Elastic Debye Temperature Research Articles

First-principles study of structural, elastic, and electronic properties of CeB6 under pressure

We performed a first-principles study of the electronic, elastic, and thermal properties of the rareearth hexaboride CeB6 using the local density approximation (LDA) in consideration of the effective onsite Coulomb parameter Ueff. To systemically evaluate the effect of Ueff on the structure of the material, the dependences of the lattice parameter a0 and bulk modulus B on Ueff were examined in the framework of the LDA+U and GGA(PBE)+U scheme. We obtained a lattice constant a0, elastic constants Cij, and a bulk modulus B at 0 K and 0 GPa that were in good agreement with the experimental results and other theoretical findings. We focused on the electronic structure by analyzing the variation of the density of states with different Ueff values and pressures, which indicates the metallic characteristic of CeB6. Interestingly, the effect of high pressure was similar to that of increasing Ueff, as the peaks at the bottom of the conduction band moved to the high-energy region in both cases. The elastic constants Cij, bulk modulus B, shear modulus G, Young’s modulus E, shear-sound velocity VS, and longitudinal-sound velocity VL were calculated from 0 to 120 GPa. Additionally, the Debye temperature ΘD and elastic Debye temperature ΘE were systematically calculated using the thermodynamic methods in the range of 0–100 GPa. This research may provide a comprehensive understanding of the Kondo compound CeB6 and similar rare-earth hexaborides.

Self-interaction corrected LDA + U investigations of BiFeO3 properties: plane-wave pseudopotential method

The structural, electronic, elastic, and optical properties of BiFeO3 were investigated using the first-principles calculation based on the local density approximation plus U (LDA + U) method in the frame of plane-wave pseudopotential density functional theory. The application of self-interaction corrected LDA + U method improved the accuracy of the calculated properties. Results of structural, electronic, elastic, and optical properties of BiFeO3, calculated using the LDA + U method were in good agreement with other calculation and experimental data; the optimized choice of on-site Coulomb repulsion U was 3 eV for the treatment of strong electronic localized Fe 3d electrons. Based on the calculated band structure and density of states, the on-site Coulomb repulsion U had a significant effect on the hybridized O 2p and Fe 3d states at the valence and the conduction band. Moreover, the elastic stiffness tensor, the longitudinal and shear wave velocities, bulk modulus, Poisson’s ratio, and the Debye temperature were calculated for U = 0, 3, and 6 eV. The elastic stiffness tensor, bulk modulus, sound velocities, and Debye temperature of BiFeO3 consistently decreased with the increase of the U value.

Pressure induced stiffening, thermal softening of bulk modulus and brittle nature of mercury chalcogenides

The pressure and temperature dependent elastic properties of mercury chalcogenides ( Hg X; X = S , Se and Te ) with pressure induced structural transition from ZnS -type (B3) to NaCl -type (B1) structure have been analyzed within the framework of a model interionic interaction potential with long-range Coulomb and charge transfer interactions, short-range overlap repulsion and van der Waals (vdW) interactions as well as zero point energy effects. Emphasis is on the evaluation of the Bulk modulus with pressure and temperature dependency to yield the Poisson's ratio ν, the Pugh ratio ϕ, anisotropy parameter, Shear and Young's modulus, Lamé's constant, Klein man parameter, elastic wave velocity and Debye temperature. The Poisson's ratio behavior infers that Hg X are brittle in nature. To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of elastic and thermodynamical properties explicitly the ductile (brittle) nature of Hg X and still awaits experimental confirmations.

Built-in-polarization field effect on intrinsic and extrinsic thermal conductivities of AlN/GaN/AlN quantum well

In this paper, the effect of built-in-polarization field on lattice thermal conductivity of AlN/GaN/AlN quantum well (QW) has been theoretically investigated. The built-in-polarization field at the hetero-interface of GaN/AlN modifies elastic constant, phonon velocity and Debye temperature of GaN QW. The relaxation time of acoustic phonons (AP) in various scattering processes in GaN with and without built-in-polarization field has been computed at room temperature. The result shows that combined relaxation time of AP is enhanced by built-in-polarization field and implies a longer mean free path. The revised intrinsic and extrinsic thermal conductivities of GaN have been estimated. The theoretical analysis shows that up to a certain temperature the polarization field acts as negative effect and reduces the thermal conductivities. However, after this temperature both thermal conductivities are significantly contributed by polarization field. This gives the idea of temperature dependence of polarization effect which signifies the pyro-electric character of GaN. The intrinsic thermal conductivity at room temperature for with and without polarization mechanism is found to be 491 Wm-1K-1and 409 Wm-1K-1, respectively i.e., 20% enhancement. However, the extrinsic thermal conductivity at room temperature for with and without polarization mechanism is found to be 280 Wm-1K-1and 245 Wm-1K-1, respectively i.e., 13% enhancement. The method we have developed may be taken into account during the simulation of heat transport in optoelectronic nitride devices to minimize the self-heating processes and in polarization engineering strategies to optimize the thermoelectric performance of GaN alloys.

Elasticity and Debye temperature of defected fcc crystals (AlCu3, Al, Cu): Molecular dynamics and first-principles calculations

This paper deals with the effects of vacancies on physical properties of AlCu3 intermetallic crystal investigated by means of molecular dynamics (MD) and, independently, ab initio calculations. Moreover, our computer experiments were carried out for pure Al and Cu crystals for sake of comparison. In consequence, we determined how elastic characteristics and Debye temperature relate to the content of two types of vacancies arbitrarily introduced into the investigated crystals. We found a systematic decrease in elastic stiffness components (C11, C12 and C44), bulk modulus, shear modulus, Young’s modulus and Debye temperature, according to both Anderson’s and Siethoff’s models, and increase in Poisson’s ratio with the growth of vacancy concentration. This holds for all the investigated materials approached by both MD and DFT-based methods. The computed values for the perfect crystals in the equilibrium state proved to agree with earlier literature data. We contend the results obtained for the defected structures are of importance for nanofabrication and provide valuable information on the nature of the materials of interest.

Structural, elastic, electronic and optical properties of the quaternary nitridogallate LiCaGaN2: First-principles study

First-principles density functional theory calculations were performed to predict some of the not yet explored physical properties of the monoclinic quaternary nitridogallate LiCaGaN2. The calculated lattice parameters, including the lattice constants, angle β and internal atomic coordinates, are in excellent agreement with the corresponding measured ones, proving the reliability of the chosen theoretical approach. The equation of state, pressure dependence of the lattice constants, unit cell volume, angle β and bond-lengths were explored in detail. The single-crystal and polycrystalline elastic constants and their pressure dependence were numerically estimated. The mechanical stability, ductility/brittleness, average elastic wave velocity, Debye temperature and elastic anisotropy were also assessed. The electronic structure and its evolution with external applied hydrostatic pressure were explored. The bonding character was demonstrated by calculating the site-projected density of states, charge density and effective Mulliken charges of all ions. The effective masses of the charge-carriers were numerically estimated. The complex dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron energy-loss function spectra were calculated for different polarizations of the incident light. Pressure dependence of the static dielectric constant, static refractive index and static reflectivity are also reported. To the best of our knowledge, this is the first attempt to explore the aforementioned physical properties for the title material.

Electronic, mechanical, phase transition, and thermo-physical properties of TMC (TM = V, Nb, and Ta): high pressure ab initio study

The structural, electronic, mechanical, phase transition, and thermo-physical properties of refractory carbides, viz. VC, NbC, and TaC have been computed in stable B1 and high pressure B2 phases by means of two different ab initio calculations using pseudo- and full-potential schemes. These materials have mixed covalent-, metallic-, and ionic-type bonding. The calculations of elastic constants show the mechanical stability of these materials in B1 phase only. The brittle nature and anisotropy is observed in these materials in B1 phase. Non-central forces are present in both the phases. Elastic wave velocities and Debye temperature have also been calculated. The present results on structural, phase transition, elastic, and other properties are in reasonably good agreement with the available experimental and theoretical data. The calculations in high pressure phase need experimental verification.

Ab Initio Studies on Structural, Elastic, Thermodynamic and Electronic Properties of FeCrAs under Pressures

The structural, elastic, thermodynamic and electronic properties of nonmetallic metal FeCrAs are studied within density function perturbation theory. The thermodynamic properties of FeCrAs were deduced based on phonon frequencies within the framework of the quasiharmonic approximation. The calculated elastic modulus under various pressures indicates that FeCrAs is mechanically stable under pressure. The pressure-dependence of bulk and shear modulus, transverse and longitudinal sound velocities V (i.e. VS and VL), elastic Debye temperature ΘE of FeCrAs have also been investigated. The calculated values of B/G indicate that FeCrAs presents high ductility under pressure. However, it is interesting that the value of B/G reaches a maximum under 40 GPa and almost remains unchanged when the pressure is above 70 GPa. The calculations show that the heat capacity CV of this material is close to the Dulong Petit limit 3R (about 224.61 J mol−1 K−1) at high temperature regime. The analysis of electronic properties nd that as the pressure increases, the absolute value of charge for As and Fe atom increases while Cr remains nearly a constant, indicating that the mechanic properties of FeCrAs under pressure should be mostly attributed to the interaction between Fe and As atoms.

Predicting crystal structures and physical properties of novel superhard p-BN under pressure via first-principles investigation

The first-principles calculations were carried out to investigate the electronic structure, elastic, hardness and thermodynamics properties of novel superhard p-BN under pressure. The calculated lattice parameters are in good agreement with previous theoretical results. The band structure, the density of states and the partial density of states are analyzed, which reveals the insulator character of p-BN. In addition, the elastic constants, polycrystalline modulus and Debye temperature under pressure are also successfully obtained. It is observed that the p-BN should be classified as brittle materials and possesses elastically anisotropic. The calculated Vicker’s hardness of p-BN was 55.4GPa, which indicates that it is a superhard material. According to the calculated polycrystalline modulus, a new modified Clarke-type equation is used to calculate the minimum thermal conductivity. This work provides a useful guide for designing novel borides materials having excellent mechanical performance.

Effect of built-in-polarization field on intrinsic and extrinsic thermal conductivity of InN

The effect of built-in-polarization field on intrinsic and extrinsic lattice thermal conductivity of InN has been investigated theoretically. The built-in-polarization field modifies elastic constant, phonon velocity and Debye temperature of InN. Using these modified parameters, the relaxation time of phonons in various scattering processes at room temperature has been computed as a function of phonon frequency for with and without built-in-polarization field. The result shows that built-in-polarization field enhances the relaxation time of phonons. This implies longer mean free path of phonons. The Callaway model of phonon thermal conductivity has been used to estimate the effect of built-in-polarization field on intrinsic and extrinsic thermal conductivity of InN. The theoretical analysis shows that up to a certain temperature, the polarization field acts as negative effect and reduces the intrinsic and extrinsic thermal conductivity. However, after this temperature, both thermal conductivity are significantly contributed by polarization field. This gives the idea of temperature dependence of polarization effect, which signifies pyro-electric character of InN. The intrinsic thermal conductivity of InN at room temperature including built-in-polarization field is found to be enhanced by 28 %. However, at room temperature, the extrinsic thermal conductivity with built-in-polarization mechanism is found to be enhanced by 15 %. The method, we have developed may be taken in the simulation of heat transport in optoelectronic nitride devices to minimize the self-heating processes and in polarization engineering strategies to optimize the thermoelectric performance of InN alloys.

First-principle calculations of the structural, elastic and bonding properties of Cs2NaLnCl6 (Ln=La–Lu) cubic elpasolites

For the first time the structural, elastic and bonding properties of 15 elpasolite crystals Cs2NaLnCl6 (Ln denotes all lanthanides from La to Lu) were calculated systematically using the CRYSTAL09 program. Several trends in the variation of these properties in relation to the atomic number Z of the Ln ions were found; in particular, the lattice parameter of these compounds decreases with Z (which can lead to the increased crystal field splittings of the 5d states for the heavier Ln ions), whereas the elastic constants and Debye temperature increase. The degree of covalency of the Ln–Cl chemical bonds is increased toward the end of the lanthanide series.

Ab initio investigation of the intermetallics in the Nb–Sn binary system

The elastic and thermodynamic properties of the stable intermetallics in the Nb–Sn system were studied by the first-principles pseudo-potential plane-wave method based on density functional theory. The elastic constants, Debye temperatures, bulk, shear and Young’s moduli and Poisson’s ratios were calculated for all the phases. Nb3Sn has the highest bulk, shear and elastic moduli of the Nb–Sn intermetallics. The elastic properties at 0K and the Debye temperatures of the Nb6Sn5 and NbSn2 phases are reported for the first time. The enthalpies of formation were calculated for all the intermetallics. The finite-temperature elastic properties of the Nb and Nb3Sn were calculated using the quasi-static approximation. The thermal expansion coefficients of Nb and Nb3Sn were also obtained.

Third Order Elastic Constants and Debye Temperature of MgB2 Under Different Pressure: First-Principles Methods

The second-order elastic constants (SOECs) and third-order elastic constants (TOECs) of superconducting MgB2 are investigated by first principles methods combined with homogeneous deformation theory. The Anderson-Gruneisen parameter is obtained from the SOECs and TOECs. The obtained value is 2.13, that is in consistent with the experimental result. The effective elastic constants and Debye temperature are also predicted from SOECs and TOECs, which increase with the increasing of pressure.

Ab initio prediction of the structural, electronic, elastic and thermodynamic properties of the tetragonal ternary intermetallics XCu2Si2 (X = Ca, Sr)

Structural parameters, electronic structure, elastic constants and thermodynamic properties of the tetragonal ternary intermetallics CaCu2Si2 and SrCu2Si2 are investigated theoretically for the first time using the plane-wave ultra-soft pseudopotential method based on the density functional theory. The calculated equilibrium structural parameters agree well with the existing experimental data. Pressure dependence of the structural parameters is also explored. Analysis of the band structure, total and site-projected l-decomposed densities of states and valence charge distributions reveals the conducting character of both considered materials with a mixture of ionic-covalent chemical bonding character. Pressure dependences of the single-crystal elastic constants C ij for CaCu2Si2 and SrCu2Si2 are explored. The elastic wave velocities propagating along the principal crystallographic directions are numerically estimated. The elastic anisotropy is estimated and further illustrated by 3D-direction-dependent of the Young’s modulus. A set of some macroscopic elastic moduli, including the bulk, Young’s and shear moduli, Poisson’s coefficient, average elastic wave velocities and Debye temperature, were calculated for polycrystalline CaCu2Si2 and SrCu2Si2 from the C ij via the Voigt-Reuss-Hill approximations. Through the quasiharmonic Debye model, which takes into account the phonon effects, the temperature and pressure dependencies of the bulk modulus, unit cell volume, volume thermal expansion coefficient, Debye temperature and volume constant and pressure constant heat capacities of CaCu2Si2 and SrCu2Si2 are explored systematically in the ranges of 0–40 GPa and 0–1400 K.

Pressure-induced phase transition and structural behavior of MgO at high temperatures: a model study

A realistic interaction potential model approach by including temperature effects is developed to study phase transition, elastic properties and thermo-physical properties at very high pressures and temperatures. This approach is effectively able to explain the inter-atomic interaction involved at high temperature and high pressure as it includes the three-body interactions. Earlier works overlooked the three-body interactions at high temperature and pressures. Moreover, the phase-transition pressures of MgO crystal at high temperatures including the three-body interaction are computed for the first time. Elastic behavior, anisotropic factor and Debye temperature of MgO at high pressures and temperatures are also reported.

Investigation of structural, elastic, and lattice-dynamical properties of Ca2Si, Ca2Ge, and Ca2Sn based on first-principles density functional theory

The structural, elastic, phonon and thermodynamic properties of orthorhombic Ca2X (X=Si, Ge, and Sn) were systematically investigated using first-principles density functional theory (DFT). The elastic constants (cij), elastic modulus (B, G, E), Poisson’s ratio (ν), and elastic Debye temperature (ΘD) were determined and compared with the results of previous calculations. For the first time, linear response theory is used to calculate the phonon dispersion relation and phonon density of states for these compounds as well as their infrared and Raman active mode frequencies. In this study, the thermodynamic properties such as vibrational entropy (Svib) and constant-volume specific heat (Cv) were predicted theoretically and discussed.

Effect of Mo content on thermal and mechanical properties of Mo–Ru–Rh–Pd alloys

Metallic inclusions are precipitated in irradiated oxide fuels. The composition of the phases varies with the burnup and the conditions such as temperature gradients and oxygen potential of the fuel. In the present work, Mox0.7+x(Ru0.5Rh0.1Pd0.1)0.70.7+x (x=0, 0.05, 0.1, 0.15, 0.2, and 0.25) alloys were prepared by arc melting, followed by annealing in a high vacuum. The thermal and mechanical properties of the alloys such as elastic moduli, Debye temperature, micro-Vickers hardness, electrical resistivity, and thermal conductivity have been evaluated to elucidate the effect of Mo content on these physical properties of the alloys. The alloys with lower Mo contents show higher thermal conductivity. The thermal conductivity of the alloy with x=0 is almost twice of that of the alloy with x=0.25. The thermal conductivities of the alloys are dominated by electronic contribution, which has been evaluated using the Wiedemann–Franz–Lorenz relation from the electrical resistivity data. It is confirmed that the variation of the Mo contents of the alloys considerably affects the mechanical and thermal properties of the alloys.

The Strongest Particle: Size-Dependent Elastic Strength and Debye Temperature of PbS Nanocrystals.

We investigated the elastic compressibility of PbS nanocrystals (NCs) pressurized in a diamond anvil cell and simultaneously probed the structure using synchrotron-based X-ray diffraction. The compressibility of PbS NCs exhibits bimodal size dependence. The elastic modulus of small NCs increases with increasing diameter and peaks near a particle diameter of approximately 7 nm. For large NCs the elastic modulus decreases toward the bulk value with increasing NC diameter. We explain the bimodal size-dependence of the elastic modulus in terms of a core-shell model based on distinct elasticity of the crystal near the surface and in the core of the particle. We combined insights into the size-dependent elasticity and lattice spacing to determine the Debye temperature of PbS NCs as a function of particle diameter. Understanding the size-dependent elasticity of defect-free colloidal NCs provides new insights into their crystal structure and mechanical properties.

Pressure Effect on Structural and Phonon Properties of Mixed Alloys Compounds

We report pressure induced structural phase transition, elastic and thermal properties of concentrations of UXLa1-XS (x= 0.80, 0.60 and 0.50) compound, using modified inter-ionic potential theory (MIPT), which parametrically includes the effect of coulomb screening. The calculated equation of state, phase transition pressure, bulk modulus and volume collapse are agree well with the available theoretical or experimental findings. We have also reported the second order elastic constants and Debye temperature of this compound for first time. We have also reported the Phonon properties of UxLa1-xS (x = 0.80, 0.60, 0.50) compounds by using breathing shell model (BSM). The present model includes breathing motion of electron shells of rare earth atom due to f-d hybridization. The calculated phonon dispersion curves of UxLa1-xS are presented follow the same trend as observed in uranium chalcogenides. We have reported doping effect of La on phonon frequencies at X and L-points for the first time. The LO-TO splitting increases as decreasing concentration of La.Keywords: Phase Transition, Elastic Properties, Debye Temperature, Phonon Dispersion

Elastic constants and Debye temperature of wz-AlN and wz-GaN semiconductors under high pressure from first-principles

First-principles calculations were performed to study the elastic stiffness constants (C ij ) and Debye temperature (𝜃 D) of wurzite (wz) AlN and GaN binary semiconductors at high pressure. The lattice constants were calculated from the optimized structure of these materials. The band gaps were calculated at Γ point using local density approximation (LDA) approach. The unit cell volume, lattice parameters, c/a, internal parameter (u), elastic constant (C ij ), Debye temperature (𝜃 D), Hubbard parameter (U) and band gap (E g) were studied under different pressures. The bulk modulus (B 0), reduced bulk modulus ( $B_{0}^{\prime })$ and Poisson ratio (υ) were also calculated. The calculated values of these parameters are in fair agreement with the available experimental and reported values.