Ab Initio DFT Calculations Research Articles

High pressure stability of lithium metatitanate and metazirconate: Insight from experiments & ab-initio calculations

Lithium metatitanate (LTO) and lithium metazirconate (LZO) are lithium rich ceramics which can be used as tritium breeder materials for thermonuclear reactors. In-situ x-ray diffraction and ab-initio studies at high pressure show that LTO has a higher bulk modulus than that of LZO. In fact these studies indicate that they are the least compressible of the known lithium rich ceramics like Li2O or Li4SiO4, which are potential candidates for blanket materials. These studies show that the TiO6 octahedra are responsible for the higher bulk modulus of LTO when compared to that of LZO. It has also been shown that the compressibility and distortion of the softer LiO6 octahedra can be controlled by altering the stacking sequence of the more rigid covalently bonded octahedra. This knowledge can be used by chemists to design new lithium based ceramics with higher bulk modulus. It was observed that LTO was stable upto 34 GPa. Ab initio DFT calculations helped to understand the anisotropy in compressibility of both LZO and LTO. This study also shows, that even though the empirical potentials developed by Vijaykumar et al. successfully determine the ambient pressure structure of lithium metatitanate, they cannot be used at non ambient conditions like high pressure [1].

Theoretical molecular design of hexasilabenzene analogues aiming for the thermodynamic and kinetic stabilization

A variety of synthesis strategy of hexasilabenzene (HSB) - the silicon analogue of benzene - is suggested on the basis of ab initio molecular orbital and DFT calculations. To overcome the difficulty of the isolation and to increase the thermodynamic and kinetic stabilities of HSB, various ideas are proposed by making efficient use of the substituent effects - theoretical molecular designing. For the purpose, the stability of HSB (BE-type) relative to the well-known valence isomers such as hexasilabenzvalene (BV-type), hexasilaprismane (PR-type) and hexasila-Dewar-benzene (DB-type) and the mechanism of isomerization between them have been investigated and compared for the introduction of various substituents into the Si6 skeleton. As a result, it was found that some substituents consisting of hydrogen-bonding and stacking interaction between the substituents have remarkable effect to make HSB exclusively stable compared to the other isomers. Furthermore, some crosslinking substituents were found to be highly effective to achieve both thermodynamic and kinetic stabilization. For the HSB analogues with crosslinking substituents, the promising synthesis strategy was also discussed to realize experimental synthesis and isolation.

Computational Study of the Molecular Structure and Hydrogen Bonding in the Hamilton Wedge/Cyanuric Acid Binding Motif

Intermolecular binding between a Hamilton wedge and cyanuric acid, a frequently employed binding motif to achieve self-assembly of single-chain polymer nanoparticles, is studied by means of ab initio quantum-chemical and DFT calculations. A conformational analysis of the isolated compounds and the hydrogen-bonded complex reveals several low-energy structures, which are characterized based on their nonplanarity and binding energy. New hypothetical binding motifs are proposed and proven to possess a superior binding energy, upon becoming planar. Comparison to experimentally measured NMR spectroscopy data is made and cases of disagreement are traced back to solvent effects for isolated substituents or to thermally stretched intermolecular bonds for hydrogen-bonded species. Strong linear correlations between NMR chemical shifts and hydrogen-bond lengths are established for all compounds investigated.

Sc solubility in p-type half-Heusler (Ti1-cScc)NiSn thermoelectric alloys

Global climate changes, due to rise of combustion of fossil fuels and greenhouse gas emissions, are becoming a major environmental concern, which can be approached by utilizing of waste heat into usable electrical energy, as in thermoelectric, TE, heat to electricity conversion. Among the recently developed TE materials classes, half-Heusler, HH, alloys gain a lot of attention due to their combined high electronic TE potential with a high temperature stability. So far the most efficient HH compositions are n-type. In the current research, based on ab-initio DFT and statistical thermodynamic calculations as well as an experimental validation, the temperature dependent solubility limit of Sc in HH (Ti1-cScc)NiSn alloys is reported. It is shown that below the solubility limit p-type TiNiSn rich single phases are expected, while beyond the solubility limit a phase separation into ScNiSn and p-type TiNiSn rich phases is favored. This finding is of a high significance upon designing of TE devices with similar n- and p- type legs, as required for practical applications, with an addition of very interesting sub-micron phase-separation features, enabling scattering of phonons and enhancing of the TE performance.

Bright luminescence of diamonds with Ge-V centers

We report on the quantum yield (η) and decay time (τ) measurements at room temperature for the bright red-orange (602nm) luminescence from new germanium-vacancy (Ge-V) centers in nano- and microcrystalline diamonds synthesized at high pressure and high temperature. The values η = 3 ± 1% and τ = 6.2±0.2ns were found. The Stokes shift measured as the energy difference between the maxima of the luminescence and luminescence excitation spectra is negligible. The relative intensity of the zero-phonon line constitutes up to 70% of the total intensity of the luminescence. Results of our ab initio DFT calculations for the ground-state electronic and vibrational structure of (Ge-V)- in diamond are presented and discussed.

High-pressure phase diagram, structural transitions, and persistent nonmetallicity of BaBiO3 : Theory and experiment

${\mathrm{BaBiO}}_{3}$ is a mixed-valence perovskite, which escapes the metallic state through a Bi valence (and Bi-O bond) disproportionation or CDW distortion, resulting in a semiconductor with a gap of 0.8 eV at zero pressure. The evolution of structural and electronic properties at high pressure is, however, largely unknown. Pressure, one might have hoped, could reduce the disproportionation, making the two Bi ions equivalent and bringing the system closer to metallicity or even to superconductivity, such as is attained at ambient pressure upon metal doping. We address the high-pressure phase diagram of pristine ${\mathrm{BaBiO}}_{3}$ by ab initio DFT calculations based on GGA and hybrid functionals in combination with crystal structure prediction methods based on evolutionary algorithms, molecular dynamics, and metadynamics. The calculated phase diagram from 0 to 50 GPa indicates that pristine ${\mathrm{BaBiO}}_{3}$ resists metallization under pressure, undergoing instead at room temperature structural phase transitions from monoclinic I2/m to nearly tetragonal P-1 at 7 GPa, possibly to monoclinic C2/m at 27 GPa, and to triclinic P1 at 43 GPa. Remarkably, all these phases sustain and in fact increase the inequivalence of two Bi neighboring sites and of their Bi-O bonds and, in all cases except semimetallic C2/m, the associated insulating character. We then present high-pressure resistivity data, which generally corroborate these results, and show that the insulating character persists at least up to 80 GPa, suggesting that the C2/m phase is probably an artifact of the small computational cell.

Effect of electron doping on lattice instabilities in single-layer 1H−TaS2

Recent ARPES measurements of single-layer 1H-TaS2 grown on Au(111) suggest strong electron doping from the substrate. In addition, STM/STS measurements on this system show suppression of the charge-density-wave (CDW) instability that occurs in bulk 2H-TaS2. We present results from ab initio DFT calculations of free-standing single-layer 1H-TaS2 to explore the effects of doping on the CDW. In the harmonic approximation, we find that a lattice instability along the Gamma-M line occurs in the undoped monolayer, consistent with the bulk 3x3 CDW ordering vector. Doping removes the CDW instability, in agreement with the experimental findings. The doping and momentum dependence of both the electron-phonon coupling and of the bare phonon energy (unscreened by metallic electrons) determine the stability of lattice vibrations. Electron doping also causes an expansion of the lattice, so strain is a secondary but also relevant effect.

Negative thermal expansion of quartz glass at low temperatures: An ab initio simulation study

Using a mixed classical Molecular dynamics (MD)/ab initio simulation scheme combined with a quasi-harmonic approximation, we calculate the linear thermal expansion coefficient αL(T) in vitreous silica glasses. The systems are first cooled down by classical MD simulations. Then they are structurally relaxed by ab initio DFT calculations. The vibrational properties are calculated employing the frozen phonon method, and these results are finally used to calculate the Helmholtz free energy as a function of volume. In agreement with experiments, our simulations predict that αL(T) is negative at low temperatures up to T ≈ 150 K. In this low-temperature regime, the simulation results are in quantitative agreement with experiments. To elucidate the origin of the negative thermal expansion, we analyze in detail the microscopic mode Grüneisen parameters in the system and show that the anomalous behavior of αL(T) can be related to the fact that the Grüneisen parameters for the lowest modes become negative at low temperatures – i.e., the lowest eigenfrequencies become stiffer with increasing volume.

Infrared spectroscopy of europium borates EuM 3(BO3)4 (M = Al, Cr, Fe, Ga) with a huntite mineral type of structure

Huntite-like EuM 3(BO3)4 crystals (M = Al, Cr, Fe, or Ga) are synthesized by means of spontaneous crystallization from a solution in a melt. The infrared spectra of the compounds are measured. The experimental data and the results from ab initio DFT calculations are compared. The comparison shows that the compounds have a primarily noncentrosymmetric rhomboherdal structure with the R32 space group. This indicates these compounds could exhibit magnetoelectric and nonlinear optical properties.

Mechanical properties of non-centrosymmetric CePt3Si and CePt3B

Elastic moduli, hardness (both at room temperature) and thermal expansion (4.2–670 K) have been experimentally determined for polycrystalline CePt3Si and its prototype compound CePt3B as well as for single-crystalline CePt3Si. Resonant ultrasound spectroscopy was used to determine elastic properties (Young’s modulus E and Poisson’s ratio ν) via the eigenfrequencies of the sample and the knowledge of sample mass and dimensions. Bulk and shear moduli were calculated from E and ν, and the respective Debye temperatures were derived. In addition, ab initio DFT calculations were carried out for both compounds. A comparison of parameters evaluated from DFT with those of experiments revealed, in general, satisfactory agreement. Positive and negative thermal expansion values obtained from CePt3Si single crystal data are fairly well explained in terms of the crystalline electric field model, using CEF parameters derived recently from inelastic neutron scattering. DFT calculations, in addition, demonstrate that the atomic vibrations keep almost unaffected by the antisymmetric spin–orbit coupling present in systems with crystal structures having no inversion symmetry. This is opposite to electronic properties, where the antisymmetric spin–orbit interaction has shown to distinctly influence features like the superconducting condensate of CePt3Si.

Band structure, electronic and optical features of Tl4SnX3 (X = S, Te) ternary compounds for optoelectronic applications

Abstract In the present work, details of the crystal growth (Bridgeman method) and optical reflection spectroscopy for two single crystals (X = S, Te) were reported. Detailed ab-initio DFT calculations of the structural, electronic and optical properties of the crystals were performed for a deeper understanding of the experimentally obtained optical data. The special attention is paid to the electron charge density distribution, carrier mobility, optical reflectivity spectra and finally the anisotropy of the corresponding optical functions were studied. Were established a relations with carrier mobility, the optical anisotropy and electronic band structures for the titled crystals and compared with the optical reflectance spectra, covering the range of principal inter-band transitions. The origin of the calculated bands and effects of the anion substitution (Se by Te) on the crystal properties are studied. The role of screening effects is discussed by a comparison of the GGA and LDA approaches, including the electronic band dispersion and effective masses anisotropy. The possible changes of the inter-band transition are considered. The relation between the space charge density distribution and optical features is established. Finally was explained the origin of the observed discrepancies between the experimental and theoretical data.

Stop-Frame Filming and Discovery of Reactions at the Single-Molecule Level by Transmission Electron Microscopy.

We report an approach, named chemTEM, to follow chemical transformations at the single-molecule level with the electron beam of a transmission electron microscope (TEM) applied as both a tunable source of energy and a sub-angstrom imaging probe. Deposited on graphene, disk-shaped perchlorocoronene molecules are precluded from intermolecular interactions. This allows monomolecular transformations to be studied at the single-molecule level in real time and reveals chlorine elimination and reactive aryne formation as a key initial stage of multistep reactions initiated by the 80 keV e-beam. Under the same conditions, perchlorocoronene confined within a nanotube cavity, where the molecules are situated in very close proximity to each other, enables imaging of intermolecular reactions, starting with the Diels–Alder cycloaddition of a generated aryne, followed by rearrangement of the angular adduct to a planar polyaromatic structure and the formation of a perchlorinated zigzag nanoribbon of graphene as the final product. ChemTEM enables the entire process of polycondensation, including the formation of metastable intermediates, to be captured in a one-shot “movie”. A molecule with a similar size and shape but with a different chemical composition, octathio[8]circulene, under the same conditions undergoes another type of polycondensation via thiyl biradical generation and subsequent reaction leading to polythiophene nanoribbons with irregular edges incorporating bridging sulfur atoms. Graphene or carbon nanotubes supporting the individual molecules during chemTEM studies ensure that the elastic interactions of the molecules with the e-beam are the dominant forces that initiate and drive the reactions we image. Our ab initio DFT calculations explicitly incorporating the e-beam in the theoretical model correlate with the chemTEM observations and give a mechanism for direct control not only of the type of the reaction but also of the reaction rate. Selection of the appropriate e-beam energy and control of the dose rate in chemTEM enabled imaging of reactions on a time frame commensurate with TEM image capture rates, revealing atomistic mechanisms of previously unknown processes.

Estimation of graphene surface stability against the adsorption of environmental and technological chemical agents

The ability of graphene to resist the chemical transformations caused by chemisorption of several environmental and technological chemical agents (H2, O2, H2O, H2O2) was studied using DFT ab initio calculations of potential energy surfaces of adsorption on perfect and defect (single vacancy and Stone–Wales defect) graphene with zero and finite curvature. Beside the evident result that structural imperfections such as curvature and atomic defects make graphene chemically vulnerable, it was found that the two kinds of impact of vacancies and Stone–Wales defects on the chemical stability of a sp2‐hybridized carbon lattice are essentially different. In the latter case, chemisorption barriers for any kind of chemical agents decrease with increasing curvature but, in the case of single vacancies the finite curvature lowers the adsorption probability for oxygen‐based molecules. In general, it was shown that single‐layered graphene could be used as an effective protecting coating against environmental and technological chemical agents.

Magnetic structure and properties of centrosymmetric twisted-melilite K2CoP2O7.

Twisted-melilite dipotassium cobalt pyrophosphate (K2CoP2O7, P42/mnm, #136), originally reported by Gabelica-Robert (1981), was synthesized in powder form by a standard solid-state reaction route. The magnetic properties of the material were studied by magnetometry and its magnetic structure determined using neutron powder diffraction for the first time. Below TN = 11 K, the material adopts a G-type antiferromagnetic structure with moments aligned in the ab-plane (magnetic space group Pn'nm, #58.3.473). Ab initio calculations were performed to examine the isotropic magnetic spin exchange parameters as well as the preferred direction of magnetic moments due to spin-orbit coupling. The relationship between crystal structure geometry and the strength of the magnetic interactions was examined and compared to those of melilite-type Sr2CoGe2O7.

Comparison of structural dynamics and coherence of d-d and MLCT light-induced spin state trapping.

Light-induced excited spin state trapping (LIESST) in FeII spin-crossover systems is a process that involves the switching of molecules from low (LS, S = 0) to high spin (HS, S = 2) states. The direct LS-to-HS conversion is forbidden by selection rules, and LIESST involves intermediate states such as 1,3MLCT or 1,3T. The intersystem crossing sequence results in an HS state, structurally trapped by metal-ligand bond elongation through the coherent activation and damping of molecular breathing. The ultrafast dynamics of this process has been investigated in FeN6 ligand field systems, under MLCT excitation. Herein, we studied LIESST in an FeIIN4O2 spin-crossover material of lower symmetry, which allowed for quite intense and low-energy shifted d-d bands. By combining ab initio DFT and TD-DFT calculations and fs optical absorption measurements, we demonstrated that shorter intermediates enhanced coherent structural dynamics, and d-d excitation induced faster LS-to-HS switching, compared to MLCT.

Magnetic Structures of Heterometallic M(II)-M(III) Formate Compounds.

A study of the magnetic structure of the [NH2(CH3)2]n[FeIIIMII(HCOO)6]n niccolite-like compounds, with MII = CoII (2) and MnII (3) ions, has been carried out using neutron diffraction and compared with the previously reported FeII-containing compound (1). The inclusion of two different metallic atoms into the niccolite-like structure framework leads to the formation of isostructural compounds with very different magnetic behaviors due to the compensation or not of the different spins involved in each lattice. Below TN, the magnetic order in these compounds varies from ferrimagnetic behavior for 1 and 2 to an antiferromagnetic behavior with a weak spin canting for 3. Structure refinements of 2 and 3 at low temperature (45 K) have been carried out combining synchrotron X-ray and high-resolution neutron diffraction in a multipattern approach. The magnetic structures have been determined from the difference patterns between the neutron data in the paramagnetic and the magnetically ordered regions. These difference patterns have been analyzed using a simulated annealing protocol and symmetry analysis techniques. The obtained magnetic structures have been further rationalized by means of ab initio DFT calculations. The direction of the magnetic moment of each compound has been determined. The easy axis of the MII for compound 1 (FeII) is along the c axis; for compound 2 (CoII), the moments are mainly within the ab plane; finally, for compound 3 (MnII), the calculations show that the moments have components both in the ab plane and along the c axis.

The Modified Debye–Grüneisen Model for Highly Compressed Diamond

A new modification of the Debye–Gruneisen model is proposed. It takes explicitly into account the anharmonicity of phonon modes, which leads to the nonlinear dependence of energy levels on quantum numbers. The low-temperature version of this model considering anharmonic displacement of only the lowest energy levels of both longitudinal and transverse phonons is developed. This model, calibrated on results of DFT ab initio calculations, reproduces all predicted low-temperature thermodynamic properties of diamond at pressures up to 1 TPa, including the region of its negative thermal expansion.

Phonon spectra of eulytite crystals Bi4M3O12 (M = Ge,Si): ab initio study

In this paper we present the results of ab initio DFT calculation of phonon spectra for bismuth ortho-germanate Bi4Ge3O12 and bismuth ortho-silicate Bi4Si3O12 crystals, in the center of the first Brillouin zone. First, the geometry optimization was performed using the analytical energy gradients, with respect to atomic coordinates and unit cell parameters. Vibrational frequencies and normal modes were calculated within the harmonic approximation by diagonalizing the mass-weighted Hessian matrix. The IR and Raman spectra of both crystals were simulated with the periodic ab initio Crystal 09 code and B3LYP hybrid functional and the two sets of Transverse-Optical and Longitudinal-Optical frequencies are generated, together with their intensities. Also, the influence of isotopic substitution for Bi, Ge and O in phonon modes and the picture with values of frequencies shift in each mode by isotopic substitution were calculated. The obtained results are discussed and the comparision between the computed spectra and experimental data is quite satisfactory, which justifies the model and simulation scheme used for the title systems.

A joint experimental and computational study on ethylammonium nitrate-ethylene glycol 1:1 mixture. Structural, kinetic, dynamic and spectroscopic properties

The intrinsic molecular polarity of a molecule, i.e. its dipole moment, seems to have an important role in the mixing process of an Ionic Liquid and a Molecular Liquid. In this work we report a complete study on the overall organization of a 1:1 binary mixture of Ethylammonium Nitrate and Ethylene Glycol by means of Wide Angle X-Ray Scattering, Raman Spectroscopy and Far Infrared Spectroscopy. The interpretation of all the experimental data was aided by a variety of computational models obtained via Classical Molecular Dynamics, ab initio Molecular Dynamics and DFT calculations. We observe that in the samples examined the Nitrate anion is strongly solvated by glycol molecules. This interaction has notable consequences on experimental observations.

Stereochemistry of nitrogen E lone pair in NH3E, NOFE, N2O3E2, AgNO2E, and NCl3E

We revisit nitrogen based simple fundamental molecules in their solid state structures, with the purpose of casting new light on the stereoactivity of valence lone pairs (LPs)—formally N(2s2)—in different crystal geometries. Based on coupled investigations of crystal chemistry and ab initio DFT calculations providing the electron localization function (ELF), LP behavior is analyzed precisely by finding its position E, orientation and “volume of influence” which consists in an electronic cloud generated around the so-called ‘centroid’ Ec of the electronic doublet. The results show the paramount importance of the role of N(2s2) LP in the crystal network architecture through the different case studies pertaining to ammonia (NH3), nitrosyl fluoride (NOF), nitrosyl nitrite (N2O3), silver nitrite (AgNO2), and nitrogen trichloride (NCl3). An unexpected direct ionic interaction between [NO]+ or Ag+ and the centroid Ec of the [NO2Ec]− nitrite group has been evidenced in N2O3E2 and AgNO2, respectively.