Plane-wave Pseudopotential Density Functional Theory Research Articles

Phase Transition and Electronic Properties of LiBH<sub>4</sub> via First-Principles Calculations

The transition phase and electronic properties of LiBH4 were investigated by ab initio plane-wave pseudopotential density functional theory method. According to the theoretical calculation, the phase sequence Pnma → P21/c → Cc is obtained. The phase transitions Pnma → P21/c and P21/c → Cc are at the pressure of 1.64 GPa and 2.83 GPa, respectively, by total energy-volume data. As the pressure increases, the value of the band gap energy is reduced from 7.1 (Pnma) to 6.1 eV (Cc). Moreover, the electronic properties of the high pressure phases are discussed. The electronic properties are linked to the band gap energy, total (partly) density of states and atoms (bond) populations.

Theoretical predictions of the structural, mechanical and lattice dynamical properties of XW2 (X = Zr, Hf) Laves phases

We have investigated the structural, mechanical and lattice dynamical properties of ZrW2 and HfW2 compounds in cubic C15 (space group Fd-3m), hexagonal C14 (space group P63/mmc) and C36 (space group P63/mmc) phases using generalized gradient approximation within the plane-wave pseudo-potential density functional theory. We have found that ZrW2 and HfW2 in cubic C15 phase are the most stable among the considered phases. From calculated elastic constants, it is shown that all phases are mechanically stable according to the elastic stability criteria. The related mechanical properties, such as bulk, shear and Young moduli, Poisson’s ratio, Debye temperature and hardness have been also calculated. The results show that ZrW2 and HfW2 compounds are ductile in nature with respect to the B/G and Cauchy pressure analysis. The phonon dispersion curves, phonon density of states and some thermodynamic properties are computed and discussed exhaustively for considered phases.

PHASE TRANSITION AND THERMODYNAMIC PROPERTIES OF MAGNESIUM FLUORIDE BY FIRST PRINCIPLES

The structural stabilities, phase transitions and thermodynamic properties of MgF 2 under high pressure and temperature are investigated by first-principles calculations based on plane-wave pseudopotential density functional theory method within the local density approximation. The calculated lattice parameters of MgF 2 in all four phases under zero pressure and zero temperature are in good agreement with the existing experimental data and other theoretical results. Our results demonstrate that MgF 2 undergoes a series of structural phase transitions from rutile (P42/mnm)→ CaCl 2-type (Pnnm)→ modified fluorite (Pa-3)→ cotunnite (Pnam) under high pressure and the obtained transition pressures are in fairly good agreement with the experimental results. The temperature-dependent volume and thermodynamic properties of MgF 2 in the rutile phase at 0 GPa are presented and the thermodynamic properties of MgF 2 in the rutile, CaCl 2-type, modified fluorite and cotunnite phases at 300 K are also predicted using the quasi-harmonic approximation model (QHA) and the quasi-harmonic Debye model (QHD), respectively. Moreover, the partial density of states and the electronic density of the four phases under the phase transition are also investigated.

First Principles Studies on Structural, Electronic and Optical Properties of SnO<sub>2</sub>

The structural, electronic, and optical properties of rutile-type SnO2 are studied by plane-wave pseudopotential density functional theory (DFT) with GGA, LDA, B3LYP and PBE0 respectively. The computing results show that the band gap getting from PBE0 and B3LYP is much more consistent with the available experimental data than that from GGA and LDA, no matter what the latter use ultra-soft pseudopotential or norm conserving pseudopotential. However, the density of state, real part and imaginary part of dielectric function calculating from every type is basically similar in qualitative analysis.

First-principles study of the electronic structure and optical properties of cubic Perovskite NaMgF3

The structural, electronic, and optical properties of cubic perovskite NaMgF3 are calculated by plane-wave pseudopotential density functional theory. The calculated lattice constant a0, bulk modulus B0, and the derivative of bulk modulus B′0 are 3.872 Å, 78.2 GPa, and 3.97, respectively. The results are in good agreement with the available experimental and theoretical values. The electronic structure shows that cubic NaMgF3 is an indirect insulator with a wide forbidden band gap of Eg = 5.90 eV. The contribution of the different bands is analyzed by total and partial density of states curves. Population analysis of NaMgF3 indicates that there is strong ionic bonding in the MgF2 unit, and a mixture of ionic and weak covalent bonding in the NaF unit. Calculations of dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, optical reflectivity, and conductivity are also performed in the energy range 0 to 70 eV.

First-principles calculations of the mechanical properties of IrB and IrB2

We have employed ab-initio plane-wave pseudopotential density functional theory to calculate the equilibrium lattice parameters, elastic constants, under the hydrostatic pressures from 0 to 100 GPa for P1 -IrB with Pnma space group and P5 -IrB2 with Pmmn structures. Results show that the P1 -IrB structure is stable, and the incompressibility is enhanced with the increase of pressure. And the elastic constants, bulk modulus, shear modulus for P5 -IrB2 structure exhibit the regular changes under the hydrostatic pressures from 0 to 100 GPa. But when the pressure becomes 50 GPa, the Young's modulus and the lattice constant in the direction b for P5 -IrB2 structure will change exceptionally. Results show that both are not of obvious band gaps in P1 -IrB and P5 -IrB2 electronic structures under zero pressure, because of the covalent effect between Ir and B atoms. The analysis of band structure and the figure of density of states for P1 -IrB and P5 -IrB2 indicate that the two kinds of structure have metal properties.

First-prinicples study of Mn-N co-doped p-type ZnO

Based on first-principles plane-wave ultrasoft pseudopotential density functional theory method,the lattice structure, formation energy, density of states and charge density of the ZnO:(Mn,N) system are calculated and studied theoretically. Results show that Mn and N co-doped ZnO system is more suitable for doping into a p-type system, for it has a lower impurity formation energy and higher chemical stability; Mn and N in a proportion of 1:2 doping system can effectively reduce the formation energy of the system and so it is more stable; when the system forms a double acceptor level defects, the p-type characteristic of the system is more obvious, for the solubility of impurities and the number of carriers in the system are increased. In addition, it is found that more impurities can go through the Fermi level density of states in the Mn-N co-doped system, while the 2p state density of N is widened and effective mass of holes is smaller and more delocalized.Moreover,compared with the Mn-N-doped system, the density of states of Mn-2N co-doped system is more dispersed near the Fermi level, and the non-localized characteristics are distinctive, thus it is expected to be a more effective means of p-type doping.

First-principles study of high-pressure stability, structure, and elasticity of FeS2 polymorphs

The pressure-dependent elastic properties of the Fe–S system are important to understand the dynamic properties of the Earth’s interior. We have therefore undertaken a first-principles study of the structural and elastic properties of FeS2 polymorphs under high pressure using a method based on plane-wave pseudopotential density function theory. The lattice constants, elastic constants, zero-pressure bulk modulus, and its pressure derivative of pyrite are in good agreement with the previous experiments and theoretical approaches; the lattice constants of marcasite are also consistent with the available experimental data. Calculations of the elastic constants of pyrite and marcasite have been determined from 0 to 200 GPa. Based on the relationship between the calculated elastic constants and the pressure, which can provide the stability of mineral, it would appear that pyrite is stable, whereas marcasite is unstable when the pressure rises above 130 GPa. Static lattice energy calculations predict the marcasite-to-pyrite phase transition to occur at 5.4 GPa at 0 K.

Compositional effect of zirconium on structural, electronical, elastic and thermodynamic properties of PbTi(1−x)ZrxO3

The structural, electronic, elastic properties and the Debye temperature of PbTi(1−x)ZrxO3 compounds are investigated by using the first-principles plane-wave pseudopotential density function theory within the generalized gradient approximation (GGA). The lattice constants of the quaternary compounds PbTi(1−x)ZrxO3 increase with the increasing of Zr composition. The calculated lattice constants are found to agree with the available experimental data. The band structures show that those compounds have indirect band gap except x=0.65 and 0.75, and the composition dependence of band gap energy exhibits increasing trend. The calculated elastic constants indicate that all of the PbTi(1−x)ZrxO3 compounds are mechanically stable. The shear modulus, Young’s modulus Poisson’s ratio σ and the ratio B/G are also calculated. Finally, The averaged sound velocity (vm), the longitudinal sound velocity (vl), transverse sound velocity (vt) and the Debye temperature (θD) are obtained.

Thermodynamic Properties of Nb<sub>4</sub>AlC<sub>3</sub>

We use ab initio plane-wave pseudopotential density functional theory method within local density approximation (LDA) to investigate the thermodynamic properties of Nb4AlC3 under high pressure (0-100 GPa) and high temperature (0-3000 K). Through the quasi-harmonic Debye model, we investigate the thermodynamic properties of Nb4AlC3.The variation of the relative volume, bulk modulus, entropy parameter S, the thermal expansion coefficient α , the heat capacity and the Gruneisen parameter γ with pressure P and temperature T, and many other thermodynamic parameters of Nb4AlC3 are obtained systematically. The bulk modulus of Nb4AlC3 is 250 GPa under zero temperature and zero pressure.

Thermodynamic properties of 3C—SiC

In the present paper, we report on the results of various thermodynamic properties of 3C—SiC at high pressure and temperature using first principles calculations. We use the plane-wave pseudopotential density functional theory as implemented in Quantum ESPRESSO code for calculating various cohesive properties in ambient condition. Further, ionic motion at a finite temperature is taken into account using the quasiharmonic Debye model. The calculated thermodynamic properties, phonon dispersion curves, and phonon densities of states at different temperatures and structural phase transitions at high pressures are found to be in good agreement with experimental and other theoretical results.

Ab initio calculations of structure and thermodynamic properties of tetragonal-TiH2under high temperatures and pressures

The structural and thermodynamic properties of tetragonal-TiH 2 under high temperatures and pressures are investigated by Ab initio calculations based on pseudo-potential plane-wave density functional theory method within using the generalized gradient approximation (GGA) and quasi-harmonic Debye model. Some ground state properties such as lattice constants, bulk modulus and elastic constants are good agreement with the available experimental results and other theoretical data. Through the quasiharmonic Debye model, in which the phononic effects are considered, the thermodynamic properties of tetragonal-TiH 2 such as thermal expansion coefficient, Debye temperature, heat capacity and Gruneisen parameters dependence of temperature and pressure in the range of 0–1000 K and 0–10 GPa are also presented, respectively.

PHASE TRANSITION, ELASTIC AND THERMODYNAMIC PROPERTIES OF BERYLLIUM VIA FIRST PRINCIPLES

The phase transition, elastic and thermodynamic properties of beryllium (Be) have been studied at high pressures by plane-wave ultrasoft pseudopotential density functional theory (DFT) within the generalized gradient approximation (GGA). It is found that the hcp → bcc phase transition of Be occurs at 506 GPa (T = 0 K ) and occurs at 1200 K (P = 0 GPa ). The coefficients of linear thermal expansion of the hexagmal close-packed (hcp), bcc and orthorhombic Be have been calculated. The hcp → orthorhombic → bcc phase transitions do not occur in all range of pressures, that is to say, the orthorhombic Be is not an intermediate phase between the hcp and bcc Be. The obtained bulk modulus (B0) are 113.2 GPa (for hcp Be), 113.1 GPa (for bcc Be) and 70.5 GPa (for orthorhombic Be), respectively.

Pressure effect on the structural, elastic and electronic properties of Nb2AC (A = S and In) phases; ab initio study

The structural parameters, elastic and electronic properties of Nb2AC (A=S and In) phases were investigated under pressure using first-principles plane-wave pseudopotential density functional theory within the generalized gradient approximation. We find that the effect of pressure on the crystal structure reflects in a compression of the unit cell-volume, mainly along the c-axis. On the other hand, the elastic constants, elastic modulus and the Debye temperature θD increase monotonically as the pressure increases. The relationship between brittleness and ductility shows that Nb2AC are brittle at 0GPa and only the Nb2SC phase tended to be ductile under pressure from 4GPa to 10GPa. Finally, we find that the density of states at the Fermi level decrease in the Nb2InC phase and increase in Nb2SC phase under pressure.

Study of Thermodynamic Properties of Superhard <i>w</i>-BC<sub>2</sub>N

We investigate the thermodynamic properties of superhard w-BC2N by using ab initio plane-wave pseudopotential density functional theory method within local density approximation (LDA). Through the quasi-harmonic Debye model, we investigate the thermodynamic properties of w-BC2N. The variation of the thermal expansion, the heat capacity and the Gruneisen parameter γ with pressure P and temperature T, and many other thermodynamic parameters of w-BC2N are obtained systematically.

Theoretical study of elastic and thermodynamic properties of η-Ta2N3

Abstract We present a detailed theoretical study of the elastic properties of η-Ta2N3 under pressure ranging from 0 GPa to 80 GPa by plane-wave pseudopotential density functional theory with the generalized gradient approximation in this paper. The results show that η-Ta2N3 is mechanically unstable at ambient conditions because of a negative C66, and it will be stable when the pressure is up to 10.7 GPa. The thermodynamic properties of η-Ta2N3 are also investigated through quasi-harmonic Debye model in a pressure range of 0–80 GPa and a temperature range of 0–1500 K. The Debye temperature, heat capacity and thermal expansion with pressure and temperature are successfully obtained and discussed.

Theoretical calculations for structural, elastic, and thermodynamic properties of c-W3N4 under high pressure

We have investigated the structural and elastic properties of c-W3N4 at high pressures using generalized gradient approximation within the plane-wave pseudopotential density functional theory. The obtained normalized volume dependence of the resulting pressure is in excellent agreement with our experimental data investigated using high-P synchrotron x-ray diffraction. The elastic constants as a function of the applied pressure, the bulk modulus, Young's modulus and shear modulus on the pressure P are also successfully obtained. The superior mechanical properties indicate that c-W3N4 is a potential candidate structure to be one of the ultra-incompressible and hard materials. The high-pressure elastic constants indicate that c-W3N4 is mechanically stable up to 50 GPa. By the elastic stability criteria, it is predicted that c-W3N4 is not stable above 110 GPa. In addition, the calculated B/G ratio indicated that c-W3N4 possesses ductile nature in the range of pressure from 0 to 50 GPa. The calculated elastic anisotropic factors suggest that c-W3N4 is elastically anisotropic. Through the quasi-harmonic Debye model, we also investigate the thermodynamic properties of c-W3N4.

First-principles calculations of structural and electronic properties of Ta2N3 under high pressures

We present a detailed theoretical study for the structural and electronic properties of the new synthesized hard material Ta2N3 under pressure ranging from 0GPa to 100GPa using the plane-wave pseudopotential density functional theory by CASTEP code in this paper. The results at zero pressure are in good agreement with the available theoretical and experimental values. We find that the Ta2N3 will undergo phase transition from tetragonal to orthorhombic phase at a pressure of 10.6GPa. There are strong covalent bonds between N and Ta atoms in Ta2N3, and the high pressure will reduce the hybridization energy.

Searching for Stable SinCn Clusters: Combination of Stochastic Potential Surface Search and Pseudopotential Plane-Wave Car-Parinello Simulated Annealing Simulations

To find low energy SinCn structures out of hundreds to thousands of isomers we have developed a general method to search for stable isomeric structures that combines Stochastic Potential Surface Search and Pseudopotential Plane-Wave Density Functional Theory Car-Parinello Molecular Dynamics simulated annealing (PSPW-CPMD-SA). We enhanced the Sunders stochastic search method to generate random cluster structures used as seed structures for PSPW-CPMD-SA simulations. This method ensures that each SA simulation samples a different potential surface region to find the regional minimum structure. By iterations of this automated, parallel process on a high performance computer we located hundreds to more than a thousand stable isomers for each SinCn cluster. Among these, five to 10 of the lowest energy isomers were further optimized using B3LYP/cc-pVTZ method. We applied this method to SinCn (n = 4–12) clusters and found the lowest energy structures, most not previously reported. By analyzing the bonding patterns of low energy structures of each SinCn cluster, we observed that carbon segregations tend to form condensed conjugated rings while Si connects to unsaturated bonds at the periphery of the carbon segregation as single atoms or clusters when n is small and when n is large a silicon network spans over the carbon segregation region.

First-principles investigations of the physical properties of binary uranium silicide alloys

The structural, elastic properties and the Debye temperature of binary Uranium Silicide (U-Si) alloys are investigated by using the first-principles plane-wave pseudopotential density function theory within the generalized gradient approximation (GGA). The ground states properties are found to agree with the available experimental data. The mechanical properties like shear modulus, Young’s modulus, Poisson’s ratio σ and ratio B/G are also calculated. Finally, The averaged sound velocity (vm), the longitudinal sound velocity (vl), transverse sound velocity (vt) and the Debye temperature (θD) are obtained. However, the theoretical values are slightly different from few existed experiment data because the latter was obtained at room temperature while the former one at 0K.