Raman spectroscopic studies of LixSiy compounds

The phase diagram of the Li–Si system contains several phases with Li and Si in well defined ratios. So far, only the Raman spectrum of LiSi has been reported. In this work, we present experimental Raman scattering results for the crystalline lithium silicide phases Li₁₂Si₇, Li₇Si₃, Li₁₃Si₄, and Li₂₁Si₅/Li₂₂Si₅, which show clearly distinguishable Raman modes. The experimental results are compared with theoretical data obtained by density functional theory calculations. Copyright © 2013 John Wiley & Sons, Ltd.     

Determination of the tautomeric composition of adenine in the gas phase by vibrational spectroscopy methods: II. Analysis of resonance Raman spectra

The resonance Raman spectra of adenine in the gas phase under excitation with laser radiation at wavelengths of 266, 218, and 200 nm have been investigated experimentally. The quantum-mechanical calculations of the intensity distribution in the resonance Raman spectra of three adenine tautomers are performed in the Herzberg-Teller approximation with the inclusion of the Duschinsky and frequency effects. Conclusions regarding the tautomeric composition of adenine in the gas phase are drawn from comparison of the results of quantum-mechanical calculations with experimental data.     

Conformational analysis and vibrational spectroscopic studies on dapsone

In this study, the theoretical conformation analysis of free dapsone has been performed by single point energy calculations at both semi-empirical PM3 and DFT/B3LYP-3-21G theory levels and three stable conformers were determined. Both the IR and Raman spectra of the molecule in solid phase have been recorded. The IR intensities and harmonic vibrational wavenumbers of each conformer were calculated by DFT method at B3LYP/6-31++G(d,p) theory level. For the fundamental characterization, the total energy distribution (TED) calculations of the vibrational modes were done using parallel quantum mechanic solution program (SQM) and the fundamental modes were assigned. The theoretical results are in agreement with the experimental ones.     

Raman and Hyper-Rayleigh Scattering in Lithium Tetraborate Crystals

Experimental studies of Raman scattering and the infrared reflection spectra of lithium tetraborate crystals were carried out within a broad temperature range with various polarization geometries. The crystals studied are characterized by record values of radiation resistance, transparence within a broad spectral range including the ultraviolet region, and nonlinear optical properties. A group-theoretical analysis of the vibrational spectra of this crystal was performed and fundamental vibrational terms were assigned to the symmetry types of the point group as well as the polarizations of the corresponding modes. The effective Raman cross section was measured and was found to be one order of magnitude higher than those of the known crystals, in which the stimulated Raman scattering (SRS) was observed. The nonlinear optical and electrooptical coefficients were evaluated and were found to be consistent with the results of independent measurements. The effect of a drastic increase in the intensity of quasi-elastic light scattering at 253 K was registered. It was associated with the phase transition that consists of the disordering of lithium ions with respect to the rigid skeleton. The formation energy for Frenkel defects in the lithium sublattice and the activation energy of the ionic conduction when heating the sample were calculated from the obtained temperature dependences of the intensity of quasielastic and hyper-Rayleigh light scattering.     

Laser‐induced phase changes in olivine FePO4: a warning on characterizing LiFePO4‐based cathodes with Raman spectroscopy

Raman spectroscopy is an excellent technique for probing lithium intercalation reactions of many diverse lithium ion battery electrode materials. The technique is especially useful for probing LiFePO₄‐based cathodes because the intramolecular vibrational modes of the PO₄³⁻ anions yield intense bands in the Raman spectrum, which are sensitive to the presence of Li⁺ ions. However, the high power lasers typically used in Raman spectroscopy can induce phase transitions in solid‐state materials. These phase transitions may appear as changes in the spectroscopic data and could lead to erroneous conclusions concerning the delithiation mechanism of LiFePO₄. Therefore, we examine the effect of exposing olivine FePO₄ to a range of power settings of a 532‐nm laser. Laser power settings higher than 1.3 W/mm² are sufficient to destroy the FePO₄ crystal structure and result in the formation of disordered FePO₄. After the laser is turned off, the amorphous FePO₄ compound crystallizes in the electrochemically inactive α‐FePO₄ phase. The present experimental results strongly suggest that the power setting of the excitation laser should be carefully controlled when using Raman spectroscopy to characterize fundamental lithium ion intercalation processes of olivine materials. In addition, Raman spectra of the amorphous intermediate might provide insight into the α‐FePO₄ to olivine FePO₄ phase transition that is known to occur at temperatures higher than 450 °C. Copyright © 2008 John Wiley & Sons, Ltd.     

Parametric solitons on Raman resonance and self-induced transparency in silica optical fibre

An analytical and numerical analysis of the dynamics of parametric solitons on Raman resonance is performed. The results of calculations describe an effect of self-induced transparency for parameteric solitons and the influence of the initial phase on Raman Stokes gain and anti-Stokes absorption. This can be used to maintain the process of energy transfer between pulses.     

锂离子电池电极材料的第一性原理研究进展

文章综述了第一性原理计算在锂离子电池电极材料模拟与设计方面的研究进展.电极材料的研究包括电极材料的电子结构和电子导电性的研究,嵌锂电位、锂离子输运特性、嵌锂过程中局部结构弛豫与相变以及材料表面特性研究等方面,第一性原理计算在上述诸方面的研究都取得了一定的进展.这些理论上的研究成果,可以帮助人们加深对材料性能与机理的理解,同时对材料的设计也具有指导意义.     

Calculation of absorption and resonance Raman cross sections of ClO2 in the gas‐phase and in solution: including Duschinsky effects

The time‐correlation function formalism has been used to calculate resonance Raman cross sections, excitation profiles, and electronic absorption spectra of the OClO molecule in the gas‐phase and in different solvents like cyclohexane, chloroform, and water. The multidimensional time domain integrals that arise in these calculations have been evaluated for the case in which an X²B₁ òA₁ electronic transition takes place between displaced‐distorted‐rotated harmonic potential energy surfaces. Ab initio calculations have been performed to provide the spectroscopic constants required for the evaluation of these integrals. The calculated absorption spectra and resonance Raman cross sections have been compared with the experimental results. Copyright © 2009 John Wiley & Sons, Ltd.     

DFT simulations and Vibrational spectra of 4‐chloro and 4‐bromophenylboronic acid molecules

The experimental and theoretical vibrational spectra of 4‐chloro‐ and 4‐bromophenylboronic acids (abbreviated as 4Clpba and 4Brpba) were studied. The Fourier transform Raman and Fourier transform infrared (FTIR) spectra of 4Clpba and 4Brpba molecules were recorded in the solid phase. The structural and spectroscopic analyses of the molecules were made by using Hartree–Fock and density functional harmonic calculations. In both 4Clpba and 4Brpba only one form was most stable using B3LYP level with the 6–311 + + G(d,p) basis set. Selected experimental bands were assigned and characterized on the basis of the scaled theoretical wavenumbers by their total energy distribution (TED). Finally, geometric parameters as well as infrared (IR) and Raman bands were compared with the experimental data of the molecules. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural and magnetic properties of chromium dioxide at high pressures

Ab initio calculations were performed on CrO₂ to study its behavior and possible similarity to silica under high pressures. At the rutile→CaCl₂-type phase transition, the lattice constants, cell volume and total energy change continuously, indicating the second-order nature of the phase transition, consistent with the experimental observations. The current calculations have demonstrated that the rutile→CaCl₂-type phase transition is driven by the softening of the Raman active B_1g mode, weakly coupling with the elastic shear modulus C_s. Further phase transitions of CrO₂ to denser packed phases of α-PbO₂-type and pyrite have been well predicted by total energy calculations. Our electronic calculations revealed that CrO₂ is still a half-metallic ferromagnet up to pressure of 95 GPa. The present results confirm the analogy of the phase sequence between silica and CrO₂ at high pressures.     

Hydrostatic pressure-induced phase transitions in RbMnCl<Subscript>3</Subscript>: Raman spectra and lattice dynamics

Raman scattering spectra of RbMnCl₃ are measured at room temperature under high hydrostatic pressure. The results are interpreted based on first principles lattice dynamics calculations. The experimental data obtained correlate with the calculations in the low frequency domain but disagree slightly in the region of high-frequency vibrations. The transition from the hexagonal to the cubic perovskite phase observed earlier (near 0.7 GPa) was confirmed, and new transitions to lower symmetry distorted phases were discovered (at 1.1 and 5 GPa).     

Raman spectroscopy of seized drugs and density functional theory interpretation

Abstract

The demand for a hand-held Raman spectrometer in the fast and accurate detection and identification of seized drugs is much higher than before, especially when facing unknown suspicious drugs. However, Raman spectra for the different drugs are less reported due to the inaccessibility of them. Here, we reported the experimental Raman spectra in detail of four typical drugs (such as methamphetamine, ketamine, caffeine, and magu). The Raman vibrational frequencies were also calculated by the method of density functional theory (DFT) at Becke-3-Lee-Yang-Parr (B3LYP) level with the 6-31?G and 6-31G(d,p) basis set. The results show that the experimental Raman spectra of these typical drugs are consistent with the theoretical Raman spectra. Using the potential energy distribution (PED) calculation with the GAR2PED program, the assignments of the observed Raman bands to the vibrational modes were presented. Further, methamphetamine and its camouflage N-benzylisopropylamine were analyzed by Raman spectroscopy and DFT calculations, and the result showed that the obvious differences of the Raman characteristic bands for these two samples could be found so that Raman technique could be used to identify the authenticity of methamphetamine. All the above results confirm the potential of the approach involving Raman spectroscopy combined with DFT calculations in the characterization of drugs. Based on this, the experimental spectra of seized drugs measured directly through a plastic package were studied. Raman spectroscopy has the advantage of being performed through packaging without disturbing the samples. Polypropylene transparent packaging does not alter the spectra of the drugs but will mask the corresponding bands if the Raman spectrum has a strong autofluorescence interference.     

Synthesis and Raman spectroscopic investigation of a new self‐assembly monolayer material 4‐[N‐phenyl‐N‐(3‐methylphenyl)‐amino]‐benzoic acid for organic light‐emitting devices

We have synthesized 4‐[N‐phenyl‐N‐(3‐methylphenyl)‐amino]‐benzoic acid (4‐[PBA]) and investigated its molecular vibrations by infrared and Raman spectroscopies as well as by calculations based on the density functional theory (DFT) approach. The Fourier transform (FT) Raman, dispersive Raman and FT‐IR spectra of 4‐[PBA] were recorded in the solid phase. We analyzed the optimized geometric structure and energies of 4‐[PBA] in the ground state. Stability of the molecule arising from hyperconjugative interactions and charge delocalization was studied using natural bond orbital analysis. The results show that change in electron density in the σ* and π* antibonding orbitals and E₂ energies confirm the occurrence of intramolecular charge transfer within the molecule. Theoretical calculations were performed at the DFT level using the Gaussian 09 program. Selected experimental bands were assigned and characterized on the basis of the scaled theoretical wavenumbers by their total energy distribution. The good agreement between the experimental and theoretical spectra allowed positive assignment of the observed vibrational absorption bands. Finally, the calculation results were applied to simulate the Raman and IR spectra of the title compound, which show agreement with the observed spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

Adsorption of 1,2,4‐triazole on a silver electrode: surface‐enhanced Raman spectroscopy and density functional theory studies

The pH‐dependent surface‐enhanced Raman scattering (SERS) of 1,2,4‐triazole adsorbed on silver electrode and normal Raman (NR) spectra of this compound in the aqueous solutions were investigated. The observed bands in the NR and SERS spectra were assigned with the help of density functional theory calculations for model molecules in the neutral, anionic, and cationic forms and their complexes with silver. The Raman wavenumbers and intensities were computed at the optimized molecular geometry. Vibrational assignments of the SERS and NR spectra are provided by calculated potential energy distributions. The combination of experimental SERS results and density functional theory calculations provide an insight into the molecular structure of adlayers formed by 1,2,4‐triazole on a silver surface at varying pH values and enable the determination of molecular orientation with respect to the surface. Copyright © 2012 John Wiley & Sons, Ltd.     

Revisiting lithium K and iron M₂,₃ edge superimposition: the case of lithium battery material LiFePO₄

Experimental electron energy-loss spectra are presented for FePO₄, LiFePO₄ and NaFePO₄ from 0 to 80 eV. With the help of the NaFePO₄ spectrum in the 50-80 eV range, the double peak observed in LiFePO₄ could be ascribed to the presence of Fe^II and not to the Li K edge, contrary to what was thought previously. Crystal field multiplet calculations confirm this attribution. Using VASP programme based on density functional theory, dielectric response calculations including local field effects in the Hartree approximation are then proven to properly simulate the fine structures due to the lithium K edge. By comparing absolute spectrum intensities, it is shown that the lithium K edge cannot be used to quantify lithium in such compounds. This detailed comparison between theoretical calculations and experimental spectra helps defining the relevant parameters governing intensities in the 50-80 energy range.     

Quadrupole Interaction Contribution to the Hyperfine Structure of P-States in Muonic Lithium,Beryllium, and Boron Ions

The contribution of the quadrupole interaction to the hyperfine structure of the spectrum of the energy of muonic lithium, beryllium, and boron ions is calculated within the quasipotential method in quantum electrodynamics. The numerical results obtained improve previous calculations due to the consideration of new corrections. They can be used for comparison with future experimental data.     

Potential energy distribution and variations of force constants with temperature in α-LiIO3

The potential energy distribution in the hexagonal lithium iodate crystal is calculated on the basis of a potential energy involving valence and central forces. The frequency shifts of observed modes with temperature are interpreted by force constants variations. An anomalous decrease of the IR and Raman silent B libration frequency is predicted.     

Size effects in lithium ion batteries

Size-related properties of novel lithium battery materials, arising from kinetics, thermodynamics, and newly discovered lithium storage mechanisms, are reviewed. Complementary experimental and computational investigations of the use of the size effects to modify electrodes and electrolytes for lithium ion batteries are enumerated and discussed together.Size differences in the materials in lithium ion batteries lead to a variety of exciting phenomena. Smaller-particle materials with highly connective interfaces and reduced diffusion paths exhibit higher rate performance than the corresponding bulk materials. The thermodynamics is also changed by the higher surface energy of smaller particles, affecting, for example,secondary surface reactions, lattice parameter, voltage, and the phase transformation mechanism. Newly discovered lithium storage mechanisms that result in superior storage capacity are also briefly highlighted.     

Phonon localization in Ge nanoislands and its manifestation in Raman spectra

Multilayer structures with germanium nanoislands that are formed on the silicon (111) surface upon submonolayer deposition by molecular-beam epitaxy have been investigated using Raman spectroscopy. To interpret the experimental Raman spectra, numerical calculations of the spectra have been performed for nanoislands containing from several to several hundred germanium atoms. The calculations demonstrate that the in-plane sizes of nanoislands (with sizes less than 2–3 nm) substantially affect the frequencies of phonons localized in these nanoislands. The experimental Raman spectra confirm the occurrence of the quantum size effect.