Papers by Keyword: Lattice Dynamics

Abstract: Understanding and controlling the phonon, the dominant heat carrier of semiconductor materials, is essential to developing a wide variety of applications. This article studies the theoretical and computational approach of the calculation of lattice thermal conductivity of semiconducting materials. Despite having different methods to calculate the lattice thermal conductivity, first-principle estimates predict more accurately in most applications. This motivates to present the descriptive explanation on first-principle calculation with the combination of lattice dynamics and Boltzmann transport equation. Finally, we summarized an overview of the recent achievements and opportunities.

Abstract: The phonon dispersion curve is determined by model (BSM) for the crystals transition metal mono-nitride, Zirconium Nitride ZrN and compared with the experimental phonons, which forms perhaps the best test for this purpose. The eight numbers of parameters of the model are determined by using the some set of input data. The results and degrees of agreement obtained in each case show clearly the superiority of the “charge-transfer mechanism” for representing the deformations of electron shells used in the BSM. The phonon dispersion curve and phonon density of state ZrN have been measured in high-symmetry directions Δ, Σ and Λ by breathing shell model (BSM). Anomalies in the dispersion of the acoustic branches and optical branches have been detected which are well described by experimental results.

Abstract: Lattice dynamics and stability of fcc crystal of Ni under isotropic (hydrostatic) tensile loading are studied from first principles using supercell method and a harmonic approximation. According to the results, strength of the crystal is determined by occurrence of an instability related to soft phonons with finite wave vector. On the other hand, the critical strains and stresses associated with such instabilities are only slightly lower than those related to the volumetric instability.

Abstract: Recently, many investigations were devoted to the study of family of perovskite-type ABO₃ oxides. The material belongs to ABO3 perovskite oxides family like SrZrO₃, BaZrO₃, PbZrO₃ have many characteristics which are suitable for high-voltage and high-reliability capacitor applications. Many acceptor-doped perovskite-type oxides show high protonic conductivity at elevated temperatures. In addition to their reduced temperature operation relative to traditional oxide ion conductors such as Y-stabilized ZrO₂, these perovskites, because of their proton transport properties, offer the possibility of application in a number of arenas including hydrogen sensors for molten metals and hydrogen pumps. In this work we are reporting the results of our theoretical investigation on the phonon properties of ABO₃ mainly BaZrO₃, PbZrO₃ & SrZrO₃ in cubic phases. The phonon properties are calculated by using lattice dynamical simulation method based on de Launey angular force (DAF) constant model to understand the role of phonon in these systems. The phonon dispersion curves of these proton conductors in cubic phase are also drawn.

Abstract: The site preference and thermodynamic properties of UT_xAl_12-x (T = Zr, Nb, Mo and Fe) and their related hydrides are studied based on the pair potentials obtained by the lattice inversion method. The calculated result demonstrates that the stabilizing elements Zr, Nb, or Mo prefer to substitute for Al in 8i sites; and Fe atom preferentially substitutes for Al in the 8f site. The interstitial H atoms only occupy 2b interstitial sites in UT_xAl_12-x. The calculated lattice parameters coincide with the experimental values. In addition, the total and partial phonon densities of states are first evaluated for these compounds.

Abstract: An atomistic simulation is presented on the phase stability and lattice parameters of the new actinide intermetallic compounds A₃Ni₅Al₁₉ (A = Th, U). Calculations are based on a series of interatomic pair potentials related to the actinides and transition metals, which are obtained by lattice inversion method. Calculated lattice constants are found to agree with a report in the literature. It is noted that, the total and partial phonon densities of states are first evaluated for the A₃Ni₅Al₁₉ (A = Th, U) compounds. The analysis for the inverted potentials explains qualitatively the contributions of different atoms to the vibrational modes.

Abstract: Lithium imide (Li2NH) and amide (LiNH2) belong to the Li-H-N system, which has been recently considered for on-board hydrogen storage applications. However the imide low-temperature crystal structure is still highly controversial, with at least six options compatible with the diffraction experimental findings. A complementary study on low-temperature Li2NH and LiNH2 has been recently accomplished by the authors using neutron spectroscopy (with energy transfer in the 3-500 meV range). The rationale of these measurements was that crystal structures (especially their proton arrangements) affect in a strong way the neutron scattering spectra, so that a combined use of computer ab-initio simulations and inelastic neutron scattering could be a stringent validation method for the various models. Data analysis has pointed out broad and almost featureless proton-projected phonon densities of states for lithium imide, with large differences in the data sets derived from forward scattering and backscattering detector banks. On the contrary, a sharp phonon spectrum and much less discrepancy was found applying the same analytic procedure to lithium amide. This Li2NH peculiarity has been interpreted as an effect of the fast proton jump diffusion among the available lattice sites, which smears out the phonon vibrational excitations in a momentum transfer-dependent way.

Abstract: Shrinking the size of a solid down to nanometer scale is indeed fascinating, which makes all the otherwise constant physical quantities to be tunable such as the Young’s modulus, dielectric constant, melting point, etc. The variation of size also generates novel properties that can hardly be seen in the bulk such as the conductor-insulator and nonmagnetic-magnetic transition of noble metals at the nanoscale. Although the physics of materials at the nanoscale has been extensively investigated, the laws governing the energetic and dynamic behavior of electrons at such a scale and their consequences on the tunable physical properties of nanostructures have not been well understood [C. Q. Sun, Prog Solid State Chem 35, 1-159 (2007); Prog Mater Sci 54, 179-307 (2009)]. The objective of the contribution is to update the recent progress in dealing with the coordination-resolved energetic and dynamic behavior of bonds in the low-dimensional systems with consideration of the joint effect of temperature and pressure. It is shown that the broken-bond-induced local strain and the associated charge and energy quantum trapping at the defect sites perturbs the atomic cohesive energy, electroaffinity, the Hamiltonian and the associated properties of entities ranging from point defects, surfaces, nanocavities and nanostructures. Application of the theories to observations has led to consistent understanding of the behavior of nanometer-sized materials and the interdependence of these entities as well as the means of determining the bond energy through the temperature-dependent measurements.

Abstract: In this paper a model was proposed to calculate the interface potential of a non ideal finite crystal. Most of the research in this issue usually assume ideal conditions to work with infinite perfect crystals. The model includes a perturbative potential to consider an effect associated to finite size crystal and superficial atomic rearrangement. This effect is considered to be in a first order. The model was applied to graphite , as an example, mainly because of its theoretical interest for wastewater electrochemical treatment.

Abstract: The lattice dynamics and thermodynamic properties of MgS and related II-VI compounds are studied by the first-principles linear-response function calculation in the framework of densityfunctional perturbation theory. The ab initio structural, mechanic and dielectric parameters of these phases are presented. From the theoretical phonon dispersion relations, the linear thermal expansion coefficient and its temperature dependence are calculated. The differences in structural and thermodynamic behaviors of these compounds are explained from their phonon dispersion characters.