Anharmonic Parameters Research Articles

Oscillating states of driven Langevin systems under large viscous drives.

We provide a perturbative scheme that is suitable to study oscillating states in driven Langevin systems in the large viscous regime. We explicitly determine the oscillating state distribution of an underdamped Brownian particle driven by a time-dependent periodic potential. Apart from the harmonic and anharmonic parameters of the potential, the noise strength and the viscous parameter (or equivalently their ratio referred to as the thermal parameter), which appear in the dynamics of the Brownian particle, are also driven periodically. We specify various nonequilibrium observables, relevant to characterize the oscillating states, and evaluate them to linear order in anharmonic perturbation. We find that the effect of viscous drives on oscillating states is measurable even at leading order and show that the thermodynamic properties of the system in these states are significantly distinct from those in equilibrium or even from those exhibited by oscillating states of overdamped driven systems.

Calculation of spectroscopic parameters of hydrogen and halogen bounded CH3CHO···HF and CH3COH···FH complexes

In this work, equilibrium geometry, bond energy, harmonic and anharmonic spectral parameters of the monomers of CH3CHO, HF molecules and CH3CHO···HF, CH3COH···FH complexes were calculated using the set of bases MP2/6-311++G(3df, 3pd). Anharmonic calculations were performed using second-order vibration theory. The binding energies of CH3CHO ··· HF and CH3COH ··· FH complexes are 10.1 kcal/mol and 1.7 kcal/mol, respectively.

Thermal expansivity and Raman spectra of natural cordierite: A potential cordierite-garnet elastic thermometer

Cordierite is widely found in metamorphic rocks, and has important application for geothermometry and geobarometry, as well as a monitor of volatile concentrations in fluids or melts. In this study, we measured high-temperature powder X-ray diffraction (XRD) patterns on a natural sample (Na0.06[(Mg1.87Fe0.19)Al3.94Si5.00O18.00](CO2)0.09(H2O)0.22) up to 1500 K. The thermal expansion coefficient is fitted with an average value of 7.39(19) × 10−6 K−1. In situ high-temperature and high-pressure Raman spectra were also obtained on this sample, on the basis of which the isobaric and isothermal Grüneisen parameters are calculated. The anharmonic correction has positive contribution to the heat capacity, since the intrinsic anharmonic parameters show systematically negative values. Next, the isothermal bulk modulus (KT) at high temperatures is modeled by density functional theory (DFT), which is essential for constructing the P-V-T equation of state (EOS) as well as thermodynamic Grüneisen parameter (γV) for cordierite. The axial thermoelastic parameters and strain evolution along isochors at high temperatures are also evaluated. Cordierite is substantially softer with weaker anisotropy for axial compressibilities as compared with zircon and rutile, and consequently, significantly smaller strain effect is expected on the vibrational frequencies. Due to its smaller thermal expansivity and larger compressibility than garnets, the assemblage of cordierite trapped in garnet minerals can potentially be an elastic thermometer in the pressure range below 1 GPa. Entrapment isomekes are calculated with negative slopes of (∂P/∂T)V(inc-host), and positive residual inclusion pressures are expected at the ambient condition.

Thermal transport and thermoelectric properties of alkali-metal telluride Na2Te from first-principles study

The excellent thermoelectric performance of alkali-metal telluride Na2Te has been systematically studied from the electronic and phonon transport properties within the density functional theory combined with the Boltzmann transport theory framework. The phonon dispersion without any imaginary mode demonstrates that the Na2Te compound is dynamically stable. The calculated lattice thermal conductivity is as small as 3.01 W m−1 K−1 at room temperature, suggesting that Na2Te can serve as a promising thermoelectric material. The related anharmonic effect parameters, phonon lifetime and Grüneisen parameter, are discussed to further explain the lattice thermal conductivity. Also, the electronic transport parameters, such as Seebeck coefficient S, electrical conductivity σ, and electronic thermal conductivity κe, are investigated to determine the thermoelectric performance by utilizing the semi-classical Boltzmann transport theory. The final figure of merit ZT can be evaluated based on the lattice thermal conductivity and the corresponding electronic transport coefficients. The maximum ZT of 1.53 can be achieved at a carrier concentration n = 3.08 × 1020 cm−3 for p-type Na2Te at T = 900 K, greatly larger than that of the n-type Na2Te (0.86), suggesting that the Na2Te can be a potential p-type thermoelectric material.

Spectroscopic valuation of phonon lifetime and surface characteristics of Zn0.96Mg0.04S thin film.

The anharmonic properties of the longitudinal optical (LO) phonon mode of Mg-doped ZnS (Zn0.96 Mg0.04S) are investigated using the Balkanski and Klemens models on the temperature-dependent Raman spectra. The variation in the position of the Raman line, peak width, and phonon lifetime with temperature was fitted using three and four phonon decay mechanisms. The values of the anharmonic fitting parameters indicated low anharmonicity. A lifetime of ∼0.17ps at 90K indicated a fast phonon decay. In addition, the thin film is analyzed to evaluate its surface characteristics using Raman mapping that showed chemical homogeneity over a large area of the film. Furthermore, we analyzed spatial variations of Raman line intensity, peak area, linewidth, and line position of the LO phonon mode. Raman analysis helped in understanding the phonon-phonon interaction mechanism in Zn0.96 Mg0.04S thin films.

Thermodynamic properties of geikielite (MgTiO3) and ilmenite (FeTiO3) derived from vibrational methods combined with Raman and infrared spectroscopic data

We present a model for the lattice vibrational density of states of MgTiO3 (geikielite) and FeTiO3 (ilmenite) that predicts thermodynamic properties, in agreement with observational data. The model is based on Kieffer’s method combined with spectroscopic data. For both substances experimental data sets are influenced by non-stoichiometry. For geikielite that affects the volume, whereas for ilmenite volume and bulk modulus are affected. We show that Kieffer’s method enables predicting bulk moduli in pressure–temperature space. We demonstrate that intrinsic anharmonicity or electronic effects significantly affect the heat capacity of ilmenite, whereas that is not the case for geikielite. We use Kieffer’s method to derive multiple-Einstein models, from which we demonstrate that thermodynamic properties are insignificantly influenced by dispersion in Grüneisen, mode-q and anharmonicity parameters for both substances. We show that our results enable predicting thermodynamic properties and shear modulus of the solid solution formed from geikielite and ilmenite. Geikielite and ilmenite are added to our thermodynamic database for the system MgO–SiO2–FeO, to enable modeling phase stability and physical properties of titanium-rich reservoirs in the Earth’s Moon.

Effects of Different Phonon Scattering Factors on the Heat Transport Properties of Graphene Ribbons.

Understanding the effect of phonon scattering is of primary significance in the study of the thermal transport properties of graphene. While phonon scattering negatively affects the thermal conductivity, the exact effect of microscopic phonon scattering is still poorly understood when full phonon dispersions are taken into account. The heat transport properties of graphene ribbons were investigated theoretically by taking into account different polarization branches with different frequencies in order to understand the physical mechanism of the thermal transport phenomenon at the nanoscale. The effects of grain size, chiral angle, Grüneisen anharmonicity parameter, specularity parameter, and mass-fluctuation-scattering parameter were evaluated, taking into account of the restrictions imposed by boundary, Umklapp, and isotope scattering mechanisms. The contribution from each phonon branch was estimated, and the anisotropic coefficients were determined accordingly. The results indicated that the graphene ribbons are very efficient at conducting heat in all the cases studied. All the acoustical branches contribute significantly to the heat transport properties, and the temperature strongly affects the relative contribution of the phonon branches. The lattice thermal conductivity varies periodically with the chiral angle. The maximum thermal conductivity is achieved in the zigzag direction, and the minimum thermal conductivity is obtained in the armchair direction. The lattice thermal conductivity and anisotropic coefficient depend heavily upon the roughness of the edges and the width of the ribbons. The specularity parameter and mass-fluctuation-scattering parameter significantly affect the lattice thermal conductivity, and the effect arising from isotope scattering is significant in the context of natural isotopic abundance. The dependence of the Grüneisen anharmonicity parameter on phonon branches must be taken into account when making predictions. The results have significant implications for the understanding of the relations between phonon scattering and thermal properties.

In situ Raman vibrational spectra of siderite (FeCO3) and rhodochrosite (MnCO3) up to 47 GPa and 1100 K

Abstract Siderite (FeCO3) and rhodochrosite (MnCO3) are two interesting carbonate minerals, which normally occur in hydrothermal deposits on deep-sea altered oceanic crust. Despite the ubiquity of carbonates in the slab, little is known of the physicochemical behavior of siderite and rhodochrosite at high-pressure (P) and high-temperature (T) conditions during slab subduction. In this study, we characterized the Raman vibrational spectra of natural siderite and rhodochrosite up to 47 GPa and 1100 K in an externally heated diamond-anvil cell (DAC). Experimental results show that the Raman frequency shifts (νi) for siderite and rhodochrosite are a function of both P and T, and the effect of the P-T cross derivative term cannot be neglected, especially at high-P and high-T conditions. Based on the functional relationship of νi-P-T, the P-T calibrants of siderite and rhodochrosite are developed, respectively. This is significant for studying the water-carbonate interaction at high P-T conditions in a DAC because the undesired change of the experimental system from traditional pressure sensors (e.g., ruby, quartz) in a reaction chamber can be avoided. Like previous studies, we observed a sharp spin transition at ~45 GPa in siderite and a phase transition from MnCO3-I to MnCO3-II at ~46 GPa for rhodochrosite at room temperature. Furthermore, we determined the isobaric and isothermal equivalents of the mode Grüneisen parameter (γiT, γiP) and the anharmonic parameter (ai) for each Raman mode of siderite and rhodochrosite. The δνi/δP, δνi/δT, γiT, γiP, and ai span a much larger value range for the external lattice modes (T, L) than internal modes (ν4, ν1) in both siderite and rhodochrosite. Combining Raman frequency shifts and the first-order Murnaghan equation of state, we also developed a method to calculate the temperature dependence of the bulk modulus (KT) for siderite and rhodochrosite, respectively.

Anharmonic Motion in the Crystal Structure of 2-Mercaptopyridone

With the present study, we revise earlier results about the title compound 2-mercaptopyridone in which the deformation density was determined from an X–N study in 1982 (X-ray, neutron diffraction). In the current X–X study (X-ray, X-ray), anharmonic motion parameters for the atoms were included and it resulted in a very clean deformation density map without the unexplainable features of the literature results. The presence of anharmonic effects is supported by a study of the thermal expansion in the temperature range 100–260 K. A topological study of the electron density confirms a bond length alternation in the six-membered ring. The calculation of the electrostatic potential indicates that the N–H⋯S hydrogen bond is an electrostatic interaction, and that other attractive intermolecular contacts are probably dispersive.

Zero-field 57Fe NMR in BiFeO3 based compounds: Problems, solutions and application to Bi1-xSrxFeO3

In this paper, we present a protocol designed to avoid typical problems occurring in the process of the spatial spin modulated structure (SSMS) investigation in BFO-based (BiFeO3) materials by zero-field nuclear magnetic resonance (ZF NMR) spectroscopy on 57Fe nuclei. We have analyzed the significant discrepancies in the parameters of the spectra in the literature and developed the measurement protocol to improve the quality and unify the obtained spectra. The study of the 57Fe spectra of Sr-doped BFO at 4.2 K samples was carried out to illustrate the protocol. The obtained spectra demonstrate suppression of the SSMS by heterovalent substitution. The Sr-concentration dependent evolution of the SSMS anharmonicity parameter was observed as well.

Elucidating the theoretical evolution of the IR spectral density and the potential energy surfaces of hydrogen bonded complexes: A quantum Dynamical study

Concerted potentials are proposed herein for the description of vibrational modes in hydrogen bonded systems to consistently determine the evolution of the infrared absorption spectrum from weak to strong hydrogen bonds. A double well potential constructed using two crossed Morse curves is particularly employed to describe the high stretching mode whereas the H-bond Bridge is designated using harmonic potential. Within the circumstances of the strong anharmonic coupling theory, a strenuous coupling between the high frequency (ω∘) and the H-bond Bridge (ω∘∘) modes is introduced. We consider in this work theoretical IR spectral density of the high frequency stretching mode of hydrogen bonds of the type X-H…Y within the frame of the linear response theory. For this purpose, several experimental correlations are introduced, namely the two angular frequencies ω∘(R) and ω∘∘(R), the barrier height V∘(R), the distance D(R) between the minima and the distance X-H, re (R), which are correlated to the R = RX−Y distance between the two bulks X and Y. The relaxation of the fast mode is incorporated using the theory rationalized by Rosh and Ratner. Fourier transform of the autocorrelation function of the dipole moment operator of the fast mode is employed to determine the IR spectral density. The evolution of the infrared absorption spectrum as well as the evolution of the energy surface potential from weak to strong H-bonds are demonstrated. Realistic evolution of the spectral density is found when weak anharmonic coupling parameters between the fast and slow modes are used. Altogether, our findings highlight the importance of the usage of crossed Morse potentials sampling in the quantum dynamics simulations of the IR spectral density of hydrogen bonded systems with different strengths.

Determination of vibrational parameters of -=SUP=-76-=/SUP=-GeH-=SUB=-4-=/SUB=- from precision experimental data

Harmonic frequencies, anharmonic parameters, and tetrahedral splitting parameters have been determined for the 76GeH4 molecule, based on experimental data of band centers. As a result, the set of 3 harmonic frequencies, 8 anharmonic parameters, and 8 tetrahedral splitting parameters reproduces 31 initial experimental band centers with a root mean square deviation 0.054 cm-1. Keywords: GeH4, vibrational parameters, local mode model.

Определение колебательных параметров молекулы -=SUP=-76-=/SUP=-GeH-=SUB=-4-=/SUB=- из высокоточных экспериментальных данных

Harmonic frequencies, anharmonic parameters, and tetrahedral splitting parameters have been determined for the 76GeH4 molecule, based on experimental data of band centers. As a result, the set of 3 harmonic frequencies, 8 anharmonic parameters, and 8 tetrahedral splitting parameters reproduces 31 initial experimental band centers with a root mean square deviation 0.054 cm-1.

Few Excited State Energies and Chemical Potential Energy of 87Rb Condensate by Many-Body Approach

We consider experimental 87Rb condensate which is trapped by harmonic plus quartic trap [V(r) = ½mω2r2 + λr4]. Keeping similarity with experiments, the anharmonic parameter (λ) is considered as a controllable parameter. The excited state energies of stable Bose-Einstein condensate are strongly influenced by the presence of an anharmonic term, even when the interatomic interaction is repulsive. The necessary dependencies of excited state energies of the trapped condensate on λ are discussed in detail. In addition, the variation of chemical potential energy as a function of λ is also investigated to explore the role of interaction. The many-particle Schrödinger equation is solved by the potential harmonic expansion method, where all possible two-body correlations are considered by utilizing the correlated two-body basis function. Specifically, we present a clear physical explanation of excited state energies and chemical potential energy of the experimental repulsive condensate confined by the anharmonic trap.

The phase composition and structure of the BiFeO-=SUB=-3-=/SUB=- film grown on MgO(001) substrate by high frequency cathode deposition in oxygen atmosphere

The crystal structure and Mossbauer spectroscopy studies results for BiFeO3 film growth on the MgO(001) single crystal substrate are present in the paper. It been shown that film have high crystal perfection and low defectiveness which results in appearing of narrow lines during the theta-2theta and φ scanning and the small (lower than 0.7o) disorientation of film and substrate crystal axes. It is been revealed that unit cell of BiFeO3/MgO(001) heterostructure possess monoclinic symmetry and deformation of unit cell is negligible. The Mossbauer study shows that magnetic subsystem of film has spatial spin-modulated structure with zero value of anharmonicity parameter (m). This indicate that at room temperature the magnetic anisotropy changes from the "easy axis" type to "easy plane" type. Keywords: thin films, bismuth ferrite, Mossbauer effect.

Фазовый состав и структура пленки BiFeO-=SUB=-3-=/SUB=-, выращенной на подложке MgO(001) методом ВЧ-катодного распыления в атмосфере O-=SUB=-2-=/SUB=-

The crystal structure and Mossbauer spectroscopy studies results for BiFeO3 film growth on the MgO(001) single crystal substrate are present in the paper. It been shown that film have high crystal perfection and low defectiveness which results in appearing of narrow lines during the θ-2θ and φ scanning and the small (lower than 0.7°) disorientation of film and substrate crystal axes. It is been revealed that unit cell of BiFeO3/MgO(001) heterostructure possess monoclinic symmetry and deformation of unit cell is negligible. The Mossbauer study shows that magnetic subsystem of film has spatial spin-modulated structure with zero value of anharmonicity parameter (m). This indicate that at room temperature the magnetic anisotropy changes from the "easy axis" type to "easy plane" type.

Implementation of Morse potential for approximation of vibrational terms of diatomic molecules

We analysed the problem of approximation of the potential function of a diatomic molecule by a Morse model function with constant anharmonicity ν x using the Birge-Sponer extrapolation. The analysis of the approximations used in the derivation of the Morse equation shows that the solution of this problem is ambiguous. A scheme for optimizing the selection of initial parameters is proposed, which is illustrated by examples taken from the literature. The advantages of delineation of anharmonicity in the excitation of vibrational levels by deviations of the value ν x from the constant value according to Morse are demonstrated. An attempt is made to use the dimensionless anharmonicity parameter x* as a universal characteristic of the shape features of the electronic term of the molecule. Keywords: Morse potential, diatomic molecule, Birge-Sponer extrapolation, anharmonicity, electronic terms, vibrational structure.

Laser-induced fluorescence experimental spectroscopy and theoretical calculations of uranium monoxide

As a model molecule of actinide chemistry, UO molecule plays an important role in understanding the electronic structure and chemical bonding of actinide-containing species. We report a study of the laser-induced fluorescence spectra of the U16O and U18O using two-dimensional spectroscopy. Several rotationally resolved excitation spectra were investigated. Accurate molecular rotational constants and equilibrium internuclear distances were reported. Low-lying electronic states information was extracted from high resolution dispersed fluorescence spectra and analyzed by the ligand field theory model. The configuration of the ground state was determined as U2+(5f37s)O2−. The branching ratios, and the vibrational harmonic and anharmonic parameters were also obtained. Radiative lifetimes were determined by recording the time-resolved fluorescence spectroscopy. Transition dipole moments were calculated using the branching ratios and the radiative lifetimes. These findings were elucidated by using quantum-chemical calculations, and the chemical bonding was also analyzed. The findings presented in this work will enrich our understanding of actinide-containing molecules.

Estimation of anharmonic parameters of nano-crystalline Sc2O3 and Nd2O3

We report anharmonic characteristics of cubic Scandium Oxide (Sc2O3) and hexagonal Neodymium Oxide (Nd2O3) through temperature dependent phonon mode variation in the temperature range 80–440 K. The samples were characterized as nanocrystalline and stoichiometric with crystallite sizes of 31 nm for Sc2O3 and 59 nm for Nd2O3. Under ambient conditions, Sc2O3 was found to have 10 phonon modes out of expected 22 Raman modes while Nd2O3 showed all 4 predicted phonon modes. The temperature dependent shift in wavenumber of Raman modes and broadening of the phonon modes has been analysed in light of thermal and anharmonic effects. The lattice expansion and strong anharmonic interactions were observed with an increase in temperature that resulted in measurable phonon softening. However, both samples were found to have stable phases within the studied temperature range. The total anharmonicity and anharmonic parameters along with phonon lifetimes are estimated. For the most intense phonon modes of Sc2O3 and Nd2O3, the total anharmonicity is calculated as − 3.44 × 10−5 K−1 and − 5+.47 × 10−5 K−1 respectively. The estimated anharmonic constants revealed the predominance of the three phonon process for decay as a function of temperature.

Study of Zero Temperature Ground State Properties of the Repulsive Bose-Einstein Condensate in an Anharmonic Trap

The zero-temperature ground state properties of experimental 87Rb condensate are studied in a harmonic plus quartic trap [ V(r) = ½mω2r2 + λr4 ]. The anharmonic parameter (λ) is slowly tuned from harmonic to anharmonic. For each choice of λ, the many-particle Schrödinger equation is solved using the potential harmonic expansion method and determines the lowest effective many-body potential. We utilize the correlated two-body basis function, which keeps all possible two-body correlations. The use of van der Waals interaction gives realistic pictures. We calculate kinetic energy, trapping potential energy, interaction energy, and total ground state energy of the condensate in this confining potential, modelled experimentally. The motivation of the present study is to investigate the crucial dependency of the properties of an interacting quantum many-body system on λ. The average size of the condensate has also been calculated to observe how the stability of repulsive condensate depends on anharmonicity. In particular, our calculation presents a clear physical picture of the repulsive condensate in an anharmonic trap.