Pseudopotential Study Research Articles

Ab initio pseudopotential study of vacancies and self-interstitials in hcp titanium

By means of an ab initio plane-wave pseudopotential method, monovacancy, divacancy and self-interstitials in hcp titanium are investigated. The calculated monovacancy formation energy is 1.97 eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results. The relaxation of the atoms around a single vacancy is observed to be small. Two divacancy configurations, the in-plane and the off-plane, have also been shown to be equally stable. With regards to the interstitials, of the eight configurations studied, two (octahedral and basal octahedral) have relatively lower formation energies and are, thus, the most likely stable configurations. We find small energy differences between them, suggesting their possible co-existence. It is also observed that the tetrahedral configuration decays to a split dumbbell configuration, whereas both the basal tetrahedral and the basal pseudocrowdion interstitials decay to the basal octahedral configuration. Using the nudged elastic band method (NEB), we determine a possible minimum energy path (MEP) for the diffusion of self-interstitial titanium atoms from an octahedral site to the nearest octahedral site. The energy barrier for this migration mechanism is shown to be about 0.20 eV.

Ab initiocalculation of phonons for bulk TiC andTiC(001)(1×1)

We present an ab initio pseudopotential study, within the local density functional approximation, of the structural and elastic properties of rock-salt TiC. The calculated structural and elastic properties of TiC are in good agreement with reported experimental values. Using the density-functional perturbation theory, the phonon spectrum and density of states are calculated. Good agreement has been found between the calculated and reported experimental dispersion curves. Following these bulk properties, we have made theoretical investigations of the atomic geometry and lattice dynamics of the $\text{TiC}(001)(1\ifmmode\times\else\texttimes\fi{}1)$ surface. The calculated surface structural parameters compare very well with available experimental values. Using our calculated surface phonon spectrum, we provide a detailed analysis of available experimental data from electron energy-loss spectroscopy.

Ab-initio pseudopotential study of electronic structure and chemisorption of oxygen on aluminium surface

Chemisorption of oxygen atom on aluminium (1 1 1), (1 1 0) and (1 0 0) surfaces is studied using ab- initio plane wave pseudopotential method based on density functional theory (DFT). Oxygen atom chemisorbed on three different high symmetry sites; top, short-bridge and hollow sites on the aluminium surfaces are examined. It has been found that the O-adatom adsorbed at the hollow site on aluminium (1 1 1), (1 1 0) and (1 0 0) plane yield energetically most stable structure. Calculation of chemisorption energies of O-adatom on aluminium surfaces shows that oxygen is most strongly bound to aluminium atoms on Al(1 1 1) plane and the calculated value of the chemisorption energy of O-adatom at the hollow site on Al(1 1 1) surface is 4.8 eV. In this work, the chemisorption energies calculated for O-adatom on Al(1 1 0) and Al(1 0 0) surfaces are reported for the first time. The electronic structures and the electronic charge density distributions of the oxygen chemisorbed aluminium surfaces are also investigated. Calculations show that for aluminium, p orbitals also contribute significantly along with the s orbitals during the bond formation with oxygen atom. Therefore, the possibilities of hybridizations lead to the strong bonding configurations.

Ab-initio vibrational properties of SiGe alloys

We report on the vibrational properties of Si 1 − x Ge x alloys by means of a pseudopotential density-functional study. We employ randomly generated supercells, and a wide range of Ge concentration ( x = 19, 50, 81%) to investigate the alloys. Accordingly, Si atoms were occasionally replaced by Ge atoms in an otherwise perfect 32 Si atom supercell, with the atomic and volumetric degrees of freedom being allowed to relax. The dynamical matrix of 32 atom supercell was determined directly from the ab-initio code. Calculated Raman spectra were obtained averaging over 15 SiGe supercell configurations for each Ge concentration. The experimental three main wide bands were reproduced. The calculated Si–Si and Ge–Ge frequencies shift linearly with Ge concentration. The Si–Si frequency downward shift agrees well with experimental data, while the agreement is not so good for Ge–Ge mode. The highest Si–Ge frequency occurs when Ge composition reaches 50%, which agrees with experimental data. However the asymmetry of the downward frequency shift is not reproduced, which may be due to the limited size of the supercell.

Empirical pseudo-potential studies on electronic structure of semiconducting quantum dots

Theoretical investigations of electronic structure of quantum dots is of current interest in nanophase materials. Empirical theories such as effective mass approximation, tight binding methods and empirical pseudo-potential method are capable of explaining the experimentally observed optical properties. We employ the empirical pseudo-potential to calculate the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as a function of shape and size of the quantum dots. Our studies explain the building up of the bulk band structure when the size of the dot is much larger than the bulk Bohr exciton radius. We present our investigations of HOMO-LUMO gap variation with size, for CdSe, ZnSe and GaAs quantum dots. The calculated excitonic energies are sensitive to the shape and size of quantum dots and are in good agreement with experimental HOMO-LUMO gaps for CdSe quantum dots. The agreement improves as experimentally observed lattice contraction is incorporated in pseudo-potential calculations for ZnSe quantum dots. Electronic structure evolution, as the size of quantum dot increases, is presented for CdSe, ZnSe and GaAs quantum dots.

All-electron Douglas–Kroll–Hess and pseudopotential study on the low-lying states of uranium hydride UH

Multireference configuration interaction (MRCI) calculations combined with the all-electron scalar-relativistic Douglas–Kroll–Hess (DKH) Hamiltonian, as well as a relativistic energy-consistent small-core pseudopotential (SPP) for uranium, have been performed to study the low-lying electronic ΛS states of uranium monohydride UH with term energies below ≈0.5 eV. After taking spin–orbit effects into account both DKH/MRCI and SPP/MRCI calculations predict a 4I 9/2 ground state. The calculated ground state molecular constants of both approaches show a good agreement with each other (MRCI + Q, DKH: R e = 2.021 Å, ω e = 1483 cm −1, D e = 2.79 eV; SPP: R e = 2.011 Å, ω e = 1497 cm −1, D e = 2.85 eV), as well as with available experimental data ( ω e = 1424 cm −1 in argon matrix).

Quasirelativistic 5f-in-core pseudopotential study of the actinocenes An(C 8H 8) 2, An = Th–Pu

Recently developed quasirelativistic energy-consistent 5f-in-core pseudopotentials as well as core-polarization potentials corresponding to tetravalent actinides have been used to study the molecular structures, ionic metal–ring binding energies, and Mulliken orbital populations of the actinocenes An(C 8H 8) 2 (An = Th–Pu). The results of Hartree–Fock calculations show only small deviations from 5f-in-valence calculations, although the 5f orbitals contribute to the covalent actinide–ring bonding. For thorocene and uranocene, second-order Møller–Plesset perturbation theory, coupled-cluster theory with single and double excitation operators and a perturbative estimate of triple excitations, and density functional theory yield actinide–ring distances which are in good agreement with the experimental values.

First-principles study of the energy-gap composition dependence ofZn1−xBexSeternary alloys

We report the results of a self-consistent pseudopotential study including structural, optical, and thermodynamic properties of cubic ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Be}}_{x}\mathrm{Se}$ semiconductor alloy. The system is modeled in various possible configurations using a large 64-atom supercell. The evaluated band-gap bowing is in good agreement with the experimental data. The different roles of structural and chemical effects on the gap bowing and its variation with composition are identified and discussed. It is found that structural effect, i.e., the relaxation of atomic bonds, overwhelms the other contributions to the gap bowing. Lattice parameter fulfills Vegard's law with a small downward bowing deviation of about $0.04\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Our calculated bond lengths appear to be more reliable compared with those obtained from previous theoretical studies using smaller atomic supercell. Besides, a regular-solution model was used to investigate the thermodynamic stability of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Be}}_{x}\mathrm{Se}$, which mainly indicates a wide phase miscibility gap.

Theoretical investigation of the alkaline-earth dihydrides from relativistic all-electron, pseudopotential, and density-functional study

Highly precise ground state geometries, harmonic vibrational frequencies and force constants of alkaline-earth dihydrides from CaH2 to RaH2 are obtained using relativistic small-core energy-consistent effective core potentials at the coupled-cluster level. The results are compared with all-electron as well as density functional calculations. All-electron results, in particular, clearly show the importance of relativistic effects in the properties considered in this paper. The monotonic trends in the geometries are explained in terms of second-order perturbation theory. Trends in the force constants are monotonic except for the bending mode where an anomaly occurs from BaH2 to RaH2. It is rationalized in terms of reduced s-d hybridization due to relativity, which is shown to be an energy effect attributed to the stabilization of the s orbital. The pseudopotentials show an excellent performance in comparison with all-electron methods and are therefore successfully transferred to molecular cases. The density functional methods, however, suffer from functional dependencies with B3LYP performing the best in this case.

Structural parameters and transition pressures of ZnO: ab‐initio calculations

AbstractThe structural parameters of ZnO in three phases, namely wurtzite, zincblende and rocksalt as well as transition pressures from wurtzite and zincblende to rocksalt structures are presented. Besides, the elastic constants in the three considered phases are reported. The calculations are performed using an ab‐initio pseudopotential study within the framework of the density‐functional theory (DFT) with both the local density approximation (LDA) and the generalized gradient approximation (GGA). Detailed comparisons are made with published experimental and theoretical results and show generally good agreement. Moreover, in most cases the present results are improved with respect to experiment when compared with previous calculations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ground state properties of zinc‐blende and wurtzite phases of MgS

AbstractWe present an ab initio pseudopotential study, within the generalised gradient approximation, of the structural and electronic properties of zinc‐blende and wurtzite phases of MgS. The calculated structural parameters are in good agreement with previous experimental and theoretical results. Using the density‐functional perturbation theory, the phonon spectrum and density of states for the two phases have also been calculated. The calculated phonon modes for the zinc‐blende phase compare very well with a Raman scattering data at the zone‐centre. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A pseudopotential density functional theory study of native defects and boron impurities inFeAl

The structures and energies of point defects and point defect complexes inB 2 iron aluminium FeAl are calculated using a local density functional theory based methodwith large supercells. Particular emphasis is given to pseudopotential quality, choice ofchemical potentials used to calculate defect formation energies, and how these are affectedby magnetism. Both purely native defects and those containing boron atomsare considered. It is found that the relative stabilities of isolated point defectsversus defect complexes depends on whether the material contains excess iron oraluminium. The situation in material containing boron is further complicated by theexistence of iron borides in more than one form. We propose that the interactionbetween point defects, dislocations, and antiphase boundaries, where the localatomic environment has some similarities with antisite defects, also depends on thealloy composition. It is likely that these interactions are part of the underlyingmechanism responsible for the unusual mechanical properties of iron aluminides.

Ab initio study of high-pressure phases of gallium nitride

An ab initio pseudopotential study within both the local density approximation (LDA) and the generalised gradient approach (GGA) of the high-pressure phases of GaN is presented. The wurtzite, zinc blende and rocksalt structures are analysed. The structural parameters of these compounds as well as the transition pressure and relative volume changes from both the wurtzite and zinc blende to the rocksalt phases are determined. Detailed comparisons are made with the available measured values and with results obtained in previous theoretical studies. These comparisons reveal generally better agreement between our results and the available experimental values when compared with previous simulations. Differences between LDA and GGA results for the different phases are also examined.

Ab initio pseudopotential studies of the pressure dependences of structural, electronic and optical properties for GaN

The structural, electronic and optical properties of GaN across its pressure induced phase transformation from the B4 (wurtzite structure) to the B1 (NaCl structure) phase have been studied. The calculations are based on the ab initio plane-wave pseudopotential density functional theory within the generalized gradient approximation for the exchange-correlation potential. The predicted phase transition pressure 45.8 GPa and the optimized lattice constants of B4 GaN under ambient pressure are in very good agreement with experimental and theoretical results. We find that the electronic and optical properties of GaN under high pressure are quite different from those under ambient pressure.

Half-metallic ferromagnetism of Cr-doped rutile TiO 2: A first-principles pseudopotential study

Using the first-principles plane-wave pseudopotential method, the electronic structure and the ferromagnetism of Cr-doped rutile TiO 2 have been investigated. We find that the Cr-doped rutile TiO 2 shows half-metallic behavior in both generalized gradient approximation and local density approximation for the exchange-correlation potential. The half-metallicity originates from the hybridization of Cr 3d states and nearest-neighboring O 2p states. It may be useful in semiconductor spintronics and other applications.

A theoretical and experimental study of lead substitution in calcium hydroxyapatite

Characterization of lead substitution for calcium in hydroxyapatite (CaHA) is carried out, using experimental techniques and Density Functional theoretical (DFT) analyses. Theoretical modeling is used to obtain information of the Pb chemical environment for occupancy at either Ca(I) or Ca(II) sites of CaHA. Effects of the larger ionic radius of Pb(+2) compared to Ca(+2) are apparent in embedded cluster calculations of local chemical bonding properties. DFT periodic planewave pseudopotential studies are used to provide first-principles predictions of local structural relaxation and site preference for Pb(x)Ca(10-x)HA over the composition range x< or = 6. General characteristics of the polycrystalline material are verified by X-ray diffraction and FTIR analysis, showing the presence of a single phase of CaHA structure. A short range structure around lead is proposed in order to interpret the Pb L-edge EXAFS spectrum of the solid solution Ca(6.6)Pb(3.4)HA. In this concentration we observe that lead mainly occupies Ca(II) sites; the EXAFS fit slightly favors Pb clustering, while theory indicates the importance of Pb-Pb avoidance on site (II).

Adsorption and diffusion of Pt and Au on the stoichiometric and reducedTiO2rutile (110) surfaces

A comparative first principles pseudopotential study of the adsorption and migration profiles of single Pt and Au atoms on the stoichiometric and reduced TiO2 rutile (110) surfaces is presented. Pt and Au behave similarly with respect to (i) most favorable adsorption sites, which are found to be the hollow and substitutional sites on the stoichiometric and reduced surfaces, respectively, (ii) the large increase in their binding energy (by ~1.7 eV) when the surface is reduced, and (iii) their low migration barrier near 0.15 eV on the stoichiometric surface. Pt, on the other hand, binds more strongly (by ~2 eV) to both surfaces. On the stoichiometric surface, Pt migration pattern is expected to be one-dimensional, which is primarily influenced by interactions with O atoms. Au migration is expected to be two-dimensional, with Au-Ti interactions playing a more important role. On the reduced surface, the migration barrier for Pt diffusion is significantly larger compared to Au.

All-electron and relativistic pseudopotential studies for the group 1 element polarizabilities from K to element 119

Two-component and scalar relativistic energy-consistent pseudopotentials for the group 1 elements from K to element 119 are presented using nine electrons for the valence space definition. The accuracy of such an approximation is discussed for dipole polarizabilities and ionization potentials obtained at the coupled-cluster level as compared to experimental and all-electron Douglas-Kroll results.

Stability and electronic properties ofSn3P4

We present a first-principles pseudopotential study on the stability and electronic properties of ${\mathrm{Sn}}_{3}{\mathrm{P}}_{4}$. Similar to other group-IV phosphides with stoichiometry 3:4, the pseudocubic-${\mathrm{Sn}}_{3}{\mathrm{P}}_{4}$ was found energetically stable among the fourteen structures considered. It was also found that the high-pressure phases and the defect-NaCl phases of ${\mathrm{Sn}}_{3}{\mathrm{P}}_{4}$ have metallic characteristics, while other structures show semiconducting properties within GGA. Therefore, if bulk crystalline ${\mathrm{Sn}}_{3}{\mathrm{P}}_{4}$ can be synthesized under ambient pressure, our calculations predict that it is a semiconductor with a sizable indirect energy gap. However, phase transition from the pseudocubic phase to the $\mathrm{Ca}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$ and $\mathrm{Ca}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$-type structures are possible at pressure of about 11.7 and $12.8\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, respectively. High-pressure phases which may also be grown on appropriate substrate are likely to be metallic.

First‐principles study of electronic and dynamical properties of AuAl 2

We present an ab initio pseudopotential study within the local density functional approximation of the structural and electronic properties of AuAl2. The calculated lattice constant and electronic structure are in good agreement with recent experimental results. Using the density-functional perturbation theory, the phonon spectrum and density of states for this material are calculated. The highest optical phonon mode compares very well with a Raman scattering data at the zone-center. We also analize the atomic displacement patterns of selected acoustic and optical phonon mode at the symmetry points. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)