First-principles Pseudopotential Calculations Research Articles

Structural Phase Stability and Electronic Properties of Magnesium under High Pressure from First-Principles Calculations

First-principles pseudopotential calculations have been performed to investigate the structural stability and electronic properties of magnesium considering three possible structures under high pressure. The results show that magnesium crystallizes in the hcp structure is to be the most stable structure at the ground state, because of the lowest total energy. Magnesium undergoes a pressure-induced phase transition from the hcp structure to bcc structure at 65 GPa. And no further transition is found up to 220 GPa. The electronic structure properties of three structures of magnesium are also calculated and discussed. The structural stability mechanism is also explained through the electronic structures of three phases.

Phase competition mediated by composition and pressure in Zr2Cu1−xNix system

Phase competition between C11b, C16 and E93 phases under pressure (0–60GPa) in Zr2Cu1−xNix (x=0, 0.125, 0.25, 0.50, 0.75, 0.875 and 1) alloys has been investigated by first-principles pseudopotential calculations. The whole picture of phase competition mediated by both external pressure and composition has been unambiguously established by modeling using both supercell (SC) method and virtual crystal approximation (VCA) method, which sheds light on the pressure dependent glass stability and crystallization behavior of related alloy systems. It is revealed that both pressure and increasing Ni content stabilize the C16 phase over the other two. The underlying mechanism of external pressure and composition stabilizing the C16 phase is also analyzed. A feasible procedure for achieving metallic glass composites with tunable precipitated phases is proposed.

D0 Ferromagnetism in oxygen-doped CuCl: First principles study

Based on our first-principles all-electron and pseudopotential calculations, CuCl doped by a nonmagnetic d0 oxygen element is predicted to be ferromagnetic. O-doped (3%) CuCl shows half-metallic ferromagnetism with a net total magnetic moment of 1.0μB. Furthermore the unusual d0 ferromagnetism in O-doped CuCl can be explained in terms of O(2p)–Cu(3d) like hybridization mechanism. These results suggest that O doped CuCl may be a promising half-metallic material free from magnetic precipitate and may find application in the field of spintronics.

Mechanical and thermodynamic properties of α-UH3 under pressure

The mechanical and thermodynamic properties of α-UH3 under pressure have been examined by first-principles pseudopotential plane-wave calculations based on the density functional theory. In order to describe the strong on-site Coulomb repulsion among the localized 5f electrons, the generalized gradient approximation and local density approximation plus a Hubbard parameter (GGA+U and LDA+U) formalisms have been adopted for the exchange correlation term. The pressure evolution of elastic constants of α-UH3 has been revealed, and α-UH3 is found to be mechanically stable at least up to 20GPa. Under pressure, the low-frequency vibration modes of α-UH3 change slightly, while the high-frequency vibration modes shift upward. Our study is expected to facilitate the understanding of uranium hydrides and their applications.

Phase transition and elastic properties of NbN under hydrostatic pressure

First-principles pseudopotential calculations are performed to investigate the phase transition and elastic properties of niobium nitrides (NbN). The lattice parameters a0 and c0/a0, elastic constants Cij, bulk modulus B0, and the pressure derivative of bulk modulus B0′ are calculated. The results are in good agreement with numerous experimental and theoretical data. The enthalpy calculations predict that NbN undergoes phase transition from NaCl-type to NiAs-type structure at 13.4 GPa with a volume collapse of about 4.0% and from AsNi-type to CW-type structure at 26.5 GPa with a volume collapse of about 7.0%. Among the four types of structures, CW-type is the most stable structure. The elastic properties are analyzed on the basis of the calculated elastic constants. Isotropic wave velocities and anisotropic elasticity of NbN are studied in detail. The longitudinal and shear-wave velocities, VP, VS and Vm increase with increasing pressure, respectively. The Debye temperature ΘD increases monotonically with increasing pressure except for NiAs-type structure. Both the longitudinal velocity and the shear-wave velocity increase with pressure for wave vector along all the propagation directions, except for VTA([100]) and VTA[001]([110]) with NaCl structure and VTA[010]([100]) with the other three types of structures.

Ab initioSternheimer-GW method for quasiparticle calculations using plane waves

We report on the extension and implementation of the Sternheimer-GW method introduced by Giustino et al. [Phys. Rev. B 81, 115105 (2010)] to the case of first-principles pseudopotential calculations based on a plane-waves basis. The Sternheimer-GW method consists of calculating the GW self-energy operator without resorting to the standard expansion over unoccupied Kohn-Sham electronic states. The Green's function is calculated by solving linear systems for frequencies along the real axis. The screened Coulomb interaction is calculated for frequencies along the imaginary axis by using the Sternheimer equation. Analytic continuation to the real axis is performed using Pad\'e approximants. The generalized plasmon-pole approximation is avoided by performing explicit calculations at multiple frequencies using Frommer's multishift solver. We demonstrate our methodology by reporting tests on common insulators and semiconductors, including Si, diamond, LiCl, and SiC. Our calculated quasiparticle energies are in agreement with the results of fully converged calculations based on the sum-over-states approach. As the Sternheimer-GW method yields the complete self-energy $\ensuremath{\Sigma}(\mathbf{r},{\mathbf{r}}^{\ensuremath{'}},\ensuremath{\omega})$ and not only its expectation values on Kohn-Sham states, this work opens the way to nonperturbative GW calculations and to direct calculations of spectral functions for angle-resolved photoemission spectroscopy. As an example of the capabilities of the method we calculate the G${}_{0}$W${}_{0}$ spectral functions of silicon and diamond.

Defect formation in chlorine-doped zinc oxide

First-principles pseudopotential calculations have been performed to investigate the chorine-doped zinc oxide. From the calculated results, it is concluded that the ClO is energetically favorable and easy to form under the O-poor condition. Therefore, Cl-doping is an effective method to eliminate the oxygen vacancy in ZnO during the growth. However, the corresponding energy levels of ClO and Cli in band gap are rather deep so that they cannot play a role of donor or acceptor.

Surface Electronic Structure of Si(111): Role of Reconstruction and Adsorbed Atoms

Various phases of the Si(111) surface are surveyed on the results of first-principle pseudopotential calculations for electronic structure. The 7×7 reconstruction on the clean surface and the √3 × √3 superlattice structures induced by adatoms are taken up. Some new findings are presented for Sb and Ag adatom systems. Calculated results are interpreted from a standpoint that the electronic properties are closely related to the configuration of surface atoms and vice versa. Surface electronic structures of these surfaces for an appropriate atomic configuration are consistent with spectroscopic data.

Hydrothermal synthesis of ZnSe:Cu quantum dots and their luminescent mechanism study by first-principles

An one-pot synthesis of aqueous ZnSe:Cu nanocrystals (NCs) is realized in aqueous solution by a facile yet efficient hydrothermal technique. The dopant emission spectrum of the NCs is tunable, spanning a wide range from 438 to 543nm. Room-temperature quantum yield for the NCs prepared at the optimal conditions reaches as high as 20% without any post-treatment. The ZnSe:Cu NCs prepared in a neutral aqueous solution (pH=8) are remarkably stable and exhibit comparatively high photoluminescent quantum yield (PL QY) as high as 17%. First-principles pseudopotential calculations using plane-wave basis functions have been performed. The formation energies of copper ions occupied in the interstitial octahedron and substitutional tetrahedral Zn2+ sites have been calculated. The occupation of copper ions in the interstitial octahedral site is found to be more thermodynamics-facilitated by −0.98eV. The density of state analysis indicates that the Cu-related emission is primary dominated by the substitutional tetrahedral Cu ions, and the large dopant related emission width of ZnSe:Cu NCs originated from the corresponding Cu 3d impurity band.

Dynamical stability of the lanthanum dihydride under high pressure: A density functional lattice dynamics approach

We report first-principles pseudopotential method and density functional linear response theory calculations of the structural, electronic, vibrational and thermodynamical properties of LaH2 in fluoride phase at zero and high pressures. The electronic band structure and first time calculated pressure dependent lattice dynamic properties show that the LaH2 is metallic in nature at zero pressure. The pressure dependent electronic band structure calculations show that the bands are closing and reopening with pressure. The phonon mode softens at 11 GPa indicating an instability in lattice structure and pressure induced transverse acoustic phonon mode driven displacive type structural phase transition. The LaH2 also shows weak superconductivity at zero pressure. The valence charge electron density at 46 GPa shows a cage type charge contour and a space where hydrogen can be absorbed and filled easily. There is a considerable charge accumulation in the bonding region of La and H atoms, which strengthens covalent character of LaH2, and henceforth this material can be one of the best materials for hydrogen storage application at high pressures.

First-Principles Study of Electronic, Elastic, and Lattice Vibrational Properties of Pbnm Orthorhombic SrHfO3

Through first-principles pseudopotential calculations based on density functional theory, the electronic structure and lattice vibrational properties of Pbnm orthorhombic SrHfO3 were investigated in the framework of standard functional approximation and density functional perturbation theory, respectively. The calculated equilibrium lattice constants of Pbnm orthorhombic SrHfO3 are in good agreement with available experimental and theoretical results. The results show that Pbnm orthorhombic SrHfO3 is an insulator with a direct band gap of 3.9 eV and 4.0 eV within the calculations using local density approximation (LDA) and generalized gradient approximation (GGA), respectively. Use of the screened exchange local density approximation (sX-LDA) as a functional in a successive band calculation has also been performed. The band gap is predicted to be 6.7 eV within sX-LDA, somewhat higher than the gap values of 6.1 ± 0.1 eV and 6.5 eV obtained from recent x-ray photoelectron spectroscopy. The phonon dispersion curves of Pbnm orthorhombic SrHfO3 were also calculated. All-positive phonon frequencies were observed in the whole Brillouin zone, indicating stability of the Pbnm orthorhombic SrHfO3 structure. In addition, the infrared-active and Raman-active vibrational modes of SrHfO3 were calculated and compared with available theoretical and experimental investigations.

STRUCTURAL TRANSITIONS OF BiI3 UNDER PRESSURE

High pressure studies of BiI 3 at 0 K are performed using first-principles pseudopotential calculations within the framework of density functional theory. The calculations indicate that BiI 3 undergoes a structural transition from rhombohedral R-3 phase to monoclinic P2 1/c phase at 7 GPa which is accompanied by a 5.8% volume collapse. In addition, we find that P2 1/c phase prevails about 60 GPa range and transforms to cubic Fm-3m phase at 68 GPa, and finally takes the orthorhombic Pnma phase at high pressures up to 133 GPa. The structural and electronic properties of four competitive structures are also calculated. The analysis of density of states reveals that BiI 3 has semiconductor-metal transition at about 61 GPa, which also demonstrates the metallic nature of both Fm-3m and Pnma phases.

Structural and elastic properties of ternary metal nitrides TixTa1 − xN alloys: First-principles calculations versus experiments

First-principles pseudopotential calculations of the lattice constants and of the single-crystal elastic constants for TixTa1−xN (0≤x≤1) alloys with B1-rocksalt structure were first carried out. These calculations were performed using density functional perturbation theory (DFPT) within the virtual crystal approximation (VCA) for the disordered alloys and the supercell method (SC) for the ordered alloys, with the ABINIT program. For disordered structures, partial comparisons of the lattice constants and of the bulk modulus are provided with calculations that used the coherent potential approximation (CPA) with the exact muffin-tin orbitals (EMTO). For the exchange-correlation potential we used the generalized gradient methods (GGA). The calculated equilibrium lattice parameters by VCA are in good agreement with the stress-free lattice parameters a0 and exhibit a positive deviation from Vegard's rule corresponding to a positive bowing parameter while the calculated single-crystal stiffness c12 and c44, gradually increase when c11 decreases from TaN to TiN. In a second stage, we have estimated by homogenization methods the averaged stiffnesses <cij>, the Young modulus and Poisson ratio of polycrystalline TixTa1−xN (0≤x≤1) alloys considering a random orientation of crystallites. Finally, comparisons are made with the experimental effective out-of-plane shear elastic modulus C44 and the effective out-of-plane longitudinal elastic constant C33 measured by Brillouin light scattering and picosecond ultrasonics, respectively, on thin films elaborated by magnetron sputtering.

First-Principles Study on Cd Doping in Cu2ZnSnS4 and Cu2ZnSnSe4

To quantitatively evaluate the substitution energies of Cd atom for Cu, Zn, or Sn atom in indium-free photovoltaic semiconductors Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe), first-principles pseudopotential calculations using plane-wave basis functions were performed. The substitution energies of Cd atom in kesterite-type CZTS and CZTSe were calculated in consideration of the atomic chemical potentials of the constituent elements of Cu, Zn, Sn, and the doping atom of Cd. During the chemical bath deposition (CBD) of the CdS layer on the CZTS or CZTSe layer, Cu, Zn, and Cd atoms dissolved in the ammonia aqueous solution and formed [Cu(NH3)2]+, [Zn(NH3)4]2+, and [Cd(NH3)4]2+ complex ions. Therefore, the chemical potentials of Cu, Zn, and Cd atoms in [Cu(NH3)2]+, [Zn(NH3)4]2+, and [Cd(NH3)4]2+ complex ions were calculated. We found that the substitution energies of n-type CdCu and charge-neutral CdZn in CZTS and CZTSe are smaller than that of p-type CdSn. The substitution energies of CdCu in CZTS and CZTSe are smaller than that in chalcopyrite-type CuInSe2 (CIS). However, the substitution energies of CdCu, CdZn, and CdSn are positive values. The formation energy of charge-neutral Cd doping with the Cu vacancy (CdCu + VCu) pair is a negative value and greatly smaller than those of donor-type CdCu and neutral CdZn in CZTS and CZTSe. These results indicate that the charge-neutral (CdCu + VCu) vacancy pair is easily formed during the CBD of the CdS layer on the CZTS or CZTSe layer. A small amount of n-type CdCu and neutral CdZn would also be formed.

First-Principles Study on Cd Doping in Cu2ZnSnS4and Cu2ZnSnSe4

To quantitatively evaluate the substitution energies of Cd atom for Cu, Zn, or Sn atom in indium-free photovoltaic semiconductors Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe), first-principles pseudopotential calculations using plane-wave basis functions were performed. The substitution energies of Cd atom in kesterite-type CZTS and CZTSe were calculated in consideration of the atomic chemical potentials of the constituent elements of Cu, Zn, Sn, and the doping atom of Cd. During the chemical bath deposition (CBD) of the CdS layer on the CZTS or CZTSe layer, Cu, Zn, and Cd atoms dissolved in the ammonia aqueous solution and formed [Cu(NH3)2]+, [Zn(NH3)4]2+, and [Cd(NH3)4]2+ complex ions. Therefore, the chemical potentials of Cu, Zn, and Cd atoms in [Cu(NH3)2]+, [Zn(NH3)4]2+, and [Cd(NH3)4]2+ complex ions were calculated. We found that the substitution energies of n-type CdCu and charge-neutral CdZn in CZTS and CZTSe are smaller than that of p-type CdSn. The substitution energies of CdCu in CZTS and CZTSe are smaller than that in chalcopyrite-type CuInSe2 (CIS). However, the substitution energies of CdCu, CdZn, and CdSn are positive values. The formation energy of charge-neutral Cd doping with the Cu vacancy (CdCu + VCu) pair is a negative value and greatly smaller than those of donor-type CdCu and neutral CdZn in CZTS and CZTSe. These results indicate that the charge-neutral (CdCu + VCu) vacancy pair is easily formed during the CBD of the CdS layer on the CZTS or CZTSe layer. A small amount of n-type CdCu and neutral CdZn would also be formed.

Structure, phase stability and elastic properties in the Ti1–xZrxN thin-film system: Experimental and computational studies

The composition-dependence of the structure and elastic properties of ternary Ti1–xZrxN alloys is systematically investigated by combining thin film growth and ab initio calculations. Single-phase Ti1–xZrxN thin films (0⩽x⩽1) with a rocksalt structure have been deposited using dc reactive magnetron sputtering at Ts=300°C in Ar/N2 plasma discharges. The structure, stress state and phase stability upon thermal annealing were studied by X-ray diffraction (XRD), while the acoustic and elastic properties were measured using Brillouin light spectroscopy, picosecond ultrasonics and nanoindentation. First-principles pseudopotential calculations of the total energy, lattice constants, bulk modulus, and single-crystal elastic constants Cij for several cubic ordered structures of Ti1–xZrxN alloys were also carried out. The positive values of the computed formation energies indicate that the homogeneous Ti1–xZrxN alloys can be only stabilized at high temperatures. However, the magnetron-sputtered thin films were found to retain their as-grown single-phase cubic structure during post-deposition annealing at 850°C for 3h. The calculated equilibrium lattice parameters are in good agreement with the stress-free lattice parameters a0 determined experimentally from XRD using the sin2ψ method: they both exhibit a positive deviation from Vegard-like linear interpolation. The calculated bulk modulus, elastic constants and Poisson’s ratio gradually decrease from TiN to ZrN. These computed values were used to interpret the experimentally derived elastic constants and Young’s modulus as functions of composition.

Magnetic and electronic properties of silicane with hydrogen vacancies on the surface

By using first-principles pseudopotential calculations, we investigate the magnetic and electronic properties of hydrosilicon honeycomb structure (silicane) with hydrogen vacancies on its surface. It is found that a single vacancy created on the nonmagnetic silicane can induce a magnetic moment of one μB. When a domain of vacancies is considered, the magnetization is determined by the size and geometry of the domain, and whether it is single- or double-sided. Compared with the energy band structure of silicane, the creation of domain causes very flat bands around the Fermi level and tends to reduce the band gap. It is interesting to find that the systems with single-sided domain all exhibit a band gap of about 1.0eV while those of double-sided domain exhibit strong size dependence.

Ab initio-based study for adatom kinetics on AlN(0001) surfaces during metal-organic vapor-phase epitaxy growth

The kinetics of Al and N adatoms on reconstructed AlN(0001) surfaces under growth conditions is investigated by performing first-principles pseudopotential calculations. Our calculations reveal that the adsorption of Al adatom strongly depends on the surface reconstruction while its diffusion is not affected by the reconstruction: the adsorption of Al adatom on the surface under N-rich conditions is much easier than that under H-rich conditions. These results indicate that the growth of AlN during metal-organic vapor-phase epitaxy is prominent under N-rich conditions rather than H-rich conditions, consistent with experimentally reported growth rate difference.

Imposing changes of band and spin–orbit gaps in GaNBi

We present first-principles pseudopotential plane-wave calculations to explore the effects of alloying of non conventional III–V compound GaN with bismuth. We found a highly nonlinear reduction of the energy gap of GaN for small Bi composition. Consequently the optical band gap bowing is found extremely important and composition dependent. The stronger contribution is due principally to structural and, to less extent, to charge transfer effects. Moreover, because of strong relativistic effects caused by bismuth, we found a giant bowing for the spin–orbit splitting energy of valence band, by far the largest of any III–V ternary alloys.

Reconstructions on AlN Polar Surfaces under Hydrogen Rich Conditions

Structures and stability of AlN(0001) and (000) surfaces under hydrogen rich conditions are theoretically investigated by performing first-principles pseudopotential calculations. The calculated surface energies demonstrate that several hydrogen incorporated structures are stabilized depending on the chemical potentials of constituting elements. Using surface phase diagrams, which are obtained by comparing the calculated adsorption energy with vapor-phase chemical potentials, we find that H atoms tend to desorb from AlN(0001) surface even under high H2 pressures. In contrast, N–H bonds on AlN(000) surface are found to be favorable over the wide range growth conditions. These results imply that the growth processes on AlN(0001) surface could be changed by growth conditions such as temperatures and pressures.