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In this research, the mechanical and phonon properties of UO in the rock-salt phase have been calculated systematically using density functional theory and ab-initio method in projector augmented wave pseudopotentials parameterization. The obtained lattice constant has good accordance with the experimental one at null pressure and temperature. The mechanical stability is confirmed because the positive elastic constants meet the Born-Huang criterion. The mechanical properties including bulk modulus, shear modulus, and Young’s modulus are evaluated using the Voigt-Reuss-Hill approximation. The value of Poisson’s ratio (v) is obtained to be nearly 0.36, which shows that this compound has an ionic bond and interatomic forces within the crystal are central. The elasticity analysis of the Kleinman parameter reveals that bond bending is dominant within the sublattice of UO. The vibrational density of states and phonon dispersion are investigated through the linear response approach. The results shows the dynamical stability of crystalline structure due to the inexistence of virtual frequencies in dispersion relations.

In situ thermo‐Raman spectroscopy is used to study a single crystal of natural orthorhombic cubanite from the Talnakh copper‐nickel sulfide deposit (Norilsk, Russia). The dependence of the wavenumbers and FWHMs of vibrational modes on temperature, including the cubanite → isocubanite phase transition region, is studied in the range of 83–873 K. For the first time, the polarized Raman spectra of cubanite have been interpreted according to ab initio calculations performed using the density functional theory (DFT) implemented in the VASP package. In spite of the tetrahedral environment of Cu and Fe atoms in cubanite, the spectra demonstrate the presence of different types of bending (twisting, wagging, scissoring, etc.) and stretching vibrations in two different triatomic (S2‐Cu‐S2) and (Fe‐S1‐Fe) fragments for Ag and B2g symmetries, and in three‐ and four‐atomic (S1‐Cu‐S1) and FeS(2)3 fragments, respectively, for B1g and B3g symmetries. [ABSTRACT FROM AUTHOR]

In situ thermo‐Raman spectroscopy is used to study a single crystal of natural orthorhombic cubanite from the Talnakh copper‐nickel sulfide deposit (Norilsk, Russia). The dependence of the wavenumbers and FWHMs of vibrational modes on temperature, including the cubanite → isocubanite phase transition region, is studied in the range of 83–873 K. For the first time, the polarized Raman spectra of cubanite have been interpreted according to ab initio calculations performed using the density functional theory (DFT) implemented in the VASP package. In spite of the tetrahedral environment of Cu and Fe atoms in cubanite, the spectra demonstrate the presence of different types of bending (twisting, wagging, scissoring, etc.) and stretching vibrations in two different triatomic (S2‐Cu‐S2) and (Fe‐S1‐Fe) fragments for Ag and B2g symmetries, and in three‐ and four‐atomic (S1‐Cu‐S1) and FeS(2)3 fragments, respectively, for B1g and B3g symmetries. [ABSTRACT FROM AUTHOR]

Proposal of hydrogen production as an alternative energy source via water dissociation is one of the possible ways forward to cope with the challenge of continuous decline of conventional energy sources and its environmental hazards. Besides, in this regard, low-cost photocatalyst with improved energy efficiency is highly desirable. On the other hand, materials with pronounced thermoelectric performance are promising candidates for thermoelectric source of alternative energy applications. Hereby, we report on the strain-assisted calculated phononic and electronic properties, thermoelectric performance and photocatalytic capacity of monolayer transition-metal dichalcogenide (TMDC) SnSSe. For the first-principles quantum computations based on density functional theory, we employed PBE (Perdew–Burke–Ernzerhof) exchange correlation functional. To account for the interatomic van der Waals Forces and empirical dispersion correction, we used the method of Grimme DFT-D2. As strain is one of the most suitable techniques on tuning the chemical and physical performance of materials for promising renewable energy demands, hereby, we present a theoretical proposal on the strain-assisted tuning on the electronic properties and photocatalytic performance of monolayer SnSSe TMDC. Besides, we confirm the stability of the material with and without strain through quantum calculations of phononic spectrum. Moreover, to predict on the thermoelectric performance of the materials, we report on the calculated temperature-dependent Seebeck effect, power factor, electrical and thermal conductivity, and figure of merit. The suitability of oxidation regarding the photocatalytic performance of the materials based on valence and conduction energy band edge potentials is guaranteed at pH = 0. [ABSTRACT FROM AUTHOR]

The density functional theory has been used to investigate the structural, thermodynamic, electronic, and optical properties of two-dimensional (2D) ZrNI monolayers without and with doped states formed by P, Cl and Br. All three acoustic and optical modes have non-zero frequencies, demonstrating the stability of the ZrNI monolayer. The separated optical and acoustic modes at lower frequencies are proportional to the crystal momentum, representing that the phase velocity of the monolayer is nearly constant in wavelength. The thermodynamically stable 2D ZrNI monolayer has a comparatively low heat capacity and moderate Deby temperature. The 2D ZrNI monolayer is an indirect bandgap semiconductor with a gap width of 0.80 eV. Halogen (Cl and Br) doping increases the gap width to 0.85 and 0.84 eV, while P doping reduces the bandgap to 0.67 eV. Upon doping, all the materials maintained the indirect nature of the band diagram. The total density of states of the pristine and doped 2D ZrNI monolayer exposed contemptible electronic accessibility at the Fermi level, thereby envisaging the semi-metallic nature of the monolayer. The band structure analysis indicated the non-dispersive behaviour of the monolayer in the first Brillouin zone (BZ) and the dispersive nature in the k -space through the energy range from −2.00 eV to 3.00 eV. Pristine ZrNI monolayer absorbs light up to the mid-visible spectrum, and Br and P doping raise the absorption capability to the entire visible range with sharp peaks at the UV range. • A DFT analysis is used to investigate the structural, thermodynamic, electronic and optical properties of 2D ZrNI monolayer. • Thermodynamic stability of the monolayer has been predicted via phonon calculations. • All the monolayers due to Cl, Br and P doping are found to be semiconducting. • The electronic properties of pristine and doped monolayers were compared by employing GGA-PBE and HSE functionals. • The P-doped ZrNI monolayer is more optically sensitive than the others. [ABSTRACT FROM AUTHOR]

The density functional theory is used to calculate vibrational spectra, suggest dependencies of total energies and frequencies on BaF2 and PbF2 crystal lattice constants, and obtain the density of two phonon states in PbF2 crystal weighted by thermal occupation numbers. Due to the order-of-magnitude difference between metal and fluorine masses, acoustic and optical branches are determined by metal and fluorine vibrations, respectively. Therefore, phonon spectra can be demonstrated in two Brillouin zones, namely fluorine lattice and sublattice, the number of optical branches being decreased twice. Based on the suggested dependencies of total energies in the crystal and squared frequencies on the lattice parameter, anharmonicity constants are identified. It is shown that with increasing temperature the frequency attenuates. For PbF2 crystal, anharmonicity constants are shown to be higher than for BaF2 crystal, and this fact is supported by the experiment. The model of the density of two phonon states in PbF2 crystal shows that the temperature broadening of Raman frequency is more substantial than that of the infrared active phonon frequency. [ABSTRACT FROM AUTHOR]

Vibrational spectra of β-LiNH4SO4 crystals in the ferroelectric phase (space group No. 33) are studied. The vibrational symmetry is classified using group-theoretical analysis. Vibrational frequencies of crystal structural elements are calculated ab initio within the framework of electron density functional theory and perturbation theory. Raman scattering spectra of β-LiNH4SO4 crystals are calculated and compared with the experimental data. The main structural components playing one of the major roles in the optical studies are discussed based on the theoretical calculations. [ABSTRACT FROM AUTHOR]

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]

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]

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 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]

In this study, we focus on structural, electronic, elastic, lattice dynamic and optic properties of MgGeP 2 and MgGeAs 2 using ab-initio density-functional theory (DFT) within Armiento-Mattson 2005 (AM05) scheme of the generalized gradient approximation (GGA) for the exchange-correlation potential. Our computed structural results are in reasonable agreement with the literature. The band gap of these compounds is predicted to be direct. Our elastic results prove that these compounds are mechanically stable. The obtained phonon spectra of MgGeP 2 and MgGeAs 2 do not exhibit any significant imaginary branches using GGA-AM05 for the exchange-correlation approximation. Further analysis of the optical response of the dielectric functions, optical reflectivity, refractive index, extinction coefficient and electron energy loss delves into for the energy range of 0–22.5 eV. It motivated that there exists an optical polarization anisotropy of these compounds for optoelectronic device applications. [ABSTRACT FROM AUTHOR]

Thermodynamic performances of MgjSn metal compounds were calculated by first principle calculations. This study, based on density functional theory, calculated lattice constants, band structure and elastic constants to detail the thermal stability of Mg2Sn structures, and then focused on the phonon density of states, constant volume and Debye temperature to deal with thermodynamic properties. Additionally, the calculations were compared with available experimental value and previous theoretical data, and a favorable result has been obtained in the work. Furthermore, the results manifested that Mg2Sn has better capability when adding the alloying element Sn into Mg alloys, and demonstrated that quasi-harmonic Debye model with first-principles is an efficient approach to study the behavior of intermetallic compounds, which provided more valuable instructional information for the design and application of Mg2Sn compounds. [ABSTRACT FROM AUTHOR]

This work reports the results of DFT-based calculations of phonon spectra of TlGaTe2. The dispersion of phonon bands was calculated along the directions of Brillouin zone (BZ) that include symmetry points. The calculated phonon frequencies at the centre of BZ were compared with those obtained by Raman spectroscopy with the aid of a confocal laser microscopy system. A fairly good agreement between the calculated and experimental data was found. Complimentary, molar heat capacity at constant volume and Debye temperature were calculated in the range 5÷500 K on the base of the obtained phonon density of states. The obtained temperature dependencies were compared with available experimental data.The results of comparison were satisfactory. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

This work reports the results of DFT-based calculations of phonon spectra of TlGaTe2. The dispersion of phonon bands was calculated along the directions of Brillouin zone (BZ) that include symmetry points. The calculated phonon frequencies at the centre of BZ were compared with those obtained by Raman spectroscopy with the aid of a confocal laser microscopy system. A fairly good agreement between the calculated and experimental data was found. Complimentary, molar heat capacity at constant volume and Debye temperature were calculated in the range 5÷500 K on the base of the obtained phonon density of states. The obtained temperature dependencies were compared with available experimental data.The results of comparison were satisfactory. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Due to the high lattice symmetry and special three-dimensional crystal channel, sulvanite Cu 3 VSe 4 is predicted to be a new type of thermoelectric material with good performance. In this work, the energy band diagram and phonon spectrum of Cu 3 VSe 4 with sulvanite structure were calculated using density functional theory, and Cu 3 VSe 4 pure phase was experimentally synthesized through the combination of the elements using conventional solid-state reactions. Surprisingly, the thermoelectric characterization reveals that Cu 3 VSe 4 is a semiconductor with electron-hole co-transport feature, which results in the poor thermoelectric performance due to the recombination of electrons and holes at low temperature. While holes gradually dominate with the increase of temperature and Cu 3 VSe 4 possesses a high Seebeck coefficient of 558 μV/K at 571 K. Our results may serve as the groundwork for future research on the structure and thermoelectric properties of Cu 3 VSe 4. • Cubic Cu 3 VSe 4 is a semiconductor with an indirect band gap of 0.84 eV. • Pure Cu 3 VSe 4 was synthesized by traditional solid-state reaction. • Cu 3 VSe 4 is a semiconductor with electron-hole co-transport feature. • The Seebeck coefficients increase from 20 μV/K at 325 K to 558 μV/K at 571 K. [ABSTRACT FROM AUTHOR]