LCAO Research Articles

Local structure of perovskites ReO3 and ScF3 with negative thermal expansion: interpretation beyond the quasiharmonic approximation

We propose an approach beyond the quasiharmonic approximation for interpretation of EXAFS and XRD data and for ab initio calculations of electronic and vibration properties of materials with negative thermal expansion. Ab initio electronic structure and lattice dynamics calculations for cubic and distorted ScF3 were performed using the linear combination of atomic orbitals (LCAO) method. The band gap obtained in calculations for ScF3 is equal to 10.54 eV and agree well with the expected value. The calculated infrared spectra of F displaced (FD) cubic ScF3 allow us to predict that its mean Sc-F-Sc angle within NTE deviates from 180 degree.

Influence of external electric field on the transfer of protons in hydrogen-bonded systems

The LCAO MO self-consistent field method in combination with the 6-31** basis set is used to calculate sections of the potential energy surface for a proton in the NH3…HCl system in the presence of an external electrostatic field. The field strength is varied in the range of 0.000 to 0.017 a. u. In the absence of the field, the potential of the proton in the isolated complex has one well, the N–Cl distance is equal to 2.92 A, and the NH3…H–Cl bond is a hydrogen bond. With increasing distance between the N and Cl atoms, a second well appears in the potential, with the well near the Cl atom remaining deeper. In the presence of an external electric field, with increasing its strength, the depth of the well near the N atom increases, while the height of the barrier for proton transfer from the chlorine to the nitrogen atom decreases. At a certain field strength, the well near the nitrogen atom becomes deeper than that near the chlorine atom, so the proton moves to the nitrogen, making the complex ionic. Thus, the external electric field can influence the type of chemical bond in NH3…HCl system.

Synthesis and electrical properties of CaO doped BZT lead free piezo ceramics

Effect of CaO on ferroelectric, dielectric, piezoelectric properties of Barium Zirconate Titanate (BZT) lead free piezo ceramics was studied. The powders were prepared by mixed-oxide route, calcined at 1000°C for 4 h. XRD patterns of the calcined powders show phase pure perovskite structure. The calcined powders were de-agglomerated, granulated, pressed into pellets and sintered at 1150°C for 3 h. Scanning Electron Microscopy (SEM) studies of thermally etched sintered pellets show sub-micron sized grains for undoped BZT which increases up to 5–8 μm sized for 0.015 mol CaO doped BZT. Energy Dispersive X-ray analysis (EDS) of CaO doped BZT sample confirms the presence of dopants very close to their stoichiometric compositions. The piezoelectric charge constant (d33 = 248 pC/N), dielectric constant (K = 2521) and remnant polarization (Pr = 4.22 μC/cm2) were maximum for BZT samples containing 0.015 mole of CaO.

High energy Compton spectroscopy and electronic structure of Laves phase ZrFe 2

Abstract We present the first-ever experimental Compton profile of Laves phase ZrFe2 using indigenous 20 Ci 137Cs Compton spectrometer. To annotate the experimental electron momentum density, we have calculated the theoretical Compton profiles using density functional theory (DFT) and hybridization of Hartree–Fock and DFT within linear combination of atomic orbitals (LCAO) method. The spin-polarized energy bands and density of states are computed using LCAO and full potential-linearized augmented plane wave methods. The revised Perdew-Burke-Ernzerhof functional (for solids) based theoretical profile gives a marginally better agreement with the experimental profile as compared to other approximations considered in the present work. The Fermi surface topology of ZrFe2 is explained in terms of majority- and minority-spin energy bands.

Electronic properties of mixed molybdenum dichalcogenide MoTeSe: LCAO calculations and Compton spectroscopy

We have employed linear combination of atomic orbitals (LCAO) method to compute the Mulliken’s population (MP), energy bands, density of states (DOS) and Compton profiles for hexagonal MoTeSe. The density functional theory (DFT) and hybridization of Hartree-Fock with DFT (B3LYP) have been used within the LCAO approximation. Performance of theoretical models has been tested by comparing the theoretical momentum densities with the experimental Compton profile of MoTeSe measured using 137Cs Compton spectrometer. It is seen that the B3LYP prescription gives a better agreement with the experimental data than other DFT based approximations. The energy bands and DOS depict an indirect band gap character in MoTeSe. In addition, a relative nature of bonding in MoTeSe and its isovalent MoTe2 is discussed in terms of equal-valence-electron-density (EVED) profiles. On the basis of EVED profiles it is seen that MoTeSe is more covalent than MoTe2.

Structure, electronic, mechanical and optical properties of ternary YAl3C3 carbide

The electronic structure, mechanical and optical properties of ternary yttrium aluminum carbide (YAl3C3) has been studied by first principles approach. The crystal structure and elastic properties are studied by using Vienna ab initio simulation package (VASP). An orthogonalized linear combination of atomic orbitals (OLCAO) method based on the density functional theory (DFT) is implemented to elucidate the electronic structure and optical properties of ternary YAl3C3 carbide. The YAl3C3 carbide exhibits a narrow indirect band gap, Eg=0.12eV which shows its poor metallic and/or semiconductor behavior. The effective charge (Q*) calculation reveals more charge transfer from Al-sites as compared to Y-sites which indicates dominant ionic character of Al-sites. The analysis of structure and bond order (BO) calculations show that the Al–C bonds in the basal plane are much stronger as compared to Al–C bonds along the c-axis. The Al–C bonds lying in the basal plane have main contribution into the overall stiffness of YAl3C3 carbide. The effective mass of charge carriers (electrons and holes) and inter-band optical properties (complex dielectric function and optical conductivity) are also studied which show high degree of anisotropy in YAl3C3.

Fabrication of Metal-Deposited Indium Tin Oxides: Its Applications to 385 nm Light-Emitting Diodes.

We report performance improvements in near-ultraviolet (NUV) light-emitting diodes (LEDs) using various metal-doped indium tin oxide (ITO/metals). Metals with an orbital energy gap greater than that of an In atom (e.g., Ti, Ga, Ge, and Al) are deposited on ITO, and subsequent annealing is performed to improve optical transmittance of ITO due to effective bandgap increase via the linear combination of atomic orbitals, as well as electrical conductivity; thus, current spreading via metal-doping effect at the surface of ITO. As a result, the ITO/metals (annealed at 550 °C, 1 min) exhibit 90.5-94.7% transmittance at 385 nm and a specific contact resistance of 2.1-3.0 × 10(-3) Ω cm(2), whereas the reference ITOs exhibit 76.5-89.5% and 3.2-4.5 × 10(-3) Ω cm(2), respectively. Compared to NUV LEDs using conventional ITO (60 nm), the InGaN/AlGaInN NUV LED using ITO (110 nm)/metal (3 nm) on average exhibits a 70% increase in light output power at 100 mA and a 2% decrease in forward voltage at 20 mA, with more uniform and brighter emission images. We also identified the origin for the improvement by analyzing the surface of ITO/metals using X-ray photoelectron spectroscopy and Auger electron spectroscopy. This approach could offer a simple, effective way to enhance the overall efficiency of conventional NUV LEDs using ITO.

Reduction of molecular oxygen on the surface of transition metal complex oxide

Ab initio quantum‐chemical calculations have been carried out for the electronic structures of molecular clusters and adsorption complexes to model physical and chemical adsorption of molecular oxygen on the surface of nickel‐cobalt and iron‐cobalt mixed oxides (spinels). The quantum‐chemical calculations were done using the Hartree‐Fock‐Roothan self‐consistent field MO LCAO formalism in the 6–31 basis set of Gaussian functions. It was shown that the value of adsorption energy of hydrogen peroxide influences the mechanism of oxygen reduction: at lower adsorption energies oxygen is reduced by the two‐electron mechanism, which is followed by desorption of the peroxide molecule formed. At higher adsorption energies the reduction of oxygen occurs via the four‐electron mechanism up to water formation. An explanation was proposed that the catalytic reaction of oxygen reduction proceeds on average via a 3.7‐electron mechanism, which is confirmed experimentally.

The Crystal Structure of Impurity Centers Tm $$^{2+}$$ 2 + and Eu $$^{2+}$$ 2 + in SrCl $$_{2}$$ 2 : Ab Initio Calculations

Ab initio calculations of the impurity centers Tm\(^{2+}\) thulium and europium Eu\(^{2+}\) in SrCl\(_{2}\) and MeF\(_{2}\) (Me = Ca, Sr, Ba) were carried out at low (zero) temperature. The crystal structure of impurity centers was investigated. Charge density maps show that the bonds formed by the rare-earth ions have an ionic character. The crystal structures, lattice dynamics, and band structures of MeF\(_{2}\) and SrCl\(_{2}\) were calculated at low temperature. Ab initio calculations were performed in periodic CRYSTAL code within the framework of the MO LCAO approach by using hybrid DFT functionals.

Electronic and optical properties of ceramic Sc2O3 and Y2O3: Compton spectroscopy and first principles calculations

We present the first-ever experimental Compton profiles (CPs) of Sc2O3 and Y2O3 using 740 GBq 137Cs Compton spectrometer. The experimental momentum densities have been compared with the theoretical CPs computed using linear combination of atomic orbitals (LCAO) within density functional theory (DFT). Further, the energy bands, density of states (DOS) and Mulliken's population (MP) data have been calculated using LCAO method with different exchange and correlation approximations. In addition, the energy bands, DOS, valence charge density (VCD), dielectric function, absorption coefficient and refractive index have also been computed using full potential linearized augmented plane wave (FP-LAPW) method with revised functional of Perdew–Becke–Ernzerhof for solids (PBEsol) and modified Becke Johnson (mBJ) approximations. Both the ab-initio calculations predict wide band gaps in Sc2O3 and Y2O3. The band gaps deduced from FP-LAPW (with mBJ) are found to be close to available experimental data. The VCD and MP data show more ionic character of Sc2O3 than Y2O3. The ceramic properties of both the sesquioxides are explained in terms of their electronic and optical properties.

Electronic structure, mechanical, and optical properties of CaO·Al2O3 system: a first principles approach

A comprehensive study of the structure, bonding, mechanical and optical properties of five stable phases within the calcium aluminate (Ca–Al–O) series with different CaO to Al2O3 proportions has been carried out using the density functional theory based orthogonalized linear combination of atomic orbitals (OLCAO) method. The phases are C3A, C12A7-crystal, CA, CA2, and CA6 and the oxygen deficient C12A7-electride phase. These five stable phases are wide band gap insulators with energy gap values ranging from 3.85 to 4.62 eV. The charge neutral C12A7-crystal has localized defective states in the gap whereas the C12A7-electride phase has a region of metallic bands of about 2 eV wide in the gap. Effective charge and bond order calculations reveal intimate details of electronic structure and bonding in relation to the aluminate contents in the series. It is shown that Al–O bonds dominate the Ca–O bonds in determining the crystal strength with CA6 having the highest and C12A7 having the lowest bond order density. Calculations of elastic coefficients and mechanical properties in these crystals show a high degree of diversity and anisotropic behavior consistent with the bond order calculations. The refractive index values from optical properties calculations are in good agreement with available literature. Other results furnish more insights for the Ca–Al–O series and provide the opportunity for further investigations on similar or more complicated quaternary systems with potential novel properties.

Calculated electronic, transport, and bulk properties of zinc-blende zinc sulphide (zb-ZnS)

We present the results from ab initio, self-consistent, local density approximation (LDA) calculations of electronic and related properties of zinc-blende zinc sulphide (zb-ZnS). We employed the Ceperley and Alder LDA potential and the linear combination of atomic orbital (LCAO) formalism. Our calculations are non-relativistic. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). The BZW-EF method includes a methodical search for the optimal basis set that yields the minima of the occupied energies. This search entails increasing the size of the basis set and related modifications of angular symmetry and of radial features. Our calculated, direct band gap of 3.725 eV, at the Γ point, is in excellent agreement with experiment. We have also calculated the total (DOS) and partial (pDOS) densities of states, electron and hole effective masses and the bulk modulus that agree very well with available, corresponding experimental results.

Effect of Pressure and Temperature on the Local Structure and Lattice Dynamics of Copper(II) Oxide

Microcrystalline and nanocrystalline (6 nm) CuO were studied in situ by the Cu K-edge X-ray absorption spectroscopy as a function of pressure (0-20 GPa) and temperature (10-300 K). Pressure dependence of X-ray absorption near edge structure (XANES) was interpreted within the full-multiple-scattering formalism based on the relaxed atomic structure determined by ab initio linear combination of atomic orbital (LCAO) calculations. Temperature dependence of the mean-square relative displacement (MSRD) for the four shortest Cu–O distances was obtained from the analysis of extended X-ray absorption fine structure (EXAFS) and described by the correlated Einstein model with the characteristic temperature θE =589 K. It was found that the thermal motion of copper and four oxygen atoms forming square-planar coordination is strongly correlated.

Compton profiles and Mulliken’s populations of cobalt, nickel and copper tungstates: Experiment and theory

We present the first ever studies on Compton profiles of AWO4 (A=Co, Ni and Cu) using 661.65keV γ-rays emitted by 137Cs source. The experimental momentum densities have been employed to validate exchange and correlation potentials within linear combination of atomic orbitals (LCAO) method. Density functional theory (DFT) with local density approximation and generalized gradient approximation and also the hybridization of Hartree-Fock and DFT (B3LYP and PBE0) have been considered under LCAO scheme. The LCAO-B3LYP scheme is found to be in better agreement with the experimental data than other approximations considered in this work, suggesting applicability of B3LYP approach in predicting the electronic properties of these tungstates. The Mulliken’s population (MP) data show charge transfer from Co/Ni/Cu and W to O atoms. The experimental profiles when normalized to same area show almost similar localization of 3d electrons (in real space) of Ni and Cu which is lower than that of Co in their AWO4 environment.

Вивчення електронної будови та реакційної здатності похідних 5-(адамантан -1-іл)-4r-1,2,4-тріазол-3-тіолів

<p>Резюме. Вивчено електронні спектри поглинання 5-(адамантан-1-іл)-4-феніл-1,2,4-тріазол-3-тіолу (І) і 5-(адамантан-1-іл) -4-метил-1,2,4-тріазол-3-тіолу (ІІ) в розчинниках різної полярності (вода, 95 % етанол, 0,1 М HCl, 0,1 М H<sub>2</sub>SO<sub>4</sub>, 0,1 М NaOH, н-гексан). Встановлено, що смуга поглинання сполуки І характеризується наявністю одного максимуму середньої інтенсивності в межах від 250 до 262 нм, а сполуки ІІ – в межах від 236 до 255 нм. Враховуючи хімічну будову досліджуваних сполук і характер їх спектрів поглинання, можна зробити висновок, що смуга поглинання сполуки І є результатом накладення p → p<sup>x</sup> – переходів 1,2,4-тріазолового циклу на <sup>1</sup>L<sub>b</sub> – смугу фенільного радикала. Смуга поглинання сполуки ІІ зумовлена лише π→π<sup>x</sup> – переходами в родоначальному хромофорі, а саме в 1,2,4-тріазолі. Використання методу Extended Hückel на підставі лінійної комбінації атомних орбіталей відповідно до програми Chemoffice Free Trial 11.0.1 дозволило нам провести прогнозування реакційної здатності досліджуваних сполук на основі квантово-хімічних розрахунків зарядів їх атомів. Проведені розрахунки дозволили зробити висновок, що 1,2,4-тріазоловий цикл сполуки І є π – надлишковим за рахунок впливу субстітуєнтів, які знаходяться у положеннях 3-SH – (-Ι, + М), 4-феніл – (-Ι, + М), 5-адамантил – (+ І). Кислотний характер SH-центру виникає за рахунок електронакцепторного впливу тріазолового кільця. Атом сірки при цьому проявляє негативний індуктивний ефект (-І), а мезомерний ефект є позитивним (+ М) і + М ˃ -І. На атомі сірки спостерігається дефіцит електронної густини (+ 0,0314774), а на атомі водню тіольного угрупування зосереджено заряд (+0,0232612), в результаті чого реакція алкілування (SF) відбувається за SH-кислотним центром. Враховуючи квантово-хімічні розрахунки зарядів атомів сполуки ІІ, можна зробити висновок, що наявність метильного субстітуєнта в положенні 4 впливає на розподіл її електронної щільності. Таким чином, метильний радикал проявляє позитивний індуктивний ефект (+І).</p><p><strong>Ключові слова:</strong> УФ-спектри, електронна будова, рекційна здатність, похідні 5-(адамантан-1-іл)-4R-1,2,4-тріазол-3-тіолів</p>

Ab initio prediction of electronic, transport and bulk properties of Li2S

In this paper, we present results from ab initio, self-consistent, local density approximation (LDA) calculations of electronic and related properties of cubic antifluorite (anti-[Formula: see text]) lithium sulfide [Formula: see text]. Our nonrelativistic computations implemented the linear combination of atomic orbital (LCAO) formalism following the Bagayoko, Zhao and Williams method, as enhanced by Ekuma and Franklin (BZW–EF). Consequently, using several self-consistent calculations with increasing basis sets, we searched for the smallest basis set that yields the absolute minima of the occupied energies. The outcomes of the calculation with this basis set, called the optimal basis set, have the full physical content of density functional theory (DFT). Our calculated indirect band gap, from [Formula: see text] to [Formula: see text], is 3.723 eV, for the low temperature experimental lattice constant of 5.689 Å. The predicted indirect band gap of 3.702 eV is obtained for the computationally determined equilibrium lattice constant of 5.651 Å. We have also calculated the total density of states (DOS) and partial densities of states (pDOS), electron and hole effective masses and the bulk modulus of [Formula: see text]. Due to a lack of experimental results, most of the calculated ones reported here are predictions for this material suspected of exhibiting a high temperature superconductivity similar to that of [Formula: see text].

Electronic and optical response of zirconium sulphoselenides: Compton spectroscopy and first-principles calculations

We present the first-ever experimental Compton profiles (CPs) of ZrSSe2 and ZrS1.5Se1.5 using 100 mCi 241Am Compton spectrometer. To analyze the experimental momentum densities, we have computed for the first-time the CPs, energy bands and density of states using linear combination of atomic orbitals (LCAO) method. To model the exchange and correlation effects within LCAO approach, we have considered Hartree–Fock (HF), density functional theory (DFT) with revised functional of Perdew–Becke–Ernzerhof (PBEsol) and hybridization of HF and DFT. Going beyond computation of electronic properties using LCAO method, we have also derived electronic and optical properties using the modified Becke–Johnson (mBJ) potential within full potential linearized augmented plane wave (FP-LAPW) method. There is notable decrease in the energy band gap on replacing S by Se atoms in ZrSSe2 to obtain ZrS1.5Se1.5 composition, which is mainly attributed to readjustment of Zr–4d, S–3p and Se–4p states. It is seen that the CPs based on hybridization of HF and DFT show a better agreement with the experimental profiles than those based on individual HF and DFT-GGA-PBEsol schemes. The optical properties computed using FP-LAPW-mBJ method unambiguously depict feasibility of using both the sulphoselenides in photovoltaics and also utility of ZrS1.5Se1.5 in the field of non-linear optics.

Ab initio simulations on N and S co-doped titania nanotubes for photocatalytic applications

In this paper we present the results of quantum chemical modeling for energetically stable anatase (001) TiO2 nanotubes, undoped, doped, and codoped with N and S atoms. We calculate the electronic structure of one-dimensional (1D) nanotubes and zero-dimensional (0D) atomic fragments cut out from these nanotubes, employing hybrid density functional theory with a partial incorporation of an exact, nonlocal Hartree–Fock exchange within the formalism of the linear combination of atomic orbitals, as implemented in both CRYSTAL and NWChem total energy codes. Structural optimization of 1D nanotubes has been performed using CRYSTAL09 code, while the cut-out 0D fragments have been modelled using the NWChem code. The electronic properties of the studied systems prove that the band structure of the pristine TiO2 nanotube can be substantially modified by introducing substitutional impurity defects. The N-doped nanotube creates a midgap state that largely has a nitrogen character. The S-doped nanotube has a defect state that almost coincides with the top of the valence bond for the pristine material. For nanotubes codoped with both S and N, we observe a downward shift of the gap state of nitrogen relative to the purely N-doped state by about 0.3 eV. This results in a system with a filled gap state about 0.3 eV below the O2/H2O oxidation level, making it a very promising candidate for photocatalytic hydrogen generation under visible light, because due to the presence of sulfur, the bottom of the conduction band is only about 2.2 eV above the occupied midgap state, and also, clearly above the standard hydrogen electrode level.

Compton profiles and electronic structure of monoclinic zinc and cadmium tungstates

Abstract We report the first ever Compton scattering study of ZnWO 4 and CdWO 4 using 20 Ci 137 Cs Compton spectrometer at momentum resolution of 0.34 a.u. To compare the experimental Compton profiles, we have also deduced the momentum densities using density functional theory (DFT) within linear combination of atomic orbitals (LCAO) methods. It is seen that the experimental Compton profiles of both the tungstates give a better agreement with LCAO–DFT calculations within generalized gradient approximation (GGA) employing Perdew–Becke–Ernzerhof (PBE) exchange and correlation energies than other approximations included in the present work. Further, energy bands, density of states (DOS) and band gaps have also been calculated using LCAO–DFT–GGA–PBE scheme and full potential linearized augmented plane wave method. Both the computational schemes show a semiconducting nature of both the tungstates, with a direct band gap at Y point of Brillouin zone. Further, a relative nature of bonding on equal-valence-electron-density scale shows more covalent character in ZnWO 4 than CdWO 4 which reconciles with the conclusions drawn using integrated DOS and Mulliken’s population data.

Supercritical electric dipole and migration of electron wave function in gapped graphene

We study the Dirac equation for quasiparticles in gapped graphene with two oppositely charged impurities by using the technique of linear combination of atomic orbitals and variational Galerkin-Kantorovich method. We show that for sufficiently large charges of impurities the wave function of the occupied electron bound state of the highest energy changes its localization from the negatively charged impurity to the positively charged one as the distance between the impurities increases. This migration of the electron wave function of supercritical electric dipole is a generalization of the familiar phenomenon of the atomic collapse of single charged impurity to the case where electron-hole pairs are spontaneously created from vacuum in bound states with charge impurities thus partially screening them.