Wave Pseudopotential Density Functional Theory Research Articles

First-principles study on the mechanism of Hf and Ti doping promoting hydrogen release in ZrCoH3

This paper reveals the mechanism of Hf and Ti doping promoting hydrogen release in ZrCoH3. Different from previous experimental methods, this study employs the plane wave pseudopotential method and density functional theory to predict the relationship between the formation energy and electronic structure of the ZrCoH3-Hf/Ti system, and uses theoretical calculations to guide the modification design of ZrCoH3. Firstly, by calculating the thermodynamic properties of the ZrCoH3-Hf/Ti system and combining the density of states theory, the bonding characteristics and dehydrogenation preference between H atoms and neighboring metal atoms in the ZrCoH3-Hf/Ti system are determined. Additionally, the electronic structure of the ZrCoH3-Hf/Ti system is calculated to elucidate the hybridization of bonding and antibonding orbitals between atoms in the ZrCoH3-Hf/Ti system. Moreover, DCo-d is utilized to measure the strength of antibonding interactions in the ZrCoH3-Hf/Ti system. The main cause of antibonding interactions in the ZrCoH3-Hf/Ti system is attributed to the variation in formation energy, and the change in antibonding interactions is sensitive to the variation in formation energy. Finally, a feasible method to improve the properties of ZrCoH3 is proposed, which involves modifying the electronic structure to increase the partial density of states at the Fermi energy level, making the formation energy of hydrides less negative and forcing the structure to be more unstable. This method has potential application value in the modification of ZrCoH3.

First principle study on modulating of Schottky barrier at metal/4H-SiC interface by graphene intercalation

In the production of SiC electronic devices, one of the main challenges is the fabrication of good Ohmic contacts due to the difficulty in finding the metals with low Schottky barriers of wide band gap SiC. Therefore, reducing the Schottky barrier height (SBH) at the metal/SiC interface is of great importance. In this paper, the effects of graphene intercalation on the SBH in different metals (Ag, Ti, Cu, Pd, Ni, Pt)/4H-SiC interfaces are studied by combining the average electrostatic potential and local density of states calculation methods based on first-principles plane wave pseudopotential density functional theory. The calculation results show that single-layer graphene intercalation can reduce the SBH of metal/4H-SiC contact. When the two layers of graphene are inserted, the SBH are further reduced. Especially, the contact between Ni and Ti exhibits negative SBH values, inferring that good Ohmic contacts are formed. When layers of graphene continue to increase, the SBH no longer changes obviously. By analyzing the differential charge density and the local density of states of the interface, the mechanism of SBH reduction may be that the dangling bonds on the SiC surface are saturated by the graphene C atoms and the influence of the metal-induced energy gap state at the interface is reduced, thereby reducing the interface state density. In addition, graphene and the corresponding new phases at the interface have low work functions. Moreover, an interfacial electric dipole layer may be formed at the SiC/graphene interface which also contributes to barrier reduction.

Regulated microarchitecture, spin polarization state, and observed charge transfers for cerium boride CeB6 under electrical Field

The thermal stabilities, microarchitectures as well as the electronic states of the cubic CeB6 under electrical field have been systematically investigated in detail within the framework of plane wave pseudo-potential density functional theory calculational method. Specifically, theCeB6 under electrical field of 0.0002 V/Å has lower total energy and enthalpy of formation. The cell of theCeB6 under electrical field is slightly expanded and the lattice expansion is isotropic, along with microarchitecture regulations. The distances between B are 2.523 Å, 1.707 Å and 1.784 Å, showing covalent bonds. There are 5 sub-bands for CeB6, the energy widths of bands are regulated with the conduction band and the first valence band moving downwards under electrical field. There are 3 band valleys and 5 band peaks for the CeB6, with metallic transition and decreased band gaps under electrical field. The middle conduction band and the first valence band are formed by f, d and p orbital electrons, respectively. The Ce s and the B orbital electrons tend to polarize under electrical field. The Ce f, Ce s and B orbital electrons are responsible for the ferrimagnetic magnetism under electrical field. There are charge transfers among these orbitals for CeB6 under electrical field.

First-principles investigation of the structural, dynamical, electronic, and elastic properties of WGe2 and W5Ge3

ABSTRACTWe have investigated the structural, dynamical, elastic, and electronic properties of WGe2 and W5Ge3 compounds in different phases. We have considered the C11b (tetragonal, space group I4/mmm) and C23 (orthorhombic, space group Pnma) strukturbericht phases for WGe2 compound and D81 (tetragonal, space group I4/mcm), D8m (tetragonal space group I4/mcm) strukturbericht phases for W5Ge3 compound. The structural parameters, formation enthalpies, phonon dispersion curves, elastic constants, mechanical modulus, anisotropic factors, thermal conductivities, and electronic structures have been investigated using generalised gradient approximation within in the plane wave pseudopotential density functional theory. The calculated lattice constants are in a good agreement with the experimental data. The considered phases for WGe2 and W5Ge3 compounds have a metallic character. The results indicated that all phases for compounds are both mechanically stable and dynamically stable except for W5Ge3-D81. The anisotropy in some mechanical modulus has been investigated using several elastic anisotropy indexes and directional dependence of compressibility, Young’s moduli, shear moduli, and Poisson’s ratio.

High Pressure Effects on Structural, Elastic and Thermodynamic Properties of Tantalum Mononitride

The pressure effects on the structural, elastic and thermodynamic properties of tantalum mononitride in WC-type phase (i.e. WC–TaN) are investigated by the first-principles plane wave pseudo-potential density functional theory method and the quasi-harmonic Debye model. The obtained equilibrium structure parameters and ground state properties are in excellent agreement with the experimental and other theoretical results. The calculations of the phonon dispersion curve and the density of phonon states verify that the WC–TaN is dynamically stable. A full elastic tensor and anisotropies behavior of the WC–TaN is also evaluated and discussed in the wide pressure range. The results show that WC–TaN is elastic anisotropy and mechanically stable up to 100 GPa, and the compression along c-axis direction is more difficult than along a-axis. The obtained superior mechanical properties show that WC–TaN is a promising candidate structure to be one of the ultra-incompressible and hard materials. Finally, by using the quasi-harmonic approximation model, we predicted the thermodynamic properties of WC–TaN under pressure and temperature. The heat capacity CV, Debye temperature θ, the thermal expansion α and the Gruneisen constant γ are obtained successfully in the ranges of 0–100 GPa and 0–2000 K.

ZnO nanorods decorated with nanocrystalline (nc) Au Particles:Electronic structure and magnetic behaviours

Electronic structures and magnetic behaviours of zinc oxide nanorods (ZnO-NRs) decorated with nanocrystalline “Au”-particles (nc-Au) have been studied using different spectroscopies, superconducting quantum interference device-type magnetometer and plane wave pseudopotential density functional theory calculations. The presence of intrinsic defects viz. zinc interstitials (Zni) and/or oxygen vacancies (VO) are established using different spectroscopies measurements. Ultraviolet photoelectron spectroscopy and valence band photoemission spectroscopy suggests that the valence band density of states of O 2p - Zn 4sp hybridized states of nc-Au/ZnO-NRs is higher than pure ZnO-NRs that could be correlated with its higher ferromagnetic behaviour. An enhanced room temperature ferromagnetism has been observed when nanocrystalline (nc) gold (Au) particles are decorated on the surface of ZnO-nanorods (ZnO-NRs). This enhanced magnetic moment in nc-Au/ZnO-NRs comes from the metallic electrons of nc-Au that induces large orbital magnetic moment at the nc-Au/ZnO-NR interface. Theoretical calculations shows the enhancement of the density of states at the valence band region of ZnO with a decoration of optimum content nc-Au particles along with the increase of oxygen vacancies in nc-Au/ZnO-NRs; that further confirms the enhancement of magnetization.

Effect of tellurium concentration on the structural, electronic and mechanical properties of beryllium sulphide: A DFT approach

The structural, electronic and mechanical properties of are studied within the concentration range of using first-principles plane–wave Pseudopotential density functional theory (DFT) approach. We have used generalized gradient approximation (GGA) to treat the exchange-correlation potentials. The elastic constants, bulk, shear and Young’s moduli, Poisson’s ratio, and Zener’s anisotropic factors are calculated. The results were found to be in agreement with other available theoretical and experimental values. It was also observed that the existence and increase of Tellurium concentration decreases the hardness of the alloy.

Satellite valleys and strained band gap transition of bulk Ge: Impact of pseudopotential approximations on quasiparticle levels

A first principles study of the electronic structure of strained Ge is carried out within the framework of plane wave pseudopotential density functional theory (DFT) combined with the GW approximation to the self-energy. Systematic investigations of the variations of high symmetry quasiparticle gaps over for a range of pseudopotential approximations are carried out. It is found that the quasiparticle corrections can be at least as sensitive to the choice of DFT approximation as to pseudopotential generation scheme, level of core-valence interaction, and pseudopotential electronic configuration. The latter can be decisive in the resulting prediction of the band gap transition strain, as the magnitudes of exchange and correlation terms involved in the quasiparticle corrections at the conduction valleys are largely determined by the arrangement of orbitals at those regions of the bandstructure, which in turn is determined by the pseudized electronic configuration.

First principle calculation of physical properties of barium based chalcogenides BaM4S7 (M = Ga, Al); a DFT, DFT-D and hybrid functional HSE06 study

The electronic structure, elastic and optical properties have been calculated for the novel nonlinear optical (NLO) crystals BaQ4S7 (Q = Ga, Al) using plane wave pseudo-potential density functional theory (DFT) method as implemented in CASTEP and ABINIT codes. In this study we used both hybrid HSE06 and DFT-D functionals with GGA approximation. These NLO compounds, which belong to the mm2 point group, are particularly interesting because of their transparency in the mid-infrared region and wide energy band gap. We present results for electronic structure, elastic tensor coefficients, refractive indices and second order nonlinear optical susceptibilities. The calculated energy band gap and frequency dependent refractive indices as well as the NLO coefficients of BaGa4S7 are in good agreement with the experimental values. With no reported theoretical or experimental energy band gap and optical properties of BaAl4S7, we present for the first time its electronic structure and above mentioned optical coefficients. This compound has higher direct band gap with 3.74eV, better optical birefringence and second-order NLO coefficients than most NLO compounds. The second-order NLO coefficients for BaAl4S7 have been calculated as d31=3.15pm/V, d31=2.20pm/V, d33=−6.31pm/V.

Structural, Thermodynamic, Elastic, and Electronic Properties of α-SnS at High Pressure from First-Principles Investigations

Abstract SnS has potential technical applications, but many of its properties are still not well studied. In this work, the structural, thermodynamic, elastic, and electronic properties of α-SnS have been investigated by the plane wave pseudo-potential density functional theory with the framework of generalised gradient approximation. The calculated pressure-dependent lattice parameters agree well with the available experimental data. Our thermodynamic properties of α-SnS, including heat capacity C P , entropy S, and Gibbs free energy relation of –(G T –H 0) curves, show similar growth trends as the experimental data. At T=298.15 K, our C P =52.31 J/mol·K, S=78.93 J/mol·K, and –(G T –H 0)=12.03 J/mol all agree very well with experimental data C P =48.77 J/mol·K and 49.25 J/mol·K, S=76.78 J/mol·K, and –(G T –H 0)=12.38 J/mol. The elastic constants, together with other elastic properties, are also computed. The anisotropy analyses indicate obvious elastic anisotropy for α-SnS along different symmetry planes and axes. Moreover, calculations demonstrate that α-SnS is an indirect gap semiconductor, and it transforms to semimetal with pressure increasing up to 10.2 GPa. Combined with the density of states, the characters of the band structure have been analysed in detail.

First-principles investigations on the elastic and thermodynamic properties of cubic ZrO2 under high pressure

We have investigated the elastic and thermodynamic properties of ZrO 2 under pressure up to 120 Gpa by the plane wave pseudopotential density functional theory with the generalized gradient approximation (GGA) method. The elastic constants of ZrO 2 are calculated and meet the generalized stability criteria, suggesting that ZrO 2 is mechanically stable within this pressure range. The pressure effects on the elastic properties reveal that the elastic modulus B, shear modulus G and Young's modulus Y increase linearly with the pressure increasing, implying that the resistance to deformation is enhanced. In addition, by analyzing the Poisson's ratio ν and the value of B/G, we notice that ZrO 2 is regarded as being a ductile material under high pressure and the ductility can be improved by the pressure increasing. Then, we employ the quasi-harmonic Debye model considering the phononic effects to obtain the thermodynamic properties of ZrO 2. Debye temperature ΘD, thermal expansion coefficient α, heat capacity Cp and Grüneisen parameter γ are systematically explored at pressure of 0–80 Gpa and temperature of 0–1000 K. Our results have provided fundamental facts and evidences for further experimental and theoretical researches.

Pressure effects on structural, elastic and electronic properties of BaZnO2: first-principles study

The structural, elastic and electronic properties of BaZnO2 under pressure are investigated by the plane wave pseudopotential density functional theory (DFT). The calculated lattice parameters and unit cell volume of BaZnO2 at the ground state are in good agreement with the available experimental data and other theoretical data. The pressure dependences of elastic constants Cij, bulk modulus B, shear modulus G, B/G, Poisson’ s ratio σ, Debye temperature Θ and aggregate acoustic velocities VP and VS are systematically investigated. It is shown that BaZnO2 maintains ductile properties under the applied pressures. Analysis for the calculated elastic constants has been made to reveal the mechanical stability and mechanical anisotropy of BaZnO2. At the ground state, the calculated compressional and shear wave velocities are 8.26 km/s and 1.81 km/s, respectively, and the Debye temperature Θ is 240.8 K. The pressure dependences of the density of states and the bonding property of BaZnO2 are also investigated.

Investigations on the structural, electronic, elastic and thermodynamic properties of niobium silicide under high temperature and pressure

A first principles investigation on the structural, electronic and elastic properties of NbSi2 has been calculated using the plane wave pseudo-potential density functional theory (DFT) method. The lattice constants and elastic constants are obtained. The characters of the band structure and the density of states of niobium silicide (NbSi2) are analyzed, which shows that NbSi2 is metallic. The bulk, shear and Young’s modulus for NbSi2 are also calculated. The Debye temperature is calculated from the average elastic wave velocity obtained from the shear and bulk modulus. The thermodynamic properties including the heat capacity CV and CP, thermal expansionα, entropy S and Grüneisen parameterγhave been calculated at temperatures from 0 to 2100K and pressures from 0 to 30GPa using the quasi-harmonic Debye model. We also found that the present work can give a reference to those not to be experimentally investigated.

Structural, elastic and electronic properties of CuYO2 from first-principles study

The structural, elastic and electronic properties of CuYO2 have been investigated by the plane wave pseudopotential density functional theory (DFT) within the local density approximation (LDA) and the LDA+U methods. The strong on-site Coulomb repulsion of the localized Cu 3d electrons and Y 4d electrons are amended by LDA+U formalism, showing that the properties of CuYO2 are quite sensitive to the change of the value of U; the optimized choice of U is 6eV plusing to the 3d state of the Cu atoms. Through the LDA+U correction, the calculated lattice constants, insulating gap and bulk modulus agree well with the available experimental and other theoretical data. Moreover, the anisotropies, the compressional and shear wave velocities, Young’s modulus, Poisson’s ratio, as well as the Debye temperature have been calculated for U=6eV, which are compared with those for U=0eV. It has been found that the predicted elastic constants, bulk modulus, shear modulus, acoustic velocities, anisotropy factor and Debye temperature of CuYO2 are slightly fluctuant with the increase of the U value. The calculated results also show that, the valence band of CuYO2 mainly composed of the 3d state of the Cu and the 2p state of the O, while the conduction band mainly composed of the 4d state of the Y.

Structural and Vibrational Properties of Manganese Sulfide

Structural properties of MnS have been studied using plane wave pseudopotential density functional theory as implemented in Quantum Espresso code. Local density approximation (LDA) along with ultrasoft pseudopotential has been used for total energy calculations. The calculated total energies are fitted to Murnaghan equation of state to calculate equilibrium lattice constant, isothermal bulk modulus and pressure derivative of isothermal bulk modulus for NaCl-type structure of MnS and compared with previous experimental and theoretical calculations and good agreement is achieved with those results. Phonon frequencies have also been derived for B1 phase of MnS along high symmetry directions using the density functional perturbation theory at ambient condition.

Fast plane wave density functional theory molecular dynamics calculations on multi-GPU machines

Plane wave pseudopotential (PWP) density functional theory (DFT) calculation is the most widely used method for material simulations, but its absolute speed stagnated due to the inability to use large scale CPU based computers. By a drastic redesign of the algorithm, and moving all the major computation parts into GPU, we have reached a speed of 12s per molecular dynamics (MD) step for a 512 atom system using 256GPU cards. This is about 20 times faster than the CPU version of the code regardless of the number of CPU cores used. Our tests and analysis on different GPU platforms and configurations shed lights on the optimal GPU deployments for PWP-DFT calculations. An 1800 step MD simulation is used to study the liquid phase properties of GaInP.

New acentric materials constructed from aminopyridines and 4-nitrophenol

Co-crystallization of 4-nitrophenol (I) with five aminopyridines (4-aminopyridine 1, 3,4-diaminopyridine 2, 2,3-diaminopyridine 3, 3-aminopyridine 4, 2-amino-6-methylpyridine 5) and 2,4-diaminopyrimidine 6 resulted in six adducts with the ratio of components 2 : 1 in five and 1 : 1 in one final compounds. Single crystals were grown by slow evaporation technique using ethanol as a solvent. Five adducts with 1–5 crystallize in acentric P21 and Pna21 space groups, and one, 2(I)·6 – in centrosymmetric P21/c space group. Compounds 2(I)·1, 2(I)·2, 2(I)·3 are isomorphous, and demonstrate similar H-bonding patterns despite the differences in aminopyridine molecules. Compound 2(I)·5 is isomorphous to two previously reported compounds. Adducts 2(I)·1, 2(I)·2, 2(I)·3, 2(I)·5, 2(I)·6 represent organic salts composed of pyridinium/pyrimidinium cation, 4-nitrophenolate anion, and 4-nitrophenol neutral molecule. The H-bonded 4-nitrophenol–4-nitrophenolate anionic dimers were found in all compounds with 2 : 1 molar ratio. In adduct I·4 both molecules are in neutral form. The IR spectral data support crystallographic conclusions on salts formation. Plane wave pseudopotential density functional theory calculations were used to predict hyperpolarizability tensor components. Our calculations suggest 2(I)·3 as the best candidate for nonlinear optical materials (14 times more active than urea).

Structural assessment of anhydrous sulfates with high field 33S solid state NMR and first principles calculations

A combination of solid state NMR, first principles calculations and single crystal XRD was applied to relate solid state 33S NMR parameters obtained from a series of anhydrous sulfates of the elements of groups I–III. Operating at high magnetic field of 21.14 T provides a dramatic improvement in the quality of spectra due to significantly enhanced sensitivity and a reduction of second order quadrupolar effects. Experimental 33S NMR spectra for most studied sulfates are dominated by quadrupolar interactions with the quadrupolar parameters unique for each compound. Magnetic shielding constants and quadrupolar parameters for sulfur were calculated using plane wave pseudo-potential density functional theory as implemented in the CASTEP computational package. The calculated NMR parameters are in very good agreement with the experimental results and help in assignment of the stationary spectra. The results demonstrate that such a combined computational – experimental solid state NMR approach can aid in an assessment and improved interpretation of the crystallographic data.

Structural and elastic properties of orthorhombic CaGeO3 under pressure from first principles

The structural and elastic properties of the orthorhombic CaGeO3 under pressure are investigated by the plane wave pseudopotential density functional theory (DFT) within the local density approximation (LDA) and the generalized gradient approximation (GGA). The calculated lattice parameters and unit-cell volume V0 of the orthorhombic CaGeO3 at 0GPa and 0K are in good agreement with the available experimental data and other theoretical data. The pressure dependences of elastic constants, bulk modulus, shear modulus, compressional and shear wave velocities are systematically investigated for the first time. It is found that the orthorhombic CaGeO3 maintains brittle properties under the applied pressures. The analysis for the calculated elastic constants has been made to reveal the mechanical stability and mechanical anisotropy of the orthorhombic CaGeO3 up to 100GPa.

The analysis of a plane wave pseudopotential density functional theory code on a GPU machine

Plane wave pseudopotential (PWP) density functional theory (DFT) calculation is the most widely used material science simulation, and the PWP DFT codes are arguably the most important material science codes. We have implemented a PWP DFT code PEtot on a multi-node GPU machine. Starting from a previous work, we have further improved the speed of the code, and achieved x13-x22 speedups over the CPU calculations for a typical 512 atom system. Such speedups are much higher than other similar works for this important class of material simulation codes on GPU clusters. The current achievement is obtained by (1) moving the calculation fully into the GPU; (2) adopting a new algorithm to reduce the data amount for MPI communication; and (3) using new GPU and CPU numerical libraries. We have also provided a detail quantitative analysis of the computational times for different physical systems and number of GPU units, which helps one to understand the challenges and bottlenecks of the PWP DFT simulations on GPU machines. Based on the analysis, we listed the machine and library requirements in order to further improve the performances of the PWP DFT calculations.