements have been performed in order to provide experimental evidence supporting the hypothesis. The structural requirements for an anthraquinone molecule not exhibiting peroxidating ability by the above mechanism have been postulated. They include maximum symmetry of electron density distribution (symmetry of the molecule), a decrease of the electron density of the pi electron system and an increase in the rigidity of the molecule.