Linear Augmented Plane Wave Calculations Research Articles

Ferromagnetism induced by calcium vacancies in Ca3BiP anti-perovskite: An ab-initio calculation

We study all possibilities of vacancy that can induce the magnetism in Ca3BiP anti-perovskite using a full-potential linearized-augmented plane-wave calculation within the generalized gradient approximation (PBE-GGA) and a modified version proposed by Becke and Johnson (TB-mBJ) for the exchange-correlation potential. The effects of spin-orbit coupling (SOC) are also considered in this study. We have examined the effect of all cation and/or anion vacancies on the magnetic structure in this system. For Ca3BiP bulk non-magnetic, the calculated band structures and densities of states show a semi-metallic and semi-conductor character obtained by PBE-GGA and TB-mBJ approximations respectively. Without spin-orbit interactions, we find that Ca3-xBiP and Ca3-xBiP1-x relaxed systems have magnetic moments of 2.44701 μB and 3.96894 μB respectively. Both approaches results show that the calcium and phosphorus vacancies are responsible for the magnetism in anti-perovskite Ca3BiP due to the spin-polarization that comes mainly from the valence states of the nearest atoms surrounding these vacancies. In the case of the other studied vacancies, the magnetism was not induced and all materials adopt a metallic character.

Electron Localization Function and Compton Profiles of Cu2O.

The nature of bonding in the cubic cuprous oxide is studied by means of the theoretical tools, namely, the electron localization function and Compton profiles. The isotropic Compton profiles together with the anisotropies in the directional Compton profiles are presented. Taking free-atom Compton profiles, the charge-transfer model is also applied. The first-principles calculations based on the GGA are performed, and the self-interaction correction is incorporated, adopting the GGA+U approach. Both types of calculations are performed deploying the linearized augmented plane-wave (LAPW) method. The effect of self-interaction correction on the electron localization function, Compton profiles, and anisotropies is discussed. The electron localization function reveals ionic behavior in the (110) plane and covalent nature in the Cu-O bond intersecting plane. The GGA+U exhibits more covalent nature. The two LAPW calculations of the Compton profiles show better agreement with the available experimental data than the free-atom profiles. Among all of the calculations undertaken, the GGA+U shows the best agreement with the experiment. The GGA+U calculation shows more anisotropic behavior in directional Compton profiles.

Fundamental properties of scandium chalcogenides and their alloys: DFT study

The full-potential linearized-augmented plane wave calculations based on density functional theory are performed to study the structural, electronic, optical and thermodynamic properties of scandium chalcogenides ScX (X = S, Se, Te) and their ternary alloys at equilibrium as well as under pressure. The revised Perdew–Burke–Ernzerhof generalized gradient approximation (GGA) is used to calculate the structural properties. The electronic and optical properties are calculated employing the GGA and the modified Becke–Johnson (mBJ) approaches. Moreover, the calculated lattice parameters agree well with the experiment results. The structure NaCl-type (B1) of the scandium chalcogenides undergoes under pressure a structural phase transition to CsCl-type (B2) and ZnS-type (B3). The binary and ternary alloys indicate a metallic behavior using GGA and mBJ scheme. The interband contribution to the optical properties is investigated by calculating the dielectric parameters e1(ω), e2(ω) and the index of refraction n(ω). A quasi-harmonic Debye model is applied to calculate the thermal properties.

Full potential linear augmented plane wave calculations of Electronic and Optical properties in ZnO

         In this work we present self-consistent calculations for the electronic and optical properties of Zinc oxide. A theoretical investigation of the electronic properties (band structure, density of charge and contour map) and optical properties (refractive index, absorption coefficient, dielectric constants and reflectivity) of Zinc oxide semiconductor ZnO. A full-potential linearized augmented plane-wave (FPLAPW) method was used within the density functional theory (DFT) along with the generalized gradient approximation (GGA96) exchange correlation potential. The results are compared with the experimental data available and some other theoretical work. We found that the GGA approximation yields only a small improvement to the band gap, however, if we allow for a rigid shift of the band structure, the so-called scissor’s operator, the optical properties are excellently reproduced.

Band structure and optical properties of hexagonal In-rich InxAl1−xN alloys

Full potential linear augmented plane wave calculations have been performed to study the electronic and optical properties of In-rich InxAl1−xN alloys in the hexagonal wurtzite structure. Compositions of x = 0.9375, 0.8125 and 0.6875 are considered which follow from replacing one, three and five In atoms by Al in the 32-atom supercell. The new form of exchange correlation, i.e. Engel–Vosko’s generalized gradient approximation within density functional theory, is employed. The calculations yield the band structure and total density of states as well as the imaginary part ε2(ω) of the ordinary and extraordinary dielectric function. The calculated dependence of the bandgap on the composition is in good agreement with recent experimental studies. A reversal of the valence band ordering is found between x = 0.8125 and 0.6875. The absorption features in the high-energy range of ε2(ω) are related to critical points of the band structure. The transition energies for these van Hove singularities are determined and their bowing parameters are discussed.

Tight-binding Hamiltonian for LaOFeAs

First-principles electronic-structure calculations have been very useful in understanding some of the properties of the new iron-based superconductors. Further explorations of the role of the individual atomic orbitals in explaining various aspects of research in these materials, including experimental work, would benefit from the availability of a tight-binding (TB) Hamiltonian that accurately reproduces the first-principles band-structure results. In this work we have used the Naval Research Laboratory-TB method to construct a TB Hamiltonian from linearized augmented plane wave (LAPW) results. Our TB model includes the $\text{Fe}\text{ }d$ orbitals and the $p$ orbitals from both As and O for the prototype material LaOFeAs. The resulting TB band structure agrees well with that of the LAPW calculations from 2.7 eV below to 0.8 eV above the Fermi level, ${\ensuremath{\epsilon}}_{F}$, and the Fermi surface matches perfectly to that of the LAPW. The TB densities of states (DOSs) are also in very good agreement with those from the LAPW in the above energy range, including the per orbital decomposition. We use our results to provide insights on the existence of a pseudogap in the DOS just above the Fermi level. We have also performed a separate TB fit to a database of LAPW results as a function of volume and with variations in the As positions. This fit although less accurate regarding the band structure near ${\ensuremath{\epsilon}}_{F}$, reproduces the LAPW total energies very well and has transferability to nonfitted energies.

Vacancy defects in strontium titanate: Ab initio calculation

A full-potential linearized-augmented plane-wave calculation was used to study structural and electronic properties of defected SrTiO 3 with two concentrations of oxygen vacancies. These properties were found to remarkably depend on the oxygen vacancy concentration. The defect states associated with oxygen vacancies are identified within the energy gap. The ground state properties, equilibrium lattice constants, bulk moduli, the formation energy, densities of electron states, band structures and charge density are determined and discussed for this compound. It is found that an oxygen vacancy creates more localized in-gap states. With increasing oxygen deficiency, the electrons left behind by oxygen removal not only localize on Sr (Ti) 3 d orbitals but also on the vacancy sites. Model structures of 2 × 2 × n ( n = 1, 2, 3 and 4) supercells are used.

Electronic structure of BaFe2As2 as obtained from DFT/ASW first‐principles calculations

AbstractWe use ab‐initio calculations based on the augmented spherical wave method within density functional theory to study the magnetic ordering and Fermi surface of BaFe2As2, the parent compound of the hole‐doped iron pnictide superconductors (K,Ba)Fe2As2, for the tetragonal I4/mmm as well as the orthorhombic Fmmm structure. In comparison to full potential linear augmented plane wave calculations, we obtain significantly smaller magnetic energies. This finding is remarkable, since the augmented spherical wave method, in general, is known for a most reliable description of magnetism.

First-principle study of full Heusler [formula omitted] using PBE0 hybrid functional

First-principle self-consistent full potential linear augmented plane wave calculations based on density functional theory using hybrid functional PBE0 are performed to study magnetic moments, density of states and half-metallicity of L2 1 type full Heusler alloys with formula Co 2Y Si. Y is Ti, V, Mn and Fe. We have compared these results with those of the PBE-GGA exchange correlation functional and the LDA + U method. The results for Co 2FeSi and Co 2MnSi are completely different; using the PBE0 hybrid functional for Co 2FeSi predicts experimental magnetic moment and also predicts this material to be half-metallic ferromagnet, while using PBE-GGA predicts it not to be half-metal. The results of PBE0 are more similar to the ones obtained by LDA + U method.

Structural, electronic and optical properties of high pressure stable phases of ZnTe

Abstract We have performed full potential linear augmented plane wave calculations to investigate the pressure induced phase transition in ZnTe. Total energies of three phases (zinc-blende, cinnabar and Cmcm) are calculated using density functional theory formalism under generalized gradient approximation and Engel–Vosko generalized gradient approximation for the exchange correlation potential approximation. The pressure stability corresponding to zinc-blende, cinnabar and Cmcm phases of ZnTe are computed. We find that cinnabar phase could be formed as a metastable phase by releasing pressure from the high pressure Cmcm phase. The obtained structural, electronic and optical results are compared with previous calculations and available experimental data. Overall good agreement is found.

Mn2CoZ(Z=Al,Ga,In,Si,Ge,Sn,Sb)compounds: Structural, electronic, and magnetic properties

We study the electronic structures and magnetic properties of ${\mathrm{Mn}}_{2}\mathrm{Co}Z$ $(Z=\mathrm{Al},\mathrm{Ga},\mathrm{In},\mathrm{Si},\mathrm{Ge},\mathrm{Sn},\mathrm{Sb})$ compounds with ${\mathrm{Hg}}_{2}\mathrm{Cu}\mathrm{Ti}$-type structure using first-principles full-potential linearized-augmented plane-wave calculations. It is found that the compounds with $Z=\mathrm{Al}$, Si, Ge, Sn, and Sb are half-metallic ferrimagnet. Experimentally, we successfully synthesized the ${\mathrm{Mn}}_{2}\mathrm{Co}Z$ $(Z=\mathrm{Al},\mathrm{Ga},\mathrm{In},\mathrm{Ge},\mathrm{Sn},\mathrm{Sb})$ compounds. Using the x-ray diffraction method and Rietveld refinement, we confirm that these compounds form ${\mathrm{Hg}}_{2}\mathrm{Cu}\mathrm{Ti}$-type structure instead of the conventional $L{2}_{1}$ structure. Based on the analysis on the electronic structures, we find that there are two mechanisms to induce the minority-spin band gap near the Fermi level, but only the $d\text{\ensuremath{-}}d$ band gap determines the final width of the band gap. The magnetic interaction is quite complex in these alloys. It is the hybridization between the $\mathrm{Mn}(C)$ and Co atom that dominates the magnitude of magnetic moment of the Co atom and the sign of the $\mathrm{Mn}(B)\text{\ensuremath{-}}\mathrm{Co}$ exchange interaction. The ${\mathrm{Mn}}_{2}\mathrm{Co}Z$ alloys follow the Slater-Pauling rule ${M}_{H}={N}_{V}\ensuremath{-}24$ with varying $Z$ atom. It was further elucidated that the molecular magnetic moment ${M}_{H}$ increases with increasing valence concentration only by decreasing the antiparallel magnetic moment of $\mathrm{Mn}(C)$, while the magnetic moments of $\mathrm{Mn}(B)$ and Co are unaffected.

Electronic Structures of CaAlSi with Different Stacking AlSi Layers by First-Principles Calculations

The full-potential linear augmented plane-wave calculations have been applied to investigate the systematic change of electronic structures in CaAlSi due to different stacking sequences of AlSi layers. The present ab-initio calculations have revealed that the multistacking, buckling and 60° rotation of AlSi layer affect the electronic band structure in this system. In particular, such a structural perturbation gives rise to the disconnected and cylindrical Fermi surface along the M–L lines of the hexagonal Brillouin zone. This means that multistacked CaAlSi with the buckling AlSi layers increases degree of two-dimensional electronic characters, and it gives us qualitative understanding for the quite different upper critical field anisotropy between specimens with and without superstructure as reported previously.

Full-potential study of d-electrons effects on the electronic structure of wurtzite and zinc-blende InN

In this paper, we focus on the role played by the relativistic and d-electron effects on the electronic structure of both wurtzite (α-InN), as well as the zinc-blende (β-InN) modifications of indium nitride. Here we perform all-electron full-potential linearized-augmented plane-wave calculations with spin–orbit interaction. The In 4d electrons are treated both as core and as valence electrons. We compare the performance of the local-density approximation (LDA), the generalized gradient approximation (GGA) and the recently proposed non-empirical meta-generalized gradient approximation (Meta-GGA) in calculations of the structural and electronic properties of InN. Band structures, densities of states, orbital-resolved densities of states, charge densities and ionicity factors are analyzed in great detail. The calculated values of the energy gaps, bandwidths, spin–orbit and crystal-field splittings and the correct band degeneracies are compared to experimental and/or ab-initio results. The role played by relativistic effects and meta-GGA functional on the band structure is discussed. For the structural properties meta-GGA is more accurate and gives the best description of InN. We found that several features of α-InN resemble those of β-InN. Most of the calculated band parameters, of band gaps, total valence bandwidths for α-InN are close to those of β-InN to within 1%. The charges distributions have similar features meaning that this material has the same ionicity factor in both structures even when In 4d states were treated both as core and as valence electrons. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Osmium is not harder than diamond

A recent measurement found Os has a larger bulk modulus than diamond, in agreement with an earlier calculation. We have made fully relativistic linear augmented plane-wave calculations of osmium and diamond and found diamond to have the larger bulk modulus, independent of whether the local-density- or generalized-gradient-approximation (GGA) exchange-correlation potentials are used. With the GGA our bulk modulus lies between two very recent and more accurate experimental values.

Phase stability in the Fe–Ni system: Investigation by first-principles calculations and atomistic simulations

First-principles calculations of the energy of various crystal structures of Fe, Ni and ordered Fe–Ni compounds with different stoichiometries have been performed by the linearized augmented plane wave (LAPW) method in the generalized gradient approximation. The most stable compounds are L1 2–Ni 3Fe, L1 0–FeNi, C11 f–Ni 2Fe and C11 f–Fe 2Ni. The L1 2–Ni 3Fe compound has the largest negative formation energy, which is consistent with the experimental Fe–Ni phase diagram. The L1 0–FeNi compound has also been observed experimentally in meteorite samples as a metastable phase. It is suggested here that the C11 f compounds could also form in Fe–Ni alloys at low temperatures. A new semi-empirical interatomic potential has been developed for the Fe–Ni system by fitting to experimental data and the results of the LAPW calculations. Recognizing the significance of the covalent component of bonding in this system, the potential is based on the embedded-atom method (EAM) but additionally includes a bond-angle dependence. In comparison with the existing modified EAM method, our potential form is simpler, extends interactions to several (3–5) coordination shells and replaces the screening procedure by a smooth cutoff of the potential functions. The potential reproduces a variety of properties of Fe and Ni with a reasonable accuracy. It also reproduces all stability trends across the Fe–Ni system established by the LAPW calculations. The potential can be useful in atomistic simulations of the phases of the Fe–Ni system.

Electronic structure and superconductivity of CaAlSi and SrAlSi

We report full-potential linear augmented plane-wave calculations for CaAlSi and SrAlSi in ordered structures and in the virtual-crystal approximation, at normal and elevated pressures. We also estimate the electron-phonon coupling using frozen-phonon calculations at the zone center, and the rigid muffin-tin approximation. We conclude that there is no simple way to explain the recently reported qualitative disparity in the superconducting properties of the two compounds. An assumption of an ultrasoft phonon mode, on the other hand, allows to reconcile the experimental findings with the theory in a reasonable way.

Interatomic potentials for atomistic simulations of the Ti-Al system

Semi-empirical interatomic potentials have been developed for Al, alpha-Ti, and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large database of experimental as well as ab-initio data. The ab-initio calculations were performed by the linear augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide reasonable description of the lattice thermal expansion, demonstrating their usefulness in the molecular dynamics or Monte Carlo studies at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in gamma-TiAl calculated with the EAM potential is in a fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in gamma-TiAl are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that gamm-TiAl is an antisite disorder compound in agreement with experimental data.

Magnetic properties of SmCo5 and YCo5

We have studied the magnetic moments and magnetic anisotropy energy (MAE) of YCo5 and SmCo5 using full-potential linear augmented plane wave (LAPW) electronic structure calculations. Most previous calculations of the MAE for YCo5, using local density approximation (LDA) for the exchange-correlation potential, have found values significantly smaller (∼0.6 meV/f.u.) than experiment (∼3.8 meV/f.u.). The rest of the MAE is attributed to many body corrections. Our LAPW calculations using the generalized gradient approximation (GGA) instead of LDA and including nonspherical corrections give values ∼1.5 meV/f.u. The Co magnetic moment of YCo5−xCux, unlike the prediction of the virtual crystal approximation, decreases slowly with impurity concentration until dropping suddenly to zero at a critical dopant concentration. Correlation effects were found to be crucial for the MAE in SmCo5. While GGA calculations give MAE for SmCo5 of the wrong sign, including the LDA+U, correction brings it to ∼21 meV/f.u., in good agreement with the experimental value of 13–16 meV/f.u.

Cohesive properties of group-III nitrides: A comparative study of all-electron and pseudopotential calculations using the generalized gradient approximation

We compare the performance of generalized gradient approximations (GGA's) and the local-density approximation (LDA) in density-functional calculations of the cohesive properties of cubic AlN, GaN, and InN. Employing the widely adopted pseudopotential approach, the calculated data are found to depend significantly on the treatment of the core states of the group-III ions, hampering a conclusive assessment of the GGA and LDA. Here we perform all-electron full-potential linearized-augmented plane-wave calculation, which we use to (i) scrutinize the results of pseudopotential calculations, and (ii) provide a proper distinction between the GGA and LDA functionals. We show that the accuracy of pseudopotential calculations is comparable to that of all-electron calculations only if the Ga and In semicore d states are treated as valence rather than core states. We also show that the use of an f-like local component can further improve the transferability of the Ga and In pseudopotentials. Regarding the PBE- (Perdew-Burke-Enzerhof-) GGA [Phys. Rev. Lett. $77,$ 3865 (1996)] we find that the cohesive energies of the group-III nitride crystals (and those of the elemental metals) agree closely with experimental data whereas they are overestimated within the LDA. Lattice parameters are described with similar accuracy within the PBE-GGA and LDA. On the other hand we find that the heats of formation of the group-III nitrides are underestimated by the PBE-GGA and given more accurately by the LDA. For the PBE-GGA, the underestimate is mainly due to the fact that it still overestimates the bond strength of the ${\mathrm{N}}_{2}$ molecule. For the LDA, the heat of formation turns out only slightly too large, because of a fortuitous cancellation of the (larger) errors in the ${\mathrm{N}}_{2}$ molecule and the bulk crystals. Several other GGA functionals are able to improve over the PBE-GGA for molecules like ${\mathrm{N}}_{2}$ due to stronger gradient corrections. Here we find that such more nonlocal GGA's significantly underestimate the cohesive energies of the group-III nitride (and metal) crystals and even further underestimate their heats of formation.

Application of a tight-binding total-energy methodfor FeAl

The Naval Research Laboratory tight-binding (TB) methodaccurately describes the electronic and mechanical properties ofmany elemental systems, including semiconductors, simple metals,and transition metals. We report an extension of the method toan intermetallic compound, FexAl1-x. We found a set ofnon-orthogonal TB parameters which reproduced the electronicstructure and total energy versus volume of a set offirst-principles linearized augmented-plane-wave (LAPW)calculations for the B1 (NaCl), B2 (CsCl), D03 (Fe3Al),C11b (MoSi2), and B32 (NaTl) structures. Structuralproperties, such as lattice constants and bulk moduli, andelectronic properties, such as the densities of states and energybands, are reproduced well. In addition, quantities that werenot fitted, such as elastic constants, are found to be in verygood agreement with independent LAPW calculations.