Ab Initio DFT Calculations Research Articles

Alloying effect on the lattice thermal conductivity of MNiSn half-Heusler alloys.

The lattice thermal conductivity of MNiSn (M = Ti, Zr, Hf) half-Heusler (HH) alloys was studied. Ab initio DFT calculations were used for the calculation of the material physical properties. A combination of the Slack model and Klemens analytical alloying model was used to simulate the lattice thermal conductivity as a function of composition and temperature. Our results emphasize the major role of point defect scattering in a single-phase state of HH alloys because of the mixing of elements in the M-sub-lattice, especially at the high working temperature of the thermoelectric material. We performed a series of calculations from pure unalloyed compounds to multicomponent compositions with five elements in the M sub-lattice of (Ti, Zr, Hf, Al, Sc)NiSn.

Changes in the Optical Properties of an M-Doped (M = Pt, Ti) hBN Sheet and CO2 Capturing

We performed ab initio DFT calculations to explore the optical properties of a hexagonal boron nitride (hBN) monolayer, doped with a Ti or a Pt atom. Ti doping increases the adsorption capability of the boron nitride surface for capturing CO2. Both doping types increase the optical absorption and reflectivity of the hBN surface in the infrared and visible regions. For the UV region, a B vacancy increases the absorption of the hBN sheet. Captured CO2 bears substantial changes in the optical absorption and reflectivity spectra of the system considered.

(Invited, Digital Presentation) Excited State Properties of Low-Dimensional Materials: Insight By Ab-Initio DFT + Mbpt Simulations

The use of emerging low-dimensional materials in technological applications presupposes a detailed knowledge of their chemical and physical properties. In this context ab-initio theoretical methods and simulations are playing a fundamental role.Here I aim to show how the use of parameter-free atomistic simulations can contribute to improve the microscopic understanding of the opto-electronic properties of nanomaterials and to predict new ones. I will show how ab-initio DFT and post-DFT (GW and BSE) calculations, based on Many-Body Perturbation Theory (MBPT), provide a very useful scheme to study i) giant electronic band-gaps renormalization due to low dielectric screening and large quantum confinement ii) strong light-matter interaction iii) fine structure of strongly bound excitons. iv) the influence of electron-phonon interaction on the electronic and optical spectra, v) how doping, molecular functionalization or strain can tune their opto-electronic properties. Main focus will be on different semiconducting 2D materials discovered after graphene, but other dimensionality examples will be discussed.

Structure and Surface Relaxation of CeO2 Nanoparticles Unveiled by Combining Real and Reciprocal Space Total Scattering Analysis.

We present a combined real and reciprocal space structural and microstructural characterization of CeO2 nanoparticles (NPs) exhibiting different crystallite sizes; ~3 nm CeO2 NPs were produced by an inverse micellae wet synthetic path and then annealed at different temperatures. X-ray total scattering data were analyzed by combining real-space-based Pair Distribution Function analysis and the reciprocal-space-based Debye Scattering Equation method with atomistic models. Subtle atomic-scale relaxations occur at the nanocrystal surface. The structural analysis was corroborated by ab initio DFT and force field calculations; micro-Raman and electron spin resonance added important insights to the NPs’ defective structure. The combination of the above techniques suggests a core-shell like structure of ultrasmall NPs. These exhibit an expanded outer shell having a defective fluorite structure, while the inner shell is similar to the bulk structure. The presence of partially reduced species testifies to the high surface activity of the NPs. On increasing the annealing temperature, the particle dimensions increase, limiting disorder as a consequence of the progressive surface-to-volume ratio reduction.

Valley selectivity of soft x-ray excitations of core electrons in two-dimensional transition metal dichalcogenides

Optical properties of semiconducting monolayer transition metal dichalcogenides have received a lot of attention in recent years, following the discovery of the valley selective optical population of either ${\text{K}}_{+}$ or ${\text{K}}_{\ensuremath{-}}$ valleys at the direct band gap, depending on the polarization of the incoming light. We use group theoretical selection rules, as well as ab initio DFT calculations, to investigate whether this valley selectivity effect is also present in x-ray optical transitions from the flat core level of the transition metal atom to the valence and conduction band K valleys. Valley selectivity is predicted for $s, {p}_{1/2}$, and ${p}_{3/2}$ edges in transitions to and from the valence band edges with circularly polarized radiation. Possible novel applications to the diagnostics of valleytronic properties and intervalley dynamics are investigated and the feasibility of ultrafast pump-probe and Kerr rotations experiments with suitable soft-x-ray free-electron laser sources is discussed.

Dynamics and disorder: on the stability of pyrazinamide polymorphs.

This article focuses on the structure and relative stability of four pyrazinamide polymorphs. New single crystal X-ray diffraction data collected for all forms at 10 K and 122 K are presented. By combining periodic ab initio DFT calculations with normal-mode refinement against X-ray diffraction data, both enthalpic and entropic contributions to the free energy of all polymorphs are calculated. On the basis of the estimated free energies, the stability order of the polymorphs as a function of temperature and the corresponding solid state phase transition temperatures are anticipated. It can be concluded that the α and γ forms have higher vibrational entropy than that of the β and δ forms and therefore they are significantly more stabilized at higher temperatures. Due to the entropy which arises from the disorder in γ form, it overcomes form α and is the most stable form at temperatures above ∼500 K. Our findings are in qualitative agreement with the experimental calorimetry results.

Structural, electronic and optical properties of [formula omitted]: First principle study

We computed structural, elastic, electronic and optical properties of Be2X(X=C,Si,Ge,Sn) family of antifluorite with ab initio DFT calculations using the generalized gradient approximation (GGA). The different parameters such as geometry optimization, band structure, density of states, elastic constants, dielectric functions have been studied. We also calculated bandgap using PBE0 and HSE hybrid functionals to compare experimental bandgap of Be2C. Although three of the compounds are hypothetical in nature, their formation energy found to be negative. The calculated values of elastic constants indicates antifluorite Be2X are mechanically stable. The graph of real part of epsilon shows negative value giving promising result for blanket behaviour of Be2X from radiation damage.

Arsenite oxidation and (thio)arsenates formation in arsenite- and sulfide-containing solution under air atmosphere

Thioarsenic species are widely existed in transitional environments with varying redox potential such as hot springs and paddy fields, where arsenite, sulfide and thioarsenites may be exposed to ambient air. However, arsenic speciation, especially the kinetics of arsenic transformation in such scenarios are not well studied. Here we present results from both kinetic experiments and computational information on the formation mechanisms of thioarsenic species from the sulfide-driven oxidation of arsenite (13.3 μM) under air atmosphere at circumneutral pH (6.5–10.5, covering the usual groundwater pH range). Ion chromatography-hydride generation atomic fluorescence spectrometry (IC-HG-AFS) was used for the determination of non-thiolated arsenic and thioarsenate species as well as indirect identification of thioarsenites at environmentally relevant As concentration. Results showed that the ambient air facilitated the thiolation and oxidation of arsenite, during which thioarsenites were served as intermediates. It was supported by our ab initio DFT calculation showing that the activity towards oxidation was in the order of: monothioarsenite > dithioarsenite > trithioarsenite > arsenite. The thiolation of arsenite was decreased with pH with a rate constant of 0.2–1.7 h−1; while the oxidation of thioarsenites by air was increased with pH with a rate constant of 0.1–0.45 h−1. The results of this study shall improve our mechanistic understanding on the formation of thioarsenic, and shed light on the potential intermediate roles played by thioarsenites in environments with varying redox potential or intermittent anoxia, such as paddy fields, peatlands, hot springs etc.

A facile method for Tauc exponent and corresponding electronic transitions determination in semiconductors directly from UV–Vis spectroscopy data

In this work, a facile method allowing for estimation of the exponent in the Tauc equation directly from the UV–vis spectra is presented. It is based on the Taylor expansion of the logarithmic version of the Tauc equation. The Tauc exponent is calculated from the tangent slope of the absorption data. Knowledge of this coefficient provides information about the optical transition types and is used as an input for the calculations of the band gap. As an exemplar material, TiO2 in form of thin film and nanotubes as well as their nanocomposites with other metal oxides are chosen. For the transparent thin 15 nm film of TiO2, two linear ranges with n = 2.0 and 1.2 are found. The first is assigned to the well-established indirect (allowed) transition. In the case of the second one, the n does not correspond to typically used values. Therefore, it is speculated that more than one transition is probable. This consideration is supported with ab-initio DFT calculations of the band structure of TiO2 with Hubbard U correction. Similar results are found for TiO2 nanotubes using diffuse reflectance spectroscopy indicating that the presented method applies both to transmission and reflection.

Probing the Origin of Ferro-/Antiferromagnetic Exchange Interactions in Cu(II)-4f Complexes.

The mechanistic investigations between Cu(II) and the anisotropic lanthanides (Ln(III)) are not much explored to date. This is due to the complicated energy spectrum which arises due to the orbital angular momentum of anisotropic lanthanides. Interestingly, the exchange coupling J in Ln(III)-Cu(II) systems was found to be antiferromagnetic for <4f7 metal ions and ferromagnetic for ≥4f7 metal ions, while the net magnitude of JTotal strength gradually decreases moving from f1 to f13. While this is established in several examples, the reason for this intriguing trend is not rationalized. In this article, we have taken up these challenging tasks by synthesizing a family of complexes with the general molecular formula [Cu2Ln(HL)4(NO3)](NO3)2, where Ln = La (1-La), Ce (2-Ce), Pr (3-Pr), Gd (4-Gd), Tb (5-Tb), Dy (6-Dy), and Ho (7-Ho) and HL = C15H15N1O3; (2-methoxy-6-[(E)-2'-hydroxymethyl-phenyliminomethyl]-phenolate) is a monodeprotonated tridentate Schiff base ligand. Detailed dc magnetic susceptibility measurements performed for all the complexes reveal that the Cu(II) ion is coupled ferromagnetically to the respective Ln(III) ion, which has more than seven electrons in the 4f shell, while an antiferromagnetic coupling is witnessed if Ln(III) has less than seven electrons. The strength of the exchange coupling constant was quantitatively determined for representative complexes from the high-field/high-frequency electron paramagnetic resonance spectroscopy which follows the order of 4-Gd (1.50(10) cm-1) > 5-Tb (1.18(10) cm-1) > 6-Dy (0.56(10) cm-1 based on the spin Hamiltonian. The increased axiality in 5-Tb and 6-Dy due to the presence of 3d ions in the near vicinity of an oblate ion and the increased exchange coupling strength between Cu(II) and Tb(III) or Dy(III) is the ideal combination to stabilize magnetic bistability in these complexes in the absence of an external magnetic field with the effective energy barrier of 15.7 K (τo = 2.49 × 10-6 s) and 12.6 K (τo = 1.70 × 10-5 s), respectively. To rationalize this experimental trend, we have performed ab initio CASSCF and DFT calculations. To compute the J values, we have employed POLY_ANISO routines and utilized the computed data to establish the generic mechanism of magnetic coupling in {Cu-Ln-Cu} motifs. These mechanistic findings reveal the importance of 5d orbitals and their energy with respect to the dx2-y2 orbital of Cu(II) ions in controlling the magnetic coupling of {Cu-4f} complexes.

New insights into the piezoelectric, thermodynamic and thermoelectric properties of lead-free ferroelectric perovskite Na0.5Bi0.5TiO3 from Ab initio calculations

Ab initio DFT calculations have been performed to investigate a complete set of structural, electronic, elastic, piezoelectric, thermodynamic and thermoelectric properties of lead-free perovskite Na0.5Bi0.5TiO3 (NBT) crystalizes in rhombohedral (R3c), tetragonal (P4bm) and cubic (Pm3̅m) phases. The lattice stabilities in Na0.5Bi0.5TiO3 crystals are studied from the formation energy and phonon dispersions. Electronic band profiles obtained using new KTBmBJ+so potential reveal that R3c, P4bm and Pm3̅m phases of NBT are indirect band gap semiconductors with energy values of ~ 3.29, 3.05 and 3.09 eV, respectively. Analysis of the calculated elastic constants (Cij) indicates that NBT systems are elastically stable. Ferroelectric tetragonal NBT crystal shows relatively high piezoelectric coefficients [d15 = 101.0 pC/N, d31 = 51.3 pC/N and d33 = 81.1 pC/N], compared to the rhombohedral system [d15 = 96.4 pC/N, d31 = 8.93 pC/N and d33 = 21.2 pC/N and d22 = 21.2 pC/N]. Thermodynamic quantities for rhombohedral NBT compound were calculated using the quasi-harmonic Debye model. Thermoelectricity in terms of electrical conductivity σ/τ, thermal conductivity κ/τ, Seebeck coefficient S, figure of merit ZT and thermo power for NBT crystals were examined using BoltzTraP2 code. P4bm-NBT structure showed a largest magnitude of S ~ 201.42 µV/K (at T = 500 K), following by Pm3̅m ~ 173.6 µV/K (800 K) and R3c ~ 158.6 µV/K (100 K). ZT maximized to high values ~ 2.76 (at 700 K), 2.16 (900 K) and 1.96 (400 K) for P4bm, Pm3̅m and R3c structures, respectively. Na0.5Bi0.5TiO3 (NBT) compounds could be a promising candidate for use in manufacturing high-performance piezoelectric devices and developing high-power thermoelectric generators.

Electrical Regulation of CO2 Adsorption in the Metal-Organic Framework MIL-53.

Active regulation of pore accessibility in microporous materials by external stimuli has aroused great attention in recent years. Here, we show the first experimental proof that guest adsorption in a dielectric microporous material can be regulated by a moderate external E-field below the gas breakdown voltage. CO2 adsorption capacity in MIL-53 (Al) was significantly reduced, whereas that of NH2-MIL-53 (Al) changed insignificantly under a direct current E-field gradient of 286 V/mm. Ab initio DFT calculations revealed that the E-field decreased the charge transfer between the CO2 molecule and the adsorption site in the MIL-53 framework, which resulted in reduced binding energy and consequently lowered CO2 adsorption capacity. This effect was only observed in the narrow pore state MIL-53 (Al) but not in its large pore configuration. Our results demonstrate the feasibility of regulating the adsorption of gas molecules in microporous materials using moderate E-fields.

Uncoupling chloride and acidification attack on the naturally formed corrosion scales

A combination of an in-situ microzone injection test and ab-initio DFT calculations was employed to investigate the effect of H+ and Cl- ions on the degradation of the FeCO3 corrosion products. The experimental results show that an acidic environment promoted the dissolution kinetics of the FeCO3 scales while the presence of Cl- ions induced FeCO3 surface reconstruction. The DFT calculations validate the experimental results and indicate that both H+ and Cl- corroded FeCO3 surface through weakening of the Fe-O bond and increased the corrosion active sites.

Ab-initio DFT Calculations on Elastic Coefficients, (001) Surface Energy, Stability Limit of Pure Metals and Separation Energy of Bimetal Interface

Ab-initio DFT Calculations on Elastic Coefficients, (001) Surface Energy, Stability Limit of Pure Metals and Separation Energy of Bimetal Interface

The behavior of zeolites wairakite and phillipsite at high P-T parameters

In situ investigation of mineral behavior in water medium at simultaneously high P-T parameters can be applied to modelling of mineral transformation processes in lithospheric plates. The behavior of zeolites wairakite and phillipsite under the P-T conditions of «cold» slab subduction, corresponding to the start of oceanic plate diving or ocean floor near geothermal sources, was studied by in situ Raman spectroscopy. During compression in water medium, phillipsite initial phase is stable up to T = 350 °C, P = 1.7 GPa and with further increase of P-T parameters, phillipsite undergoes amorphization and partially dissolves in water. Wairakite compressed in water medium has a polymorphic transformation at T ≈ 300 °C and P ≈ 0.4 GPa. At 300–450 °C and P = 1 GPa the Raman spectrum almost disappears due to the amorphization of wairakite. Zeolite wairakite partially dissolves, and other zeolite phillipsite grows out of the fluid at T = 450 °C and P = 1 GPa. This transformation indicates the higher stability of phillipsite in comparison to wairakite. The in situ observed high P-T stability of phillipsite, which does not transform to other zeolites, and its formation from wairakite may indicate ф possible widespread distribution of this zeolite in marine sediments.By using the plane-wave pseudo-potential method, ab initio DFT calculations of Raman and FTIR spectra of wairakite were carried out. Comparing theoretical and experimental spectra, interpretation of the vibrational spectra of both zeolites was suggested.

Molecular dynamics studies of entropic elasticity of condensed lattice networks connected with uniform functionality f = 4.

To study the linear region of entropic elasticity, we considered the simplest physical model possible and extracted the linear entropic regime by using the least squares fit and the minimum of the mean absolute error. With regard to the effect of the fluctuation of the strand length Ns, the strand length with fluctuation was set to a form proportional to (1.0 + C (R - 0.5)), where R is a uniform random number between 0 and 1 and C is the amplitude of fluctuation. This form enabled us to analytically calculate the fluctuation dependence of the elastic modulus G. To reveal the linear regions of entropic elasticity as a function of the strand length between neighboring nodes in lattices, molecular dynamics (MD) simulations of condensed lattice networks with harmonic bonds without the excluded volume interactions were performed. Stress-strain curves were estimated by performing uniaxial stretching MD simulations under periodic boundary conditions with a bead number density of 0.85. First, we used a diamond lattice with functionality f = 4. The linear region of the entropic elasticity was found to become larger with the increasing number of beads in a strand Ns. For Ns = 100, the linear region had a strain of up to 8 for a regular diamond lattice. We investigated the effect of strand length fluctuation on the diamond lattice, and we confirmed that the equilibrium shear modulus G increases as the obtained analytical prediction and the linear entropic region in the stress-strain curves becomes narrower with increasing fluctuation of Ns. To investigate the difference in network topology with the same functionality f and uniform strand length Ns, we performed MD simulations on regular networks of the BC-8 structure with f = 4 prepared from the ab initio DFT calculations of carbon at high pressure. We found that the elastic behavior depends on the network connectivity (i.e., topology). This indicates that the network topology plays an important role in the emergence of nonlinearity owing to the crossover from entropic to energetic elasticity.

Revisiting properties of CaCoSinO2n+2. Crystal and electronic structure

In a public space there are several reports of materials with general stoichiometry CaCoSinO2n+2. Pyroxene CaCoSi2O6 is probably the best-known representative for n = 2 but not much is known about materials with n = 3 and n = 4. In this study, attempts were carried out to synthesize those phantom materials and it was found that they do not exist as a single phase. A quantitative XRD analysis revealed that their chemical composition is correct but the formula should be written as CaCoSi2O6 + (n-2)SiO2. Similar qualitative conclusions were drawn from investigation of magnetic (DC magnetometry) and electronic properties using X-ray Photoelectron Spectroscopy (XPS) and Si K edge X-ray Absorption Spectroscopy (XAS). Additionally, the DFT ab initio calculations were carried out to obtain electronic signature from band structure of CaCoSi2O6.The apparent influence of the excess of SiO2 on magnetic properties of this “series” can be understood in terms of presence and suppression of secondary phases like Ca2CoSi2O7, which form when the starting materials are not homogenized properly. Addition of surplus SiO2 suppresses their formation leaving clear signature from CaCoSi2O6, which also can be synthesized from stoichiometric mixture using proper techniques.

Method for assessing atomic sources of flicker noise in superconducting qubits

Flicker noise causes decoherence in Josephson junction-based superconducting qubits, thus limiting their practical potential as building blocks for quantum computers. This is due to limited length and complexity of executable algorithms, and increased dependency on error-correcting measures. Therefore, identifying and subsiding the atomic sources of flicker noise are of great importance to the development of this technology. We developed a method that combines ab initio DFT calculations and quantum dynamics to model charge transport across a Josephson junction, by which it is possible to more accurately assess different defects as sources of flicker noise. We demonstrate the use of our method in an investigation of various atomic defects, including vacancies, trapping, and substitutions, in an Al|Al2O3|Al Josephson junction. This demonstration both reveals weaknesses in previous attempts to pinpoint the atomic sources of flicker noise and highlights new candidates.

On the interaction of solute atoms with vacancies in diluted Al-alloys: A paradigmatic experimental and ab-initio study on indium and tin

To study the vacancy-solute-atom interaction, diluted Al-In and Al-Sn alloys with 0.005 and 0.025 at.% indium/tin cast from very high purity elements were investigated by positron annihilation spectroscopy (PAS). Therefore, the alloys have been solution heat treated at temperatures ranging from 320 to 620 °C and then rapidly quenched into ice water freezing-in most thermal vacancies. Positron annihilation lifetime (PALS) and coincidence Doppler broadening spectroscopies (CDBS) were combined to unambiguously identify vacancy-solute-atom complexes. For enabling a direct comparison to experiment, we did employ ab-initio DFT calculations of vacancy-solute-atom complexes providing relaxed atomic coordinates, which are used to calculate PAS annihilation parameters. In the as-quenched state vacancy-solute-atom pairs as well as vacancy clusters were observed in both alloys for all concentrations. During isochronal annealing vacancy clusters, formed during quenching, dissolved at about 130 °C, leaving vacancy-solute-atoms complexes as the only remaining defects. Thus, we could unambiguously identify those by a combination of PALS and CDBS with ab-initio calculations. Employing isothermal annealing the binding energy EB of vacancies to In and Sn solute atoms was determined experimentally by PALS as well as by ab-initio calculations. We find from our experiments EB = (0.20 ± 0.03) and (0.32 ± 0.10) eV for In and Sn, respectively, which is in very good agreement with our ab-initio calculations giving 0.23 and 0.26 eV, respectively. Our results clearly identified vacancy-In/Sn complexes as responsible for the retardation of vacancy migration after quenching. The vacancies bound to In and Sn solute-atoms are released around 150 °C shifting, when added to AlCu alloys as trace elements, the transport of copper atoms to higher temperatures necessary for the formation of finely distributed ″precipitates″, which efficiently strengthen this kind of alloys.

Surface Electron-Hole Rich Species Active in the Electrocatalytic Water Oxidation.

Iridium and ruthenium and their oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotentials observed for Ru- and Ir-based anodes and the high corrosion resistance of Ir-oxides. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nanofabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrOx centers (i.e., oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally IrV species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (μ1-O and μ1-OH) that are responsible for water activation and oxidation. Oxygen bound to three iridium centers (μ3-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a μ1-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface-to-bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.