CASTEP

First-principles calculations of structural, electronic and optical properties of ZnGa_xAl_2?xO₄ spinel-type oxides

The structural, electronic and optical properties of the perfect ZnGa_xAl_2?xO₄oxides have been computed by density functional theory. It is found that Al substituted by Ga gives rise to the formation of the pseudo-cubic spinel-type structure. With the increase of Ga concentration, average bond lengths of O?Al and O?Ga are greater than that of Al₂O₃and Ga₂O₃, respectively. The zinc aluminate has direct band structure of 3.895eV while the zinc gallate has indirect structure due to forming hybridization between Ga-3d and O-2p states. The threshold of interband transitions related with adsorption spectrum decreases according to the imaginary part?₂(ω) of dielectric function. The real part?₁(ω) of the dielectric function located at zero energy has a square fit relationship with refractive indexn(0) (from 1.77 to 2.01). The absorption shoulder appears in the UV region of 350nm forx=0, with a red shift fromx=0 tox=2. Energy loss function flattens out to a large extent with the enlargement of Ga concentration.

http://dx.doi.org/10.1016/j.jallcom.2013.05.040

Theoretical investigations on the elastic, electronic and thermal properties of orthorhombic Li₂CdGeS₄ under pressure

The structural, elastic, electronic, optical and thermal properties of orthorhombic Li₂CdGeS₄have been performed by the first-principles plane-wave pseudopotential method. The calculated structural parameters and elastic constants at zero pressure and temperature are in good agreement with the available theoretical result. The dependence of the elastic constantsC_ij, the aggregate elastic modulusB,Gand the anisotropies of Li₂CdGeS₄under pressure have been investigated. By the elastic stability criteria, it is predicted that orthorhombic Li₂CdGeS₄is not stable above 8.6GPa. The electronic band structures and optical properties of Li₂CdGeS₄under pressure are studied. It is found that a direct band gap at zero pressure induced by the G?G transition is presented, which is 2.421eV (LDA). Moreover, the direct energy band gap (G?G) transforms to the indirect energy gap (along G?X point) at about 4GPa. The refractive and the absorption indexes under pressure suggest that the strong absorption spectrum appears mostly in the ultra-violet region, and the optical absorption decreases with photon energy in the high energy range. Finally, by using a quasi-harmonic Debye model, the heat capacity and Grüneisen parameter are also obtained successfully.

http://dx.doi.org/10.1016/j.jallcom.2013.07.043

Synthesis, structure, elastic properties, lattice dynamics and thermodynamics of YVO₄ polymorphs from experiments and density functional theory calculation

Pure zircon and scheelite YVO₄were prepared by solid state reaction and high-pressure route, respectively. Calculated structural parameters, bulk modulus and elastic constants show good agreement with experimental results. Calculated phonon dispersions show that zircon and scheelite YVO₄are dynamically stable. Raman frequencies were determined and assigned to different modes according to the calculated results. After phase transition from zircon to scheelite, V?O bond length becomes longer and shows more ionic characteristic, which leads to smaller discrepancy of V atoms’ Born effective charge from their nominal charge and the shift downward of the internal modes frequencies. Thermodynamic properties of YVO₄polymorphs were accurately predicted by quasi-harmonic approach.

http://dx.doi.org/10.1016/j.jallcom.2013.07.031

Anisotropic elastic and thermal properties of titanium borides by first-principles calculations

The anisotropic elastic and thermal expansions of the titanium borides (TiB₂, Ti₃B₄, TiB_Pnma and) are calculated from first-principles using density functional theory. All borides show different anisotropic elastic properties; the bulk, shear and Young’s moduli are consistent with those determined experimentally. The temperature dependence of thermal expansions is mainly caused by the restoration of thermal energy due to phonon excitations at low temperature. When the temperature is higher than 500K, the volumetric coefficient is increased linearly by increasing temperature. Meanwhile, the heat capacities of titanium borides are obtained based on the knowledge of thermal expansion coefficient and the elasticity, the calculations are in good agreement with the experiments.

http://dx.doi.org/10.1016/j.jallcom.2013.06.119

Electronic and optical properties of kesterite Cu₂ZnSnS₄ under in-plane biaxial strains: First-principles calculations

The electronic structures and optical properties of Cu₂ZnSnS₄(CZTS) under in-plane biaxial strain were systematically investigated using first-principles calculations based on generalized gradient approximation and hybrid functional method, respectively. It is found that the fundamental bandgap at the Γ point decreases linearly with increasing tensile biaxial strain perpendicular toc-axis. However, a bandgap maximum occurs as the compressive biaxial strain is 1.5%. Further increase of compressive strain decreases the bandgap. In addition, the optical properties of CZTS under biaxial strain are also calculated, and the variation trend of optical bandgap with biaxial strain is consistent with the fundamental bandgap.

http://dx.doi.org/10.1016/j.physleta.2013.06.046

Effect of non-metal elements (B, C, N, F, P, S) mono-doping as anions on electronic structure of SrTiO₃

We present first-principles density-functional calculation for the electric properties of boron (B)-, carbon (C)-, nitrogen (N)-, fluorine (F)-, phosphorus (P)-, and sulfur (S)-doped SrTiO₃. The obtained results indicate that the bands originating from B (C, N, F) 2p or P 3p states appear in the band gap of SrTiO₃, but the mixing of B (C, N, F) 2p or P 3p states with O 2p states is too weak to produce a significant band gap narrowing. Only in S-doped SrTiO₃case, the S 3p states mix well with the O 2p states and increase the width of valence-band (VB) of SrTiO₃which can produce the really band gap narrowing. Our results fully explain the absorption of visible light is due to the B (C, N, F) 2p or P 3p isolate states above the VB maximum of SrTiO₃, while for S-doped SrTiO₃the visible light absorbance originating from the mixing of S 3p states with O 2p states which causing the band gap narrowing. We also propose that N (S)-doped SriO₃would be the best choice for single anion doping SrTiO₃, while the B (C, P) elements may be the better candidates for co-doping.

http://dx.doi.org/10.1016/j.commatsci.2013.06.009

The first-principles study on the mechanical and electronic properties about rim phase and hard phase of Ti(C,N) based cermets

The first-principle total energy calculations based on density functional theory and ultrasoft pseudopotentials were carried out, within the generalized gradient approximation (GGA) for the exchange and correlation potential, to investigate the mechanical and electronic properties of hard phase (TiC, TiN and TiC_0.75N_0.25), inner rim phase (Ti_0.75Mo_0.25C and Ti_0.75W_0.25C) and outer rim phase (Ti_0.75Mo_0.25C_0.75N_0.25and Ti_0.75W_0.25C_0.75N_0.25). According to the formation energy, all the compounds are stable and hard phase possesses the best stability with the largest formation energy and the minimum lattice parameters. Molybdenum and tungsten are observed significantly to increase the bulk modulusB, Young’s modulusE, Poisson’s ratioυandB/G, namely, the rim phase has the better above properties than those of hard phase. From the analysis ofB/G, Poisson’s ratio and calculated hardness, comparing with hard phase, the rim phase possesses the better ductility and lower hardness. Based on the electronic properties of these compounds, the higher hardness for hard phase may attribute to the interactions between Ti-3d and non-metal (C or N) 2p electrons, meanwhile, the relatively low hardness of rim phase may be the results of some metallic d?d interactions near the Fermi level.

http://dx.doi.org/10.1016/j.commatsci.2013.06.031

First-principle study of interfacial properties of Ni? Ni₃Si composite

In order to explore the interfacial structure of Ni/Ni₃Si (001) interfaces and clarify the heterogeneous nucleation potential of α-Ni grains on Ni₃Si particles in Ni?Ni₃Si eutectic alloys, the work of adhesion, electronic structure, and interface energy have been studied by first-principle method based on density functional theory (DFT). Surface convergence tests show that Ni₃Si (001) slabs with more than eight layers exhibit bulk-like interior feature. The values of adhesion work and interfacial distance suggest the Ni+Si-terminated interface with hollow site stacking is the most thermodynamic stable and has the largest critical stress for crack propagation among the four models, indicating the precipitates of Ni₃Si can improve stability and tensile strength of alloys. This interface has covalent feature, which is mainly contributed by the hybridization of Ni-3d and Si-3p orbital electron. The calculated interface energy of Ni/Ni₃Si (001) interface is smaller than that between the α-Ni and nickel melts, elucidating the excellent nucleation potency of Ni₃Si particles for α-Ni grains from thermodynamic considerations.

http://dx.doi.org/10.1016/j.commatsci.2013.06.030

Theoretical study of elastic and thermodynamic properties of η-Ta₂N₃

We present a detailed theoretical study of the elastic properties ofη-Ta₂N₃under pressure ranging from 0GPa to 80GPa by plane-wave pseudopotential density functional theory with the generalized gradient approximation in this paper. The results show thatη-Ta₂N₃is mechanically unstable at ambient conditions because of a negative C66, and it will be stable when the pressure is up to 10.7GPa. The thermodynamic properties ofη-Ta₂N₃are also investigated through quasi-harmonic Debye model in a pressure range of 0?80GPa and a temperature range of 0?1500K. The Debye temperature, heat capacity and thermal expansion with pressure and temperature are successfully obtained and discussed.

http://dx.doi.org/10.1016/j.commatsci.2013.07.045

Bonding, stability, and electronic properties of the BC₃ honeycomb monolayer structure on NbB₂(0001)

Two-dimensional graphenelike BC₃honeycomb structures, which have been grown on the NbB₂(0001) surface, possess intriguing properties that offer new opportunities for applications in nanoelectronics, mechanical materials, and superconductivity. The bonding configuration of BC₃on the substrate has not yet been determined, however. We reportab initiothermodynamics calculations and analysis that lead to a prediction about the most stable bonding configuration of a monolayer BC₃on NbB₂, namely theC-topconfiguration in which carbon is bonded directly on the top of the niobium atoms. We also find that the BC₃monolayer on NbB₂is thermodynamically stable, accounting for the experimental observation that the BC₃sheet can be grown on NbB₂by carbon substitution and surface segregation of carbon impurities. We further provide detailed information about the atomic structures and electronic properties of the BC₃-bonded NbB₂surface.

http://dx.doi.org/10.1103/PhysRevB.88.11543

Double-shell C₆₀/C₂₄₀fullerenes with Stone-Wales defects for hydrogen storage: An ab initio study

We presentab initiocalculationsto find the migration pathways of the hydrogen atom through Stone-Wales defects into the inside of the double-shellfullerene.We report that the most favorable pathway consists of thetunnelingpathway through Stone-Wales defects on the double-shellC₆₀/C₂₄₀fullerene.Thistunnelingpathway gives rise to three barrier heights of 0.54?eV, 0.47?eV, and 0.7?eV. The driving force for the hydrogen atom diffusion through thetunnelingpathway towards the inside of the double-shellfullereneis 0.82?eV. Our findings lead to a relatively lowenergypathway, which provides a practical route to develop newly inexpensive solid-statehydrogen storages.

http://dx.doi.org/10.1063/1.482128

Ab initio study of dielectric function of C-substituted single walled boron nanotubes

We report dielectric function related optical properties namely dielectric constant, static dielectric constant, and absorption coefficients of C-substituted hexagonal boron nanotubes. The optical properties were computed for parallel and perpendicular polarized light in the framework of density functional theory. In this regard, three models of BNTs namely armchair (3,3), zigzag (5,0), and chiral (4,2) have been undertaken for probing the effect of carbon impurity. Our calculations show high dielectric constant of armchair and chiral BNTs for parallel polarized light and magnitude becomes smaller for higher impurity concentration, while zigzag BNT exhibits reverse trend for high impurity concentration. For perpendicular polarized light, the magnitude of dielectric constantε₁(ω) is decreased and shifts at higher frequencies. The absorption is revealed highest for armchair followed by zigzag and chiral BNTs independent of impurity concentration. The intensity of absorption gets weaken for higher concentration. The chiral BNTs show smaller but uniform absorption in smaller frequency range results in uniform field emission. These findings are also compared with available experimental and theoretical results. These metallic nanotubes are promising candidate as interconnects for nanodevices as well as field emission devices.

http://dx.doi.org/10.1140/epjb/e2013-40739-1

CASTEP + DMol3

Crystal structure, stability and spectroscopic properties of methane and CO₂ hydrates

Methane hydrates are highly present in sea-floors and in other planets and their moons. Hence, these compounds are of great interest for environment, global climate change, energy resources, and Cosmochemistry. The knowledge of stability and physical?chemical properties of methane hydrate crystal structure is important for evaluating some new green becoming technologies such as, strategies to produce natural gas from marine methane hydrates and simultaneously store CO₂as hydrates. However, some aspects related with their stability, spectroscopic and other chemical-physical properties of both hydrates are not well understood yet. The structure and stability of crystal structure of methane and CO₂hydrates have been investigated by means of calculations with empirical interatomic potentials and quantum-mechanical methods based on Hartree?Fock and Density Functional Theory (DFT) approximations. Molecular Dynamic simulations have been also performed exploring different configurations reproducing the experimental crystallographic properties. Spectroscopic properties have also been studied. Frequency shifts of the main vibration modes were observed upon the formation of these hydrates, confirming that vibration stretching peaks of C?H at 2915cm^?1and 2905cm^?1are due to methane in small and large cages, respectively. Similar effect is observed in the CO₂clathrates. The guest?host binding energy in these clathrates calculated with different methods are compared and discussed in terms of adequacy of empirical potentials and DFT methods for describing the interactions between gas guest and the host water cage, proving an exothermic nature of methane and CO₂hydrates formation process.

http://dx.doi.org/10.1016/j.jmgm.2013.06.006

DMol3

Theoretical study on the structure and dehydrogenation mechanism of mixed metal amidoborane, Na[Li(NH₂BH₃)]₂

This study is based on the synthesis of a mixed metal amidoborane, Na[Li(NH₂BH₃)]₂(SLAB), the first example of an inorganic sodium?lithium compound. This paper is the first systematic study of its structure and dehydrogenation mechanism, and the results obtained were consistent with the experimental results. The first principle method was used to study the structure of SLAB in solid phase, while the second Moller?Plesset Perturbation Theory was used for gas-phase kinetic studies. Potential energy curves were obtained by CCSD(T) method. Three mechanistic pathways were designed to study its dehydrogenation, which include A pathway (without the cleavage of N?B bond (S and S′ pathways)), B pathway (with the cleavage of the intramolecular N?B bond before dehydrogenation), and D pathway (with the formation of direct dihydrogen bonds). Na⁺cation movement was proved to play a very important role in the hydrogen-transfer process. Finally, a possible dissociation pathway (A pathway) was confirmed and dehydrogenation rates similar to the experimental values were obtained.

http://dx.doi.org/10.1016/j.jallcom.2013.07.022

A first-principles study of structure, orbital interactions and atomic oxygen and OH adsorption on Mo-, Sc- and Y-doped nickel bimetallic clusters

Density functional theory (DFT) has been used to study the stability, orbitals interactions and oxygen and hydroxyl chemisorption properties of Ni_nM (1?n?12) clusters. A single atom doped-nickel clusters increase the stability, and icosahedral Ni₁₂Mo cluster is the most stable structure. Molybdenum atom prefers to exhibit center at the cluster (n?10) rather than edge, while Sc and Y atom remain at the edge. The Ni?Mo bond lengths are smaller than the Ni?Sc and Ni?Y. The pDOS results show that thed?dorbitals interactions are mainly dominating on the stability of clusters, whileporbitals have a small effect on the stability. The Mo-doped nanoclusters have the highest oxygen and OH chemisorption energy, and the most favorable adsorption site is on the top Mo site. The larger cluster distortion is found for the Sc- and Y-doped structures compared to other clusters. The oxygen 2porbitals are strongly hybridizing with the Mo 4dorbitals (n<9) and a little interaction between oxygen 2pand Ni 3d, 4sand Mo 5sorbitals. The Mo-doped clusters are significantly increased the chemisorption energies that might improve the passive film adherence of nanoalloys.

http://dx.doi.org/10.1016/j.jallcom.2013.05.034

Can all nitrogen-doped defects improve the performance of graphene anode materials for lithium-ion batteries?

The electronic and adsorption properties of graphene can be changed significantly through substitutional doping with nitrogen and nitrogen decoration of vacancies. Hereab initiodensity functional theory with a dispersion correction was used to investigate the stability, magnetic and adsorption properties of nine defects in graphene, including both nitrogen substitutional doping and nitrogen decoration of vacancies. The results indicate that only pyridinic N₂V₂defect in graphene shows a ferromagnetic spin structure with high magnetic moment and magnetic stabilization energy. Not all nitrogen-doped defects can improve the capacity of the lithium-ion batteries. The adsorption energies of a lithium atom on nitrogen-substituted graphenes are more positive, indicating that they are meta-stable and no better than the pristine graphene as anode materials of lithium-ion batteries. Nitrogen-decorated single and double vacancy defects, especially for the pyridinic N₂V₂defect in graphene, can greatly improve the reversible capacity of the battery in comparison with the pristine graphene. The theoretical prediction of the reversible capacity of the battery is 1039 mA h g^?1for the nitrogen-doped graphene material synthesized by Wuet al., which is in good agreement with the experimental data (1043 mA h g^?1). The theoretical computations suggest that nitrogen-decorated single and double vacancy defects in graphene are the promising candidate for anode materials of lithium-ion batteries. Each nitrogen atom in the decoration can improve the reversible capacity of the battery by 63.3?124.5 mA h g^?1in a 4 × 4 supercell of graphene. The present work provides crucial information for the development of N-doped graphene-based anode materials of lithium-ion batteries.

http://dx.doi.org/10.1039/C3CP51689J

Statin Inhibition of HMG-CoA Reductase by Quantum Biochemistry Computations

Hundreds of millions of adults have high cholesterol, which has generated a billionaire market of drugs. Patents covering the leading statins have expired recently, pressuring the development of new drugs. Statins act by inhibiting the HMG-CoA reductase in the process of converting HMG-CoA to mevalonate, a committed step in the biosynthesis of cholesterol. It is observed in clinical trials that this action decreases by 20 to 60% the low density protein (LDL) cholesterol levels, reducing coronary events by up to one-third over a five years period. In this work, considering the crystallographic data of HMGR complexed with statins, we perform a computer simulation within an ab-initio quantum mechanical approach, based on the density functional theory, to investigate the details of the binding interaction energies of the statins atorvastatin, rosuvastatin, fluvastatin, and simvastatin to the HMGR enzyme. Our purpose is to elucidate why statins have differences in their efficiency to reduce cholesterol levels, by obtaining and comparing the interaction energy between the HMGR residues and the ligand atoms. The main advantage of our methodology is the possibility to evaluate what amino acid residues contribute more intensely to the stabilization of the statin-HMGR complex, a very helpful information for drug design.

http://dx.doi.org/10.1109/SBEC.2013.88

DMol3 + Forcite

New gluconamide-type cationic surfactants: Interactions with DNA and lipid membranes

New linear cationic surfactants ? 2-(alkyldimethylammonio)ethylgluconamide bromides, denoted as C_nGAB,n=10, 12, 14 and 16 ? were synthesized from natural resources and characterized with respect to their potential as gene-delivery agents in gene therapy applications. Interactions with plasmid DNA and with model membranes were studied both experimentally and theoretically. The compounds withn=12, 14 and 16 show exponentially increasing ability to fully condense DNA. C₁₆GAB condenses DNA at 1:1 surfactant to nucleotide molar ratio. Furthermore, C_nGABs interact with model membrane, slightly lowering the temperature of the main phase transitionT_mof the DPPC bilayer. C₁₀GAB is found to interact only at the membrane surface. C₁₆GAB reducesT_mless than C₁₂GAB and C₁₄GAB, and forms domains in the bilayer at the surfactant/DPPC molar ratio of 0.1 and higher. The results suggest that C₁₆GAB can be a promising candidate for building gene-delivery carrier systems.

http://dx.doi.org/10.1016/j.bpc.2013.06.010

DMol3 + GULP

Molecular dynamics simulation on the failure mechanism of Y-junction single-walled carbon nanotubes

Molecular dynamics simulation based on the second generation Brenner potential has been employed to investigate the mechanical properties and failure mechanism of Y-junction carbon nanotubes (Y-CNTs) under tensile loading. It was found that due to the existence of junction heptagonal defects, shear bands induced by the external load may expand outward in the form of spiral and promote the formation of Stone?Wales transformation, which therefore constitutes an effective way to dissipate the energy. We also study the temperature effect on the mechanical properties of Y-CNTs. Our simulation results provide useful insights to the design and fabrication of Y-CNTs based nanostructures and devices.

http://dx.doi.org/10.1016/j.commatsci.2013.06.035

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