Full Potential Linearized Augmented Plane Wave Calculations Research Articles

Electronic structure of monoclinic α-KY(WO 4) 2 tungstate as determined from first-principles FP-LAPW calculations and X-ray spectroscopy studies

Total and partial densities of states of the constituent atoms of potassium yttrium double tungstate, KY(WO 4) 2 (KYW), have been calculated using the first-principles self-consistent full potential linearized augmented plane wave (FP-LAPW) method. The results derived reveal that the O 2p-like states are the dominant contributors into the valence band of KYW, while the conduction band of the compound is dominated by contributions of the W 5d-like states. The FP-LAPW calculations render that KYW is an indirect-gap material with the band-gap value of 3.645 eV. The X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) methods were also used in the present work to study experimentally electronic properties of potassium yttrium double tungstate. For the above compound, the XES bands reflecting the energy distribution of the valence W d-, O p- and K p-like states were derived and compared on a common energy scale with the X-ray photoelectron valence-band spectrum. Measurements of the energy shift of the XAS W L III edge of KYW reveal that tungsten atoms are in the formal valence state 6+. A rather good agreement of the experimental XES and theoretical FP-LAPW data concerning electronic properties of KYW has been obtained in the present paper.

X-ray Diffraction, X-ray Photoelectron Spectra, Crystal Structure, and Optical Properties of Centrosymmetric Strontium Borate Sr2B16O26

We report results of X-ray diffraction (XRD) and valence band X- ray photoelectron (VB-XPS) spectra for strontium borate Sr(2)B(16)O(26). The X-ray structural analysis shows that the single crystals of Sr(2)B(16)O(26) crystallize in the monoclinic space group P2(1)/c with a = 8.408(1) A, b = 16.672(1) A, c = 13.901(2) A, beta = 106.33(1) degrees , and Z = 4. The crystal structure consists of a 3D network of the complex borate anion [B(16)O(20)O(12/2)](4-), formed by 12 BO(3) triangles and four BO(4) tetrahedra, which can be viewed as three linked [B(3)O(3)O(4/2)](-) triborate groups bonded to one pentaborate [B(5)O(6)O(4/2)](-) group and two BO(3) triangles. Using this structure, we have performed theoretical calculations using the all-electron full potential linearized augmented plane wave (FP-LAPW) method for the band structure, density of states, electron charge density, and the frequency-dependent optical properties. Our experimental VB-XPS of Sr(2)B(16)O(26) is compared with results of our FP-LAPW calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Gamma of the Brillouin zone (BZ) resulting in a direct energy gap of about 5.31 eV. Our measured VB-XPS show reasonable agreement with our calculated total density of states for the valence band that is attributed to the use of the full potential method.

First-principles calculations and X-ray spectroscopy studies of the electronic structure of CuWO 4

First-principles self-consistent band-structure calculations of copper tungstate, CuWO 4, have been made using the full potential linearized augmented plane wave (FP-LAPW) method. Total and partial densities of states of the constituent atoms of CuWO 4 have been derived. The results obtained reveal that the valence band of CuWO 4 is dominated by contributions of the O 2p-like states, while the W 5d-like states are the main contributors into the conduction band of the tungstate. Additionally, the FP-LAPW calculations render that the W 5d- and Cu 3d-like states contribute mainly at the bottom and at the top of the valence band of CuWO 4, respectively. In the present work, the X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) methods were also employed to investigate experimentally the electronic structure of copper tungstate. For the mentioned compound, the XES bands reflecting the energy distribution of mainly the W 5d-, Cu 3d- and O 2p-like states were derived and compared on a common energy scale with the X-ray photoelectron valence-band spectrum. A rather good agreement of the experimental and theoretical data concerning electronic properties of CuWO 4 has been obtained in the present paper. Measurements of the energy shift of the XAS W L III edge of CuWO 4 clearly demonstrate that tungsten atoms in copper tungstate are in the formal valence state +6.

Electronic structure of KTiOAsO 4: A comparative study by the full potential linearized augmented plane wave method, X-ray emission spectroscopy and X-ray photoelectron spectroscopy

First-principles self-consistent band-structure calculations of potassium titanyl arsenate, KTiOAsO 4 (KTA), have been made using the full potential linearized augmented plane wave (FP-LAPW) method. Total and partial densities of states of the constituent atoms of KTA have been derived. The results obtained show that the valence band of KTA is dominated by contributions of the O 2p-like states, while the Ti 3d-like states are the main contributors into the conduction band of the compound. Additionally, the FP-LAPW calculations have revealed that potassium atoms are highly ionized in KTA. In the present work, the X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) methods were also employed to investigate experimentally the electronic structure of potassium titanyl arsenate. For the mentioned compound, the XES K Ll, Ti Lα, As Kβ 2 and O Kα bands reflecting the valence K s-, Ti s,d-, As p- and O p-like states, respectively, were derived and compared on a common energy scale with the XPS valence-band spectrum. A rather good agreement of the experimental XES and XPS results and the theoretical FP-LAPW data for electronic properties of KTA has been obtained in the present paper.

Computational quantum magnetism: Role of noncollinear magnetism

We are witnessing today a golden age of innovation with novel magnetic materials and with discoveries important for both basic science and device applications. Computation and simulation have played a key role in the dramatic advances of the past and those we are witnessing today. A goal-driving computational science—simulations of every-increasing complexity of more and more realistic models has been brought into greater focus with greater computing power to run sophisticated and powerful software codes like our highly precise full-potential linearized augmented plane wave (FLAPW) method. Indeed, significant progress has been achieved from advanced first-principles FLAPW calculations for the predictions of surface/interface magnetism. One recently resolved challenging issue is the role of noncollinear magnetism (NCM) that arises not only through the SOC, but also from the breaking of symmetry at surfaces and interfaces. For this, we will further review some specific advances we are witnessing today, including complex magnetic phenomena from noncollinear magnetism with no shape approximation for the magnetization (perpendicular MCA in transition-metal overlayers and superlattices; unidirectional anisotropy and exchange bias in FM and AFM bilayers; constricted domain walls important in quantum spin interfaces; and curling magnetic nano-scale dots as new candidates for non-volatile memory applications) and most recently providing new predictions and understanding of magnetism in novel materials such as magnetic semiconductors and multi-ferroic systems.

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.

Electronic structure of face-centred cubic MoO 2: A comparative study by the full potential linearized augmented plane wave method, X-ray emission spectroscopy and X-ray photoelectron spectroscopy

X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) methods were employed in the present paper to investigate the electronic structure of face-centred cubic (fcc) molybdenum dioxide, fcc-MoO 2. For the mentioned compound, the XES O Kα and Mo Lβ 2,15 bands reflecting the valence O p- and Mo s,d-like states, respectively, were derived and compared on a common energy scale with the XPS valence-band spectrum. For comparison, the similar experimental studies of the electronic structure were made for a usual orthorhombic form of molybdenum trioxide, MoO 3. Band-structure calculations of fcc-MoO 2 were made using the full potential linearized augmented plane wave (FP-LAPW) method. A rather good agreement of the experimental XES and XPS results and the theoretical FP-LAPW data for the electronic properties of fcc-MoO 2 has been achieved in the present paper. A new near-Fermi sub-band was detected on both the XES Mo Lβ 2,15 band and the XPS valence-band spectrum when going from orthorhombic MoO 3 to fcc-MoO 2. The FP-LAPW calculation reveals that the main contributors into the aforementioned sub-band of fcc-MoO 2 are the Mo 4d(e g) states. Further, the FP-LAPW data indicate that the O 2p and Mo 4d(t 2g) states contribute into both the central part and the bottom of the valence band of fcc-MoO 2, while the Mo 4d(e g) states contribute almost exclusively into the bottom of the valence band of the oxide. A significant portion of density of states (mainly Mo 4d(e g) states) detected by the FP-LAPW calculation at the Fermi energy of fcc-MoO 2 indicates that the oxide is rather unstable.

Semiconductor–metal transition of pyriteFeS2 under high pressure by full-potential linearized-augmented plane wave calculations

The effects of hydrostatic pressure on the electronic band structure of the semiconductor mineral ironpyrite FeS2 have been investigated theoretically by an ab initio full-potential linearized-augmented planewave (FPLAPW) method within a local approximation (LDA/GGA) to the density functionaltheory. The calculations predict that at a pressure of 94.1 GPa the indirect band gap of pyriteFeS2 vanishes and the material becomes a metal. This is due to the presence of the S–S and Fe–Sbonds, which provide novel energy band distortions in the process of attaining the metallicstate. Analysis indicates that, under increasing high pressure, the conduction bands(3pz ofsulfur and 3dx2−y2+3dxy of iron) intrude downwards into the valence bands, which are predominantly 3d in nature.At normal pressure, the lattice constant, the bulk modulus, sulfur position parameteru, S–S bond length, and the indirect band gap of pyriteFeS2 are calculated using a fully relaxed unit cell and found to be equal to 541.8 pm, 159.7 GPa,u = 0.383, 219.5 pm and 0.45 eV, respectively. Apart from the gap, which is too small (the usual ‘LDAerror’), these results agree well with recent experiments. The effective masses of an electronat selected points in the conduction band are reported.

All-electron first-principles calculations of clean surface properties of low-Miller-index Al surfaces

We report a systematic theoretical study of the clean surface properties of the Al(111), Al(100), and Al(110) surfaces as function of the film thickness employing slabs with thicknesses up to 32 \AA{}. Our calculations are based on density functional theory employing the all-electron full-potential linearized augmented plane-wave (FLAPW) method. Our results show clearly a periodic oscillatory behavior of the surface energies, work functions, interlayer relaxations, total density of states at the Fermi level as function of the slab thickness, however, similar behavior is not found for the occupied bandwidth at the $\ensuremath{\Gamma}$ point. The magnitude of the oscillations decrease with an increase of the number of layers in the slab, as expected. We found that the period of the oscillations are almost the same for Al(111) and Al(110), however, the work functions and interlayer relaxations obtained for Al(100) show oscillations with a larger period (almost by a factor of two) compared to the Al(111) and Al(110) surfaces, which are explained in terms of the deep penetration of the surface states into the bulk region of the Al(100) surface. This new physical result, as well as the agreement between our FLAPW calculations and the available experimental results, are discussed in this paper.

Electronic and Magnetic Properties of AlFe3 and AlFe3N Nitride

AbstractFor Abstract see ChemInform Abstract in Full Text.

First-principles investigation of the multilayer relaxation of stepped Cu surfaces

We performed density-functional theory calculations, employing the all-electron full-potential linearized augmented plane-wave (FLAPW) method, for the multilayer relaxations of the vicinal, high-Miller-index Cu(210), Cu(211), and Cu(331) surfaces, as well as for the flat, low-Miller-index Cu(100), Cu(110), and Cu(111) surfaces. Generally, it is expected that the interlayer relaxation-sequence at stepped metal surfaces with $n$ surface atom rows in the terraces exposed to the vacuum show $n\ensuremath{-}1$ contractions (indicated by $\ensuremath{-}$) followed by one expansion (indicated by $+$). However, recent studies based on low-energy electron diffraction (LEED) intensity analysis and all-electron FLAPW calculations suggested that the multilayer relaxation-sequence of the stepped Cu(331) surface, for which $n=3$, behaves anomalously, i.e., $\ensuremath{-}++\ensuremath{\cdots}$, instead of the expected $\ensuremath{-}\ensuremath{-}+\ensuremath{\cdots}$. From the results presented in this work, we did not find any indication of such anomalous behavior for Cu(331) or for any of the investigated stepped Cu surfaces. For the flat surfaces we obtained the expected contraction of the topmost interlayer distance. In the particular case of the Cu(110) surface, a pronounced alternating oscillatory behavior extending over six interlayer distances was found, i.e., $\ensuremath{-}+\ensuremath{-}+\ensuremath{-}+$. For all studied Cu surfaces in the present work, we found a good quantitative agreement between our interlayer relaxations and those obtained by LEED intensity analysis.

Electronic and magnetic properties of AlFe3 and AlFe3N nitride

Self-consistent band structure calculations were performed for the metal aluminide AlFe3 (in the fcc structure) and the nitride AlFe3N (perovskite-like nitride) at several lattice parameters in order to obtain the electronic structure and magnetic properties of these compounds. In this study we have employed the full potential linear augmented plane wave (FPLAPW) and the linear muffin-tin orbital (LMTO-ASA) methods. Our results show that the modeled AlFe3 is ferromagnetic (FM) with 2.07μB as the magnetic moment at Fe sites. On the other hand, AlFe3N is non-magnetic (NM) contrary to other perovskite nitrides that show magnetic order. Hence as the stable phase of AlFe3 is magnetic with DO3 structure a crystallographic transformation occurs accompanied by a magnetic transition from a non-magnetic perovskite structure to a magnetic DO3 structure as AlFe3N lose nitrogen atoms. The LMTO and FPLAPW total energy calculations give the equilibrium volume for both compounds. For AlFe3N, the FPLAPW calculations give the equilibrium volume that agrees well with the experimental lattice spacing, which is not achieved by LMTO calculations within the same accuracy. The calculated magnetic moment as function of the lattice parameter for the AlFe3 shows a collapse of the magnetism at certain critical lattice parameter whose FPLAPW value (2.962Å) differs only slightly from the LMTO value (3.041Å).

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.

Structural properties of boron compounds at high pressure

The present work employs the full potential linearized augmented plane wave(FP-LAPW) method to calculate the elastic and the bonding propertiesof the boron compounds BN, BP, BAs and BSb at high pressures. Anumerical first principles calculation of the elastic constants is used to obtainC11,C12 and C44 for these compounds. The equilibrium lattice constant and the bulk modulus are alsocalculated. Our FP-LAPW calculations were performed to evaluate the totalenergy as a function of strain, after which the data were fitted to a polynomialfunction of strain to determine the elastic modulus. The experimental elasticconstants are only available for BN and BP. Hence for BAs, the results are onlycompared with the available theoretical works. To our knowledge the elastic constantsof BSb have not yet been measured or calculated, hence our results serve as aprediction for future study. In addition, we discuss the bonding parameter in terms ofcharge density at equilibrium and at transition volumes, which suggests that thebonding of BP, BAs and BSb are less ionic than in other zinc-blende compounds.

On the anisotropic optical response of Al(110)

In a recent paper, good agreement was reported between the results of an ab initio full-potential linear augmented plane wave (FP-LAPW) calculation of the linear optical response of Al(1 1 0) and experimental measurements of the reflectance anisotropy. In the present work, our FP-LAPW calculations are extended to develop a microscopic picture of the anisotropic optical response at the Al(1 1 0) surface. Evidence for an anisotropic intraband transition (Drude) contribution in the infrared is presented, and the anisotropy is explained by considering the redistribution of charge that occurs when an Al(1 1 0) surface is created. The interband transitions that make the dominant contribution to the reflectance anisotropy at higher energy are identified, and the symmetries and the surface or bulk character of the initial and final states are determined. Changes in the relative energies and occupations of electronic states due to surface charge redistribution are a possible mechanism for the interband contribution to the reflectance anisotropy.

On the anisotropic optical response of Al(1 1 0)

In a recent paper, good agreement was reported between the results of an ab initio full-potential linear augmented plane wave (FP-LAPW) calculation of the linear optical response of Al(1 1 0) and experimental measurements of the reflectance anisotropy. In the present work, our FP-LAPW calculations are extended to develop a microscopic picture of the anisotropic optical response at the Al(1 1 0) surface. Evidence for an anisotropic intraband transition (Drude) contribution in the infrared is presented, and the anisotropy is explained by considering the redistribution of charge that occurs when an Al(1 1 0) surface is created. The interband transitions that make the dominant contribution to the reflectance anisotropy at higher energy are identified, and the symmetries and the surface or bulk character of the initial and final states are determined. Changes in the relative energies and occupations of electronic states due to surface charge redistribution are a possible mechanism for the interband contribution to the reflectance anisotropy.

Absolute deformation potential with a compressed atom model

AbstractA compressed atom model is developed to study the one‐electron energy level in the FLAPW (full‐potential linearized augmented plane wave) calculation. The reference energy level in the FLAPW calculations is shown to be dominated by the exchange‐correlation potential. Taking this into account it is possible to evaluate the absolute deformation potential.

Effective electron and hole masses in intrinsic and heavily n-typedoped GaN and AlN

We have investigated the electronic structure near the band edges inintrinsic and heavily n-type doped GaN and AlN. Both the wurtzite and thezinc-blende polytypes have been considered. The electronic structures ofthe intrinsic materials were obtained from a full-potential linearizedaugmented plane wave calculation. We show the importance of performing afully relativistic calculation. For instance, the hole mass in cubic AlN isa very large and negative quantity if spin-orbit coupling is excluded,whereas the fully relativistic calculation gives a relatively small andpositive value. The electron-phonon coupling was taken into accountaccording to the Fröhlich Hamiltonian for large polarons, resulting ineffective polaron masses. The effects on the effective electron masses dueto doping were investigated by using a Green's function formalism withinthe random phase approximation and with a local-field correction accordingto Hubbard.

Magnetism of the V(001) surface: Contradictory results from pseudopotential and linearized augmented plane-wave calculations

Recently using a pseudopotential and the generalized gradient approximation (GGA) we determined that V(001) with the experimentally determined surface relaxation has a strongly ferromagnetic surface. More recently, others also using the GGA found that with full-potential linearized augmented plane-wave (FLAPW) calculations, the unrelaxed surface is only weakly magnetic and the relaxed surface is nonmagnetic. We report here other pseudopotential and FLAPW calculations. The FLAPW results are consistent with the previous ones except that the surface relaxation is much smaller. A nonmagnetic pseudopotential calculation yields a -13.5% surface relaxation, slightly larger than the FLAPW calculation, while the ground state has a -12.5% surface relaxation with a surface magnetization of $0.75{\ensuremath{\mu}}_{B}.$ The unrelaxed surface has a $1.77{\ensuremath{\mu}}_{B}$ magnetization in good agreement with our previous calculation. Thus we are forced to conclude that what are usually assumed to be the most accurate methods of electronic structure calculation are in complete disagreement insofar as the magnetic nature of the V(001) surface is concerned. We speculate on the source of this disagreement.

Electronic structure and physical properties of early transition metal mononitrides: Density-functional theory LDA, GGA, and screened-exchange LDA FLAPW calculations

The desirable physical properties of hardness, high temperature stability, and conductivity make the early transition metal nitrides important materials for various technological applications. To learn more about the nature of these materials, first-principles density-functional theory calculations using the full-potential linearized augmented plane wave (FLAPW) method within the local-density approximation (LDA) and with the generalized gradient approximation (GGA) have been performed. We investigate the bulk electronic and physical properties of a series of early transition metal mononitrides, namely, those formed with 3d metals (ScN, TiN, VN), 4d metals (YN, ZrN, NbN), and 5d metals (LaN, HfN, TaN) in the rocksalt structure. In particular, lattice constants, bulk moduli, heats of formation, and cohesive energies as well as bulk band structures and densities of states are reported, and trends discussed. We find that the GGA yields 1%--2% larger lattice constants, 10%--20% smaller bulk moduli, and 10%--30% lower heats of formation compared to the LDA. The GGA slightly overestimates lattice constants, but leads overall to an improved agreement with experiment compared to the LDA, for which the values are too small. These materials are metallic with the exception of ScN, YN, and LaN, which appear to be semimetals within the LDA (and GGA), but are in fact semiconductors with indirect band gaps of 1.58, 0.85, and 0.75 eV, respectively, as revealed by calculations performed using the screened-exchange LDA approach. These last, relatively unexplored, refractory III-V nitrides may therefore have potential use in device applications; in particular, ScN is well lattice matched to GaN, a wide-band-gap semiconductor that is of great current interest in relation to optoelectronic devices, and high temperature and high power electronic applications.