Partial Phonon Density Of States Research Articles

Phonon density of states of single-crystalSrFe2As2across the collapsed phase transition at high pressure

To help our understanding of the structural and superconducting transitions in ferropnictides, partial phonon density of states (PDOS) of iron in a single-crystal $\mathrm{SrF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ pnictide have been investigated from both out-of-plane and in-plane polarizations with respect to the basal plane of the crystal structure using nuclear resonant inelastic x-ray scattering in a high-pressure diamond anvil cell at ambient temperature. The partial PDOS of iron in the pnictide crystal changes dramatically at approximately 8 GPa, which can be associated with the tetragonal (T) to collapsed tetragonal (CT) isostructural transition as evidenced in high-pressure x-ray diffraction measurements and theoretical calculations. Across the T-CT phase transition, analysis of the PDOS spectra shows a rapid stiffening of the optical phonon modes and a dramatic increase of the Lamb-M\ossbauer factor (${f}_{\mathrm{LM}}$) and mean force constant which can be associated with the rapid decrease of the $c$ axis and the anomalous expansion of the $a$ axis. Theoretically calculated Fe partial PDOS and lattice parameters of $\mathrm{SrF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ further reveal the strong correlation between the lattice parameters and phonons. Our results show that the T-CT transition can induce significant changes in the vibrational, elastic, and thermodynamic properties of $\mathrm{SrF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ single crystal at high pressure.

First-Principles Study of Lattice Dynamics, Structural Phase Transition, and Thermodynamic Properties of Barium Titanate

Abstract Lattice dynamics, structural phase transition, and the thermodynamic properties of barium titanate (BaTiO3) are investigated by using first-principles calculations within the density functional theory (DFT). It is found that the GGA-WC exchange-correlation functional can produce better results. The imaginary frequencies that indicate structural instability are observed for the cubic, tetragonal, and orthorhombic phases of BaTiO3 and no imaginary frequencies emerge in the rhombohedral phase. By examining the partial phonon density of states (PDOSs), we find that the main contribution to the imaginary frequencies is the distortions of the perovskite cage (Ti-O). On the basis of the site-symmetry consideration and group theory, we give the comparative phonon symmetry analysis in four phases, which is useful to analyze the role of different atomic displacements in the vibrational modes of different symmetry. The calculated optical phonon frequencies at Γ point for the four phases are in good agreement with other theoretical and experimental data. The pressure-induced phase transition of BaTiO3 among four phases and the thermodynamic properties of BaTiO3 in rhombohedral phase have been investigated within the quasi-harmonic approximation (QHA). The sequence of the pressure-induced phase transition is rhombohedral→orthorhombic→tetragonal→cubic, and the corresponding transition pressure is 5.17, 5.92, 6.65 GPa, respectively. At zero pressure, the thermal expansion coefficient α V , heat capacity C V , Grüneisen parameter γ, and bulk modulus B of the rhombohedral phase BaTiO3 are estimated from 0 K to 200 K.

On the mean kinetic energy of the proton in strong hydrogen bonded systems.

The mean atomic kinetic energies of the proton, Ke(H), and of the deuteron, Ke(D), were calculated in moderate and strongly hydrogen bonded (HB) systems, such as the ferro-electric crystals of the KDP type (XH2PO4, X = K, Cs, Rb, Tl), the DKDP (XD2PO4, X = K, Cs, Rb) type, and the X3H(SO4)2 superprotonic conductors (X = K, Rb). All calculations utilized the simulated partial phonon density of states, deduced from density functional theory based first-principle calculations and from empirical lattice dynamics simulations in which the Coulomb, short range, covalent, and van der Waals interactions were accounted for. The presently calculated Ke(H) values for the two systems were found to be in excellent agreement with published values obtained by deep inelastic neutron scattering measurements carried out using the VESUVIO instrument of the Rutherford Laboratory, UK. The Ke(H) values of the M3H(SO4)2 compounds, in which the hydrogen bonds are centro-symmetric, are much lower than those of the KDP type crystals, in direct consistency with the oxygen-oxygen distance ROO, being a measure of the HB strength.

First-Principles-Based Phonon Calculation and Raman Spectroscopy Measurement of RuGa<sub>2</sub> and RuAl<sub>2</sub> with High Thermoelectric Power Factors

TiSi2type intermetallic compounds RuGa2 and RuAl2 have narrow band gaps of ～0.3 eV and a relatively large power factor at 600900 K. However, the maximum dimensionless figure of merit, ZT, are 0.5 and 0.2 for RuGa2 and RuAl2, respectively, due to a high lattice thermal conductivity. We investigated the phonon properties of these compounds by using a firstprinciples calculation and Raman spectroscopy to develop a further reduction scheme of the lattice thermal conductivity. Phonon dispersion relations and density of states were obtained from a realspace force constants method based on supercells with finite displacements. The calculated zonecenter wavenumbers were comparable to the experimental Raman wavenumbers. Besides, the phonon group velocities of acoustic branches were in accord with the experimental transverse and longitudinal speed of sound. The differences of phonon dispersion between RuGa2 and RuAl2 are explained by the fact that Ga is heavier and weaker bonding element than Al. According to the calculated partial phonon density of states, acoustic phonon modes of RuGa2 and RuAl2 are attributed to Ga site and Ru site, respectively. Heavier atom substitution for these sites can effectively reduce averaged phonon group velocity and lattice thermal conductivity. [doi:10.2320/jinstmet.JA201510]

Yttrium aluminium garnet under pressure: Structural, elastic, and vibrational properties from ab initio studies

The structural, elastic, and vibrational properties of yttrium aluminum garnet Y3Al5O12 are studied under high pressure by ab initio calculations in the framework of the density functional theory. The calculated ground state properties are in good agreement with the available experimental data. Pressure dependences of bond length and bulk moduli of the constituent polyhedra are reported. The evolution of the elastic constants and the major elastic properties, Young and shear modulus, Poisson's ratios, and Zener anisotropy ratio, are described. The mechanical stability is analyzed, on the light of “Born generalized stability criteria,” showing that the garnet is mechanically unstable above 116 GPa. Symmetries, frequencies, and pressure coefficients of the Raman-active modes are discussed on the basis of the calculated total and partial phonon density of states, which reflect the dynamical contribution of each atom. The relations between the phonon modes of Y3Al5O12 and the internal and external molecular modes of the different polyhedra are discussed. Infrared-active modes, as well as the silent modes, and their pressure dependence are also investigated. No dynamical instabilities were found below 116 GPa.

Lattice dynamics of bismuth-deficient BiFeO3 from first principles

Lattice dynamic properties of bismuth ferrite with 6% bismuth vacancies have been investigated in present work for the first time. Hellmann–Feynman forces of each atom were calculated by direct method in the framework of density functional theory, and strong electron correlations of Fe ion were taken into account by applying the Hubbard potential U and the exchange interaction J. Based on the analysis of partial phonon density of states, effective force constants of Fe–O bond are influenced by the altered nearest coordination of oxygen due to bismuth vacancy. This behavior is connected with the soft phonon modes observed in the low energy range of phonon dispersion curve of nonstoichiometric Bi0.94FeO3. Low-temperature heat capacities are showing essential difference between nonstoichiometric and stoichiometric system that optic phonon modes begin contribute to the heat capacity from 3K in Bi0.94FeO3, earlier than 7K in BiFeO3.

Effect of ferromagnetic ordering on phonons in KCo2Se2

Results of the density functional theory studies of the phonon dynamics in the ternary layered cobalt diselenide are reported. The partial phonon densities of states due to vibrations of K, Co, and Se atoms are analysed in detail. They indicate that phonons associated with the dynamics of Co and Se ions within the [Co2Se2] structural blocks span the entire spectral range extending to 260 cm−1, whereas phonons from the K-sublattice remain limited to the frequency range of 80–150 cm−1. The phonons conform with structural features of the quasi-2D layered structure of KCo2Se2. Ferromagnetic order in the Co-sublattice is shown to determine to a great extent the phonon densities of states, the Raman and infrared spectra of KCo2Se2. The in-planar magnetic interactions are responsible for pronounced softening of the high-frequency phonon modes and lead to disappearance of the low-frequency Raman-active mode of the Eg symmetry. The observed behavior of the Raman-active and infrared-active modes suggests rather strong spin-phonon coupling in KCo2Se2. Results of the present investigations allow to clarify the origin of substantial differences between dynamical properties of the ferromagnetic Co-based and the paramagnetic Ni-based ternary layered dichalcogenides, both adopting the ThCr2Si2-type structure.

Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia

Inelastic neutron scattering measurements on monoclinic zirconia (ZrO_2) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373wK. Temperature-dependent phonon densities of states (DOS) are reported, as are Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Gruneisen parameters. These mode Gruneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.

高パワーファクターを有するRuGa2とRuAl2の第一原理フォノン計算とラマン測定

TiSi2-type intermetallic compounds RuGa2 and RuAl2 have narrow band gaps of ~0.3 eV and a relatively large power factor at 600–900 K. However, the maximum dimensionless figure of merit, ZT, are 0.5 and 0.2 for RuGa2 and RuAl2, respectively, due to a high lattice thermal conductivity. We investigated the phonon properties of these compounds by using a first-principles calculation and Raman spectroscopy to develop a further reduction scheme of the lattice thermal conductivity. Phonon dispersion relations and density of states were obtained from a real-space force constants method based on supercells with finite displacements. The calculated zone-center wavenumbers were comparable to the experimental Raman wavenumbers. Besides, the phonon group velocities of acoustic branches were in accord with the experimental transverse and longitudinal speed of sound. The differences of phonon dispersion between RuGa2 and RuAl2 are explained by the fact that Ga is heavier and weaker bonding element than Al. According to the calculated partial phonon density of states, acoustic phonon modes of RuGa2 and RuAl2 are attributed to Ga site and Ru site, respectively. Heavier atom substitution for these sites can effectively reduce averaged phonon group velocity and lattice thermal conductivity.

Structural, elastic and vibrational properties of nanocrystalline lutetium gallium garnet under high pressure.

An ab initio study of the structural, elastic and vibrational properties of the lutetium gallium garnet (Lu3Ga5O12) under pressure has been performed in the framework of the density functional theory, up to 95 GPa. Pressure dependence of the elastic constants and the mechanical stability are analyzed, showing that the garnet structure is mechanically unstable above 87 GPa. Lattice-dynamics calculations in bulk at different pressures have been performed and compared with Raman scattering measurements of the nanocrystalline Tm(3+)-doped Lu3Ga5O12 up to 60 GPa. The theoretical frequencies and pressure coefficients of the Raman active modes for bulk Lu3Ga5O12 are in good agreement with the experimental data measured for the nano-crystals. The contributions of the different atoms to the vibrational modes have been analyzed based on the calculated total and partial phonon density of states. The vibrational modes have been discussed in relation to the internal and external modes of the GaO4 tetrahedron and the GaO6 octahedron. The calculated infrared modes and their pressure dependence are also reported. Our results show that with this nano-garnet size the sample has essentially bulk properties.

Anharmonicity and atomic distribution of SnTe and PbTe thermoelectrics

The structure and lattice dynamics of rock-salt thermoelectric materials SnTe and PbTe are investigated with single crystal and powder neutron diffraction, inelastic neutron scattering (INS), and first-principles simulations. Our first-principles calculations of the radial distribution function (RDF) in both SnTe and PbTe show a clear asymmetry in the first nearest-neighbor (1NN) peak, which increases with temperature, in agreement with experimental reports (Ref. 1,2). We show that this peak asymmetry for the 1NN Sn–Te or Pb–Te bond results from large-amplitude anharmonic vibrations (phonons). No atomic off-centering is found in our simulations. In addition, the atomic mean square displacements derived from our diffraction data reveal stiffer bonding at the anion site, in good agreement with the partial phonon densities of states from INS, and first-principles calculations. In conclusion, these results provide clear evidence for large-amplitude anharmonic phonons associated with the resonant bonding leading to the ferroelectric instability.

Ab initio phonon dynamics in the layered ternary diselenide KNi2Se2

Abstract Dynamical properties of KNi 2 Se 2 lattice are investigated within density functional theory and the approximation of harmonic phonons. Partial phonon densities of states indicate that Ni and Se vibrations span the entire spectral range extending to 27 meV, whereas K vibrations are limited to 11–18 meV. It is shown that inelastic neutron scattering spectra reflect the dynamics of Ni atoms within [Ni 2 Se 2 ] blocks and the high-energy E g mode is hardly observed in the Raman spectra due to its low intensity. The electronic contribution to the heat capacity of 47 mJ mol − 1 K − 2 is estimated at very low temperatures.

Nanoscale phononic interconnects in THz frequencies.

Phononic computing is emerging as an alternative computing paradigm to the conventional electronic and optical computing. In this study, we propose and analyze various phononic interconnects, such as nano-scaled phononic resonators, waveguides and switches, on the 〈111〉 surface of 3C-SiC and 3C-GeSi with substitutional and vacancy defects. This is achieved by simultaneously introducing defects of various types, and by varying their specific locations on the surface. To calculate the intrinsic and the defect-induced vibrational properties, such as the phononic bandgap and the variation in the phonon spectra, the total phonon density of states (TPDOS) and the partial phonon density of states (PPDOS) were calculated using molecular dynamics simulations with semi-empirical potentials. The proposed phononic interconnects, in conjunction with electronic and/or photonic interconnects, can be used in the current and future devices.

A high-resolution neutron powder diffraction investigation of galena (PbS) between 10 K and 350 K: no evidence for anomalies in the lattice parameters or atomic displacement parameters in galena or altaite (PbTe) at temperatures corresponding to the saturation of cation disorder

The temperature dependences of the unit cell parameter and the atomic displacement parameters (adp) for galena (PbS) have been measured using high resolution neutron powder diffraction in the temperature interval 10–350 K. No evidence has been found for the anomalous behaviour recently reported in a total scattering study of galena, in which the temperature variation of both the unit cell and the adp for lead are reported to undergo a dramatic reduction at a temperature of ~250 K. The linear thermal expansion coefficient calculated from the powder diffraction study is found to be in excellent agreement with literature values over the entire temperature interval studied, and approximately 25% greater at room temperature than that determined by analysis of the pair distribution function (pdf) derived from the total scattering data. This discrepancy is shown to be attributable to a linear, temperature-dependent offset from the published temperatures in the total scattering study, and has arisen from the sample temperature being significantly lower than the experimental set point temperature. Applying this correction to the adps of the lead cation removes the anomalous temperature dependence and shows the pdf results are in agreement with the neutron powder diffraction results. Application of the identical temperature offsets to the results of the pdf analysis of data collected on altaite (PbTe) eliminates the anomalous behaviour in the unit cell and the adp for lead, bringing them in line with literature values. Contrary to the conclusions of the pdf analysis, adps for the lead cation in both galena and altaite can be described in terms of Debye-like behaviour and are consistent with the partial phonon density of states.

Lattice Dynamics Study of Nanocrystalline Yttrium Gallium Garnet at High Pressure

This work reports an experimental and theoretical lattice dynamics study of nanocrystalline Y3Ga5O12 (YGG) garnet at high pressures. Raman scattering measurements in nanocrystalline Tm3+-doped YGG garnet performed up to 29 GPa have been compared to lattice dynamics ab initio calculations for bulk garnet carried out up to 89 GPa. Good agreement between the theoretical vibrational modes of bulk crystal and the experimental modes measured in the nanocrystals is found. The contribution of GaO4 tetrahedra and GaO6 octahedra to the different phonon modes of YGG is discussed on the basis of the calculated total and partial phonon density of states. Symmetries, frequencies, and pressure coefficients of the Raman-active modes are discussed. Moreover, the calculated infrared-active modes and their pressure dependence are reported. No pressure-induced phase transition has been observed in nano-YGG up to 29 GPa. This is in agreement with theoretical results, which show a mechanical instability of YGG above 84 GPa, si...

Pressure-induced instability in CdO

The lattice dynamics of rocksalt CdO is studied as a function of pressure using density-functional linear-response theory. The predicted static phase-transition pressure from rocksalt (B1) to CsCl (B2) structures is 76.12 GPa. Analysis of the phonon calculations suggests that the pressure-induced instabilities at p = 78.26 GPa of the transverse acoustic (TA) modes in a large portion of the Brillouin zone are responsible for the phase transition from B1 → B2 phases of CdO. Moreover, the calculated partial phonon density of states at the precursor phase transition of p = 70 GPa reveals strong reduction of the longitudinal acoustic Cd-like modes and the existence of a large peak related to the longitudinal optic O-like mode.

Effect of nitrogen impurity on the structural, mechanical and phonon properties of diamond from first-principle study

By means of density functional theory, we investigated physical properties of nitrogen-doped diamond and compared them with those of pure diamond. We showed that the nitrogen in substitution causes the strong lattice deformation near impurity, decreases elastic moduli, hardness and their anisotropy. We have confirmed experimental data that the face (111) is harder than face (100) in diamond. The nitrogen incorporation leads to an appearance of the resonant localized modes. The total and partial phonon densities of states, the mode localization, and the Raman and IR intensities in nitrogen-doped diamond were analyzed.

Using Monte Carlo ray tracing simulations to model the quantum harmonic oscillator modes observed in uranium nitride

Recently an extended series of equally spaced vibrational modes was observed in uranium nitride (UN) by performing neutron spectroscopy measurements using the ARCS and SEQUOIA time-of-flight chopper spectrometers [A. A. Aczel et al., Nat. Commun. 3, 1124 (2012)]. These modes are well described by three-dimensional isotropic quantum harmonic oscillator (QHO) behavior of the nitrogen atoms, but there are additional contributions to the scattering that complicate the measured response. In an effort to better characterize the observed neutron scattering spectrum of UN, we have performed Monte Carlo ray tracing simulations of the ARCS and SEQUOIA experiments with various sample kernels, accounting for nitrogen QHO scattering, contributions that arise from the acoustic portion of the partial phonon density of states, and multiple scattering. These simulations demonstrate that the U and N motions can be treated independently, and show that multiple scattering contributes an approximate Q-independent background to the spectrum at the oscillator mode positions. Temperature-dependent studies of the lowest few oscillator modes have also been made with SEQUOIA, and our simulations indicate that the T dependence of the scattering from these modes is strongly influenced by the uranium lattice.

Atomistic study on the site preference and lattice vibration of Gd3−xYxCo29T4B10 (T=Al and Ge)

The effects of the Y substitution for Gd on the structural stability and the site preference of intermetallics Gd3−xYxCo29T4B10 (T=Al and Ge) are studied by using a series of interatomic pair potentials. The calculated results show Y can stabilize Gd3−xYxCo29T4B10 with the tetragonal structure, and Y substitute for Gd with a strong preference for the 2b sites. The calculated lattice parameters are in good agreement with the experimental data. Furthermore, the total and partial phonon densities of states are evaluated for the Gd3−xYxCo29T4B10 compounds with the tetragonal structure. A qualitative analysis is carried out with the relevant potentials for the vibrational modes, which makes it possible to predict some properties related to lattice vibration.

Phonons and the electronic gap in FeSi

The partial phonon densities of states of iron atoms in the intermetallic compound FeSi have been measured in the temperature range 46–297 K using nuclear resonant inelastic scattering of synchrotron radiation. A significant phonon softening with increasing temperature has been established. The greatest phonon softening for iron atoms is shown to occur in the region of long-wavelength acoustic phonons, for the acoustic branches near the boundary of the Brillouin zone, and for the low-lying weakly dispersive optical branches. The results obtained are analyzed in terms of the views that relate the change in the phonon density of states of iron atoms to the temperature evolution of the electronic density of state for the compound.