Density Functional Theory Scheme Research Articles

The crystalline structure of SrRuO3: Application of hybrid scheme to the density functionals revised for solids

The crystalline structure of ground-state orthorhombic SrRuO3 is reproduced by applying the hybrid density functional theory scheme to the functionals based on the revised generalized-gradient approximations for solidstate calculations. The amount of Hartree–Fock (HF) exchange energy is varied in the range of 5–20% in order to systematically ascertain the optimum value of HF mixing which in turn ensures the best correspondence to the experimental measurements. Such investigation allows one to expand the set of tools that could be used for the efficient theoretical modelling of, for example, only recently stabilized phases of SrRuO3, helping to resolve issues emerging for the experimentalists.

125Te NMR shielding and optoelectronic spectra in XTe3O8 (X = Ti, Zr, Sn and Hf) compounds: Ab initio calculations

We have calculated the NMR shielding, structural properties and optoelectronic spectra of XTe3O8 (X = Ti, Zr, Sn and Hf) compounds. The full potential linearized augmented plane wave (FP-LAPW) method and the modified Becke-Johnson (mBJ) are used by density functional theory schemes. The calculated shielding and measured shifts are arranged in a straight line and the tensors of magnetic shielding have a low symmetry and the shielding along the x direction is greater than the y and z directions. Obtained results show that the X ions have the most important influence on the 125Te chemical shift. Calculated chemical shielding components (σii) decrease from Ti to Sn then increases from Sn to Hf so that these behaviors are vice versa for 125Te isotropic chemical shift (δiso). Density of states spectra show that the X-p and d states play key role in the optical and NMR calculations. Optical results illustrate that there is a direct relation between the chemical shielding components for Te atom and the static dielectric function, refractive index and Plasmon energies.

Single excited molecular orbitals’ contribution in Uv-Vis absorption of dichloride-bis(5,7-dichloroquinolin-8-olato)tin(IV)

The absorption spectrum of dichloride-bis(5,7-dichloroquinolin-8-olato)tin(IV) of the chemical formula Q2SnCl2 is calculated making use of first-principles methods and is compared with the experimental data. The energy correction terms for the excitation energies are computed by considering the single excited molecular orbitals (SEMO) from ground state to excited states of the molecule. By this approach, the contributions of the Columbic interaction between an excited electron and the remained hole in the absorption spectrum during the process of optical excitation are estimated. The MO energy difference and MO wave functions calculated by the density functional theory (DFT) and SEMO energy contribution are calculated. The results show that the corrections in energies by considering the contributions of the SEMO improve significantly the theoretical optical absorption spectrum. The calculations are based on DFT and necessary parameters and integrals for the computation of SEMO are obtained in DFT scheme. This method proves to be preferred compared to other ab-initio methods for calculating excited states, due to its ability in specifying MOs for any excitation energy, and also its lower computational cost. The method is applied for the first time to the calculation of the energy of transitions and specifies the electronic transition between MOs, especially in the absorption machinery of the OLED. Taking into account the contribution of the electron-hole interaction in the optical mechanism of absorption in the molecule makes the theoretical spectrum closer to that of the experiment. As a result, the crucial role of the electron-hole interaction, i.e. the interaction between the excited electron and the remained hole, in the absorption mechanism cannot be ignored..

Selective orbital reconstruction in tetragonal FeS: A density functional dynamical mean-field theory study

Transport properties of tetragonal iron monosulfide, mackinawite, show a range of complex features. Semiconductive behavior and proximity to metallic states with nodal superconductivity mark this d-band system as unconventional quantum material. Here, we use the density functional dynamical mean-field theory (DFDMFT) scheme to comprehensively explain why tetragonal FeS shows both semiconducting and metallic responses in contrast to tetragonal FeSe which is a pseudogaped metal above the superconducting transition temperature. Within local-density-approximation plus dynamical mean-field theory (LDA+DMFT) we characterize its paramagnetic insulating and metallic phases, showing the proximity of mackinawite to selective Mott localization. We report the coexistence of pseudogaped and anisotropic Dirac-like electronic dispersion at the border of the Mott transition. These findings announce a new understanding of many-particle physics in quantum materials with coexisting Dirac-fermions and pseudogaped electronic states at low energies. Based on our results we propose that in electron-doped FeS substantial changes would be seen when the metallic regime was tuned towards an electronic state that hosts unconventional superconductivity.

Physical properties of fluorine based perovskites for vacuum-ultraviolet-transparent lens materials

Density functional theory (DFT) is used to study the bonding nature as well as the structural, electronic, and optical properties of the SrLiF3, SrNaF3, SrKF3 and SrRbF3 fluoroperovskite compounds. The findings are based on the total energy calculations where the Khon Sham (KS) equation is solved by means of an ab-initio full-potential linearized augmented plane wave (FP-LAPW) method. The structural properties including the lattice constant (ao), ground state energy (E), bulk modulus (B), and their pressure derivative (B′) are evaluated using the Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA) exchange correlation potential. Furthermore, to avoid the underestimation of the band profile by other DFT schemes we highlight the GGA plus Trans-Blaha modified Becke–Johnson (TB-mBJ) potential in lieu of attaining opto-electronic trends close to the expected experimental findings. Detailed analysis of the band dispersion curves is done with five different exchange and correlation schemes at the (Γ-Γ), (R-R), (M-M) and (X-X) symmetry points. A predominant characteristic associated with cation replacement shows that Li by Na, Na by K, and K by Rb significantly reduce the direct bandgap in these compounds. This crucial variation is responsible for working in different UV regions of the spectrum. The calculations of the band structure show that these compounds have a wide and direct energy bandgap at (Γ-Γ). The total and partial density of state curves are used to define the contributions of the different bands. In addition to this, contour plots of the electron density verify that the ionic behavior increases as we go from SrLiF3 to SrRbF3 while the brittleness, which dominates in them, decreases from SrLiF3 to SrRbF3. To verify the opto-electronic behavior bandgap dependent optical parameters, such as the complex dielectric function Ԑ(ω), optical conductivity σ(ω), energy loss function L(ω), and effective number of electrons neff, the sum rules are analyzed. All the calculated results are in favorable agreement with the previous theoretical and existing experimental data. Furthermore, the opto-electronic properties from the (TB-mBJ) potential are reported, which reveals that the fluorine based strontium series of perovskites are wide and direct band gap ionic insulators. The comprehensive present methodology represents an effective and influential tool for calculating the whole set of opto-electronic parameters, which can provide support for the understanding of various physical phenomena and empower device engineers for implementing these materials in UV based devices.

Electron momentum distribution and electronic response of ceramic borides

Isotropic Compton profiles of transition metal based ceramics TaB and VB have been measured using 137Cs (661.65keV) γ-ray Compton spectrometer. The experimental momentum densities are compared with those deduced using linear combination of atomic orbitals (LCAO) with Hartree-Fock (HF), density functional theory (DFT) with Wu-Cohen generalized gradient approximation (WCGGA) and also the hybridization of HF and DFT (namely B3PW and PBE0) schemes. It is found that LCAO-DFT-WCGGA scheme based profiles give an overall better agreement with the experimental data, for both the borides. In addition, we have computed the Mulliken's population (MP) charge transfer data, energy bands, density of states and Fermi surface topology of both the borides using full potential-linearized augmented plane wave (FP-LAPW) and LCAO methods with DFT-WCGGA scheme. Cross-overs of Fermi level by the energy bands corresponding to B-2p and valence d-states of transition metals lead to metallic character in both the compounds. Equal-valence-electron-density profiles and MP analysis suggest more ionic character of VB than that of TaB.

First-principles calculations of momentum distributions of annihilating electron–positron pairs in defects in UO2

We performed first-principles calculations of the momentum distributions of annihilating electron–positron pairs in vacancies in uranium dioxide. Full atomic relaxation effects (due to both electronic and positronic forces) were taken into account and self-consistent two-component density functional theory schemes were used. We present one-dimensional momentum distributions (Doppler-broadened annihilation radiation line shapes) along with line-shape parameters S and W. We studied the effect of the charge state of the defect on the Doppler spectra. The effect of krypton incorporation in the vacancy was also considered and it was shown that it should be possible to observe the fission gas incorporation in defects in UO2 using positron annihilation spectroscopy. We suggest that the Doppler broadening measurements can be especially useful for studying impurities and dopants in UO2 and of mixed actinide oxides.

Electronic Transport along Hybrid MoS2 Monolayers

Molybdenum disulfide (MoS2) is a two-dimensional material in which a semiconducting and a metallic phase can coexist. In this work, we investigate the electronic and transport properties of a hybrid MoS2 monolayer composed of a metallic strip embedded in the semiconducting MoS2 phase. Using quantum mechanical calculations within the density functional theory scheme together with the non-equilibrium Greens functions approach, we study in detail the structural and electronic properties of this hybrid material and its metal–semiconductor interface. A single point-defect analysis is performed in order to assess the stability of the hybrid system. Focus is given to the electronic transport properties of the hybrid MoS2 monolayer extracted from the electronic transmission spectra. These are linked to the local current across the monolayer. A clear asymmetry of the current flowing across the hybrid monolayer was found and was attributed to the atomistic characteristics of the material’s interfaces. The results s...

Equilibrium zinc isotope fractionation in Zn-bearing minerals from first-principles calculations

Isotopic composition analysis contributes significantly to the investigation of the biogeochemical cycle of zinc with its economical, environmental and health implications. Interpretation of isotopic measurements is however hindered by the lack of a set of equilibrium isotopic fractionation factors between Zn-bearing minerals. In this study, equilibrium mass-dependent Zn isotope fractionation factors in Zn-bearing minerals are determined from first-principles calculations within the density functional theory (DFT) scheme. A wide range of minerals belonging to sulfide, carbonate, oxide, silicate, sulfate and arsenate mineral groups are modelled to account for the natural diversity of Zn crystal-chemical environment. Calculated reduced partition function ratios (β-factors) span a range smaller than 2‰ at 22°C. All studied secondary minerals (adamite, gahnite, gunningite, hemimorphite, hydrozincite, zincite) but zinc carbonate (smithsonite) are isotopically heavier than zinc sulfide minerals (sphalerite and wurtzite) from which they could form through supergene processes. Zinc–aluminium spinel and zinc silicate are the isotopically heaviest minerals. The investigation of the crystal-chemical parameters at the origin of differences in isotopic properties shows an excellent linear correlation between lnβ and Zn interatomic force constants. β-factors are also observed to increase when the Zn-first neighbour bond lengths decrease and charges on atoms involved in the bonding increase and vice versa. These findings are in line with the observation of heavy isotope enrichment in systems having the largest bond strength.

(Invited) Computational Design of Lanthanide-Doped Optical Materials By a Combined DFT and Crystal-Field Approach: Recent Developments and Applications

In the last decades, lanthanide activated luminescent materials for lighting and display applications have been explored and optimized mainly by the cumbersome and costly trial-and-error experimental approach. Nowadays, the quick development of lanthanide spectroscopy theory together with an increase of the computational resources have made it possible to propose a systematical calculation strategy to understand the physics picture behind experiment and build up the link between the structural and optical properties of lanthanide-doped materials. It is well known that density functional theory (DFT) calculations employing the single-particle ground-state picture can provide very sufficiently accurate structural relaxation and the chemical trends that determine the emission energies and the luminescence mechanisms of optical dopants [1], whereas crystal-field (CF) theory calculations can very well reproduce experimentally observed optical spectra, which involve transitions between multi-electronic energy states of lanthanide ions [2]. Therefore, a combined theoretical scheme of modern DFT and conventional CF models has been proposed to give full play to their advantages [3]. In this talk, we will demonstrate how such a combined theoretical scheme supports the recent experimental studies of lanthanide phosphors. Ce3+ and Eu2+ ions will be paid more attention to due to their applications for white LED phosphors. The real reason resulting in the red or blue shift of the lowest 5d-4f emissions of Eu2+ or Ce3+ ions [4] will be analyzed by considering the combined effects of electronic structure and Stokes shift. Moreover, the misunderstanding about the position of the lowest 4f65d energy level in the 4f-5d excitation spectra of Eu2+ ions will be clarified, and thus the Stokes shift can be properly reproduced and further fed into understanding the electron-phonon coupling effect of Eu2+ ions with host lattices [5]. The successful application to the solid solution phosphors Ca2(Al1-x Mg x )(Al1-x Si1+x )O7: Eu2+will be shown in order to highlight the role of the theoretical work in the discovery of novel lanthanide phosphors [6]. Final, the recent developments and future challenges of the combined theoretical scheme will be given and commented, which include the first-principles structural determination of the 5d excited states of lanthanide ions and reproduction of the lowest charge transfer transition energy and its Stokes shift. Acknowledgements: The work is financially supported by National Natural Science Foundation of China (Grant No. 11204393), Natural Science Foundation Project of Chongqing (Grant No. CSTC2014JCYJA50034) and National Training Program of Innovation and Entrepreneurship for Undergraduates (Grant No. 201410617001).

Accurate ionization potential of semiconductors from efficient density functional calculations

Despite its huge successes in total-energy-related applications, the Kohn-Sham scheme of density functional theory cannot get reliable single-particle excitation energies for solids. In particular, it has not been able to calculate the ionization potential (IP), one of the most important material parameters, for semiconductors. We illustrate that an approximate exact-exchange optimized effective potential (EXX-OEP), the Becke-Johnson exchange, can be used to largely solve this long-standing problem. For a group of 17 semiconductors, we have obtained the IPs to an accuracy similar to that of the much more sophisticated $GW$ approximation (GWA), with the computational cost of only local-density approximation/generalized gradient approximation. The EXX-OEP, therefore, is likely as useful for solids as for finite systems. For solid surfaces, the asymptotic behavior of the ${v}_{xc}$ has effects similar to those of finite systems which, when neglected, typically cause the semiconductor IPs to be underestimated. This may partially explain why standard GWA systematically underestimates the IPs and why using the same GWA procedures has not been able to get an accurate IP and band gap at the same time.

Electronic and magnetic properties of an iron perovskite slab

From the [001] grown Sr2FeMoO6 double perovskite (SFMO) it was excised a slab with (001) free surfaces and half c wide. A supercell was constructed with that slab, leaving a 10 Å vacuum region along the [001] direction, in such a way that it is possible to deal with the slab as an insulated system. After the corresponding geometry optimization, the electronic and magnetic properties of the oriented slab were calculated. All calculations were made within the Density Functional Theory (DFT) scheme, with the Generalized Gradient Approximation (GGA), using the Perdew-Burke-Ernzerhof functional (PBE). The calculated values for each atomic magnetic moment were such that Fe atoms have the higher spin value (2.947ħ) for the SFMO system and are ferromagnetically aligned. Also the Density of States (DOS) and band structure were calculated, because the main SFMO bulk feature is the half-metallic behavior.

Fully self-consistent calculations of momentum distributions of annihilating electron-positron pairs in SiC

We performed calculations of momentum distributions of annihilating electron-positron pairs in various fully relaxed vacancy defects in SiC. We used self-consistent two-component density functional theory schemes to find the electronic and positronic densities and wave functions in the considered systems. Using the one-dimensional momentum distributions (Doppler-broadened annihilation radiation line shapes) we calculated the line-shape parameters $S$ and $W$. We emphasize the effect of the experimental resolution and the choice of the integration ranges for the $S$ and $W$ parameters on the distributions of the points corresponding to different defects in the $S(W)$ plot. We performed calculation for two polytypes of SiC, $3C$, and $6H$ and showed that for silicon vacancies and clusters containing this defect there were no significant differences between the Doppler spectra. The results of the Doppler spectra calculations were compared with experimental data obtained for $n$-type $6H$-SiC samples irradiated with 4-MeV Au ions. We observed a good general agreement between the measured and calculated points.

Effect of the transition metal ratio on bulk and thin slab double perovskite Sr2FeMoO6

Double perovskites are promising materials for multiple applications on microelectronics, specially on magnetic devices development. Perhaps the most interesting one is the double perovskite Sr2FeMoO6 since its magnetic properties differ from that of other related simple perovskites: SrFeO3 and SrMoO3. In this work the evolution of the electronic properties and the magnetic moment distribution as a function of the Fe/Mo ratio in bulk and a thin slab of Sr2FeMoO6 was studied. The thin slab was constructed keeping free surfaces parallel to the (001) crystalline planes with different thickness and compositions. All calculations were made in the Density Functional Theory scheme in the Generalized Gradient Approximation, using the Perdew-Burke-Ernzerhof functional, as implemented in the DMol3 code. After being geometry optimized, the electronic Density of States and band structure were calculated, as well as the magnetic moment distribution, for each modeled system. Essential results are as follows: for the bulk cases it was found that half-metallic behavior which characterizes the stoichiometric double perovskite changes if the compound becomes molybdenum or iron rich; for the slab is remarkable the induction of magnetic moments, owed to the corresponding to iron atoms, over their neighbor atoms.

Determination of the basic physical properties of semiconductor chalcopyrite type MgSnT2 (T = P, As, Sb) from first-principles calculations

Abstract

Perovskite-Type Thin Slabs: A First-Principles Study of Their Magnetic and Electronic Properties

The development of spintronic devices is the most promising and interesting way to improve conventional electronics. They will allow us to continue diminishing the devices' dimensions as well as optimizing the energy requirements for their operation. Perovskites stand out among materials that could be used in those devices because their variety permits combinations of optical, electronic, and magnetic properties. The bulk double perovskite Sr 2FeMoO6 (SFMO) is a half-metallic system that has become very attractive after the Kobayashi report about their high Curie temperature (about 470 K). SFMO has been used to build solid oxide fuel cell electrodes, but this compound could be used as a laminar system in the development of spintronic devices because of its high spin polarization. This proposal has been motivated by the exploitation of the magnetoresistive response in manganite applications and the reported experimental work on perovskite thin films. The inclusion of SFMO laminar systems in spintronic devices could affect magnetic-tunnel-junction-like devices. From a crystalline SFMO model system, thin slabs were excised and a supercell was built for each system, setting the appropriate initial separation between image slabs along the [001] direction. The geometry optimization and magnetic and electronic properties (density of states, band structures, and magnetic moment distribution) were calculated in the density functional theory scheme using the local density approximation with the Hubbard parameter (LDA + U).

From micro-to macroscopic: Understanding optical properties in zinc-blend-derived materials Cu2ZnYX4(X = S, Se, Te, Y = Si, Ge, Sn) by means of the quantum chemical topology analysis

Intensive calculations in the framework of the density functional theory (DFT) scheme have been carried out in order to predict accurately the dynamical stability and electronic properties of the zinc-blend-derived Cu2ZnYX4(X = S, Se, Te, Y = Si, Ge, Sn) compounds. Emphasis is also placed on the optical properties of the defect chalcopyrite compounds. Advantages of the analysis of the topology of the electron localization function (ELF) to provide microscopic point of view of the chemical bonding in solids are illustrated. A relationship between polarization of bonds and origin-independent atomic contributions to electric dipoles in the Cu2ZnYX4(X = S, Se, Te, Y = Si, Ge, Sn) defect chalcopyrite compounds is highlighted. In fact, what makes these compounds most interesting is a rich collection of S–Y long-distance bond paths. The ELF attractors do not systematically localize on the bond midpoint and their localization depends on electronegativity differences between the atoms forming the X–Y bond and on the nature of the chemical environment. Particularly, we show that the charge transfer remains generally the main contribution to the local bond dipole contribution. The compound which exhibits perturbed S lone pair is the more promising for nonlinear optical properties.

Elastic properties of 5d transition-metal carbides: An ab initio study

We have systematically studied the mechanical stability of group V transition metal carbides TMC$_2$ (TM = Hf, Ta, W, Re, Os, Ir, Pt, and Au) in the pyrite and fluorite phase, by calculating their elastic constants within the density functional theory scheme. It was found that all but ReC$_2$ and OsC$_2$ are stable in pyrite phase. On the other hand, all metal carbides studied were unstable in the fluorite phase.

Europium-doped silicon clusters EuSi n (n = 3–11) and their anions: structures, thermochemistry, electron affinities, and magnetic moments

The structures, electron affinities, and dissociation energies of EuSi n (n = 3–11) and their anions have been examined by means of four hybrid and pure density functional theory (DFT) methods. Basis sets used in this work are of segmented (SEG) Gaussian valence basis sets and relativistic small-core effective core potentials (ECP) with additional diffuse 2pdfg functions, denoted aug-SEG/ECP for Eu atoms and aug-cc-pVTZ for Si atoms. The geometries are fully optimized with each DFT method independently. The ground-state structures for all of these species are found to be substitutional type, which can be regarded as being derived from the ground-state structure of Si n+1 (and/or Si +1 − ) by replacing a Si atom with a Eu atom. The theoretical adiabatic electron affinities (AEAs) of EuSi n predicted by the four DFT schemes are in excellent agreement with the experimental data, especially the AEAs of TPSSh and B2PLYP. The average absolute deviations from experiment are by 0.10, 0.06, 0.07, and 0.05 eV, and the largest deviations are 0.16, 0.12, 0.18, and 0.10 eV at the B3LYP, TPSSh, PBE, and B2PLYP levels, respectively. The AEA of EuSi n (n = 3–11) is less than that of Si n . With the increase in silicon cluster size, the AEA of EuSi n may be close to that of Si n , but cannot be larger than that of Si n . The Eu atom acts as an electron donor, and the bonding between Eu and silicon clusters is ionic in nature. The bond between Eu and silicon clusters of neutral EuSi n (n = 3–11) is stronger than that of the anions. The total magnetic moments of EuSi n /EuSi (n = 3–11) and the magnetic moments on the Eu atom do not quench, and the total magnetic moments are contributed by Eu atom. The dissociation energies of Eu atom from EuSi n and their anions have also been calculated to examine relative stabilities.

Phonon dispersion and Raman spectra of wurtzite InAs under pressure

The authors have systematically studied the vibrational properties of wurtzite InAs at high pressure within the Density Functional Theory scheme. It is found that pressure significantly affects the phonon dispersion curves and Raman spectra. We observed an indication of phase transition for WZ-InAs at about 10GPa. The elastic constants calculation show mechanical stability for WZ-InAs. The calculated values of structural parameters are in good agreement with available data. There is a quadratic increase in optical modes with pressure while the LO–TO splitting and effective charge decrease linearly with pressure.