Pseudopotential Density Functional Theory Research Articles

Theoretical calculations for structural, elastic, and thermodynamic properties of RuN2 under high pressure

The structural and elastic properties of RuN2 were investigated through the first-principles calculation using generalized gradient approximation (GGA) and local density approximation (LDA) within the plane-wave pseudopotential density functional theory. The obtained equilibrium structure and mechanical properties are in excellent agreement with other theoretical results. Then we compared the elastic modulus of RuN2 with several other isomorphic noble metal nitrides. Results show that RuN2 can nearly rival with OsN2 and IrN2, which indicate RuN2 is a potentially ultra-incompressible and hard material. By the elastic stability criteria, it is predicted that RuN2 is stable in our calculations (0–100 GPa). The calculated B/G ratios indicate that RuN2 possesses brittle nature at 0 GPa and when the pressure increases to 13.4 GPa (for LDA) or 20.8 GPa (for GGA), it begins to prone to ductility. Through the quasi-harmonic Debye model, we also investigated the thermodynamic properties of RuN2.

The elastic and thermodynamic properties of ZrMo2 from first principles calculations

The elastic and thermodynamic properties of ZrMo2 under high temperature and pressure are investigated by first-principles calculations based on pseudopotential plane-wave density functional theory (DFT) within the generalized gradient approximation (GGA) and quasi-harmonic Debye model. The calculated lattice parameters are in good agreement with the available experimental data. The calculated elastic constants of ZrMo2 increase monotonically with increasing pressure, and the relationship between the elastic constants and pressure show that ZrMo2 satisfies the mechanical stability criteria under applied pressure (0–65GPa). The related mechanical properties such as bulk modulus (B), shear modulus (G), Young’s modulus (E), and Poisson’s ratio (v) are also studied for polycrystalline of ZrMo2. The calculated B/G value shows that ZrMo2 behaves in a ductile manner, and higher pressure can significantly improve the ductility of ZrMo2. The pressure and temperature dependencies of the relative volume, the bulk modulus, the elastic constants, the heat capacity and the thermal expansion coefficient, as well as the Grüneisen parameters are obtained and discussed by the quasi-harmonic Debye model in the ranges of 0–1800K and 0–65GPa.

Doping the transition metal atom Fe, Co, Ni into C48B12 fullerene for enhancing H2 capture: A theoretical study

The feasibility of transition metal coated fullerene cages M12C48B12(M = Fe, Co, and Ni) for hydrogen storage is investigated by the pseudopotential density functional theory. Fe12C48B12(Co12C48B12 and Ni12C48B12) adsorbs 60(48 and 48) H2 with moderate average adsorption energy of 0.50(0.45 and 0.32) eV/H2. The gravimetric hydrogen density of Fe12C48B12(Co12C48B12 and Ni12C48B12) can reach 8.7(6.8 and 6.8) wt%. The Dewar–Kubas interaction dominates the adsorption of H2 on the outer surface of Fe12C48B12(Co12C48B12 and Ni12C48B12). Therefore, the stable M12C48B12(M = Fe, Co, and Ni) cages can be applied as candidates for hydrogen storage under near-ambient conditions.

Structural Phase Transition and Electronic Properties of MgCe under High Pressure from First-Principles Calculations

The structural stability of MgCe under high pressures has been investigated by using the first-principles plane-wave pseudopotential density functional theory within the local density approximation (LDA). The obtained results predict that MgCe in the Ba structure is predicted to be the most stable structure corresponding to the ground state, because of lowest total energy. MgCe undergoes a pressure-induced phase transition from the Ba structure to B32 structure at 36 GPa. And no further transition is found up to 120 GPa. In addition, the electronic structures of four structures of MgCe are also calculated and discussed.

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.

Structural, electronic, mechanical, and dynamical properties of graphene oxides: A first principles study

We report the results of a theoretical study on the structural, electronic, mechanical, and vibrational properties of some graphene oxide models (GDO, a-GMO, z-GMO, ep-GMO and mix-GMO) at ambient pressure. The calculations are based on the ab-initio plane-wave pseudo potential density functional theory, within the generalized gradient approximations for the exchange and correlation functional. The calculated values of lattice parameters, bulk modulus, and its first order pressure derivative are in good agreement with other reports. A linear response approach to the density functional theory is used to derive the phonon frequencies. We discuss the contribution of the phonons in the dynamical stability of graphene oxides and detailed analysis of zone centre phonon modes in all the above mentioned models. Our study demonstrates a wide range of energy gap available in the considered models of graphene oxide and hence the possibility of their use in nanodevices.

First-principles study of vibrational modes and Raman spectra in P-doped Si nanocrystals

We have studied the vibrational modes and Raman spectra of P-doped Si nanocrystals using pseudopotential density functional theory and the Placzek approximation. We find that Si nanocrystal vibrations are largely unaffected by the introduction of P dopants. However, the Raman spectra of doped nanocrystals are enhanced relative to those of pristine nanocrystals, and demonstrate a strong dependence on dopant position. Thus, Raman has the potential of being developed as a tool for probing the location of the dopant within the nanocrystal. Our analysis shows that vibrational modes involving atoms in the vicinity of the dopant give the largest contributions to the Raman spectra.

Theoretical investigation of charge accumulation layer on the Bi-induced InAs(111)-(2 × 2) surface

Based on pseudopotential method and density functional theory, we have investigated the stability, atomic geometry, and detailed electronic structures for Bi adsorbates on the InAs(111)-(2 × 2) surface with three different sites: (i) T4 (Bi trimer centered on T4 site), (ii) H3 (Bi trimer centered on H3 site), and (iii) T4–H3 (which is formed by trimers with opposite orientations: one centered on a T4 site and the other on a H3). Our total energy calculations suggest that adsorption on the T4–H3 site is the energetically most stable structure among the proposed structures. The electronic band structure calculations reveal the existence of an accumulation layer between InAs(111) surface and Bi adatoms for T4–H3. Charge density difference results indicate significant amount of the charge accumulation on the Bi/InAs interface.

The near edge structure of hexagonal boron nitride.

Hexagonal boron nitride (hBN) is a promising material for a range of applications including deep-ultraviolet light emission. Despite extensive experimental studies, some fundamental aspects of hBN remain unknown, such as the type of stacking faults likely to be present and their influence on electronic properties. In this paper, different stacking configurations of hBN are investigated using CASTEP, a pseudopotential density functional theory code. AB-b stacking faults, in which B atoms are positioned directly on top of one another while N atoms are located above the center of BN hexagons, are shown to be likely in conventional AB stacked hBN. Bandstructure calculations predict a single direct bandgap structure that may be responsible for the discrepancies in bandgap type observed experimentally. Calculations of the near edge structure showed that different stackings of hBN are distinguishable using measurements of core-loss edges in X-ray absorption and electron energy loss spectroscopy. AB stacking was found to best reproduce features in the experimental B and N K-edges. The calculations also show that splitting of the 1s to π* peak in the B K-edge, recently observed experimentally, may be accounted for by the presence of AB-b stacking faults.

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.

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.

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-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 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.

Elastic and electronic properties of MnTi2O4 under pressure: A first-principle study

The structural, magnetic, elastic and electronic properties of MnTi2O4 are investigated by using plain-wave pseudopotential density functional theory within LDA+U framework. Our results show that MnTi2O4 is indeed a face-centered cubic crystal with ferrimagnetism (FCC-MAG), and maybe have semiconductor property at 0GPa and 0K. The results consist well with the experimental results by Huang et al. The elastic constants, bulk modulus (B), shear modulus (G), and Poisson ratios (σ) of the FCC-MAG MnTi2O4 are also obtained for the first time. By analyzing its elastic constants, we find that the FCC-MAG MnTi2O4 is mechanical stable up to 25GPa. The obtained values of B/G indicate the ductility of the FCC-MAG MnTi2O4, and the calculated values of Poisson ration σ show that the inter-atomic forces in FCC-MAG MnTi2O4 is central force in the range of applied pressure. Moreover, its hardness is predicated, the pressure dependences of the electronic structures are also investigated. It is expected that our work can provide useful information to further investigate MnTi2O4 from both the experimental and theoretical sides.

Electronic properties of mixed-phase graphene/h-BN sheets using real-space pseudopotentials

A major challenge for applications of graphene is the creation of a tunable electronic band gap. Hexagonal boron nitride has a lattice very similar to that of graphene and a much larger band gap, but B-N and C do not alloy: B-C-N materials tend to phase separate into $h$-BN and C domains. Quantum confinement within the finite-sized C domains of a mixed B-C-N system can create a band gap, albeit within an inhomogeneous system. Here we investigate the properties of hybrid $h$-BN/C sheets with real-space pseudopotential density functional theory. We find that the electronic properties are determined not just by geometrical confinement, but also by the bonding character at the $h$-BN/C interface. B-C terminated carbon regions tend to have larger gaps than N-C terminated regions, suggesting that boron-carbon bonds are more stable. We examine two series of symmetric structures that represent different kinds of confinement: a graphene dot within a $h$-BN background and a $h$-BN antidot within a graphene background. The gaps in both cases vary inversely with the size of the graphenic region, as expected, and can be fit by simple empirical expressions.

3d–4f Magnetic Interaction with Density Functional Theory Plus U Approach: Local Coulomb Correlation and Exchange Pathways

The 3d-4f exchange interaction plays an important role in many lanthanide based molecular magnetic materials such as single-molecule magnets and magnetic refrigerants. In this work, we study the 3d-4f magnetic exchange interactions in a series of Cu(II)-Gd(III) (3d(9)-4f(7)) dinuclear complexes based on the numerical atomic basis-norm-conserving pseudopotential method and density functional theory plus the Hubbard U correction approach (DFT+U). We obtain improved description of the 4f electrons by including the semicore 5s5p states in the valence part of the Gd-pseudopotential. The Hubbard U correction is employed to treat the strongly correlated Cu-3d and Gd-4f electrons, which significantly improve the agreement of the predicted exchange constants, J, with experiment, indicating the importance of accurate description of the local Coulomb correlation. The high efficiency of the DFT+U approach enables us to perform calculations with molecular crystals, which in general improve the agreement between theory and experiment, achieving a mean absolute error smaller than 2 cm(-1). In addition, through analyzing the physical effects of U, we identify two magnetic exchange pathways. One is ferromagnetic and involves an interaction between the Cu-3d, O-2p (bridge ligand), and the majority-spin Gd-5d orbitals. The other one is antiferromagnetic and involves Cu-3d, O-2p, and the empty minority-spin Gd-4f orbitals, which is suppressed by the planar Cu-O-O-Gd structure. This study demonstrates the accuracy of the DFT+U method for evaluating the 3d-4f exchange interactions, provides a better understanding of the exchange mechanism in the Cu(II)-Gd(III) complexes, and paves the way for exploiting the magnetic properties of the 3d-4f compounds containing lanthanides other than Gd.

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.