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Improvements in determination of the thermal scattering law of moderator materials (measuring, calculating and validating) are important for accurate prediction of neutron thermalization in nuclear systems. In this work a methodology for producing thermal scattering libraries from the experimental data for polyethylene C 2 H 4 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 phonon spectrum for C 2 H 4 n , are created using the NJOY2016 code. Two different methods were used: direct and indirect geometry neutron scattering at ARCS and SEQUOIA, and VISION instruments, respectively, where the phonon spectrum was derived from the dynamical structure factor S(Q, ω ) obtained from the measured DDSCS. In order to compare and validate the newly created library, the experimental setup was simulated using MCNP6.1. Compared with the current ENDF/B-VII.1, the resulting RPI C 2 H 4 n libraries improved both double differential scattering and total scattering cross sections. A set of criticality benchmarks containing C 2 H 4 n from HEU-MET-THERM resulted in an overall improved calculation of K eff , although the libraries should be tested against benchmarks more sensitive to C 2 H 4 n . The DFT + oClimax method is used and is shown to be most comprehensive method for analysis of moderator materials. The importance of DFT + oClimax method lies in the fact that it can be validated against all data measured at VISION, ARCS and SEQUOIA, and experimental total scattering cross section measurements. [ABSTRACT FROM AUTHOR]

Cu6PS5Br compound belongs to the large family of complex chalcohalides crystallizing in the argyrodite structures. The main peculiarity of the copper-containing argyrodites is high ionic conductivity of Cu+ ions, which makes it possible to use as the functional electronic materials. In the present study, the crystal structure of Cu6PS5Br argyrodite have been analyzed and described using the superspace symmetry concept. The program operating under the Maple environment and suitable for theoretically calculating the phonon spectra of the CuPS5 Br argyrodite crystal has been developed. Zone structures have been calculated and presented for a series of model Cu6PS5 Br phases. The eigenvalues of the generalized dynamic matrix have been found as well as the dispersion dependences have been built for the directions G-X-M-R-G-M of the Brillouin zone. [ABSTRACT FROM AUTHOR]

The lattice thermal conductivity of lithium fluoride (LiF) is accurately computed from a first-principles approach based on an iterative solution of the Boltzmann transport equation. Real-space finite-difference supercell approach is employed to generate the second- and third-order interatomic force constants. The related physical quantities of LiF are calculated by the second- and third- order potential interactions at 30 K–1000 K. The calculated lattice thermal conductivity 13.89 W/(m K) for LiF at room temperature agrees well with the experimental value, demonstrating that the parameter-free approach can furnish precise descriptions of the lattice thermal conductivity for this material. Besides, the Born effective charges, dielectric constants and phonon spectrum of LiF accord well with the existing data. The lattice thermal conductivities for the iterative solution of BTE are also presented. [ABSTRACT FROM AUTHOR]

We study the inter-layer coupling in bulk tin selenide (SnSe) through density functional theory based calculations. Different approximations for the exchange-correlation functionals and the van der Waals interaction are employed. By performing comparison with graphite, MoS 2 and black phosphorus, we analyze the inter-layer coupling from different points of view, including the binding energy, the low frequency inter-layer optical phonons, and the inter-layer charge transfer. We find that, there is a strong charge transfer between layers of SnSe, resulting in the strongest inter-layer coupling. Moreover, the charge transfer renders the inter-layer coupling in SnSe not of van der Waals type. Mechanical exfoliation has been used to fabricate mono- or few-layer graphene, MoS 2 and black phosphorus. But, our results show that it may be difficult to apply similar technique to SnSe. [ABSTRACT FROM AUTHOR]

In this paper, first-principles calculations and density functional perturbation theory (DFPT) based on the ab-initio approach, have been utilized to investigate mechanical, elastic anisotropy, dynamical, and thermal properties of nuclear fuel UO. The results reveal that the obtained equilibrium lattice parameter at zero temperature and pressure has a good consistency with the experimental lattice constant. The elastic constants are evaluated using the strain-stress method. These coefficients are estimated to be positive and satisfied the Born-Huang relations, which is indicative of the mechanical stability of this compound. The mechanical properties such as shear modulus, bulk modulus, and Young's modulus of UO are calculated, and the results show the compound's stiffness. Pugh's criterion (B/G) is obtained to be approximately 3.5, which confirms that UO is a ductile substance. Elasticity analysis including universal anisotropy and Zener's anisotropy factor reveals that UO falls into the category of anisotropic materials. The phonon spectrum curve is calculated using DFPT along the high symmetric paths and no negative frequencies are observed in the dispersion relations, implying the dynamical stability of the crystalline structure. The quasi-harmonic Debye (QHD) model is employed to study the thermophysical properties such as Debye temperature, isochoric heat capacity, coefficient of thermal expansion, entropy, and Grüneisen parameter at high pressures and temperatures. We found that molar specific heat was more affected by the temperature than pressure. [Display omitted] • The quasi-harmonic Debye model (QHD) has been used in this study. • Phonon dispersion diagram reveals that UO is dynamically stable in cubic phase. • The obtained elastic constants have shown that this ceramic is mechanically stable. • Thermodynamic properties under high pressures and temperatures are studied. • The elastic anisotropy properties of UO alloy are evaluated for the first time. [ABSTRACT FROM AUTHOR]

Based on first-principles calculations with generalized gradient approximation as exchange-correlation functional, the structural, electronic, mechanical, thermal, and phonon properties of III-phosphide binary compounds, namely BP, AlP, GaP, and InP, with cubic zincblende structure have been investigated. The calculations were performed in the framework of density functional theory and density functional perturbation theory (DFPT) implemented in the Quantum ESPRESSO package. The results obtained for the structural and electronic properties are in good agreement with available theoretical and experimental results. The results of our electronic calculations indicate semiconducting properties for these binary compounds. Furthermore, the frequency bandgaps and phonon density of states were also investigated. The computed mechanical constants predict that BP, AlP, GaP, and InP are elastically stable. Finally, we determined the heat capacity and entropy for these binary compounds within a quasiharmonic Debye model using DFPT for comparison. [ABSTRACT FROM AUTHOR]

BaCeO 3 exhibits impressive application potentials on solid oxide fuel cell electrolyte, hydrogen separation membrane and photocatalyst, owing to its unique ionic and electronic properties. In this article, the electronic structures, phonon spectra and thermal properties of BaCeO 3 in orthorhombic, rhombohedral and cubic phases are investigated based on density functional theory. Comparisons with reported experimental results are also presented. The calculation shows that orthorhombic structure is both energetically and dynamically stable under ground state, which is supported by the experiment. Moreover, charge transfer between cations and anions accompanied with phase transition is observed, which is responsible for the softened phonon modes in rhombohedral and cubic phases. Besides, thermal properties are discussed. Oxygen atoms contribute most to the specific heat. The calculated entropy and specific heat at constant pressure fit well with the experimental ones within the measured temperature range. [ABSTRACT FROM AUTHOR]

The structural, vibrational, phonon dispersion spectrum, and thermodynamic properties of LaCoO 3 were investigated using first-principles calculations using density functional theory. Calculations were performed using the projector augmented wave (PAW) pseudopotentials within the generalized-gradient approximation (GGA) and local density approximation (LDA) models for the exchange and correlation functional. In addition, GGA + U and LDA + U were used to describe the 3 d electrons. The phonon dispersion spectrum and phonon density of states of LaCoO 3 were calculated with the finite displacement method. The related thermodynamic properties such as heat capacity, entropy, free energy, and the coefficient of thermal expansion were calculated at various temperatures from the lattice dynamical data obtained by quasi-harmonic approximation. The phonon dispersion spectrum results indicate that the LaCoO 3 structure is dynamically stable. The phonon dispersion and the peaks of the infrared (IR) and Raman spectrum in the frequency range of 0–700 cm −1 were analyzed and assigned. The calculated thermodynamic properties agree well with the experimental results. [ABSTRACT FROM AUTHOR]

The phonon spectra, phonon optical activities, thermal expansion and thermodynamic properties of ordered quaternary carbides M 2 TiAlC 2 (M = Cr, Mo, W) and their ternary counterparts Ti 3 AlC 2 and Cr 3 AlC 2 are studied by first-principles phonon calculations. Phonon eigenvectors of all Raman and infrared modes are analyzed. We find that vibrational frequencies of some Raman active modes (A 1g and E 2g ) can be correlated to the strength and reduced mass of covalent M (4f) C or metallic M (4f) Al bond in all studied structures. Phonon mode-Grüneisen parameter and macroscopic Grüneisen constants are evaluated from phonon spectra. The thermal expansion coefficients, specific heats and isothermal bulk moduli at finite temperatures are obtained under the quasiharmonic approximation (QHA). The average linear thermal expansion coefficient (LTEC) values are found to be α L = 11.30 × 10 −6 K −1 , 7.15 × 10 −6 K −1 , 8.67 × 10 −6 K −1 , 10.29 × 10 −6 K −1 and 15.70 × 10 −6 K −1 for Cr 2 TiAlC 2 , Mo 2 TiAlC 2 , W 2 Ti 2 AlC 2 , Ti 3 AlC 2 and Cr 3 AlC 2 in a range from 300 K to 1400 K, respectively. The hypothetical Cr 3 AlC 2 phase shows strong anharmonic effects in the calculated thermal expansion profile above 600 K. Using Slack-model, the thermal conductivities are also evaluated, and the values are κ = 36.1 W/m K (34.4 W/m K, 27.2 W/m K) for Cr 2 TiAlC 2 (Mo 2 TiAlC 2 , W 2 Ti 2 AlC 2 ) at room temperature. [ABSTRACT FROM AUTHOR]

The study of vibrational characteristics of chains of rare gas atoms adsorbed in the grooves between nanotubes in nanobundles is reduced to the analyses of the phonon spectrum and the vibrational characteristics of linear chains of atoms in an external field. Atoms in the chain have three degrees of freedom. The analytical expressions for the vibrational characteristics of the atoms in the chain, depending on the ratio between the interatomic distance in the chain r and the equilibrium distance between atoms in the chain $$r_0$$ , are obtained. It is shown that at $$r

The lattice thermal conductivity of CsCl crystal is theoretically investigated from a first-principles theoretical approach based on an iterative solution of the Boltzmann transport equation. Real-space finite-difference supercell approach is employed to generate the harmonic and anharmonic interatomic force constants. Phonon frequencies, velocities, and specific heat capacity as well as anharmonic properties are then obtained and applied to calculate the bulk thermal conductivity of CsCl crystal at the temperatures ranging from 20 K to 700 K. The calculated lattice thermal conductivity 1.14 W/mK of CsCl at room temperature agrees well with the experimental value, demonstrating that this parameter-free approach can provide a good description for the thermal transport of this material. The RTA and iterative solution of BTE are both presented. Our results show that both methods can obtain the thermal conductivity successfully. [ABSTRACT FROM AUTHOR]

The results from calculation of the energy spectra and densities of states of phonons in bulk and two-dimensional crystals of monomolecular thickness from molybdenum disulfide and ditelluride are presented. They demonstrate the dominant role of the interaction of atoms within a single monomolecular layer in the phonon characteristics of these materials. [ABSTRACT FROM AUTHOR]

In this paper, we study the phonon thermal properties of a ladder nanostructure in harmonic approximation. We present a model consisting of two infinite chains with different masses. Then, we investigate the effect of different masses on the phonon spectrum. Moreover, as a specific case, in the absence of the second neighbor interaction, we calculate the phonon density of states/modes. Finally, we consider the thermal conductivity of the system. The results show that the phonon spectrum shifts down to the lower frequencies by increasing the masses. Furthermore, a frequency gap appears in the phonon spectrum. By increasing the springs constants, the thermal conductance decreases.