Jahn-Teller distortion, octahedra rotations and orbital ordering in perovskites: KScF 3 as a model system.

The KScF 3 perovskite has been used as a model for investigating the relative importance of the Jahn-Teller (JT) lift of degeneracy, the ScF 6 octahedra rotation (OR), and the quadrupole-quadrupole interaction linked to different occupancy of the Sc t 2 g subshell in various sites of the unit cell (orbital ordering, OO). The group-subgroup sequence P m 3 ¯ m , P 4 m m m , P 4 m b m , and P n m a , supplemented by C m m m and I 4 m c m , has been explored by using an all electron Gaussian type basis set, hybrid functionals, and the CRYSTAL17 code. The JT lift of degeneracy provides a stabilization about 5 times larger than the sum of the OO and OR effects. The energy gained in the transition from P 4 m m m to P 4 m b m , consisting in a rotation of the octahedra around the c axis, is 1077 µ E h . From P 4 m b m to P n m a , additional rotations around the a and b axes are possible, and the d Sc electron can occupy a different t 2 g orbital, with a total energy reduction of 2318 µ E h . The rotation of the octahedra reduces the strength of superexchange: in going from P 4 m m m to P n m a the G-AFM stabilization with respect to FM shrinks by a factor 4.

Ab initio investigations of a CoBiS monolayer with and without point defects.

Through density functional theory (DFT) calculations, a new triclinic monolayer, namely CoBiS, with higher stability than that of penta-CoBiS, is predicted. Our results show that this monolayer is a nonmagnetic metallic compound. To tune its magnetic properties, we systematically investigated the formation and energetics of different point defects in the CoBiS monolayer, such as VBi, VS and VCo. We find that the monolayer becomes magnetic with the different points defects. Our calculated magnetic anisotropy energy (MAE) indicated that VBi and VS exhibit out-of-plane MAE, while the MAE is in-plane for VCo. By solving the Heisenberg model using the Monte Carlo simulation method, we obtain transition temperatures for VS and VCo systems much larger than room temperature, implying potential applications in spintronic devices.

Thermodynamic and structural properties of binary calcium silicate glasses: insights from molecular dynamics.

We study calcium silicate glass of composition (CaO)X(SiO2)(1-X), where X = 40-70 mol%, by means of molecular dynamics for different cooling rates between 1011-1013 K s-1. The thermodynamic and kinetic properties of calcium silicate materials are determined, discussed, and correlated to local structures at short and intermediate range orders and to the potential energies of the oxygen atoms. We show that the amount of non-bridging oxygens and the appearance of free oxygens are related to the increase of the glass transition temperature for an increasing CaO content. Our results are analyzed and discussed in connection with the available experimental data.

A first principle study of graphene functionalized with hydroxyl, nitrile, or methyl groups.

By means of ab initio calculations, we study the functionalization of graphene by different chemical groups such as hydroxyl, nitrile, or methyl. Two extreme cases of functionalization are considered: a single group on a supercell of graphene and a sheet of graphene fully functionalized. Once the equilibrium geometry is obtained by density functional theory, we found that the systems are metallic when a single group is attached to the sheet of graphene. With the exception of the nitrile functionalized boat configuration, a large bandgap is obtained at full coverage. Specifically, by using the GW approximation, our calculated bandgaps are direct and range between 5.0 and 5.5 eV for different configurations of hydroxyl functionalized graphene. An indirect GW bandgap of 6.50 eV was found in nitrile functionalized graphene while the methyl group functionalization leads to a direct bandgap with a value of 4.50 eV. Since in the two limiting cases of minimal and full coverage, the electronic structure changes drastically from a metal to a wide bandgap semiconductor, a series of intermediate states might be expected by tuning the amount of functionalization with these different groups.

An ab initio study of the electronic structure of indium and gallium chalcogenide bilayers.

Using first principle calculations, we have studied the structural and electronic properties of two dimensional bilayers of indium and gallium chalcogenides. With density functional theory corrected for van der Waals interactions, the different modes of stacking were investigated in a systematic way, and several of them were found to compete in energy. Then, their band structures were obtained with the GW approximation and found to correspond to indirect bandgap semiconductors with a small dependency on the mode of stacking. Finally, by analysing the electron density, it appeared that GaSe-InS is a promising system for electron-hole separation.

Dispersion Corrected Structural Properties and Quasiparticle Band Gaps of Several Organic Energetic Solids.

We have performed ab initio calculations for a series of energetic solids to explore their structural and electronic properties. To evaluate the ground state volume of these molecular solids, different dispersion correction methods were accounted in DFT, namely the Tkatchenko-Scheffler method (with and without self-consistent screening), Grimme's methods (D2, D3(BJ)), and the vdW-DF method. Our results reveal that dispersion correction methods are essential in understanding these complex structures with van der Waals interactions and hydrogen bonding. The calculated ground state volumes and bulk moduli show that the performance of each method is not unique, and therefore a careful examination is mandatory for interpreting theoretical predictions. This work also emphasizes the importance of quasiparticle calculations in predicting the band gap, which is obtained here with the GW approximation. We find that the obtained band gaps are ranging from 4 to 7 eV for the different compounds, indicating their insulating nature. In addition, we show the essential role of quasiparticle band structure calculations to correlate the gap with the energetic properties.

Structural, vibrational, and quasiparticle band structure of 1,1-diamino-2,2-dinitroethelene from ab initio calculations.

The effects of pressure on the structural and vibrational properties of the layered molecular crystal 1,1-diamino-2,2-dinitroethelene (FOX-7) are explored by first principles calculations. We observe significant changes in the calculated structural properties with different corrections for treating van der Waals interactions to Density Functional Theory (DFT), as compared with standard DFT functionals. In particular, the calculated ground state lattice parameters, volume and bulk modulus obtained with Grimme's scheme, are found to agree well with experiments. The calculated vibrational frequencies demonstrate the dependence of the intra and inter-molecular interactions on FOX-7 under pressure. In addition, we also found a significant increment in the N-H...O hydrogen bond strength under compression. This is explained by the change in bond lengths between nitrogen, hydrogen, and oxygen atoms, as well as calculated IR spectra under pressure. Finally, the computed band gap is about 2.3 eV with generalized gradient approximation, and is enhanced to 5.1 eV with the GW approximation, which reveals the importance of performing quasiparticle calculations in high energy density materials.

A DFT study on structural, vibrational properties, and quasiparticle band structure of solid nitromethane.

We report a detailed theoretical study of the structural and vibrational properties of solid nitromethane using first principles density functional calculations. The ground state properties were calculated using a plane wave pseudopotential code with either the local density approximation, the generalized gradient approximation, or with a correction to include van der Waals interactions. Our calculated equilibrium lattice parameters and volume using a dispersion correction are found to be in reasonable agreement with the experimental results. Also, our calculations reproduce the experimental trends in the structural properties at high pressure. We found a discontinuity in the bond length, bond angles, and also a weakening of hydrogen bond strength in the pressure range from 10 to 12 GPa, picturing the structural transition from phase I to phase II. Moreover, we predict the elastic constants of solid nitromethane and find that the corresponding bulk modulus is in good agreement with experiments. The calculated elastic constants show an order of C11> C22 > C33, indicating that the material is more compressible along the c-axis. We also calculated the zone center vibrational frequencies and discuss the internal and external modes of this material under pressure. From this, we found the softening of lattice modes around 8-11 GPa. We have also attempted the quasiparticle band structure of solid nitromethane with the G0W0 approximation and found that nitromethane is an indirect band gap insulator with a value of the band gap of about 7.8 eV with G0W0 approximation. Finally, the optical properties of this material, namely the absorptive and dispersive part of the dielectric function, and the refractive index and absorption spectra are calculated and the contribution of different transition peaks of the absorption spectra are analyzed. The static dielectric constant and refractive indices along the three inequivalent crystallographic directions indicate that this material has a considerable optical anisotropy.

Binding and interlayer force in the near-contact region of two graphite slabs: experiment and theory.

Via a novel experiment, Liu et al. [Phys. Rev. B 85, 205418 (2012)] estimated the graphite binding energy, specifically the cleavage energy, an important physical property of bulk graphite. We re-examine the data analysis and note that within the standard Lennard-Jones model employed, there are difficulties in achieving internal consistency in the reproduction of the graphite elastic properties. By employing similar models which guarantee consistency with the elastic constant, we find a wide range of model dependent binding energy values from the same experimental data. We attribute some of the difficulties in the determination of the binding energy to: (i) limited theoretical understanding of the van der Waals dispersion of graphite cleavage, (ii) the mis-match between the strong bending stiffness of the graphite-SiO2 cantilever and the weak asymptotic inter-layer forces that are integrated over to produce the binding energy. We find, however, that the data do support determination of a maximum inter-layer force that is relatively model independent. We conclude that the peak force per unit area is 1.1 ± 0.15 GPa for cleavage, and occurs at an inter-layer spacing of 0.377 ± 0.013 nm.

Crystal structure of the pressure-induced metallic phase of SiH4 from ab initio theory.

Metallization of pure solid hydrogen is of great interest, not least because it could lead to high-temperature superconductivity, but it continues to be an elusive goal because of great experimental challenges. Hydrogen-rich materials, in particular, CH(4), SiH(4), and GeH(4), provide an opportunity to study related phenomena at experimentally achievable pressures, and they too are expected to be high-temperature superconductors. Recently, the emergence of a metallic phase has been observed in silane for pressures just above 60 GPa. However, some uncertainty exists about the crystal structure of the discovered metallic phase. Here, we show by way of elimination, that a single structure that possesses all of the required characteristics of the experimentally observed metallic phase of silane from a pool of plausible candidates can be identified. Our density functional theory and GW calculations show that a structure with space group P4/nbm is metallic at pressures >60 GPa. Based on phonon calculations, we furthermore demonstrate that the P4/nbm structure is dynamically stable at >43 GPa and becomes the ground state at 97 GPa when zero-point energy contributions are considered. These findings could lead the way for further theoretical analysis of metallic phases of hydrogen-rich materials and stimulate experimental studies.

Cohesive properties and asymptotics of the dispersion interaction in graphite by the random phase approximation.

The structural properties of graphite, such as the interlayer equilibrium distance, the elastic constant, and the net layer binding energy, are obtained using the adiabatic-connection fluctuation-dissipation theorem in the random phase approximation. Excellent agreement is found with the available experimental data; however, our computed binding energy of 48 meV per atom is somewhat smaller than the one obtained by quantum Monte Carlo methods. The asymptotic behavior of the interlayer dispersion interaction, previously derived from analytic approximations, is explicitly demonstrated to follow a d-3 behavior at very large distances.

Correlated electrons step by step: itinerant-to-localized transition of fe impurities in free-electron metal hosts.

High-resolution photoemission spectroscopy and ab initio calculations have been employed to analyze the onset and progression of d-sp hybridization in Fe impurities deposited on alkali metal films. The interplay between delocalization, mediated by the free-electron environment, and Coulomb interaction among d electrons gives rise to complex electronic configurations. The multiplet structure of a single Fe atom evolves and gradually dissolves into a quasiparticle peak near the Fermi level with increasing host electron density. The effective multiorbital impurity problem within the exact diagonalization scheme describes the whole range of hybridizations.

Imprinting isolated single iron atoms onto mesoporous silica by templating with metallosurfactants.

HYPOTHESIS: One of the main drawbacks of metal-supported materials, traditionally prepared by the impregnation of metal salts onto pre-synthesized porous supports, is the formation of large and unevenly dispersed particles. Generally, the larger are the particles, the lower is the number of catalytic sites. Maximum atom exposure can be reached within single-atom materials, which appear therefore as the next generation of porous catalysts. EXPERIMENTS: Herein, we designed single iron atom-supported silica materials through sol-gel hydrothermal treatment using mixtures of a non-ionic surfactant (Pluronic P123) and a metallosurfactant (cetyltrimethylammoniumtrichloromonobromoferrate, CTAF) as porogens. The ratio between the Pluronic P123 and the CTAF enables to control the silica structural and textural properties. More importantly, CTAF acts as an iron source, which amount could be simply tuned by varying the non-ionic/metallo surfactants molar ratio. FINDINGS: The fine distribution of iron atoms onto the silica mesopores results from the iron distribution within the mixed micelles, which serve as templates for the polymerization of the silica matrix. Several characterization methods were used to determine the structural and textural properties of the silica material (XRD, N2 sorption isotherms and TEM) and the homogeneous distribution and lack of clustering of iron atoms in the resulting materials (elemental analysis, magnetic measurements, pair distribution function (PDF), MAS-NMR and TEM mapping). The oxidation and spin state of single-iron atoms determined from their magnetic properties were confirmed by DFT calculations. This strategy might find straightforward applications in preparing versatile single atom catalysts, with improved efficiency compared to nanosized ones.

Two-Dimensional Indium Selenides Compounds: An Ab Initio Study.

We use first-principle calculations to investigate the electronic structure of InSe and In2Se3. The interlayer binding energy is found to be in the same range as for other 2D systems, and the monolayers are found to be dynamically stable, which suggest the possibility to obtain them as isolated layers. The GW approximation including spin-orbit is used to obtain the bandgaps, which are in the range relevant for application in electronics. Also, it is shown that an electric field perpendicular to the layers can induce a semiconductor to metal transition in this family of compounds.

A rapid density gradient technique for separating polymorphonuclear granulocytes.

A gradient separation technique followed by isotonic ammonium chloride haemolysis was compared with two methods for the isolation of polymorphonuclear neutrophils from blood. This technique provided a high yield, excellent purity without lymphocyte and erythrocyte contamination, and made it possible to isolate more than 50 x 10(6) human neutrophils from 15 ml of blood. The polymorphonuclear neutrophils isolated in this way were capable of generating a large amount of reactive oxygen species. This technique for the separation of polymorphonuclear neutrophils is an effective method for in vitro studies.

Protective role of glutathione on alpha 1 proteinase inhibitor inactivation by the myeloperoxidase system. Hypothetic study for therapeutic strategy in the management of smokers' emphysema.

In smoking subjects with obvious emphysema, the interaction between neutrophil-derived MPO and H2O2 produced by alveolar inflammatory cells (alveolar macrophages (AM) and polymorphonuclear neutrophils (PMN)) has the ability to spontaneously inactivate, in vitro, the alpha 1 proteinase inhibitor (alpha 1PI). This inactivation can induce a desequilibrium of the protease-antiprotease balance in the lungs. In this study, we investigated the ability of glutathione to protect alpha 1PI. In a cellular model of alpha 1PI inactivation mimicking the effects of alveolar inflammatory cells present in the lower respiratory tract of smoking patients with emphysema, we demonstrated that glutathione can protect alpha 1PI against the oxidative inactivation by these activated cells. This protection has been computed in a cellular experimentation (AM and MPO-system) with a 50% inhibitory concentration of 62 microM. Moreover, glutathione has an important inhibitory effect directly on H2O2 released by PMA-stimulated AM (IC50 = 30 microM) or PMA stimulated PMN (IC50 = 70 microM). The mechanism, which governs glutathione may be a result of a scavenging effect on H2O2 as demonstrated in a free cellular experiment. With this in vitro demonstrated effectiveness, glutathione as a therapeutic antioxidant, via the aerosol, has been proposed, in order to prevent tissue damage, inflicted by an excess of activated phagocytic cells, in some lung diseases such as smoking patients with emphysema.

Decrease of hypochlorous acid and hydroxyl radical generated by stimulated human neutrophils: comparison in vitro of some thiol-containing drugs.

Among reactive oxygen species generated by human neutrophils during inflammatory disorders, hypochlorous acid and hydroxyl radical are especially involved in many diseases such as arteriosclerosis or emphysema. It was shown in vitro that two thiol-containing drugs, mesna and N-acetylcysteine, have antioxidant properties towards these oxidants. The 50% inhibitory concentrations (IC50s) of mesna and N-acetylcysteine for hypochlorous acid production by stimulated neutrophils were 29 and 30 mcM, respectively, and for hydroxyl radical production, IC50s were 520 and 480 mcM, respectively. With this in vitro demonstrated effectiveness, both mesna and N-acetylcysteine have been considered as therapeutic antioxidants to decrease tissue damage inflicted by an excess of activated neutrophils by scavenging hypochlorous acid and hydroxyl radical.

Comparison of in vitro effects of two thiol-containing drugs on human neutrophils hydrogen peroxide production.

During inflammatory disorders, potentially destructive reactive oxygen species, especially hydrogen peroxide, are produced by activated phagocytic cells. It was demonstrated in vitro that mesna and N-acetylcysteine (NAC), mucolytic thiols, have antioxidant properties. An estimation was made of the 50% inhibitory concentration (IC50) of mesna and NAC for PMA-induced H2O2 production by human neutrophils, the results being 70 mcM and 77 mcM, respectively. The mechanism which governs mesna and NAC reactions results from a scavenging effect of H2O2: the calculated IC50s of this effect were 30 mcM and 42 mcM, respectively, in free cellular experimentation. The results suggest that mesna and NAC might be used as antioxidants in aerosols to prevent tissue damage inflicted by this reactive oxygen species, especially in the lungs.

Pro-oxidant properties of methotrexate: evaluation and prevention by an anti-oxidant drug.

Polymorphonuclear neutrophils (PMNs) have the ability to liberate large amounts of reactive oxygen species like hydrogen peroxide. This free radicals release may have beneficial effect in chemotherapy but may also lead to cytotoxicity in case of prolongated inflammatory reaction. This in vitro study demonstrates that methotrexate (MTX), an anticancer drug, increases the amount of hydrogen peroxide released by stimulated PMNs in a dose-dependent manner with a maximum increase of 43.7% (i.e. 22 microM of hydrogen peroxide) for 500 microM of MTX. The mechanism which govern MTX reaction seems to be a result of an intracellular pro-oxidant mechanism by intervention on the oxidative metabolism of PMNs rather than a cell-free chemical interaction. Moreover, an association of MTX with mesna, an anti-oxidant drug, allowed to suppress the excess of hydrogen peroxide production. This association might be used in anticancer therapy, during oxidative burst, particularly when MTX is used in high concentrations, in order to limit toxic effects induced by free radicals.

An intuitive and efficient method for cell voltage prediction of lithium and sodium-ion batteries.

The voltage delivered by rechargeable Lithium- and Sodium-ion batteries is a key parameter to qualify the device as promising for future applications. Here we report a new formulation of the cell voltage in terms of chemically intuitive quantities that can be rapidly and quantitatively evaluated from the alkaliated crystal structure with no need of first-principles calculations. The model, which is here validated on a wide series of existing cathode materials, provides new insights into the physical and chemical features of a crystal structure that influence the material potential. In particular, we show that disordered materials with cationic intermixing must exhibit higher potentials than their ordered homologues. The present method is utilizable by any solid-state chemist, is fully predictive and allows rapid assessement of material potentials, thus opening new directions for the challenging project of material design in rechargeable batteries.