Plane-wave Density Functional Theory Research Articles

Structural and Thermodynamic Properties of TiAl intermetallics under High Pressure

The structural and thermodynamic properties of TiAl intermetallics under high pressure have been investigated by ab initio plane-wave pseudopotential density functional theory method. It is found that the ratio of lattice parameter c to a keeps almost constant with a value of 1.02 under the pressure from 0 to 20 GPa, which agrees well with the experimental results. With the pressure increasing from 20 to 45 GPa the values of c/a decrease almost linearly from 1.02 to 0.99. These calculated results indicate under low pressure the variation rate for a-axis is almost the same to that for c-axis, but under higher pressure the variation for a-axis is smaller than along c-axis. Through the quasi-harmonic Debye model, the equation of state (EOS) of TiAl intermetallics, as well as the thermal expansion and heat capacity at various pressures and temperatures are also studied.

First-principles calculations of structural, elastic and electronic properties of Nb2SnC under pressure

The structural, elastic and electronic properties of Nb2SnC under pressure were investigated by using first-principles plane-wave pseudo-potential density functional theory within the generalized gradient approximation (GGA). We find that the compressibility along the c-axis was lower than along the a-axis for pressures lower than 4GPa, while the elastic constants, elastic modulus and the Debye temperature θD of Nb2SnC increase monotonically as the pressure increases. Finally, the density of states (DOS) at the Fermi level decreases with increasing pressure, due to the decrease of the contribution of Nb 4d states at the Fermi level.

Computational study of ethanol adsorption and reaction over rutile TiO2 (110) surfaces

Studies of the modes of adsorption and the associated changes in electronic structures of renewable organic compounds are needed in order to understand the fundamentals behind surface reactions of catalysts for future energies. Using planewave density functional theory (DFT) calculations, the adsorption of ethanol on perfect and O-defected TiO(2) rutile (110) surfaces was examined. On both surfaces the dissociative adsorption mode on five-fold coordinated Ti cations (Ti(4+)(5c)) was found to be more favourable than the molecular adsorption mode. On the stoichiometric surface E(ads) was found to be equal to 0.85 eV for the ethoxide mode and equal to 0.76 eV for the molecular mode. These energies slightly increased when adsorption occurred on the Ti(4+)(5c) closest to the O-defected site. However, both considerably increased when adsorption occurred at the removed bridging surface O; interacting with Ti(3+) cations. In this case the dissociative adsorption becomes strongly favoured (E(ads) = 1.28 eV for molecular adsorption and 2.27 eV for dissociative adsorption). Geometry and electronic structures of adsorbed ethanol were analysed in detail on the stoichiometric surface. Ethanol does not undergo major changes in its structure upon adsorption with its C-O bond rotating nearly freely on the surface. Bonding to surface Ti atoms is a σ type transfer from the O2p of the ethanol-ethoxide species. Both ethanol and ethoxide present potential hole traps on O lone pairs. Charge density and work function analyses also suggest charge transfer from the adsorbate to the surface, in which the dissociative adsorptions show a larger charge transfer than the molecular adsorption mode.

First-principles calculations for the structure and mechanical properties of IrxNby

The equilibrium lattice parameters, bulk modulus, shear modulus and elastic constant of IrxNby was calculated by ad initio plane-wave pseudopotential density functional theory and agreement between experimental and calculated values is good. According to the results and the experimental criterion raised by Pugh, It was significantly improved the ductility of IrxNby with the increasing of Nb. The results of enthalpy of formation and binding energy show that the structure of IrxNby is stable.

First-principles calculations for the mechanical properties of BC5

We employ ab initio plane-wave pseudo potential density functional theory to calculate the equilibrium lattice parameters, elastic constants, anisotropies, Poisson's ratios, and the Cauchy violation under hydrostatic pressures of 060 GPa for BC5 with hexagonal P3m1 and tetragonal I4m2 structures. The results show that two structures are stable under high pressure and the incompressibility increases with pressure. In addition, the electron structures, the total and the partial densities of states are also calculated. BC5 is found to be metallic with band gap, unlike other B-C compounds. The material properties of BC5 are mainly determined by B 2p1 and C 2p2 electronic states together. All these show that BC5 is an unusual super hard material, and it may be a potential candidate for diamond at high temperature. The given density of states indicates the covalent hybridization between B and C atoms in this compound. The pressure slightly influences the density of states and the band gap, indicating stability under high pressure. We also find that pressure has no influence on the density of states near Fermi level and the band gap, except for slight shifts of the bands. It can be further inferred that BC5 will have good stability under high pressure.

First-Principles Calculations on Elasticity and Anisotropy of Tetragonal Tungsten Dinitride under Pressure

First-principles calculations of the crystal structure and the elastic properties of tetragonal WN2 have been performed with the plane-wave pseudopotential density functional theory method. The calculated structural parameters and elastic constants at zero pressure and temperature are in excellent agreement with the available theoretical results. The dependence of the elastic constants Cij, the aggregate elastic moduli B, G and the anisotropies on pressure have been investigated. WN2 is a brittle system below about 66GPa, whereas it becomes ductile under high pressure. By the elastic stability criteria, it is predicted that tetragonal WN2 are not stable above 232.1GPa. [doi:10.2320/matertrans.M2011373]

Field-effect transistors fabricated from diluted magnetic semiconductor colloidal nanowires

Field-effect transistors (FETs) fabricated from undoped and Co(2+)-doped CdSe colloidal nanowires show typical n-channel transistor behaviour with gate effect. Exposed to microscope light, a 10 times current enhancement is observed in the doped nanowire-based devices due to the significant modification of the electronic structure of CdSe nanowires induced by Co(2+)-doping, which is revealed by theoretical calculations from spin-polarized plane-wave density functional theory.

A density functional theory study of structural, mechanical and electronic properties of crystalline phosphorus pentoxide

Quantum mechanical calculations of single crystal phosphorus pentoxide (P(2)O(5)) have been conducted using the plane-wave ultrasoft pseudopotential technique based on the density functional theory (DFT), in the generalized gradient approximation, with dispersive correction (DFT-D). The implementation of the dispersive correction is shown to improve significantly the structural agreement with experiment, compared to standard plane-wave DFT. The second order elastic constants for the o'(P(2)O(5))(∞) and o(P(2)O(5)) orthorhombic phases were obtained from a polynomial fit to the calculated energy-strain relation. Both phases are shown to be highly elastically anisotropic due to structural features. Polycrystalline aggregate properties have been evaluated to give complete mechanical descriptions. Further investigation of the electronic band structure and density of states has been completed. Analysis of the complex chemical bonding has been carried out using Löwdin atomic charge and valence charge density data showing mixed ionic and covalent character in both phases.

First-principle calculations of structural, electronic and optical properties of BaHf xTi 1−xO 3

The structural, electronic and optical properties of cubic BaHf x Ti 1− x O 3 ( x = 0, 0.25, 0.5, 0.75) (BHT) are investigated using pseudo-potential plane-wave (PP-PW) density functional theory (DFT) taking the generalized gradient approximation (GGA) as the exchange–correlation energy functional. The calculated structural parameters are consistent with the experimental results. The energy band structures, density of states (DOS), Mulliken charges and bond populations are obtained. The fundamental band gaps of BHT are all indirect and the Hf substitute can induce the band gap of BaTiO 3 widening. The analysis of the structural properties, the Mulliken charges and bond populations shows that the space group of BHT becomes from Pm3 m to P4 mm and spontaneous polarization is caused by the substitution of Ti by Hf. The dielectric function, reflectivity spectrum, absorption coefficient, refractive index, and electron energy-loss spectrum are also calculated and analyzed in details. It is found that dielectric imaginary part of BHT decreases as Hf concentration increases.

Elastic properties, Debye temperature, density of states and electron–phonon coupling of ZrB 12 under pressure

The structural parameters, elastic constants and the electronic density of states of ZrB 12 under pressure are determined using first-principles calculations with plane-wave pseudopotential density functional theory, within the generalized gradient approximation. From the elastic constants the elastic parameters and Debye temperature were calculated. They increase as the pressure is increased. The density of states at the Fermi level decreases as pressure is increased, changing from 0.576 to 0.515. Using the Debye temperature and the McMillan equation, the electron–phonon coupling constant was obtained as a function of pressure. It is found that the electron–phonon coupling constant is proportional to the logarithm of the ratio between the value of the Debye temperature and the value of the superconducting critical temperature.

Structural, elastic and thermodynamic properties of Ti2SC

The structural parameters, elastic constants and thermodynamic properties of Ti2SC were investigated under pressure and temperature by using first-principles plane-wave pseudopotential density functional theory within the generalized gradient approximation. The obtained results are in agreement with the available experimental data. The bulk moduli along the a- and c-axes, B a and B c, almost linearly increase with pressure, and the former is always smaller than the latter. The ratio of B c/B a has a trend of gradual increase as the pressure increases. It is found that the elastic constants, anisotropy and Debye temperature of Ti2SC increase with pressure, while axial compressibility along the a- and c-axes decreases with pressure. The thermal properties including the equation of state, the Grüneisen parameter γ, the anisotropies Δp, Δ S1 and Δ S2, and the heat capacity are estimated at various pressures and temperatures.

First-principles investigations on elastic, phonon and thermodynamic properties of SrB 6 under pressure

The elastic, phonon and thermodynamic properties of the divalent alkaline-earth hexaboride SrB 6 are investigated by using plane-wave pseudopotential density functional theory method. The calculated structure parameters and bulk modulus are well consistent with the available experiment and theoretical data. The pressure dependences of elastic constants C ij , bulk modulus B 0, shear modulus G, Young's modulus E and Poisson's ratio σ are also presented. With these elastic parameters, we investigate the mechanical stability and compressibility of SrB 6. For the thermodynamic properties, both phonon and quasi-harmonic Debye model methods are adopted. Through the comparison with experimental and other theoretical results, we found the method of quasi-harmonic Debye model is a little better. Moreover, the phonon dispersion relations are also obtained. It is found that there are two LO/TO splitting around 5 THz and 26 THz, respectively.

Theoretical studies of adsorption property on Ir 4/MgO and Ir 4/γ-Al 2O 3

The adsorption properties of atomic and molecular species on Ir 4/MgO and Ir 4/γ-Al 2O 3 have been systematically studied by means of planewave density functional theory (DFT) calculations using the periodic boundary conditions. The binding energies of these species were ordered as follows: H 2O<C 2H 4<H<OH<S< N<O<C. The adsorption energies of adatoms on Ir 4/MgO were larger than those on Ir 4/γ-Al 2O 3 except hydrogen atom, but were in reverse for the molecules calculated. In addition, the difference of adsorption energies on MgO and γ-Al 2O 3 supports has been elucidated by analyzing the electronic properties. A detailed investigation on state density clarifies the nature of the magnitude of adsorption energy. These calculated results are consistent well with the available experimental and theoretical results.

The Nature of the Binding of Au, Ag, and Pd to Benzene, Coronene, and Graphene: From Benchmark CCSD(T) Calculations to Plane-Wave DFT Calculations.

The adsorption of Ag, Au, and Pd atoms on benzene, coronene, and graphene has been studied using post Hartree–Fock wave function theory (CCSD(T), MP2) and density functional theory (M06-2X, DFT-D3, PBE, vdW-DF) methods. The CCSD(T) benchmark binding energies for benzene–M (M = Pd, Au, Ag) complexes are 19.7, 4.2, and 2.3 kcal/mol, respectively. We found that the nature of binding of the three metals is different: While silver binds predominantly through dispersion interactions, the binding of palladium has a covalent character, and the binding of gold involves a subtle combination of charge transfer and dispersion interactions as well as relativistic effects. We demonstrate that the CCSD(T) benchmark binding energies for benzene–M complexes can be reproduced in plane-wave density functional theory calculations by including a fraction of the exact exchange and a nonempirical van der Waals correction (EE+vdW). Applying the EE+vdW method, we obtained binding energies for the graphene–M (M = Pd, Au, Ag) complexes of 17.4, 5.6, and 4.3 kcal/mol, respectively. The trends in binding energies found for the benzene–M complexes correspond to those in coronene and graphene complexes. DFT methods that use empirical corrections to account for the effects of vdW interactions significantly overestimate binding energies in some of the studied systems.

Pathways of Methanol Steam Reforming on PdZn and Comparison with Cu

PdZn alloy has been shown to catalyze methanol steam reforming (MSR), producing hydrogen gas and carbon dioxide with high selectivity. Despite many studies, the mechanism for MSR on this catalyst is still not completely understood. In this work, several possible pathways of MSR are explored using a plane-wave density functional theory. The focus is placed on the reaction network starting from a facile reaction between adsorbed formaldehyde and hydroxyl species, produced from the decomposition of methanol and water, respectively. These pathways were found to have barriers lower than the rate-limiting step, namely, the dehydrogenation of methoxyl, and they involve species that have been detected in various experiments. Interestingly, the reaction pathways share many similarities with the MSR process on copper, which is the traditional catalyst for MSR.

Investigations of phase transition, elastic and thermodynamic properties of GaP by using the density functional theory

The phase transition of gallium phosphide (GaP) from zinc-blende (ZB) to a rocksalt (RS) structure is investigated by the plane-wave pseudopotential density functional theory (DFT). Lattice constant a0, elastic constants cij, bulk modulus B0 and the pressure derivative of bulk modulus B′0 are calculated. The results are in good agreement with numerous experimental and theoretical data. From the usual condition of equal enthalpies, the phase transition from the ZB to the RS structure occurs at 21.9 GPa, which is close to the experimental value of 22.0 GPa. The elastic properties of GaP with the ZB structure in a pressure range from 0 GPa to 21.9 GPa and those of the RS structure in a pressure range of pressures from 21.9 GPa to 40 GPa are obtained. According to the quasi-harmonic Debye model, in which the phononic effects are considered, the normalized volume V/V0, the Debye temperature θ, the heat capacity Cv and the thermal expansion coefficient α are also discussed in a pressure range from 0 GPa to 40 GPa and a temperature range from 0 K to 1500 K.

Atomic hydrogen adsorption on lithium-doped graphite surfaces

The effects of lithium doping of pristine and defective graphite surfaces on hydrogen adsorption are studied by the first-principles Plane-Wave Density Functional Theory. The surface defects are simulated by a single atomic vacancy. The DFT calculation is corrected for long-range effects through semi-empirical London terms for each constituent of the system. The lithium doping of the graphite surfaces notably reinforces hydrogen atom binding. Qualitative comparison with experimental results is given using the lithium 1s energy level shifts induced by the atomic vacancy and/or hydrogen trapping.

Elastic and thermodynamic properties of CaB6 under pressure from first principles

The elastic and thermodynamic properties of CsCl-type structure CaB6 under high pressure are investigated by first-principles calculations based on plane-wave pseudopotential density functional theory method within the generalized gradient approximation (GGA). The calculated lattice parameters of CaB6 under zero pressure and zero temperature are in good agreement with the existing experimental data and other theoretical data. The pressure dependences of the elastic constants, bulk modulus B (GPa), and its pressure derivative B′, shear modulus G, Young's modulus E, elastic Debye temperature ΘB, Zener's anisotropy parameter A, Poisson ratios σ, and Kleinmann parameter ζ are also presented. An analysis for the calculated elastic constants has been made to reveal the mechanical stability of CaB6 up to 100GPa. The thermodynamic properties of the CsCl-type structure CaB6 are predicted using the quasi-harmonic Debye model. The pressure–volume-temperature (P–V–T) relationship, the variations of the heat capacity CV, Debye temperature ΘD, and the thermal expansion α with pressure P and temperature T, as well as the Grüneisen parameters γ are obtained systematically in the ranges of 0–100GPa and 0–2000K.

High-Pressure Phase Transitions and Thermodynamic Behaviors of Cadmium Sulfide

The pressure-induced phase transitions of cadmium sulfide semiconductor in both zinc-blende and wurtzite structures are investigated by ab initio plane-wave pseudopotential density functional theory with the local density approximation. On the basis of the fourth-order Birch-Murnaghan equation of state, the phase transition pressures P(t) are determined by the enthalpy criterion. It is found that the phase transitions occur at pressure of 2.57 GPa (zinc blende-rocksalt structure) and 2.60 GPa (wurtzite-rocksalt structure), respectively. The equilibrium structural parameters, elastic constants, and phase transition pressures are calculated and compared with the experimental data available and other theoretical results. According to linear-response approach, the thermodynamic properties such as the free energy, enthalpy, entropy, and heat capacity are also obtained successfully from the phonon density of state.

Methyl Formate Pathway in Methanol Steam Reforming on Copper: Density Functional Calculations

Methyl formate has been proposed to be an intermediate in methanol steam reforming (MSR) on copper catalysts. We show here using plane-wave density functional theory that methyl formate can indeed be formed by reaction between formaldehyde and methoxyl. However, this reaction competes unfavorable with that between formaldehyde and hydroxyl, which explains why methyl formate is only observed in the absence of water. Methyl formate can be further hydrolyzed by a surface OH species to produce formic acid, which can dehydrogenate to produce CO2. This process has a lower overall barrier than MSR, thus consistent with the experimental observation that the steam reforming of methyl formate is faster than MSR. However, this hydrolysis process might have difficulties competing with desorption of methyl formate, which has a small adsorption energy. Our theoretical model, which is consistent with all experimental observations related to methyl formate in MSR, thus assigns a minor role for the methyl formate pathway.