Principles Total Energy Calculations Research Articles

Influence of impurities on phase stability of martensites in titanium

The influence of H, C, N and O impurities on the phase stability of titanium was studied by first principles total energy calculations. The occupation energies of the impurities were estimated to identify their site preferences. All impurities considered prefer to occupy the octahedral site except for H, which tends to occupy the tetrahedral site in the β phase. Electronic structures were analyzed to clarify the intrinsic influence mechanisms of impurity on the stability of martensitic phases. It was found that the density of states around the Fermi energy, which was affected dramatically by impurity, and the bonding interactions between impurity and titanium were connected to the phase stability of Ti. Elastic constants of impurity-containing systems were estimated from curves of energy against strain to evaluate the mechanical stability of these systems. It was shown that the α″ phase can not be stabilized by impurities considered here, while the α′ phase (regardless of the impurities) and H- and C-containing β phase are thermodynamically stable and also satisfy the mechanical stability criteria.

Theoretical study of the formation of a GaAs bilayer on Si(1 1 1)

We have performed first principles total energy calculations, within the density functional theory, to investigate the formation of a GaAs bilayer on the Si(111) surface. For the exchange–correlation energies we have used the generalized gradient approximation, while the electron–ion interactions were treated using ultrasoft pseudo-potentials. We have first studied the adsorption of As and Ga atoms on the Si(111) surface. A monolayer of As atoms substitutes the topmost Si layer forming an almost ideally bulk terminated configuration. On the other hand, the adsorption of 1/3ML of Ga results in the formation of a √3×√3 reconstruction, with Ga atoms occupying T4 sites. The deposit of 1/3ML of Ga atoms on the As terminated Si(111) surface also results in the occupation of T4 sites. However, at full monolayer coverage, the Ga atoms occupy near top sites and form one dimensional atomic chain.

Prediction of orientational phase transition in boron carbide

The assessed binary phase diagram of boron–carbon exhibits a single intrinsically disordered alloy phase designated “B4C” with rhombohedral symmetry occupying a broad composition range that falls just short of the nominal carbon content of 20%. As this composition range is nearly temperature independent, the phase diagram suggests a violation of the third law of thermodynamics, which typically requires compounds to achieve a definite stoichiometry at low temperatures. By means of first principles total energy calculations we predict the existence of two stoichiometric phases at T = 0 K: one of composition B4C with monoclinic symmetry; the other of composition B13C2 with rhombohedral symmetry. Using statistical mechanics to extend to finite temperatures, we demonstrate that the monoclinic phase reverts to the observed disordered nonstoichiometric rhombohedral phase above T = 600 K, along with a slight reduction on carbon content.

Surface-Enhanced Decomposition Kinetics of Molecular Materials Illustrated with Cyclotetramethylene-tetranitramine

First principles total energy calculations were combined with variational transition state theory to reveal a surface-induced effect on the decomposition of a molecular material. Explored decomposition reactions were illustrated with energetic molecular solid β-cyclotetramethylene-tetranitramine. Simulated N–NO2 homolysis reactions demonstrated that initiation of molecular material’s degradation is characterized by a reduced activation barrier and significantly accelerated reaction rates for molecules that are placed on a free surface, an internal void, or a vacancy as compared to the perfect bulk crystal.

First principles calculations of the Sc adsorption on Si(001)-c(2 × 4)

We have investigated the energetics and the atomic structure of the adsorption of Sc on the Si(001)-c(2×4) surface using first principles total energy calculations, within the periodic density functional. The Sc adsorption has been studied at high symmetry sites considering different concentrations. We have first explored the one atom case and then increased the coverage up to a 0.25 of a monolayer of Sc. For the adsorption of one Sc atom we have obtained that the most stable configuration corresponds to the adsorption in the trench between two Si dimers, at the C1 (cave) site. The interaction of the adsorbed Sc with the Si dimers induces a decrease of the dimers buckling amplitude. On the other hand Si dimers without interaction with the adsorbate have buckling amplitudes similar to those of the clean Si surface. When the Sc coverage is increased to two Sc atoms, the most stable structure corresponds to the adsorption at two consecutive V (valley-bridge) sites along the trench between Si dimers, resulting in the weakening of some of the Si dimers bonds. This result indicates that the formation of one dimensional Sc chains on the silicon surface is energetically and kinetically favorable.

First principles studies of the graphene-phenol interactions

Studies of the interaction between phenol and intrinsic graphene, as well as phenol and aluminum doped graphene layer are performed using first principles total energy calculations within the periodic density functional theory. A 4x4 periodic structure is used to explore the adsorption of a phenol molecule on the intrinsic graphene and on aluminum doped graphene layer. The electron-ion interactions are modeled using ultra-soft pseudo-potentials, and the exchange-correlation energies are treated according to the generalized gradient approximation (GGA) with the PBE parameterization. We consider different molecule orientations: parallel and perpendicular to the graphene layer to relax the atomic structure. To explain the optimized atomic geometry we determine binding energies for all cases and the density of states (DOS) and partial DOS for the most relevant configurations. Results indicate that the direct interaction of oxygen with aluminum yields the ground state geometry with the phenol molecule adsorbed on the graphene layer. Binding energies and DOS structures also demonstrate that the ground state configuration is that where the O and Al atoms interact with a separation distance of 1.97 Å.

Interaction of oxygen vacancies and lanthanum in Hf-based high-k dielectrics: an ab initio investigation

The interaction between oxygen vacancies and La atoms in the La doped HfO2 dielectric were studied using first principles total energy calculations. La dopants in the vicinity of a neutral oxygen vacancy (V O) show lower formation energy compared to the La defects far from V O centres. La doping in HfO2 leads to the shift of the defect states of oxygen vacancies towards the conduction band edge. A statistical average of this shift over several possible configurations of La atoms and V O shows that the incorporation of La effectively passivates the V O induced defect states leading to the reduction of the gate leakage current and improvement of the device reliability.

Electronic structure and elastic properties of scandium carbide and yttrium carbide: A first principles study

We have studied the electronic, structural, and elastic properties of scandium carbide and yttrium carbide by means of accurate first principles total energy calculations using the full-potential linearized plane wave method (FP-LAPW). We have used the generalized gradient approximation (GGA) for the exchange and correlation potential. Volume optimization, energy band structure, and density of states (DOS) of the systems are presented. The second order elastic constants have been calculated and other related quantities such as the Zener anisotropy factor, Poisson's ratio, Young's modulus, Kleinman parameter, Debye temperature, and sound velocities have been determined. The band gap calculation shows that YC is relatively more ionic than ScC.

First principles lattice dynamical calculation of semiboride Be 2B and its ternary alloys XBeB (X=Na, Mg, Al)

We present results of first principles total energy calculations of the structure, electronic and lattice dynamics for beryllium semiboride and its three ternary alloys using generalized gradient and local density approximations under the framework of density functional theory. The generalized gradient approximation is used for all compounds except MgBeB using the Perdew–Burke–Ernzehorf exchange correlation functional while local density approximations use the Perdew–Zunger ultrasoft exchange correlation functional. The calculated ground state structural parameters are in good agreement with those of experimental and previous theoretical studies. The electronic band structure calculations show that Be 2B may transform to a semiconductor after Al substitution. A linear response approach to density functional theory is used to calculate phonon dispersion curves and vibrational density of states. The phonon dispersion curves of Be 2B and AlBeB are positive indicating a dynamical stablility of the structure for these compounds. The phonon dispersion curves of NaBeB and MgBeB show the imaginary phonons throughout the Brillouin zone, which confirms dynamical instability as indicated in band structures for these alloys. We also present the partial phonon density of states for different species of Be 2B and AlBeB to bring out the details of the participation of different atoms in the total phonon density of state, particularly the role played by Al atom. The first time calculated phonon properties are clearly able to bring out the significant effect of isoelectronic substitution in Be 2B.

On the electronic properties of two-dimensional honeycomb GaInN and GaAlN alloys: a molecular analysis

We have performed first principles total energy calculations to investigate the structural and the electronic properties of two-dimensional honeycomb GaAlN and GaInN alloys. Calculations were done using a coronene-like (C(24)H(12)) cluster and for different numbers of Ga, Al, and In atoms. The exchange and correlation potential energies were treated within the generalized gradient approximation (GGA). The bond length, dipole moment, binding energy, and gap between the HOMO and the LUMO are reported as a function of x. The stability of the structures depends on the site of the substituted atom; for example, when three Ga atoms are substituted, the GaInN alloy becomes unstable. The gap in the GaAlN increases from 3.76 eV (GaN) to 4.51 eV (AlN), and in the GaInN decreases to 2.11 eV. The biggest polarity occurs when eight and four Ga atoms are substituted, for GaAlN and GaInN, respectively.

DFT studies of the phenol adsorption on boron nitride sheets

We perform first principles total energy calculations to investigate the atomic structures of the adsorption of phenol (C(6)H(5)OH) on hexagonal boron nitride (BN) sheets. Calculations are done within the density functional theory as implemented in the DMOL code. Electron-ion interactions are modeled according to the local-spin-density-approximation (LSDA) method with the Perdew-Wang parametrization. Our studies take into account the hexagonal h-BN sheets and the modified by defects d-BN sheets. The d-BN sheets are composed of one hexagon, three pentagons and three heptagons. Five different atomic structures are investigated: parallel to the sheet, perpendicular to the sheet at the B site, perpendicular to the sheet at the N site, perpendicular to the central hexagon and perpendicular to the B-N bond (bridge site). To determine the structural stability we apply the criteria of minimum energy and vibration frequency. After the structural relaxation phenol molecules adsorb on both h-BN and d-BN sheets. Results of the binding energies indicate that phenol is chemisorbed. The polarity of the system increases as a consequence of the defects presence which induces transformation from an ionic to covalent bonding. The elastic properties on the BN structure present similar behavior to those reported in the literature for graphene.

Structural transition, dielectric and bonding properties of BeCN2

By means of first principle total energy calculations, this paper studies the structural transition, elastic, mechanical, dielectric and electronic properties of BeCN2. The calculations in total energy indicate that under ambient condition, the orthorhombic BeSiN2-type BeCN2 (space group Pna21) is a more favoured structure than the tetragonal chalcopyritetype one (space group I-42d). The results of elastic properties reveal that BeCN2 in both orthorhombic and tetragonal structure has higher bulk and shear moduli and smaller Poisson's ratio. The calculated Vicker hardness of tetragonal phase is 36.8 GPa, indicating a hard material. The analyses of electronic structure and electron density difference demonstrate that these excellent mechanical properties are attributed to the stronger covalent-bonding of CN4 and BeN4 subunits in BeCN2 crystal. Also, the orthorhombic BeCN2 phase is found to be a transparent semiconductor material with the calculated direct band gap of about 5.56 eV, superior to the indirect band gap of diamond and c-BN. Moreover, it also calculates Born effective charges and dielectric constants of BeCN2. These results suggest that BeCN2 may have some useful applications as optoelectronic, optical window and wear resistant materials.

ELECTRONIC, MECHANICAL AND OPTICAL PROPERTIES OFSi3P4ANDGe3P4: ANAB INITIOSTUDY

First principles total energy calculations within the density functional formalism have been used to investigate the electronic, mechanical, and optical properties of pseudocubic- Si3P4and Ge3P4. Considering the technological importance of the Si/Ge -Group-V elements, we have concentrated mainly on the comparatively less studied, but energetically more favorable pseudocubic- Si3P4and Ge3P4structures of the Si and Ge phosphides. We find that the electronic band structures show that pseudocubic- Si3P4and Ge3P4are both indirect band semiconductors with very low density functional band gaps of 0.24 eV and 0.13 eV, respectively. We also calculate mechanical properties of the materials, such as the bulk modulus, elastic constants, shear modulus, and Vickers hardness of the two phosphides. We find that the bulk and shear modulus of pseudocubic- Si3P4and Ge3P4are 76.18 GPa and 58.37 GPa, and 59.99 GPa and 46.92 GPa, respectively. Pseudocubic- Si3P4and Ge3P4have low Vickers hardness, nearly 18.88 and 16.86 GPa, respectively. Moreover, optical parameters, including dielectric function, refractive index, optical absorption, energy loss function, and plasma frequency are also studied.

Ab initio study of the adsorption of antimony and arsenic on the Si(110) surface

We have performed first principles total energy calculations to investigate the adsorption of Sb and As adatoms on the Si(110) surface using a (2 × 3) supercell. The energetics and atomic structures have been investigated in four atomic configurations. One structure is obtained by placing 1/3 of a monolayer (ML) of Sb (As) atoms on the Si(110) surface. The other three geometries are obtained by depositing 1 ML of Sb (As) atoms on the surface. In the first case the structure is formed by four trimers, in the second case the geometry is formed by zigzag atomic chains and in the third case the structure contains “microfacets”. The energetics results of the Sb adsorption show that for low coverage the tetrahedrons formed by the adsorption of 1/3 ML is the most stable configuration, while in the monolayer region the zigzag atomic chain is the most stable structure. However, the total energies of the trimer and microfacet structures are slightly higher, indicating that under some conditions, they may be formed. In an experimental report it has been suggested that the adsorption of 1/3 and 1 ML of Sb corresponds to the low and high coverage in the experiments of Zotov et al. [A. V. Zotov, V. G. Lifshifts, and A. N. Demidchik, Surf. Sci. 274, L583 (1992)]. On the other hand, our results of the As adsorption show that for low coverage, the tetrahedrons in the adsorption of 1/3 ML also give the most stable configuration. However, at the 1 ML coverage, a structure formed by microfacets is the most stable structure, in agreement with previous results.

Electronic properties of group III‐A nitride sheets by molecular simulation

AbstractWe have performed first principles total energy calculations to investigate the structural and reactivity parameters of novel N12X12H12 (X=B, Al, Ga, In, Tl) nitrides, in their coronene‐like (C24H12) structure to simulate these sheets. The exchange and correlations potential energies are treated in the Generalized Gradient Approximation (GGA), and the Local Density Approximation (LDA) within the parameterization of Perdew‐Wang and Perdew‐Burke‐Ernzerhof (PWC, PBE) and the double Numeric plus polarization (DNP) atomic base. The chemical potential, hardness and electrophilicity index, as well as bond length are reported. The bond length of the structures is similar to the bulk. The gap between the HOMO and LUMO decreases from BN (5.18 eV) to TlN (1.76 eV). At the same time, the polarity increases except for TlN. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Density functional theory study of lithium and fluoride doped boron nitride sheet

AbstractWe have performed first principles total energy calculations to investigate the structural and reactivity parameters of the lithium and fluoride doped graphene and boron nitride coronene‐like (C24H12) structure nanosheets. The exchange and correlations potential energies are treated in the local density approximation (LDA) within the parameterization of Perdew‐Wang (PWC) and the doubly polarized atomic base (DNP). The obtainment of non‐negative frequencies was the criterion for structural stability. Chemical potential, hardness and electrophilicy index, as well as bond length are reported. The B atom is substituted by Li and F atoms preserving the hexagonal structure, however, the stability is reached only with the Li atom. The covalence and the chemical reactivity are bigger when compared with the corresponding isolated systems. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Full potential linear augmented plane wave study of the elastic properties of XPt3 (X=V, Cr, Mn, Fe, Co, Ni)

Abstract From the first principles total energy calculations based on full-potential linear augmented plane wave method (FPLAPW), the elastic properties of XPt3 (X=V, Cr, Mn, Fe, Co, Ni) are reported here. Theoretical values of Young's modulus, shear modulus, Poisson's ratio and Debye temperature are estimated from the computed elastic constants. From the analysis of the ratio of shear to bulk modulus, it is found that these intermetallic compounds are ductile in nature except CrPt3, which is brittle. The calculated results are compared with other reported values.

Pressure induced phase transition in tin: Ab-initio calculations

First principles total energy calculations as a function of unit cell volume have been performed on α (diamond structure), β (double bct), bct, bcc and hcp structures of tin. The comparison of total energies of these structures at various volumes suggests high pressure structural transitions of α → β → bet → bct at pressures of ~ 0.15 GPa, 2.7 GPa and 28 GPa. Also, 300 K isotherm upto maximum pressures of ~ 400 GPa has been determined for this metal. Using theoretical isotherm, shock Hugoniot of tin has been derived. Further, theoretically evaluated volume dependent Gruneisen parameter in conjunction with Lindeman melting law has been used to determine the pressure dependent melting of tin. The intersection of the theoretical melting curve with theoretically determined temperature rise with shock pressure reveals that this metal will melt at shock pressure of ~ 38 GPa (temperature ~ 1428 K) in good agreement with experimental value of 39 GPa (1550+100 K).

Structural and electronic properties of zinc blend GaAs 1−xBi x solid solutions

First principles total energy calculations were carried out to investigate structural and electronic properties of zinc-blend (ZB) GaAs, GaBi and GaAs 1− x Bi x solid solutions. We have calculated lattice parameters, bulk modulus, pressure derivative and GaAs 1− x Bi x band-gap energy for zinc blend-type crystals of the compositions x=0, 0.25, 0.5, 0.75, 1. Discussions will be given in comparison with results obtained with other available theoretical and experimental results.

Boron and nitrogen functionalized diamondoids: A first principles investigation

Chemically functionalized adamantane molecules have been investigated by first principles total energy calculations. Boron and nitrogen functionalized molecules were found to be very stable, consistent with available experimental data. Two hypothetical molecular crystals, involving functionalized adamantane, were investigated. These molecular crystals presented direct electronic bandgaps and large bulk moduli, which suggested a possible road for molecular self-assembly using functionalized diamondoids.