Full-potential Linear Augmented Plane-wave Method Research Articles

Theoretical analysis of x-ray absorption spectra of Ti compounds used as catalysts in lithium amide/imide reactions

We present a theoretical analysis and interpretation of the x-ray absorption near-edge structure of x-ray absorption spectroscopy (XAS) at the titanium $K$-edge of several Ti compounds for understanding catalysis mechanism in lithium amide ${\text{LiNH}}_{2}$ and imide ${\text{Li}}_{2}\text{NH}$ systems for hydrogen storage. Our theoretical approach is based on first-principles calculations using all-electron full-potential linear augmented plane-wave method. Chemical bonding and local geometry of catalytically-active Ti states in the hydrogen desorption reaction ${\text{LiNH}}_{2}+\text{LiH}\ensuremath{\rightarrow}{\text{Li}}_{2}\text{NH}+{\text{H}}_{2}$ are investigated. It is found that XAS spectra of some compounds consisting of elements Li, N, H, and Ti are quite similar to measured ones of catalytically-active Ti compounds. We conclude that Ti ions may occupy the Li sites in ${\text{LiNH}}_{2}$ during the reaction.

Electronic and optical properties of orderedBexZn1−xSe alloys by the FPLAPW method

We have used the ab initio full potential linear augmented plane wave (FPLAPW) methodwithin the local density functional theory to calculate the dielectric function and reflectivity ofBexZn1−xSe alloys for differentcompositions x = 0.0, 0.25, 0.50, 0.75 and 1.0. The calculated dielectric function and reflectivities showreasonably good agreement with the experimental data. The valence band maximum isdominated by the Se-3p states and the conduction band minimum by Zn-4s states forx = 0.0, i.e. ZnSe, and byBe-p states for x = 1.0, i.e. BeSe. Our calculations show that the direct band gap varies linearly withx.

Spin, charge, and orbital ordering inRBaMn2O5+δ(R=Y, La;0⩽δ⩽1) and their dependence on oxygen content and size of theRconstituent

The effect of oxygen content on spin, charge, and orbital ordering in $R\mathrm{Ba}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5+\ensuremath{\delta}}$ ($R=\mathrm{Y}$, La; $0\ensuremath{\leqslant}\ensuremath{\delta}\ensuremath{\leqslant}1$) is studied by density-functional-theory-based calculations as implemented in the full-potential linearized-augmented plane-wave method. Structural optimizations using the projector augmented wave method have been performed for all the phases and the calculated structural parameters are found to be in good agreement with experimental values. Total-energy calculations have systematically been performed including force as well as stress minimization for paramagnetic, ferromagnetic, and antiferromagnetic configurations. For $\ensuremath{\delta}=0$, the ground state is found to be ferrimagnetic, whereas the variants with oxygen contents $\ensuremath{\delta}=1∕2$ and 1 give rise to an antiferromagnetic ground state, all in perfect agreement with experimental findings. The electronic-band characteristics are analyzed using total and site- and orbital-projected densities of states and the examination shows that the electronic structure undergoes a gradual change from semiconductor-to-metal behavior on going from $\ensuremath{\delta}=0$ to 1. Even the $\mathrm{GGA}+U$ approach failed to reproduce the insulating state for the phases with $\ensuremath{\delta}=1$, indicating that introduction of the experimental $CE$-type magnetic structure may be important. The charge and orbital ordering are analyzed with the help of the energy-projected-density matrices of the $d$ electrons. Very different ordering patterns have emerged for the different phases under investigation, indicating that both cation radii and oxygen stoichiometry play an important role in deciding spin, charge, and orbital ordering.

Simulation of thermal properties of Ba 1−xZrO 3 compounds for thermal barrier coating applications

The effect of vacancies on structural properties of the perovskite-type oxide was studied using the full-potential linearized augmented plane wave (FP-LAPW) method, within the density functional theory. In this approach, the generalized gradient approximation was used for the exchange-correlation potential. The ground state properties such as lattice parameter, bulk modulus and inter-atomic distances of cubic Ba 1− x ZrO 3 compounds ( x = 0, 0.125 and 0.25) were calculated. Additionally, using a set of total energy versus volume obtained with the FP-LAPW method, the quasi-harmonic Debye model was applied to determine the thermal properties including temperature dependence of bulk modulus, thermal expansion coefficient, specific heats at constant volume and constant pressure. No experimental data are available and our results are considered as purely predictive.

Cubic-to-tetragonal phase transition of HfO 2 from computational study

The structural and electronic properties of HfO 2 polymorphs were investigated using density functional theory (DFT). The Kohn–Sham equations were solved by applying the full-potential linearized augmented plane wave (FP-LAPW) method. We used the generalized gradient approximation in the Perdew–Wang formalism to the exchange and correlation energy functional. The ground state properties such as lattice parameter, bulk modulus and its pressure derivative as well as the structural phase stability were calculated and compared to the available experimental data and previous theoretical works. The FP-LAPW method correctly orders the zero temperature energies of hafnia polymorphs. The effect of distortion from the cubic to the tetragonal structure has also been studied on the basis of charge density calculations.

Electronic structure and x-ray magnetic circular dichroism of gadolinium beyond the local spin density approximation

The electronic and magnetic properties of gadolinium are studied by means of the full-potential linear augmented plane-wave method including the Hubbard interaction for describing a $4f$ electron-electron interaction in the mean-field approximation. The manner in which the $4f$ localized orbitals are approximated seems to be of great importance in elucidating the mechanisms responsible for the strong magnetic moment of gadolinium and its ordered magnetic structure as well as the relative energy positions of the occupied and empty $4f$ states. Through various calculations, including the spin-orbit coupling in a second-variation scheme, within the local-spin-density approximation (LSDA), the generalized-gradient approximation (GGA), the $\mathrm{LDA}+\mathrm{U}$, and the $\mathrm{GGA}+\mathrm{U}$, it is clearly shown that the $\mathrm{LDA}+\mathrm{U}$ or the $\mathrm{GGA}+\mathrm{U}$ are the most adequate methods for describing the electronic and magnetic structures as well as the x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) of the the strongly correlated $4f$ electrons of gadolinium. In particular, the calculated ${L}_{2,3}$ and ${M}_{4,5}$ XAS and XMCD spectra are found in good agreement with the experimental ones except that the shoulder above the principal peak of the ${M}_{4,5}$ spectra is not present in the calculation. The spin magnetic moments of the $5d$ states obtained from the XMCD spectra using the XMCD sum rules compare favorably with the self-consistent band-structure results only when the dipole magnetic term is included in the calculation.

Ground state structures in the polonium based II–VI compounds

We have performed ab-initio self-consistent calculations using the full-potential linear augmented plane-wave method to investigate the structural and the electronic properties of the less known II–VI compounds: ZnPo, CdPo, and HgPo. Total energy calculations of the cubic zinc-blende, wurtzite, rocksalt, cesuim chloride, orthorhombic Cmcm, and tetragonal PbO phases are investigated. Ground state parameters are computed, and compared with available theoretical and experimental works. The zinc-blende structure is found to be the ground state phase of ZnPo and CdPo, while HgPo prefers the tetragonal PbO structure. The calculated band structure of II-Po shows features that differ considerably from those of typical II–VI semiconductors. In particular we found an inverted band gap, reflecting a semi-metallic character for these compounds.

Electronic origin of elastic properties of titanium carbonitride alloys

We have carried out numerical ab initio calculations of the elastic constants for several cubic ordered structures modeling titanium carbonitride (TiCxN1−x) alloys. The calculations were performed using the full-potential linear augmented plane-wave method (FPLAPW) to calculate the total energy as functions of volume and strain, after which the data were fit to the traditional Murnaghan equation of state and to a polynomial function of strain to determine the formation energy; the bulk modulus; and the elastic constants C 11, C 12 and C 44. The predicted equilibrium lattice parameters are slightly higher than those found experimentally (on average by 0.2 pct). The computed formation energy indicates that the alloys are stable in the entire range of the carbon concentration x and the maximum stability is obtained for 0.5≤x≤0.75. The computed bulk modulus, the shear modulus G, and the Young’s modulus E are within approximately 2, 1, and 2 pct of the experimentally measured characteristics, respectively. The maximum deviation is observed for TiC and TiN. The moduli G, E, and Poisson’s ratio reach a maximum value at approximately the middle of the concentration range, which is due to the fact that the shear modulus C 44 shows a maximum value for a valence electron concentration (VEC) in the range of 8.25 to 8.5. The other shear modulus (C 11−C 12)/2 does not exhibit any maximum overall concentration range and instead has a flat dependence in the range mentioned previously. Such a concentration behavior of the elastic constants is related to specific changes in the band structure of TiCxN1−x alloys caused by the orthorhombic and monoclinic strains that determine the shear moduli (C 11−C 12)/2 and C 44, respectively.

Atomistic study of structural, elastic, electronic and thermal properties of perovskites Ba(Ti,Zr,Nb)O3

The full-potential linearized augmented plane waves (FP-LAPW) method based the on density functional theory (DFT) using the generalized gradient approximation (GGA) is applied to study the structural, mechanical, and electronic properties of BaTiO3, BaZrO3, and BaNbO3 cubic perovskites. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the FP-LAPW method, is applied to study the thermal and vibrational effects. Predicted temperature and pressure effects on the structural parameters, thermal expansions, heat capacities, and Debye temperatures are determined from the non-equilibrium Gibbs functions.

Structural and electronic properties of zirconia phases: A FP-LAPW investigations

The structural and electronic properties of ZrO 2 polymorphs were investigated using density functional theory (DFT). The Kohn–Sham equations were solved by applying the full-potential linearized augmented plane wave (FP-LAPW) method. We used the generalized gradient approximation (GGA) in the Perdew–Wang formalism to the exchange and correlation energy functional. The ground state properties such as lattice parameter, transition pressures, bulk modulus and its pressure derivative as well as the structural phase stability were calculated. The results were compared with previous calculations and experimental data when available. The FP-LAPW method correctly orders the zero temperature energies of all zirconia polymorphs. We have also studied the effect of distortion from the cubic to the tetragonal structure on the basis of charge density calculations. On the other hand, band structure and density of states (DOS), which allow us to discuss the features of orbital mixing, are also given. Our results suggest that the cotunnite structure should be better than the other zirconia phases as gate dielectric material.

Full-relativistic calculation of electronic structure of Zr 2AlC and Zr 2AlN

We have employed a full-relativistic version of an all-electron full-potential linearized-augmented plane-wave method in the local density approximation to investigate the electronic structure of nanolaminate Zr 2AlX (X=C and N). The Zr 4d electrons are treated as valence electrons. We have investigated the lattice parameters, bulk moduli, band structures, total and partial densities of states and charge densities. It is demonstrated that the strength and electrical transport properties in these materials are principally governed by the metallic planes.

Meta-GGA calculation of the electronic structure of group III–V nitrides

The ground state electronic structure of wurtzite AlN, GaN, and InN has been calculated using full-relativistic all-electron full-potential linearized-augmented plane-wave method. Several DFT exchange-correlation functionals, including the recently proposed non-empirical meta-generalised gradient approximation (Meta-GGA) have been used. The role played by relativistic effects and meta-GGA functional on the band structures and the density of states is discussed. We find that the meta-GGA improves the accuracy of the structural properties as well as the energies of the semicore d states in both GaN and InN. This new functional slightly outperforms the local density approximation (LDA) and the generalized gradient approximation (GGA) overall as to lattice parameters, bulk modulus, and valence band widths. We find also that the meta-GGA induced modifications of the band structure are significant, but limited to the valence band states, while leaving all other features identical to LDA and GGA calculations. Finally, our results show that GaN and InN present anomalous dependence of the valence band splitting versus pressure.

Relaxations and bonding mechanism inHg1−xCdxTewith mercury vacancy defect: First-principles study

The structural and electronic properties of the mercury vacancy defect in ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Te}$ have been studied by combining the full-potential linear augmented plane wave and plane-wave pseudopotential method base on the density functional theory. Structural relaxation, local charge density, and on-site and partial densities of states are computed to investigate the effects of the mercury vacancy on the electronic structure. The characteristics of dangling bond rehybridization due to the mercury vacancy are discussed by analysis of the valence charge density and the bonding charge density. We reveal upshift of the energy level for the $5s$ states of the nearest neighbor tellurium of the defect due to dangling bond rehybridization. The double acceptor levels introduced by the vacancy are determined by the single-particle electron energy calculations and the transition energy levels, which agree well with the experimental results.

Cubic (BN)xC2(1−x) ordered alloys: a first-principles study of the structural, electronic, and effective massproperties

We apply a first-principles method based on the density functional theory withinthe generalized gradient approximation, and the full-potential linear augmentedplane-wave method, to calculate the structural and electronic properties of cubic(BN)xC2(1−x) ordered alloys. We investigate the equilibrium lattice parameters, the bulk moduli, the density ofstates, the band-gap energies and the effective masses of the conduction and valence bands along the[111],[100] and [110] directions. The obtained results are used to provide effective-mass and Luttingerparameters, and to give an important guideline to the material’s design for optoelectronicdevices, we link the first-principles band calculations with effective mass theory.

Elastic, electronic and optical properties of ZnS, ZnSe and ZnTe under pressure

The results of first-principles theoretical study of the structural, electronic and optical properties of zinc monochacogenides ZnS, ZnSe and ZnTe, have been performed using the full-potential linear augmented plane-wave method plus local orbitals (FP-APW + lo) as implemented in the WIEN2k code. In this approach the local density approximation (LDA) is used for the exchange-correlation (XC) potential. Results are given for lattice constant, elastic constant, bulk modulus, and its pressure derivative. The band structure, density of states, pressure coefficients of elastic constants, energy gaps and refractive indices are also given. The results are compared with previous theoretical calculations and the available experimental data.

Semiconductor properties of Cu-based delafossites revealed by an electric field gradientstudy

In this paper we offer an interpretation of the previously observed trend of the electricfield gradient (EFG) values measured in a group of semiconducting delafossitesCuBO2 (B = Al,Fe, Cr, Nd) at Cd impurities which substitute either Cu or B atoms. Our theoretical studyindicates that this EFG trend reveals one of the most subtle details in the electronicspectrum of the compounds, namely whether the impurity states are formed within or outof the band gap. When Cd substitutes the Cu, it exhibits a larger EFG value inCuAlO2 andCuFeO2 thanin CuCrO2 and CuNdO2. This occurs because the Cd states form in the first two compounds a shallow band withinthe gap, but in the second two compounds they do not. When Cd substitutes the B atom itexhibits almost the same EFG in all delafossites. In this case, Cd states are not formed withinthe gap in any of the compounds. The same interpretation can be applied to the whole family ofCuBO2 delafossites, whatever the B atom is. To arrive at these conclusions we analysed and calculated various systems(Cd-doped CuAlO2 and CuCrO2 compounds, and molecular clusters) using the full-potential linear augmented plane wavemethod.

M-Doped TiO2(M=Co, Cr, Fe)의 제조 : 전자 밴드구조-(1)

The electronic band structures of Metal-doped titanium dioxide, M-doped TiO₂ (M＝Co, Cr, Fe), have been studied by using XRD, UV-vis diffuse reflectance spectrometer and FP-LAPW (Full-Potential Linearized Augmented-Plane-Wave) method. The UV-vis of M-doped TiO₂ (M＝Co, Cr, Fe) showed two absorption edges; the main edge due to the titanium dioxide at 387 ㎚ and a shoulder due to the doped metals at around 560 ㎚. The band gap energies of Co, Cr and Fe-doped TiO₂ calculated by FP-LAPW method were 2.6, 2.0, and 2.5 eV, respectively. The theoretically calculated band gap energy of TiO₂ by using FP-LAPW method was the same as experimental results. FP-LAPW method will be useful for fabrication and development of photocatalysts working under visible light.

Adhesion and bonding of Pt/Ni and Pt/Co overlayers: Density functional calculations

The electronic and energetic properties of bimetallic surfaces Pt/Ni(111) and Pt/Co(111) are examined using the FP-LAPW (Full-Potential Linearized Augmented Plane Wave) method by means of spin-polarized and non-polarized calculations. We present both the results of the shifts in the d-band centers when one metal (Pt) is pseudomorfically deposited on another with smaller lattice constant (Ni, Co) and those corresponding to the surface and adhesion energies. The surface is modeled by a seven layer slab separated in z direction by a vacuum region of six substrate layers. The results obtained for pure Ni, Co and Pt surfaces are presented in order to compare with experimental and theoretical data reported in the literature

Temperature dependence of the EFG at Cd‐doped Lu 2 O 3 : How ab initio calculations can complement PAC experiments

We report an ab initio study of the temperature dependence of the electric-field gradient (EFG) tensor at Cd impurities replacing cations in Lu2O3. Calculations were performed with the Full-Potential Linearized-Augmented Plane Wave method that allows us to treat the electronic structure and the processes induced by the impurity in the host-lattice without the use of external parameters. In this new insight, the EFG thermal dependence arises from the ionization of an impurity acceptor level introduced in the band-gap of Lu2O3 by Cd impurities, in good agreement with a previously proposed two state model. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure of and quantum size effect in III-V and II-VI semiconducting nanocrystals using a realistic tight binding approach

We analyze the electronic structure of group III-V semiconductors obtained within full potential linearized augmented plane wave (FP-LAPW) method and arrive at a realistic and minimal tight-binding model, parameterized to provide an accurate description of both valence and conduction bands. It is shown that cation sp3 - anion sp3d5 basis along with the next nearest neighbor model for hopping interactions is sufficient to describe the electronic structure of these systems over a wide energy range, obviating the use of any fictitious s* orbital, employed previously. Similar analyses were also performed for the II-VI semiconductors, using the more accurate FP-LAPW method compared to previous approaches, in order to enhance reliability of the parameter values. Using these parameters, we calculate the electronic structure of III-V and II-VI nanocrystals in real space with sizes ranging upto about 7 nm in diameter, establishing a quantitatively accurate description of the band-gap variation with sizes for the various nanocrystals by comparing with available experimental results from the literature.