Showing papers on "Debye model published in 2009"

Predicting phonon properties and thermal conductivity from anharmonic lattice dynamics calculations and molecular dynamics simulations

Abstract: Two methods for predicting phonon frequencies and relaxation times are presented. The first is based on quasiharmonic and anharmonic lattice dynamics calculations, and the second is based on a combination of quasiharmonic lattice dynamics calculations and molecular dynamics simulations. These phonon properties are then used with the Boltzmann transport equation under the relaxation-time approximation to predict the lattice thermal conductivity. The validity of the low-temperature assumptions made in the lattice dynamics framework are assessed by comparing to thermal conductivities predicted by the Green-Kubo and direct molecular dynamics methods for a test system of Lennard-Jones argon. The predictions of all four methods are in agreement at low temperature (20 K). At temperatures of 40 K (half the Debye temperature of Lennard-Jones argon) and below, the thermal-conductivity predictions from the two methods that use lattice dynamics calculations are within about 30% of those made using the more accurate Green-Kubo and direct molecular dynamics methods. The thermal-conductivity predictions using the lattice dynamics techniques become inaccurate at high temperature (above 40 K) due to the approximations inherent in the lattice dynamics framework. We apply the results to assess the validity of (i) the isotropic approximation in modeling thermal transport and (ii) the common assertion that low-frequency phonons dominate thermal transport. Lastly, we suggest approximations that can be made within the lattice dynamics framework that allow the thermal conductivity of Lennard-Jones argon to be estimated using two orders of magnitude less computing effort than the Green-Kubo or direct molecular dynamics methods.

First-principles study of the structural, electronic, vibrational, and elastic properties of orthorhombic NiSi

Abstract: We present a study of the structural, electronic, vibrational, and elastic properties of the orthorhombic NiSi structure by means of the density-functional theory and the density-functional perturbative theory, with the Perdew-Burke-Ernzerhof generalized gradient approximation of the exchange-correlation functional, within its spin-polarized version. The optimized lattice parameters, the formation energy, and vibrational properties are found in agreement with experimental data. We show that NiSi is not ferromagnetic, with a low density of states at the Fermi level. Elastic constants have been calculated by means of three different approaches for comparison. In the first two, the calculated energy $E$ is fitted as a function of the deformation, atomic positions are either relaxed or not relaxed during the simulations. Atomic relaxations are shown to modify significantly elastic constants. In the third approach we have related acoustic velocities to elastic constants. NiSi is shown to be highly anisotropic. In particular the linear bulk modulus along $b$ axis is much larger than along other axes. Polycrystalline elastic properties and Debye temperature have been also evaluated for a complete description of elastic properties.

Anomalous properties of the acoustic excitations in glasses on the mesoscopic length scale

Abstract: The low-temperature thermal properties of dielectric crystals are governed by acoustic excitations with large wavelengths that are well described by plane waves. This is the Debye model, which rests on the assumption that the medium is an elastic continuum, holds true for acoustic wavelengths large on the microscopic scale fixed by the interatomic spacing, and gradually breaks down on approaching it. Glasses are characterized as well by universal low-temperature thermal properties that are, however, anomalous with respect to those of the corresponding crystalline phases. Related universal anomalies also appear in the low-frequency vibrational density of states and, despite a longstanding debate, remain poorly understood. By using molecular dynamics simulations of a model monatomic glass of extremely large size, we show that in glasses the structural disorder undermines the Debye model in a subtle way: The elastic continuum approximation for the acoustic excitations breaks down abruptly on the mesoscopic, medium-range-order length scale of ≈10 interatomic spacings, where it still works well for the corresponding crystalline systems. On this scale, the sound velocity shows a marked reduction with respect to the macroscopic value. This reduction turns out to be closely related to the universal excess over the Debye model prediction found in glasses at frequencies of ≈1 THz in the vibrational density of states or at temperatures of ≈10 K in the specific heat.

Correction to “Modeling of the Melting Point, Debye Temperature, Thermal Expansion Coefficient, and the Specific Heat of Nanostructured Materials”

Abstract: The size-dependences of the melting point, Debye temperature, thermal expansion coefficient, and the specific heat of nanostructured materials have been modeled free of adjustable parameters. The melting point and Debye temperature drop while the thermal expansion coefficient and specific heat rise when the grain size is decreased. Relative to nanoparticles, however, the variation of the above parameters of nanostructured material is weak, dominated by the ratio of the grain boundary energy to the surface energy. Our theoretical predictions agree fairly well with available experimental and computer simulation results for semiconductors and metals.

Singular behavior of the Debye-Waller factor of graphene

Abstract: It is shown that the mean-square displacement or the exponent of the Debye-Waller factor of graphene has a singularity except at zero temperature. The zero-temperature values of the mean-square displacement are calculated separately for planar and out-of-plane phonon modes for graphene. These values give the DebyeWaller factor that can be used to model various scattering processes at temperatures much lower than the Debye temperature of graphene. Since the Debye temperature of graphene is about 2300 K for planar modes, the calculated values should provide a useful estimate of the Debye-Waller factor at temperatures of practical interest. Finally, it is shown qualitatively that the singularity can be removed by accounting for the finite size of real graphene crystals.

Temperature dependence of the thermal expansion of AlN

Abstract: The thermal expansion of wurtzite AlN bulk crystals grown by physical vapor transport was studied by high resolution x-ray diffraction in a temperature range from 20 to 1250 K. The temperature dependence of the derived anisotropic thermal expansion coefficients along the a- and c-directions could be well described over the entire temperature range within both the Debye model and the Einstein model. In comparison to GaN, larger expansion coefficients and higher characteristic temperatures have been found. The resulting thermal mismatch of AlGaN/GaN heterostructures are presented.

Phonon density of states and heat capacity of La 3 − x Te 4

Abstract: The phonon density of states (DOS) of La_(3−x)Te_4 compounds (x=00,018,032) was measured at 300, 520, and 780 K, using inelastic neutron scattering A significant stiffening of the phonon DOS and a large broadening of features were observed upon introduction of vacancies on La sites (increasing x) Heat-capacity measurements were performed at temperatures 185 ≤ T ≤ 1200 K and were analyzed to quantify the contributions of phonons and electrons The Debye temperature and the electronic coefficient of heat capacity determined from these measurements are consistent with the neutron-scattering results, and with previously reported first-principles calculations Our results indicate that La vacancies in La_(3−x)Te_4 strongly scatter phonons and this source of scattering appears to be independent of temperature The stiffening of the phonon DOS induced by the introduction of vacancies is explained in terms of the electronic structure and the change in bonding character The temperature dependence of the phonon DOS is captured satisfactorily by the quasiharmonic approximation

Mechanisms for thermal conduction in methane hydrate.

Abstract: Crystalline clathrate hydrates exhibit an unusual thermal transport with glasslike thermal conductivity close to the Debye temperature but a crystal-like temperature dependence at low temperature. Molecular dynamics calculations on structure I methane clathrate hydrate reproduced the qualitative trend in the thermal conductivity. Analysis of the heat flux and local energy correlation functions shows that both the crystal structure of the clathrate framework and guest-host interactions contribute to thermal transport processes. The lower thermal conductivity relative to ice Ih is due to differences in crystal structures. The glasslike temperature dependence is governed by the guests and the guest-host interactions.

Relative Contributions of Inelastic and Elastic Diffuse Phonon Scattering to Thermal Boundary Conductance Across Solid Interfaces

Abstract: The accuracy of predictions of phonon thermal boundary conductance using traditional models such as the diffuse mismatch model (DMM) varies depending on the types of material comprising the interface. The DMM assumes that phonons, undergoing diffuse scattering events, are elastically scattered, which drives the energy conductance across the interface. It has been shown that at relatively high temperatures (i.e., above the Debye temperature) previously ignored inelastic scattering events can contribute substantially to interfacial transport. In this case, the predictions from the DMM become highly inaccurate. In this paper, the effects of inelastic scattering on thermal boundary conductance at metal/dielectric interfaces are studied. Experimental transient thermore-flectance data showing inelastic trends are reviewed and compared to traditional models. Using the physical assumptions in the traditional models and experimental data, the relative contributions of inelastic and elastic scattering to thermal boundary conductance are inferred.

First‐principles calculations of structural, elastic and electronic properties of Ni2MnZ (Z = Al, Ga and In) Heusler alloys

Abstract: We have performed ab-initio density-functional theory self-consistent calculations using the full-potential linear muffin-tin orbital method within local spin-density approximation to study the electronic and magnetic properties of Ni2MnZ (Z = Al, Ga and In) in L21 structure. The magnetic phase stability is determined from the total energy calculations for both the nonmagnetic (NM) and magnetic (M) phases. The theoretical calculations clearly indicate that at both ambient and high pressures, the magnetic phase is more stable than the nonmagnetic phase. The elastic constants at equilibrium are also determined. We derived the bulk and shear moduli, Young's modulus, and Poisson's ratio. The Debye temperature of Ni2MnZ was estimated from the average sound velocity. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydrogen atoms in Debye plasma environments

Abstract: Plasma-screening effects are investigated on hydrogen atoms embedded in weakly coupled plasmas. In the present context, bound state wave functions are introduced related to the screening Coulomb potential (Debye model) using the Ritz variation method. The bound energies are derived from an energy equation, which contains one unknown variational parameter. To calculate the parameter numerically, fixed-point iteration scheme is used. The calculated energy eigenvalues for various Debye lengths agree well with the other available theoretical results. The radial wave functions and radial probability distribution functions are presented for different Debye lengths. The outcomes show that the plasma affects the embedded hydrogen atom.

Calculated structural, electronic and elastic properties of M 2 GeC (M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W)

Abstract: Using ab initio calculations, we have studied the structural, electronic and elastic properties of M2GeC, with M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W. Geometrical optimizations of the unit cell are in agreement with the available experimental data. The band structures show that all studied materials are electrical conductors. The analysis of the site and momentum projected densities shows that bonding is due to M d-C p and M d-Ge p hybridizations. The elastic constants are calculated using the static finite strain technique. The shear modulus C 44, which is directly related to the hardness, reaches its maximum when the valence electron concentration is in the range 8.41–8.50. We derived the bulk and shear moduli, Young’s moduli and Poisson’s ratio for ideal polycrystalline M2GeC aggregates. We estimated the Debye temperature of M2GeC from the average sound velocity. This is the first quantitative theoretical prediction of the elastic constants of Ti2GeC, V2GeC, Cr2GeC, Zr2GeC, Nb2GeC, Mo2GeC, Hf2GeC, Ta2GeC and W2GeC compounds, and it still awaits experimental confirmation.

Particle size-dependent electrical properties of nanocrystalline NiO

Abstract: Nickel oxide nanoparticles are formed by chemical precipitation and subsequent drying and calcinations at temperatures ≥523 K. Samples are characterized using X-ray diffraction and BET surface area measurements indicating the formation of a single NiO phase whose crystallite size increases with increasing calcination temperature. The electrical properties are examined by measuring DC and AC conductivities and dielectric properties as functions of temperature. Electrical conductivities first slightly increases with increasing particle size up to 7–10 nm and are about 8 orders of magnitude higher than that of NiO single crystals. Further increasing the particle size above 10 nm, leads to a monotonic decrease of conductivity. The data are discussed in view of variations of grain boundary as well as triple junction volume fractions as the particle size varies. At temperatures above θD/2 (θD is the Debye temperature), the conductivity is ascribed to a band-like conduction due to the large polaron. The activation energy of conduction was found to be minimal for the highly conducting samples of 7–10 nm, and gradually increases to ~0.5 eV with increasing the particle size above 10 nm. For T < θD/2, the conductivity is best described by variable–range–hopping models. Model parameters are thus estimated and presented as functions of particle size. Frequency as well as temperature dependencies of the AC conductivity and dielectric constant exhibit trends usually observed in carrier dominated dielectrics.

Pressure-temperature stability studies of FeOOH using x-ray diffraction

Abstract: The Mie-Gruneisen formalism is used to fit a Birch-Murnaghan equation of state to high-temperature (T), high-pressure (P) X-ray diffraction unit-cell volume (V) measurements on synthetic goethite (alpha-FeOOH) to combined conditions of T = 23-250o C and P = 0-29.4 GPa. We find the zero-pressure thermal expansion coefficient of goethite to be alpha0 = 2.3 (+-0.6) x 10-5 K-1 over this temperature range. Our data yield zero-pressure compressional parameters: V0 = 138.75 (+- 0.02) Angstrom3, bulk modulus K0 = 140.3 (+- 3.7) GPa, pressure derivative K0' = 4.6 (+- 0.4), Gruneisen parameter gamma0 = 0.91 (+- 0.07), and Debye temperature Theta0 = 740 (+- 5) K. We identify decomposition conditions for 2alpha-FeOOH --> alpha-Fe2O3 + H2O at 1 - 8 GPa and 100-400oC, and the polymorphic transition from alpha-FeOOH (Pbnm) to epsilon-FeOOH (P21mn). The non-quenchable, high-pressure epsilon-FeOOH phase P-V data are fitted to a second-order (Birch) equation of state yielding, K0 = 158 (+- 5) GPa and V0 = 66.3 (+- 0.5) Angstrom3.

Surface core level shifts of clean and oxygen covered Ir(111)

Abstract: We present the results of high resolution core level photoelectron spectroscopy employed to investigate the electronic structure of clean and oxygen covered Ir(111) surface. Ir 4f7/2 core level spectra are shown to be very sensitive to the local atomic environment. For the clean surface we detected two distinct components shifted by 550meV, originated by surface and bulk atoms. The larger Gaussian width of the bulk component is explained as due to experimentally unresolved subsurface components. In order to determine the relevance of the phonon contribution we examined the thermal behaviour of the core level lineshape using the Hedin-Rosengren theory. From the phonon- induced spectral broadening we found the Debye temperature of bulk and surface atoms to be 298 and 181K, respectively, which confirms the softening of the vibrational modes at the surface. Oxygen adsorption leads to the appearance of new surface core level components at 200meV and +230meV, which are interpreted as due to first-layer Ir atoms differently coordinated with oxygen. The coverage dependence of these components demonstrates that the oxygen saturation corresponds to 0.38ML, in good agreement with recent density functional theory calculations.

Systematic Study of Lattice Specific Heat of Filled Skutterudites

Abstract: The lattice specific heat C lat of La-based filled skutterudites La T 4 X 12 ( T = Fe, Ru and Os; X = P, As, and Sb) has been systematically studied, and both the Debye temperature Θ D and the Einstein temperature Θ E of La T 4 X 12 were carefully estimated We confirmed that a correlation exists between Θ D and the reciprocal of the square root of average atomic mass for La T 4 P 12 , La T 4 As 12 , and La T 4 Sb 12 The Θ D of filled skutterudites was found to depend mainly on the nature of the species X forming the cage The temperature dependence of C lat / T 3 for La T 4 X 12 exhibited a large broad maximum at low temperatures (10–30 K), which suggests a nearly dispersionless low-energy optical mode characterized by Einstein specific heat Since no such broad maximum exists for the unfilled skutterudite RhP 3 , the low-energy optical modes are associated with vibration involving La ions in the X 12 cage (the so-called “guest ion modes”) The Θ E of filled skutterudites was found to roughly correspon

Mechanical properties of Ti3AlX (X = C, N): Ab initio study

Abstract: Elastic properties of the alloys Ti3AlC and T i3AlN are derived from the first-principles total energy calculations based on the full-potential linear muffin-tin Orbital (FP-LMTO) method. From the computed elastic constants, theoretical values of Young's modulus, shear modulus, Poisson's ratio, sound velocities and Debye temperature are evaluated. By analysing the ratio between the bulk and shear moduli, it is found that Ti3AlN is ductile in nature, whose ductility is expected to be greater than that of Ti3Al, whereas T i3AlC is found to be brittle. The site-projected density of states and the charge density plots have been used to analyse the chemical bonding between the Ti6N and T i6C cluster and the surrounding metallic lattice of Al atoms. This further reveals that the strong covalent nature of Ti-C bonds in Ti3AlC, together with the high Young, shear and bulk moduli, make the compound more brittle than Ti3AlN.

New Antiperovskite-Type Superconductor ZnNyNi3

Abstract: We have synthesized a new superconductor ZnN y Ni 3 with T c ∼3 K. The crystal structure is of the same antiperovskite-type as MgCNi 3 and CdCNi 3 . As far as we know, this is the third superconducting material in the Ni-based antiperovskite series and the first antiperovskite nitride superconducting material. For this new superconductor, the lower critical field H c1 (0), upper critical field H c2 (0), coherence length ξ(0), penetration depth λ(0), Ginzburg–Landau parameter κ(0), electronic specific heat coefficient γ, and Debye temperature Θ D have been experimentally determined.

FP-APW + lo study of the elastic, electronic and optical properties of the filled skutterudites CeFe4As12 and CeFe4Sb12

Abstract: Using a full-relativistic version of the full-potential augmented plane wave plus local orbitals (FP-APW + lo) method within the local density approximation (LDA), we have studied the elastic, electronic and optical properties of the filled skutterudites CeFe4As12 and CeFe4Sb12. Structural parameters, including lattice constant, internal free parameters and, bulk modulus and its pressure derivative were calculated. We have determined the full set of first-order elastic constants, Young’s modulus, Poisson’s ratio and the Debye temperature of these compounds. Band structures, density of states, pressure coefficients of energy band gaps are also given. It is found that both CeFe4As12 and CeFe4Sb12 are indirect band gap semiconductors. The valence band maximum (VBM) is located at Γ point, whereas the conduction band minimum (CBM) is located at N point. Optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss were calculated for radiation up to 30 eV. This is the first quantitative theoretical prediction of the elastic and optical properties for these compounds, and it still awaits experimental confirmation.

Structural studies of some phospho-borate glasses using ultrasonic pulse–echo technique, DSC and IR spectroscopy

Abstract: Glasses in the system (95−x) [0.25 Na2O–0.75 B2O3]–x P2O5–5 Fe2O3 (0⩽x⩽15 mol%), have been prepared by the melt quenching technique. Elastic properties and FT-IR spectroscopic studies have been employed to study the role of P2O5 on the structure of the glass system. Elastic properties Poisson's ratio, micro-hardness and Debye temperature have been investigated using sound wave velocity measurements at 4 MHz (both longitudinal and shear) at room temperature. The results showed that the density and the molar volume increase as both sound velocities and the determined glass transition temperatures decrease with increasing the contents of P2O5. Infrared spectra of the glasses reveal that the borate network consists of diborate units and is affected by the increase in the concentration of P2O5 content as a second network former. These results are interpreted in terms of the replacement of the diborate units with B–O–B bridges by phosphate units with non-bridging oxygens (NBOs). Therefore, the elastic moduli are observed to decrease with the increase in P2O5 content.