Experimental Phonon Spectra Research Articles

Crystal effects in the vibrational spectra of one-dimensional molecular spin crossover crystals using molecular dynamics simulations

We use molecular dynamics simulations to model the iron phonon density of states (pDOS) of a spin crossover (SCO) crystal built from 39-nuclear [Fe(atrz)_3Cl_2] chains (atrz = 4-amino-1,2,4-triazole). This is possible by using an atomistic potential that depends on the spin state of the Fe atoms. While quantum-chemical methods are able to provide data on isolated SCO molecules, the computational costs necessitate the use of classical simulations to calculate crystal effects on the spectra. Data are provided for a high-spin (HS) crystal, in which all Fe atoms are in their HS state, and a low-spin (LS) crystal. HS and LS crystals show distinctively different spectra that are in agreement with experimental data. Compared to the single-molecule spectra, the crystal spectra show several distinctive features: (i) The spectra are smoother, and the main peaks are slightly reduced in amplitude. (ii) The effect of intermolecular interactions shows up in additional structure at long-wavelengths. Good agreement between experimental and simulated phonon spectra is observed. The lattice-dynamics analysis of the low-energy pDOS allows to extract the temperature dependence of the vibrational entropy of the LS and HS crystals. Our study thus provides novel information about the crystal effects in SCO crystals.

Raman- and Infrared-Active Phonons in Nonlinear Semiconductor AgGaGeS4

AgGaGeS4 is an emerging material with promising nonlinear properties in the near- and mid-infrared spectral ranges. Here, the experimental phonon spectra of AgGaGeS4 single crystals synthesized by a modified Bridgman method are presented. The infrared absorption spectra are reported. They are obtained from the fitting of reflectivity to a model dielectric function comprising a series of harmonic phonon oscillators. In the Raman spectra, several modes are registered, which were not detected in previous works. The analysis of the experimental vibrational bands is performed on the basis of a comparison with reported data on structurally related binary, ternary, and quaternary metal chalcogenides. The temperature dependence of the Raman spectra between room temperature and 15 K is also investigated.

Thermal scattering law for ice based on neutron time-of-flight experiments carried out at the SEQUOIA spectrometer at the Oak Ridge National Laboratory

Precise estimation of neutron thermalization in moderators relies on high-fidelity thermal scattering cross-section data governed by the thermal scattering law (TSL). The Institut de Radioprotection et de Sûreté Nucléaire (IRSN) has been working on the development of improved TSL for light water ice. Many polymorphic phases of light water ice exist depending on the thermodynamic conditions. The most common type of ice at standard pressure and temperature below water freezing point (273.15 K) is ice-Ih. It is essential to have high-resolution experimental double differential data for developing and/or validating TSL for moderator materials.Existing experimental double-differential scattering data for ice-Ih are extremely sparse and of limited quality. New high-quality double-differential measurements for ice-Ih over multiple temperatures and incident neutron energies would directly support the validation and improvement of ice-Ih TSL models for criticality safety applications. Series of time-of-flight (TOF) inelastic neutron scattering experiments on ice-Ih at temperatures starting at 271 K and down to 6 K, have been carried out at the SEQUOIA spectrometer at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL), United States. The experiments have been performed for incident neutron energies, Ei= 11, 55, 160, 250, and 600 meV, to explore different excitation energies in the vibrational phonon spectrum. This paper presents the thermodynamic conditions and the details of the TOF measurements on ice-Ih and the derived phonon spectrum from the experimentally measured double differential data. A study of the variation of the phonon spectrum of ice-Ih as a function of temperature is highlighted, and a preliminary TSL evaluation is developed based on the experimental phonon spectrum.

Strongly two-dimensional exchange interactions in the in-plane metallic antiferromagnet Fe2As probed by inelastic neutron scattering

To understand spin interactions in materials of the Cu$_2$Sb structure type, inelastic neutron scattering of Fe$_2$As single crystals was examined at different temperatures and incident neutron energies. The experimental phonon spectra match well with the simulated phonon spectra obtained from density functional theory (DFT) calculations. The measured magnon spectra were compared to the simulated magnon spectra obtained via linear spin wave theory with the exchange coupling constants calculated using the spin polarized, relativistic Korringa-Kohn-Rostoker method in Zhang et al. (2013). The simulated magnon spectra broadly agree with the experimental data although, the energy values are underestimated along the $K$ direction. Exchange coupling constants between Fe atoms were refined by fits to the experimental magnon spectra, revealing stronger nearest neighbor Fe1-Fe1 exchange coupling than previously reported. The strength of this exchange coupling is almost an order of magnitude higher than other exchange interactions despite the three-dimensional nature of the phonon interactions. The lack of scattering intensity at energies above 60 meV makes unconstrained determination of the full set of exchange interactions difficult, which may be a fundamental challenge in metallic antiferromagnets.

Phononic bandgap and phonon anomalies in HfN and HfN/ScN metal/semiconductor superlattices measured with inelastic x-ray scattering

Epitaxial metal/semiconductor superlattice heterostructures with lattice-matched abrupt interfaces and suitable Schottky barrier heights are attractive for thermionic energy conversion, hot electron-based solar energy conversion, and optical hyperbolic metamaterials. HfN/ScN is one of the earliest demonstrations of epitaxial single-crystalline metal/semiconductor heterostructures and has attracted significant interest in recent years to harness its excellent properties in device applications. Although the understanding of the mechanism of thermal transport in HfN/ScN superlattices is extremely important for their practical applications, not much attention has been devoted to measuring their phonon dispersion and related properties. In this Letter, we employ non-resonant meV-resolution inelastic x-ray scattering to determine the momentum-dependent phonon modes in epitaxial metallic HfN and lattice-matched HfN/ScN metal/semiconductor superlattices. HfN exhibits a large phononic bandgap (∼40 meV) and Kohn anomaly in the longitudinal and transverse acoustic phonon modes at q ∼ 0.73 along the [100] and [110] directions of the Brillouin zone due to the nesting of the Fermi surface by the wave vector (q). The in-plane [100] acoustic phonon dispersion of the HfN/ScN superlattices is found to be dominated by the HfN phonons, while the optical phonons exhibit both ScN and HfN characteristics. First-principles density functional perturbation theory modeling is performed to explain the experimental phonon spectra, and temperature-dependent thermal conductivity is measured using a pump-probe spectroscopic technique. These results will help understand the phonons in HfN and HfN/ScN metal/semiconductor superlattices for thermionic energy conversion.

Toward a better thermal scattering law of (C5O2H8)n: Inelastic neutron scattering and oClimax + NJOY2016

With the advancements in technology (both experimental and computational) the determination of the“true” experimental phonon spectrum became more accessible. In this work a methodology for producing thermal scattering libraries from the experimental data (namely the DFT + oClimax method) for lucite (C5O2H8)n is discussed. Double differential scattering cross section (DDSCS) experiments were performed at the Spallation Neutron Source of Oak Ridge National Laboratory (SNS ORNL). New scattering kernel evaluations, based on the phonon spectrum for (C5O2H8)n,were created using oClimax and NJOY2016 codes. In order to compare and asses the performance of the newly created library, the experimental setup was simulated using MCNP6.1. Compared to the current ENDF/B-VIII.0, the resulting RPI (C5O2H8)n library improved the calculation of both double differential scattering and total scattering cross sections. A set of criticality benchmarks containing (C5O2H8)n from HEU-MET-THERM resulted in an overall improved calculation of Keff. The DFT + oClimax method is shown to be the most comprehensive method for analysis of moderator materials, due to the fact that it can be verified against all data measured at VISION, ARCS and SEQUOIA neutron spectrometers at SNS ORNL, and experimental total scattering cross section measurements. This method also provides a new technique for calculating any phonon spectrum-related quantities such as scattering law kernel, specific heat capacity, thermal conductivity, etc. for any solid state material.

Neutron Spectroscopy of the Atomic Dynamics of La2Zr2O7 at Fluorite–Pyrochlore Structural Transformations

The formation of the structural and dynamic properties of La2Zr2O7 in the process of crystallization at the isothermal annealing of initially amorphous precursors obtained by the coprecipitation of corresponding salts has been studied by neutron spectroscopy. The existence of vibrational states characteristic of hydrogen, which is in one or another of the possible chemical states and is incorporated into a solid matrix, has been detected in the spectra of amorphous and fluorite phases. The DFT calculation of the phonon density of states has been performed to analyze the energy structure of experimental phonon spectra for various phases of the La2Zr2O7 compound. The amount of hydrogen in the fluorite phase has been estimated.

High-throughput density-functional perturbation theory phonons for inorganic materials

The knowledge of the vibrational properties of a material is of key importance to understand physical phenomena such as thermal conductivity, superconductivity, and ferroelectricity among others. However, detailed experimental phonon spectra are available only for a limited number of materials, which hinders the large-scale analysis of vibrational properties and their derived quantities. In this work, we perform ab initio calculations of the full phonon dispersion and vibrational density of states for 1521 semiconductor compounds in the harmonic approximation based on density functional perturbation theory. The data is collected along with derived dielectric and thermodynamic properties. We present the procedure used to obtain the results, the details of the provided database and a validation based on the comparison with experimental data.

Low-temperature heat capacities of Y2GaSbO7 and R2FeSbO7 (R = Y, Dy) pyrochlore compounds: Quasi-harmonic phonons calculation

Abstract Heat capacities at constant pressure, C°p,m, of three polycrystalline pyrochlore compounds, Y2GaSbO7, Y2FeSbO7 and Dy2FeSbO7, have been measured in the temperature range up to 300 K. Temperature variation of the Debye temperatures, θD(T), for Y2GaSbO7 is determined using a quasi-harmonic expression of Debye lattice heat capacity, which has been employed to estimate the phonon heat capacity, Cph, for the other two compounds, e.g., Y2FeSbO7 and Dy2FeSbO7. Theoretical analysis using phonon model shows that the acoustic and optic phonons contribute significantly to the thermodynamic properties. The effect of anharmonicity in the lattice vibrations at higher temperature (above 220 K) was also considered for Y2GaSbO7. The Cph of Y2FeSbO7, calculated using the experimental infra-red and Raman phonon spectra, are in good agreement with the measured heat capacity data and thus provides a good taste of understanding to the molecular dynamics in the pyrochlore structure. The Schottky heat capacity has been deduced from the crystal-field level scheme of Dy3+ ions and added to the Cph of Dy2FeSbO7 to estimate the total heat capacity, which is in reasonable agreement with the experimental heat capacity, C°p,m, of Dy2FeSbO7. From the integrated heat capacity data, the entropy and enthalpy relative to absolute zero have been generated for Y2GaSbO7 and Y2FeSbO7. The room-temperature thermal conductivity was estimated to be ∼2 W⋅m−1⋅K−1 for Y2GaSbO7 implying that Y2GaSbO7 could be a potential candidate for thermal barrier coating material.

Impacts of potential models on calculating the thermal conductivity of graphene using non-equilibrium molecular dynamics simulations

This work employs non-equilibrium molecular dynamics (NEMD) simulations to examine the applicability of four kinds of interatomic potential models: the Tersoff, the REBO, the opt-Tersoff and the AIREBO, which are widely used to model the thermal transport in single- and multi-layer graphene, as well as graphite crystallites. Thermal conductivities of ∼17×5nm2 and ∼50×5nm2 graphene are calculated in the temperature range of 200∼500K with the four potentials and quantum correction is applied due to an extremely high Debye temperature of about 2100K for graphene. The predicted thermal conductivities are compared with experimental data and phonon spectrum functions are calculated to quantify the degree of phonon scattering. The results show that two original potentials, the Tersoff and the REBO, as well as the AIREBO significantly underestimate thermal conductivities of single-layer graphene but they can qualitatively describe the trend of thermal conductivities with temperature. The opt-Tersoff is found to be the most suitable potential for modeling the thermal conductivity of both single- and multi-layer graphene because it predicts a larger frequency range and a larger frequency value for the high frequency peak, while appropriately capturing phonon scattering in thicker multi-layer graphene when Lennard-Jones term is added into the opt-Tersoff to describe interlayer atomic interactions.

The calculation of radial and tangential force constants for nickel using an experimental phonon spectrum

The possibility of calculating radial and tangential force constants, as well as elastic constants of face-centered cubic (FCC) metals, has been demonstrated for the case study of nickel on the basis of the experimentally measured phonon spectrum, which is theoretically described by the DeLaunay model in terms of the dynamic matrix D(q). The proposed calculation procedure is an alternative to the Born–von Karman model, which is widely applied for the description of the phonon spectrum in terms of force constants for interatomic interactions and in calculations of elastic constants using the phonon spectrum. The reliability of the obtained force constants for phonon spectrum was verified by comparison between the experimental and calculated data for the phonon spectrum and elastic constants for nickel. The obtained values of force constants for nickel have been compared with the theoretically calculated values using the pseudopotential method.

Thermal evolution of vibrational properties ofα-U

By means of ab initio molecular dynamics calculations, the thermal evolution of vibrational properties in $\ensuremath{\alpha}$-U is studied at low temperature. The phase transition undergone by this material around 50 K was previously studied extensively using ab initio calculations in the framework of the linear response at 0 K. Although these previous efforts capture successfully the complexity of the experimental phonon spectrum at room temperature, in particular the soft-phonon mode and its pressure dependence, they fail to reproduce the transition to the charge-density-wave state at ambient pressure as a function of temperature. In the present work, by going beyond the quasiharmonic approximation and taking into account the temperature effects explicitly, we are able to reproduce the behavior of both phonon spectrum and elastic constants of U-$\ensuremath{\alpha}$ as a function of temperature.

Calculation of the radial and tangential force constants of cesium from the experimental phonon spectrum

By the example of cesium, a procedure for calculating the radial and tangential force constants of the interatomic interaction of metals with the body-centered cubic (BCC) structure from the experimentally measured phonon spectrum, which can be theoretically described using elements of the dynamic matrix D(q), is proposed. This technique is an alternative to the Born–von Karman model, which is still widely applied in describing the phonon spectrum in terms of the force constants of interatomic interaction and in calculation of the elastic constants of the phonon spectrum. The reliability of the procedure and obtained values of the force constants is verified by calculating the phonon spectrum and elastic constants of cesium and by comparing the results with experimental data.

Dynamic flexoelectric effect in perovskites from first-principles calculations

Using the dynamical matrix of a crystal obtained from ab initio calculations, we have evaluated for the first time the strength of the dynamic flexoelectric effect and found it comparable to that of the static bulk flexoelectric effect, in agreement with earlier order-of-magnitude estimates. We also proposed a method of evaluation of these effects directly from the simulated phonon spectra. This method can also be applied to the analysis of the experimental phonon spectra, being currently the only one enabling experimental characterization of the static bulk flexoelectric effect.

Model for the Specific Heat of Uranium Dioxide

The existing models of the specific heat of uranium dioxide are analyzed. A new physical model of the specific heat of uranium dioxide is developed on the basis of experimental phonon spectra by the methods of the theory of solids. The model proposed is applicable in a wide temperature interval and has no adjustable parameters.

Nonharmonic phonons inα-iron at high temperatures

Phonon densities of states (DOS) of bcc $\alpha$-$^{57}$Fe were measured from room temperature through the 1044K Curie transition and the 1185K fcc $\gamma$-Fe phase transition using nuclear resonant inelastic x-ray scattering. At higher temperatures all phonons shift to lower energies (soften) with thermal expansion, but the low transverse modes soften especially rapidly above 700K, showing strongly nonharmonic behavior that persists through the magnetic transition. Interatomic force constants for the bcc phase were obtained by iteratively fitting a Born-von K\'{a}rm\'{a}n model to the experimental phonon spectra using a genetic algorithm optimization. The second-nearest-neighbor fitted axial force constants weakened significantly at elevated temperatures. An unusually large nonharmonic behavior is reported, which increases the vibrational entropy and accounts for a contribution of 35 meV/atom in the free energy at high temperatures. The nonharmonic contribution to the vibrational entropy follows the thermal trend of the magnetic entropy, and may be coupled to magnetic excitations. A small change in vibrational entropy across the $\alpha$-$\gamma$ structural phase transformation is also reported.

Negative thermal expansion in cubic ZrW2O8: Role of phonons in the entire Brillouin zone fromab initiocalculations

We report the ab initio density functional theory calculation of phonons in the cubic phase of ZrW${}_{2}$O${}_{8}$ in the entire Brillouin zone and identify specific anharmonic phonons that are responsible for large negative thermal expansion (NTE) in terms of the translation, rotation, and distortion of WO${}_{4}$ and ZrO${}_{6}$. We have used density functional calculations to interpret the experimental phonon spectra as a function of pressure and temperature as reported in literature. We discover that the phonons showing anharmonicity with temperature are not necessarily the same as those showing anharmonicity with pressure although both are of similar frequencies. Only the latter phonons are associated with NTE. Therefore, the cubic and/or quadratic anharmonicity of phonons is not relevant to NTE but just the volume dependence of frequencies. The calculations are able to reproduce the observed anomalous trends, namely, the softening of the low-frequency peak at about 4 meV in the phonon spectra with pressure and its hardening with temperature, whereas, both changes involve a compression of the lattice.

Electron-phonon interaction and superconductivity in Tl-Pb-Bi alloys from first principles: Importance of spin-orbit coupling

The importance of spin-orbit coupling for lattice dynamics, electron-phonon coupling, and superconducting properties of face-centered cubic Tl-Pb-Bi alloys is investigated from first principles based on density-functional perturbation theory and a mixed-basis pseudopotential method. The alloys are modeled by the virtual crystal approximation. Electron-phonon coupling and superconducting properties are analyzed within the framework of the strong-coupling Eliashberg theory. A large increase of the coupling constant $\ensuremath{\lambda}$ for Bi doping can be traced back to a progressive softening of the phonon spectrum, signaling an incipient lattice instability. Performing calculations with and without spin-orbit interaction, we found a profound influence of this relativistic correction on the coupling strength, enhancing in most cases the coupling constant $\ensuremath{\lambda}$ by as much as 50$%$, but reducing it for Tl-rich alloys. For the whole alloy series, spin-orbit interaction impressively improves the agreement with both experimental phonon spectra as well as with superconducting quantities obtained from tunneling spectroscopy measurements, which at the same time demonstrates the applicability of the virtual crystal approximation.

High-pressure phases of crystalline tellurium: A combined Raman andab initiostudy

A room temperature study of the high-pressure phases of crystalline Te was carried out by combining Raman spectroscopy and density functional theory (DFT)-based calculations. The pressure dependence of the experimental phonon spectrum reveals the occurrence of phase transitions, confirming the high-pressure scenario recently proposed by Hejny and McMahon. The effects of the incommensurate lattice modulation on the vibrational properties of Te are discussed. DFT-based calculations are consistent with the present and previous experimental data and show that the metallization process at 4 GPa is due to charge-bridges between atoms belonging to adjacent chains. A first-principles study of the stability of the 4 GPa phase is here reported and discussed against the insurgence of lattice modulation.

Inelastic neutron scattering an ab-initio calculation of negative thermal expansion in Ag2O

The compound Ag2O undergoes large and isotropic negative thermal expansion over 0–500K. We report temperature dependent inelastic neutron scattering measurements and ab-initio calculations of the phonon spectrum. The temperature dependence of the experimental phonon spectrum shows strong anharmonic nature of phonon modes of energy around 2.4meV. The ab-initio calculations reveal that the maximum negative Grüneisen parameter, which is a measure of the relevant anharmonicity, occurs for the transverse phonon modes that involve bending motions of the Ag4O tetrahedra. The thermal expansion is evaluated from the ab-initio calculation of the pressure dependence of the phonon modes, and found in good agreement with available experimental data.