First-principles Total-energy Calculations Research Articles

A universal trend of structural, mechanical and electronic properties in transition metal (M = V, Nb, and Ta) borides: First-principle calculations

First-principle total energy calculations are employed to provide a fundamental understanding of the structural, mechanical, and electronic properties of transition metal (M=V, Nb, and Ta) borides with different boron concentrations (M2B, M3B2, MB, M5B6, M3B4, M2B3, and MB2). The results show that a universal trend in thermodynamic stability, elastic properties (bulk modulus and shear modulus) and electronic structure exists for V, Nb, and Ta borides. With the increase of boron concentrations, thermodynamic stability and bulk modulus of V, Nb, and Ta borides present an increasing trend, whereas shear modulus reaches the maximum at M2B3 and then decreases to MB2. The variation of electronic structure and Mulliken overlap population explain the origin of the general changes of these borides and their superior mechanical properties. Also, due to the relatively higher mechanical properties, M5B6, M3B4, and M2B3 are demonstrated to be possible candidates for superhard films.

First-principles study of Si(111)[formula omitted]-In reconstruction

Using first-principles total-energy calculations, structural properties of the Si(111)31×31-In reconstruction have been studied. New refined structural model of the reconstruction has been proposed which adopts 17 In atoms and 31 Si atoms. The model is characterized by the reasonably low surface energy and demonstrates good correspondence between simulated and experimental scanning tunneling microscopy images. Calculations reveal semiconducting nature of the model structure in agreement with experiment.

First-principles study of B1 to B2 phase transition in PbS

The high pressure structural phase transition in PbS has been studied by means of first-principles total energy calculations which are based on linear combination of atomic orbitals (LCAO) method within local density approximation (LDA). In the present study, the exchange scheme of Becke and correlation functional of von-Barth-Hedin (VBH) are employed. It is observed that more stable phase for PbS is NaCl type (B1) and PbS transforms to the CsCl type (B2) structure under high pressure (22.8 GPa). The calculated value of transition pressure (Pt) from B1 to B2 structure is found in good agreement with the earlier experimental and theoretical investigations.

Electronic Structure Modulation of Graphene by Metal Electrodes

We investigated electronic structure modulation of graphene partially covered by Pd, Ag, Pt, and Au pillars by using first-principles total-energy calculations to simulate the effects of metal electrodes on the electronic structure of graphene. Our calculations show that the electrostatic potential in graphene strongly depends not only on the metal species but also on its displacement from the electrodes. This potential modulation leads to an energy shift of the Dirac point of graphene, resulting in carrier transfer between graphene and the metal pillars depending on the metal species and spatial position.

Structural and Electronic Properties of Zr x Al1−x N in Cubic Phase Using First Principles

We calculate the structural and electronic properties of the ZrxAl1−xN in cubic phase using first-principles total energy calculations. The study is based on the functional density theory, DFT, with Augmented Plane Wave Plus Local Orbitals method for Calculating Crystal Properties (FP-LAPW). Determination of the crystal lattice constant, the volume of the unit cell, the bulk modulus for concentrations x=0.25,0.50,0.75, is carried out. In addition, we also study the electronic properties for each x. The study consists of the analysis of the electrons energy versus the wave vector, in some directions of the first Brillouin zone, and the Density of States (DOS) as a function of the energy. The results show that the lattice constant increases with x, but the functional relations are not linear. On the other hand, the electronic properties vary substantially with x, and a possible semiconductor metal transition is predicted for any x<0.25.

Hydrogen spillover storage on Ca-decorated graphene

In this work, first-principles total energy calculations were performed in order to study the structure and hydrogen storage behavior on Ca-decorated graphene. On the stable structure of Ca-decorated graphene with 3×3−300 reconstruction, the first hydrogen molecule adsorbed is dissociative, with the energy barrier of only 0.05 eV. The electrons of H atoms saturate the electronic states of Ca around the Fermi level and enhance the system stability. Further adsorption of hydrogen molecules on Ca-adsorbed graphene is weak, which indicates Ca-adsorbed graphene does not suit for the hydrogen storage via physical adsorption of hydrogen molecules. On the other hand, hydrogen spillover mechanism could exist on Ca-decorated graphene. On the graphene with one Ca dimer adsorbed, one of the four H atoms adsorbed on the Ca dimer adsorbes chemically on C in graphene more stably by 0.37 eV than on the Ca dimer. With the number of hydrogen atoms adsorbed on Ca-decorated graphene increases, the binding energy of hydrogen atoms tends to increase. Thus, the spillover process is energetically favorable. The hydrogen storage capacity via the spillover mechanism in Ca-adsorbed graphene depends on the Ca content and could approach 7.7 wt.%.

First-principles study of electronic, dynamical and thermodynamic properties of Li2TiO3

The first-principles total energy calculations with the generalised gradient approximation and the plane-wave pseudopotential method have been employed to investigate the structural, electronic and dynamical properties of Li2TiO3 (lithium titanate). The atomic structure is fully relaxed, and the structural parameters are found to differ by less than 1% from the experimental data. The indirect band-gap with 3.49 eV is predicted from the band structure calculations of this compound. The calculated phonon frequencies at the Γ-point for the Raman-active and the infrared-active modes are presented and assigned. The phonon dispersion curves are also calculated along high symmetry lines in the Brillouin zone (BZ). Furthermore, the thermodynamic functions have been worked out using the corresponding phonon density of states, and the results indicate that they are in good agreement with available experimental values.

Multiscale calculations of thermoelectric properties ofn-type Mg2Si1−xSnxsolid solutions

The band structure of Mg${}_{2}$Si${}_{1\ensuremath{-}x}$Sn${}_{x}$ solid solutions with 0.250 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 0.875 is calculated using the first-principles pseudopotential method. It is found that the low-lying light and heavy conduction bands converge and the effective mass reaches a maximum value near $x$ $=$ 0.625. Using the semiclassical Boltzmann transport theory and relaxation-time approximation, we find that the system with $x$ $=$ 0.625 exhibits both higher Seebeck coefficient and higher electrical conductivity than other solid solutions at intermediate temperatures. By fitting first-principles total energy calculations, a modified Morse potential is constructed, which is used to predicate the lattice thermal conductivity via equilibrium molecular dynamics simulations. Due to relatively higher power factor and lower thermal conductivity, the Mg${}_{2}$Si${}_{0.375}$Sn${}_{0.625}$ is found to exhibit enhanced thermoelectric performance at 800 K, and additional Sb doping is considered in order to make a better comparison with experiment results.

First-principles calculations and thermodynamic re-modeling of the Hf–W system

An integrative approach coupling first-principles calculations and the CALculation of PHAse Diagram (CALPHAD) method provides a more thermodynamically accurate model of the Hf–W system when compared to previous models. A two-sublattice model is used for describing the solid solubility of the HfW2 Laves phase. The modeling of the Laves phase includes input from first-principles total energy calculations and predictions of finite temperature properties from the Debye–Grüneisen model. In addition, first-principles calculations performed on hcp and bcc special quasirandom structures (SQS) predicted a positive enthalpy of mixing in both solid solution phases. Predicting the finite temperature properties of bcc SQS with the Debye–Grüneisen model was necessary to balance the positive, asymmetric enthalpy of mixing found in the bcc solid solution. The model produced by the coupling of CALPHAD modeling with first-principles calculations agrees well with experimental data. It also reproduces the Hf–W phase diagram with fewer parameters than previous models, which were created without the aid of first-principles calculations.

Cerium-Doped Endohedral Fullerene: A Density-Functional Theory Study

First-principles total energy calculations of the structural and electronic properties of Ce-doped fullerene have been performed within the framework of the density functional theory at the generalized gradient approximation level. Among various locations, Ce atom was found to engage with the six-fold carbon ring. The total energy is found to significantly change as the Ce atom being shifted from the center of the cage toward the edge close to the six-membered ring where the total energy reaches its local minimum. Moreover, repulsive interaction between Ce atom and the cage components turns as the adatom directly interacts with the six C atoms of the ring. The lowest-energy CeC60 geometry is found to have a binding energy of approximately 5.34 eV, suggesting strong interaction of the dopant with the cage members. Furthermore, fundamental key structural parameters and the total density of states of the optimized structure have been determined and compared with the available data.

Equilibrium lines and barriers to phase transitions: the cubic diamond to beta-tin transition in Si from first principles

The phase transition between the cubic diamond (cd) and beta-tin (β-Sn) phases of Si under pressure and the region of interaction of the two phases are studied by first-principles total energy calculations. For a non-vibrating crystal we determine the pressure of the thermodynamic phase transition pt = 96 kbar, the Gibbs free energy barrier at pt of ΔG = 19.6 mRyd/atom that stabilizes the phases against a phase transition and the finite pressure range in which both phases are stable. We show that the phases in that pressure range are completely described by three equilibrium lines of states along which the structure, the total energy E, the hydrostatic pressure p that would stabilize the structure and the values of G all vary. Two equilibrium lines describe the two phases (denoted the ph-eq line, ph is cd or β-Sn phase); a third line is a line of saddle points of G with respect to structure (denoted the sp-eq line) that forms a barrier of larger G against instability of the metastable ranges of the phase lines. An important conclusion is that the sp-eq line merges with the two ph-eq lines: one end of the sp-eq line merges with the cd-eq line at high pressure, the other end merges with the β-Sn-eq line at low pressure. The mergers end the barrier protecting the metastable ranges of the two ph-eq lines, hence the lines go unstable beyond the mergers. The mergers thus simplify the phase diagram by providing a natural termination to the stable parts of all metastable ranges of the ph-eq lines. Although 96 kbar is lower than the experimental transition pressure, we note that phonon pressure raises the observed transition pressure.

Ab-initio calculations of the direct and hydrogen-assisted dissociation of CO on Fe(3 1 0)

Via the formation process of the adsorbed intermediates formyl (HCO) and hydroxy-carbene (COH), the thermodynamics of the hydrogen-assisted CO dissociation on Fe(310) is investigated by means of first-principles total-energy calculations. A comparison with direct CO dissociation in the presence of coadsorbed atomic hydrogen leads to the conclusion that the direct process is the only thermodynamically viable route for CO dissociation on Fe(310), with strongly endothermic formation energies for both intermediates, HCO and COH.

Mechanical properties and hardness of new carbon-rich superhard C11N4 from first-principles investigations

The structural, mechanical properties and hardness of the new carbon-rich material C11N4 are studied by first-principles total energy calculations based on the density-functional theory. We use the empirical equations of state (EOS) to investigate the lattice properties and bulk modulus. It is found that the calculated lattice constants and bulk modulus are in good agreement with previous calculations. And the full set elastic constants are calculated using the stress-strain method. The Voigt-Reuss-Hill approximation is used to evaluate the mechanical moduli. The elastic constants show that the two phases of C11N4 are mechanically stable. The tetragonal-C11N4 (α-C11N4) exhibits larger mechanical moduli than the orthorhombic-C11N4 (β-C11N4). The mechanical anisotropy is calculated of several different anisotropic indexes and factors, such as universal anisotropic index (AU), the percent anisotropy (AG and AB) and shear anisotropic factors (A1, A2 and A3). Furthermore, the hardness of α-C11N4 and β-C11N4 are evaluated according to the intrinsic hardness calculation theory. α-C11N4 is predicted to be a superhard material with the Vickers hardness of 67.17GPa, which is slightly higher than that of the cubic boron nitride. And the β-C11N4 is also a superhard material with the calculated Vickers hardness of 45.63GPa. C11N4 can be considered as candidate superhard compounds.

Effect of interstitial atoms on the stability and electronic structure of Re3Zn alloy: First-principles calculations

Using first-principles total energy calculations, we investigate the structural stability and electronic properties of Re3ZnNx (x = 0, 0.25, 0.5, 1) and Re3ZnX (X = H, B, C, and O) compounds. The calculations indicated that interstitial atoms including N, H, B, C, and O, always prefer the octahedral sites to the tetrahedral sites in the studied hexagonal structure, in good agreement with the experimental results. However, the calculated heat of formation and elastic properties suggest that the incorporation of nitrogen is not favorable for the formation of the Re–Zn alloy, but high pressure and high temperature lower the energy barrier to facilitate the incorporation of N into Re3Zn alloy. Furthermore, B addition displays the highest effect in strengthening Re–Zn alloy upon the formation of relatively-strong boron–boron bond and thus higher shear modulus. Also, the ductile character of these alloys is important for possible applications.

Lateral in-plane coupling between graphene nanoribbons: A density functional study

Properties brought about by lateral in-plane coupling between graphene nanoribbons (GNRs) are investigated using the first-principle total energy calculations. It is found that, when two GNRs approach each other, the lateral coupling between the two brings about edge state splitting. Between zigzag-edged graphene nanoribbons (ZGNRs), the coupling mainly results from Coulomb and spin-spin interaction, while for armchair-edged graphene nanoribbons (AGNRs), it is from Coulomb interaction only. It is further found that the maximum inter-ribbon distance for effective coupling depends on the type of ribbons, which is ∼10 Å for ZGNRs, but ∼6 Å for AGNRs. Also, displacements of the GNRs along the ribbon direction are found to affect the electronic properties of the coupled GNRs. The results may be important for the microminiaturization of future nanoelectronic and spintronic devices based on graphene.

Structural stability, phase transition, and mechanical and electronic properties of transition metal nitrides MN (M = Tc, Re, Os, and Ir): First-principles calculations

The structural stabilities, phase transition, and mechanical and electronic properties of MN (M=Tc, Re, Os, and Ir) with different structures were reported by means of first-principles total energy calculations. The calculations indicate that the NbO-type structure is more energetically preferred for TcN and ReN, and the Pmn21-type structure is more favorable for IrN and OsN. The NbO-type ReN and TcN are stable up to 51.9 and 19.5GPa, respectively, above which NiAs type structure becomes more energetically favorable. Both NiAs-type ReN and Pmn21-type OsN exhibit excellent mechanical properties due to the strong bonding between M and N atoms, similar to TiN. The electronic structure calculation suggests that the Pmn21-IrN is a semiconductor with an indirect band gap of about 0.38eV, suggesting the potential technological applications.

Design of [formula omitted] electron network in graphene using atomic Pt adsorbates

Based on a first-principles total-energy calculation, we demonstrate the possibility of π electron network design in graphene using atomic Pt adsorbate. The d state of the adsorbed Pt atoms hybridizes with the π state of graphene, which effectively creates a boundary in the two-dimensional π electron network in graphene. By adsorbing Pt atoms along the line parallel to the zigzag direction in graphene, we found that a characteristic flat dispersion band emerges at the zone boundary of the two-dimensional Brillouin zone. This flat dispersion band is partially filled by electrons, which induces a magnetic moment in this system of non-magnetic elements.

Tunable Magnetic Properties of Rhombohedral Graphite Thin Films: Effects of Insulating Substrate on Magnetic Properties

First-principles total energy calculations are performed to investigate the electronic and magnetic properties of rhombohedral graphite thin films on hexagonal boron nitride (h-BN) substrate. We find that the ferrimagnetic spin ordering of graphite thin films on h-BN substrate is robust irrespective of the adsorption arrangement. However, by enhancing the interaction between the graphite thin film and the substrate, the interaction suppresses the electron spin polarization of the bottommost graphene layer situated at the interface with the substrate. The electron spin polarization emerges at the second subsurface graphene layer in the thin film. We also find that the magnetic property of the graphite thin film on the insulating substrate is tunable by applying an external pressure.

Structure and stability of Al2Fe and Al5Fe2: First-principles total energy and phonon calculations

We employ first-principles total energy and phonon calculations to address the structure and stability of Al${}_{2}$Fe and Al${}_{5}$Fe${}_{2}$. The observed structure of Al${}_{2}$Fe, which is reported as stable in the assessed Al-Fe phase diagram, is distinguished by an unusually low triclinic symmetry. The initial crystallographic structure determination additionally featured an unusual hole large enough to accommodate an additional atom. Our calculations indicate that the hole must be filled, but predict that the triclinic structure is unstable relative to a simpler tetragonal structure based on the prototype MoSi${}_{2}$. This tetragonal structure is interesting because it is predicted to be nonmagnetic, electrically insulating, and high density, while the triclinic structure is magnetic, metallic, and low density. We reconcile this seeming contradiction by demonstrating a high vibrational entropy that explains why the triclinic structure is stable at high temperatures. Finally, we note that orthorhombic Al${}_{5}$Fe${}_{2}$ is also destabilized by the tetragonal structure but may be stabilized at high temperatures, again by vibrational entropy and partial occupancy associated with the diffusion of Al atoms along channels.

Competition between ferromagnetic and anti-ferromagnetic couplings in Co doped ZnO with vacancies and Ga co-dopants

Spin-polarized first-principles electronic structure and total energy calculations have been performed to better understand the magnetic properties of Co doped ZnO (ZnO:Co) with vacancies and Ga co-dopants. The paramagnetic state of ZnO:Co, in which Co ions lose their magnetic moments, has been found to be unstable. The total energy results show that acceptor-like Zn vacancies and donor-like Ga co-dopants render the anti-ferromagnetic (AFM) and ferromagnetic (FM) states to be more favorable, respectively. With O vacancies, ZnO:Co has been found to be in the weak FM state. These magnetic properties can be understood by the calculated O- and Zn-vacancies and Ga co-dopant induced changes of the electronic structure, which suggest that AFM and FM Co-Co couplings are mediated by O 2p-Co majority (↑)-spin 3d hybridized states in the valence band of ZnO and O-vacancy-derived p states or Ga sp states in the ZnO band gap, respectively. For ZnO:Co with Zn vacancies (Ga co-dopants) the AFM (FM) coupling outweighs the FM (AFM) coupling and results in the AFM (FM) state, while for ZnO:Co with O vacancies, both the FM and AFM couplings are enhanced by similar degrees and result in the weak FM state. This study reveals a competition between FM and AFM couplings in ZnO:Co with vacancies and Ga co-dopants, the detailed balancing between which determines the magnetic properties of these materials.