Local Dipole Moments Research Articles

Efficient "on-the-Fly" Calculation of Raman Spectra from Ab-Initio Molecular Dynamics: Application to Lithium-Sulfur Batteries

We present a novel computational method to accurately calculate Raman spectra from first principles. Together with an extension of the second-generation Car-Parrinello ab-initio molecular dynamics (AIMD) method to propagate maximally localized Wannier functions along with the nuclei, a speed-up of one order of magnitude can be obtained. Due to expression of the total polarizability as a sum of Wannier polarizabilities, the method also enable us to find partial Raman activities corresponding to local vibrations of different fragments in the system. We have used this method to investigate the Raman spectra of (Li2S4)\U0001d45b, \U0001d45b=1,4,8 clusters which are believed to be the last intermediates in the S8→Li2S transition in lithium-sulfur (Li-S) batteries, during the discharge cycle. However, it is not fully established whether or not any other products coexist with the Li2S crystal in the discharged state. In this study, we have observed a clear evidence of Li2S4→Li2S2 transition by investigating the systematic changes in the simulated Raman spectra as the cluster size increases. In line with recent experiments, we found that the Raman-active sulfur-sulfur stretching mode at 440 cm-1 can be considered as a signature of covalent bonding between two and more sulfur atoms per formula. We have also demonstrated that the transition is mainly due to the strong electrostatic interactions between polar Li2S4 monomers which result in energy lowering by arranging the local dipole moments in anti-parallel fashion. This arrangement result in the formation of crystal-like Li2S2 structures at the interfaces of Li2S4 monomers. Furthermore, through minimum-energy conformation search combined with AIMD simulations, we have shown that upon carbonization of polyacrylonitrile in early discharge stages of Li-S batteries and further exposure of the product to octasulfur, S2 groups form a bridge between two consecutive nitrogen atom on the edge of carbon fiber. Moreover, our Raman simulations reveal a characteristic Raman activity for these -N-S-S-N- groups in 300 - 400 cm-1 frequency range, in form of two distinct peaks which researchers could not experimentally assign before. Finally, we will discuss our recent studies on structure of complex cathode materials using our approach together with experimental measurements.

NaBa4(GaB4O9)2X3 (X = Cl, Br) with NLO-Active GaO4 Tetrahedral Unit: Experimental and ab Initio Studies

Two gallium borates, NaBa4(GaB4O9)2X3 (X = Cl, Br), were synthesized by high temperature solution method. The title compounds crystallize in the same space group P42nm. Their structures feature a 3D [GaB4O9]∞ framework composed of GaO4 tetrahedra and B4O9 groups. NaBa4(GaB4O9)2X3 (X = Cl, Br) compounds show the largest second harmonic generation responses among alkali metal and alkaline-earth metal gallium borates that are 1.5 and 1.1 times that of KH2PO4 (KDP), respectively, and are type I phase-matchable. Combined electronic structure and SHG density calculations as well as local dipole moment analysis have revealed that B–O groups and GaO4 tetrahedron play major roles in SHG response.

Thermal Expansion and Second Harmonic Generation Response of the Tungsten Bronze Pb2AgNb5O15.

The incorporation of transition metal element Ag was performed to explore negative thermal expansion (NTE) materials with tetragonal tungsten bronze (TTB) structures. In this study, the structure and thermal expansion behaviors of a polar TTB oxide, Pb2AgNb5O15 (PAN), were systematically investigated by high-resolution synchrotron powder diffraction, high-resolution neutron powder diffraction, transmission electron microscope (TEM), and high-temperature X-ray diffractions. The TEM and Rietveld refinements revealed that the compound PAN displays (√2a(TTB), √2b(TTB), 2c(TTB))-type superstructure. This superstructure within the a-b plane is caused by the ordering of A-site cations, while the doubling of the c axis is mainly induced by a slight tilt distortion of the NbO6 octahedra. The transition metal Ag has larger spontaneous polarization displacements than Pb, but the Pb-O covalence seems to be weakened compared to the potassium counterpart Pb2KNb5O15 (PKN), which may account for the similar Curie temperature and uniaxial NTE behavior for PAN and PKN. Powder second harmonic generation (SHG) measurement indicates that PAN displays a moderate SHG response of ∼0.2 × LiNbO3 (or ∼100 × α-SiO2) under 1064 nm laser radiation. The magnitudes of the local dipole moments in NbO6 and PbOx polyhedra were quantified using bond-valence approach. We show that the SHG response stems from the superposition of dipole moments of both the PbO(x) and NbO6 polyhedra.

Synthesis, single crystal structure, spectroscopic characterization and molecular properties of (2E)-3-(2,6-dichlorophenyl)-1-(3,4-dimethoxyphenyl)prop-2-en-1-one

A novel (2E)-3-(2,6-dichlorophenyl)-1-(3,4-dimethoxyphenyl)prop-2-en-1-one (DCPDMP) compound has been synthesized and its single crystal has been grown by slow evaporation technique. The structure of the compound has been characterized by FT-IR, FT-Raman and single-crystal X-ray diffraction techniques. The optimized molecular structure, vibrational wavenumbers, corresponding vibrational assignments of the compound have been investigated by means of the density functional theory. The molecule crystallizes in triclinic system, space group P-1 with a = 7.6179 (7), b = 8.5023 (7), c = 12.1967 (10) Å, V = 764.39 (11) Å3 and two molecules in the unit cell. The crystal structure is primarily stabilized through intramolecular C–H … Cl and C–H … O hydrogen bonds and intermolecular C–H … O and weak C–H … π interactions. These inter- and intramolecular interactions are analyzed. Moreover, the molecular electrostatic potential surface of the molecule has been constructed. Global and local reactivity descriptors and dipole moment (μ), static polarizability (α), first order hyperpolarizability (β) and optical gap (ΔE) have been also calculated to study the nonlinear optical (NLO) property of the title compound.

The stabilities and electronic structures of AlnSi12−nN12 (n = 0, 1, 2, and 4)

Abstract

Induced Dipole–Dipole Interactions Influence the Unfolding Pathways of Wild-Type and Mutant Amyloid β-Peptides

Amyloid-forming proteins undergo a structural transition from α-helical to disordered conformations and, ultimately, cross-β fibrils. The unfolding and aggregation of the amyloid β-peptide (Aβ) have been implicated in the development and progression of Alzheimer’s disease (AD) and cerebral amyloid angiopathy (CAA). However, the events underlying the initial structural transition leading to the disease state remain unclear. Although most cases are sporadic, several genetic variants exist that alter the electrostatic properties of Aβ and lead to more rapid unfolding and more severe phenotypes. In the present study, the enhanced unfolding is shown to be due to the mutated side chains altering the local peptide-bond dipole moments leading to local destabilization of the α-helix, as determined from polarizable molecular dynamics (MD) simulations of wild-type (WT) Aβ fragments and several common mutations. The local perturbation of the helix then leads to progressive unwinding of the α-helix in a cooperative fashion due to decreases in adjacent (i ± 1) and hydrogen-bonded (i + 4) peptide-bond dipole moments. Side-chain dynamics, subsequent variations in dipole moments, and ultimately the response in the peptide-bond dipole moments are all modulated by solvent dielectric properties based on simulations in water versus ethanol. The polarizable simulation results, along with simulations using the additive CHARMM36 force field, further indicate that cooperativity due to the alignment of peptide bonds leading to enhanced dipole moments is a fundamental force in stabilizing α-helices.

New niobium selenites, ANbO(SeO3)2 (A = Na, K, Rb, and Cs): Effect of alkali metal cation size on the dimensionality and intraoctahedral distortion

Four new alkali metal niobium selenites with both families of second-order Jahn-Teller distortive (SOJT) cations, ANbO(SeO3)2 (A = Na, K, Rb, and Cs), have been synthesized through standard solid state reactions using A2CO3, Nb2O5, and SeO2 in sealed fused silica tubes. Crystal structures of all four materials have been determined by single crystal X-ray diffraction. While NaNbO(SeO3)2 is configured with corner-shared zigzag chains of NbO6 octahedra and SeO3 intrachain connectors, three isostructural ANbO(SeO3)2 (A = K, Rb, and Cs) are arranged with corrugated layers that are composed of NbO6 octahedra and SeO3 trigonal pyramids. More detailed structural investigations indicate that the size and the coordination environment of alkali metal cations particularly affect the backbone geometries and the dimensions of the reported materials. With ANbO(SeO3)2 (A = K, Rb, and Cs), the alkali metal cations in between the layers also influence the extent of octahedral distortions. Full characterizations including thermal analyses, infrared and UV–vis diffuse reflectance spectroscopy, intraoctahedral distortions, and local dipole moment calculations are also presented.

Dielectric behavior in YMn1.07O3 ceramics

YMn1.07O3 ceramics were prepared by a sol–gel method, and the effect of Mn-excess on structure and dielectric properties was investigated. The X-ray diffraction pattern (XRD) shows that the sample has the hexagonal structure with space group P63cm. It is found to suppresses ferroelectric ordering and enhance antiferromagnetic ordering compared with YMnO3. Moreover, two dielectric relaxations were observed for the low and high temperature relaxations, respectively, which are considered to be associated with local dipole moments and the oxygen vacancies.

Fluctuations of local electric field and dipole moments in water between metal walls.

We examine the thermal fluctuations of the local electric field Ek (loc) and the dipole moment μk in liquid water at T = 298 K between metal walls in electric field applied in the perpendicular direction. We use analytic theory and molecular dynamics simulation. In this situation, there is a global electrostatic coupling between the surface charges on the walls and the polarization in the bulk. Then, the correlation function of the polarization density pz(r) along the applied field contains a homogeneous part inversely proportional to the cell volume V. Accounting for the long-range dipolar interaction, we derive the Kirkwood-Fröhlich formula for the polarization fluctuations when the specimen volume v is much smaller than V. However, for not small v/V, the homogeneous part comes into play in dielectric relations. We also calculate the distribution of Ek (loc) in applied field. As a unique feature of water, its magnitude |Ek (loc)| obeys a Gaussian distribution with a large mean value E0 ≅ 17 V/nm, which arises mainly from the surrounding hydrogen-bonded molecules. Since |μk|E0 ∼ 30kBT, μk becomes mostly parallel to Ek (loc). As a result, the orientation distributions of these two vectors nearly coincide, assuming the classical exponential form. In dynamics, the component of μk(t) parallel to Ek (loc)(t) changes on the time scale of the hydrogen bonds ∼5 ps, while its smaller perpendicular component undergoes librational motions on time scales of 0.01 ps.

Investigation of Raman Modes and Born‐Effective Charges in AgNb1/2Ta1/2O3: A Density‐Functional and Raman Scattering Study

Using ab initio density‐functional theory, the Born‐effective charge tensors and zone‐center phonon mode frequencies are computed for AgNb1/2Ta1/2O3 in monoclinic P2/m and orthorombic Pbcm symmetries. The experimental mode frequencies are obtained from deconvolution of Raman spectrum of prepared AgNb1/2Ta1/2O3 samples and are compared with computed mode frequencies. The Raman modes with high (>350 cm−1) and low frequencies (<90 cm−1) correspond to Ag and O vibrations, respectively. The modes in intermediate frequency band (120–350 cm−1) are dominated by Nb(Ta)–O vibrations. The computed effective charge tensors of cations at A (Ag) and B (Nb, Ta) sites are found to be diagonal. The off‐diagonal components of charge tensor are found sizeable only for O ions in orthorhombic AgNb1/2Ta1/2O3 with Pbcm symmetry. Further, charge tensor structure of O ions is found to depend on site symmetry in the unit cell. Charge tensor components for Nb, Ta, and O ions differ significantly from their nominal ionic values suggesting (1) large local dipole moments induced by off‐centering of Nb(Ta) ions and tilting(rotations) of Nb(Ta)O6 octahedra, (2) hybridization between d‐orbitals of Nb(Ta) and p‐orbitals of O atoms. Furthermore, the electronic structure, directional dependence of effective charges and performance of LDA (GGA) exchange‐correlation functionals with regard to computed values are also discussed.

Synthesis, crystal growth, single crystal X-ray analysis and vibrational spectral studies of (2E)-3-(2-chloro-4-fluorophenyl)-1-(3,4-dimethoxyphenyl)prop-2-en-1-one: A combined DFT study

A new chalcone derivative, (2E)-3-(2-chloro-4-fluorophenyl)-1-(3,4-dimethoxyphenyl)prop-2-en-1-one (a) was synthesized and single crystals were grown by slow evaporation technique. The FT-Raman and FT-IR spectra of the sample were recorded in the region 3500–100 cm−1 and 4000–400 cm−1 respectively. The spectra were interpreted with the aid of normal coordinate analysis, following structure optimizations and force field calculations based on B3LYP/6-31G (d) level of theory. Normal coordinate calculations were performed using the DFT force field corrected by a recommended set of scaling factors yielding fairly good agreement between the observed and calculated wavenumbers. The total electron density and molecular electrostatic potential surfaces of the molecule were constructed using B3LYP/6-31G (d) method to display electrostatic potential (electron + nuclei) distribution, molecular shape, size, and dipole moments of the molecule. HOMO and LUMO energies were also calculated. Stability of the molecule arising from hyperconjugative interactions and charge delocalization has been analyzed using natural bond orbital (NBO) analysis. Global and local reactivity descriptors and dipole moment (μ), static polarizability (α), first order hyperpolarizability (β) and optical gap (ΔE) were also calculated to study the NLO property of our title compound.

X-ray absorption study of the local dipole moment in SrTiO3 under uniaxial pressure

X-ray absorption spectra under uniaxial pressure were measured at the Ti K-edge of a SrTiO3 single crystal in order to obtain direct evidence for the ferroelectric transition that is expected on the basis of second harmonic generation studies. A uniaxial pressure was applied perpendicular to the ab-plane. The pre-edge peak, which corresponds to Ti 3d e g -O 2pσ bonding, shows little change across the transition temperature of 40 K at 1 GPa. This indicates that a local electric dipole moment is not induced under uniaxial pressure, as opposed to the macroscopic ferroelectric moments that are predicted to be induced along the ab-plane.

Defect-Tolerant Aligned Dipoles within Two-Dimensional Plastic Lattices.

Carboranethiol molecules self-assemble into upright molecular monolayers on Au{111} with aligned dipoles in two dimensions. The positions and offsets of each molecule's geometric apex and local dipole moment are measured and correlated with sub-Ångström precision. Juxtaposing simultaneously acquired images, we observe monodirectional offsets between the molecular apexes and dipole extrema. We determine dipole orientations using efficient new image analysis techniques and find aligned dipoles to be highly defect tolerant, crossing molecular domain boundaries and substrate step edges. The alignment observed, consistent with Monte Carlo simulations, forms through favorable intermolecular dipole-dipole interactions.

Ferroelectric Bloch-line switching: A paradigm for memory devices?

Vortices inside polar domain walls in ferroelastic materials can form ordered arrays resembling Bloch-lines in magnets. The Bloch lines are energetically degenerate with dipoles oriented perpendicular to the wall. By symmetry, these dipoles are oriented at +90° or −90° relative to the wall dipoles. These two states have the same energy and can be inverted by modest applied electric fields. As the majority of wall dipoles are oriented inside the wall, perpendicular to the Bloch line vortex, weak depolarization fields exist for the wall dipoles but not for Bloch lines. The Bloch line density depends on the density of the twin walls and the elastic anisotropy of the crystal structure. We estimate that distances between twin boundaries are as small as 50 nm and Bloch lines can form with some densities of 100 Bloch lines in an area of 100 × 100 nm2. The local dipole moment in the Bloch line is equivalent to the displacement of Ti in BaTiO3. Switchable Bloch lines can be detected by their macroscopic dipole moment and can constitute the functional part of a memory device.

Flux crystal growth of Ba2TiOSi2O7

Crystals of Ba2TiOSi2O7 (BTS) with sizes up to 18×13×5mm3 have been successfully grown from the LiF–H3BO3 flux system. The selection of the flux and the morphology of the crystals were discussed. Through our experiment, the alkali metal fluoride and boric acid system was found to be a flux system that can reduce the system temperature drastically. The crystals exhibit a maximum second harmonic generation (SHG) response about 5.5 times that of KDP at the particle size range of 88–105μm. The SHG response is mainly caused by the parallel alignment of the TiO5 polar pyramidal polyhedra, and the calculation of local dipole moments shows that the polarizations of all the TiO5 groups are well enhanced along the c-axis. The UV–vis–NIR Diffuse-Reflectance spectrum shows that the ultraviolet cutoff edge for BTS is 240nm. The powder X-ray diffraction (XRD) pattern of the grown BTS crystals has been indexed.

Local electric dipole moments for periodic systems via density functional theory embedding.

We describe a novel approach for the calculation of local electric dipole moments for periodic systems. Since the position operator is ill-defined in periodic systems, maximally localized Wannier functions based on the Berry-phase approach are usually employed for the evaluation of local contributions to the total electric dipole moment of the system. We propose an alternative approach: within a subsystem-density functional theory based embedding scheme, subset electric dipole moments are derived without any additional localization procedure, both for hybrid and non-hybrid exchange-correlation functionals. This opens the way to a computationally efficient evaluation of local electric dipole moments in (molecular) periodic systems as well as their rigorous splitting into atomic electric dipole moments. As examples, Infrared spectra of liquid ethylene carbonate and dimethyl carbonate are presented, which are commonly employed as solvents in Lithium ion batteries.

Rich structural chemistry in new alkali metal yttrium tellurites: three-dimensional frameworks of NaYTe4O10, KY(TeO3)2, RbY(TeO3)2, and a novel variant of hexagonal tungsten bronze, CsYTe3O8.

Pure polycrystalline phases and single crystals of four new quaternary alkali metal yttrium tellurites, NaYTe4O10, KY(TeO3)2, RbY(TeO3)2, and CsYTe3O8, have been prepared by solid-state and hydrothermal reactions using A2CO3 (A = Na, K, Rb, and Cs), Y(NO3)3·6H2O, Y2O3, and TeO2 as starting reagents. X-ray diffraction analyses suggest that NaYTe4O10 exhibits a highly symmetric three-dimensional (3D) framework consisting of YO8 square antiprisms and chains of TeO4 polyhedra. Within the framework, six- (6-) and eight-membered ring (8-MR) channels are observed. KY(TeO3)2 and RbY(TeO3)2 are isostructural to each other and reveal another 3D framework with structures containing YO6 octahedra and TeO3 trigonal pyramids with 4-MR and 12-MR channels. CsYTe3O8 shows a hexagonal tungsten bronze (HTB)-like topology composed of hexagonal tungsten oxide-like layers of TeO4 polyhedra and YO6 octahedral linkers with 3-MR and 6-MR channels. Thermal analyses, elemental analyses, and spectroscopic characterizations, such as UV-vis diffuse reflectance and infrared spectra, are presented, as are local dipole moment calculations for the constituent asymmetric polyhedra TeO3 and TeO4.

Cooperative Effects of Cation Size and Variable Coordination Modes of Te4+ on the Frameworks of New Alkali Metal Indium Tellurites, NaIn(TeO3)2, KIn(TeO3)2, RbInTe3O8, and CsInTe3O8

Four new alkali metal indium tellurites, NaIn(TeO3)2, KIn(TeO3)2, RbInTe3O8, and CsInTe3O8, have been prepared through hydrothermal and solid state synthesis reactions using corresponding alkali metal carbonates, In2O3 [or In(NO3)3·xH2O], and TeO2. The structures of the reported materials have been determined by powder and single crystal X-ray diffraction. The mixed indium tellurites reveal a rich structural chemistry with different channel structures. NaIn(TeO3)2 shows 8-membered rings, whereas stoichiometrically similar KIn(TeO3)2 exhibits both 8- and 12-membered rings in the frameworks. Isostructural RbInTe3O8 and CsInTe3O8 reveal three-dimensional frameworks consisting of InO6, TeO3, and TeO4 groups. Close structural examination suggests that the alkali metal cation size and variable coordination modes of Te(4+) cations cooperatively influence the framework geometries of the new mixed metal tellurites. Detailed characterizations including spectroscopic, elemental, and thermal analyses are introduced. Local dipole moments and out-of-center distortions for the constituent polyhedra are also reported.

Structures of vibrational absorption bands of the SiF4 molecule in a low-temperature nitrogen matrix

We have studied the IR absorption spectra of diluted mixtures SiF4/M = 1/6000–1/10 000 in an N2 matrix at 11 K (for comparison, the spectra of SiF4 in Ar and Xe matrices have also been studied). It has been shown that, in solid nitrogen, the appearance of doublets is observed both in the range of the ν3 band of the SiF4 (28SiF, 29SiF4, and 30SiF4) isotopologues of the SiF4 molecule and in the range of the ν1 + ν4, ν2 + ν3, and ν1 + ν3 bands of 28SiF4, whereas, in the range of the 2ν3 band of 28SiF, a triplet appears. In order to analyze the influence of the matrix on the spectrum of free SiF4 molecules, we have used a model that makes it possible to successively calculate (i) the spectrum of SiF4 in terms of the model of local modes, (ii) the structure of a matrix composed by 864 N2 molecules + a rigid SiF4 molecule using the Monte Carlo method, and (iii) the interaction of matrix particles with local dipole moments in the approximation of dipole-induced dipole and dipole-quadrupole interactions. The model describes satisfactorily the low-frequency shift of bands in the nitrogen matrix. All obtained experimental and theoretical results are consistent with the assumption that two kinds of stable trapping centers of SiF4 molecules obeying the Td symmetry are realized in the nitrogen matrix.

Sr3Bi2(SeO3)6·H2O: A novel anionic layer consisting of second-order Jahn–Teller (SOJT) distortive cations

A new layered bismuth selenite hydrate material, Sr3Bi2(SeO3)6·H2O has been synthesized through a hydrothermal reaction using SrCO3, Bi2O3, SeO2, and water as reagents. The crystal structure of the reported material has been determined by single crystal X-ray diffraction analysis. The anionic layered framework of Sr3Bi2(SeO3)6·H2O consists of polyhedra of second-order Jahn–Teller (SOJT) distortive cations, Bi3+ and Se4+. Attributable to the variable and asymmetric coordination geometry of the constituent cations, a rich structural chemistry including uni-dimensional bands and linkers is observed in the layer. The material is thermally stable up to about 390°C and starts decomposing due to the sublimation of SeO2 above the temperature. The UV–vis diffuse reflectance spectrum suggests a band gap of 3.3eV. Elemental analysis, infrared spectrum, local dipole moment calculations, and electronic structure calculations are also reported.