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The present work employs density functional theory to explore the structural, optoelectronic, and thermoelectric attributes of the halide-based double perovskite A2GeSnF6 (A = K, Rb, and Cs) compounds. The stable phonon dispersion spectrum affirms dynamical stability, whereas the enthalpy of formation and tolerance factor evaluated collectively verify structural stability. Considering the Tran Blaha modified Becke Johnson potentials (mBJ), the predicted direct band gaps along the symmetry point are 3.19 eV for K2GeSnF6, 3.16 eV for Rb2GeSnF6 and 3.12 eV Cs2GeSnF6. According to an in-depth examination of the optoelectronic features, A2GeSnF6 (A = K, Rb, and Cs), double perovskites are assuring contenders for optoelectronic devices due to their suitable bandgap. The extremely high figure of merit values (0.94–0.97) obtained from the numerical calculation of power factor and thermal conductivity suggest the intriguing prospects of these compositions for thermoelectric devices. These studies offer a perceptive comprehension of the materials for their potential applications in the future. [ABSTRACT FROM AUTHOR]

Recently, there has been renewed interest in thermoelectric materials based on germanium telluride, which demonstrate high efficiency in mid-temperature range. This paper discusses the theoretical description of the phonon spectrum and lattice thermal conductivity in GeTe using ab initio methods. Using these methods, the temperature dependence of the lattice thermal conductivity in the rhombohedral phase was calculated and effects of scattering by point defects and nanostructuring were estimated. The modification of the phonon spectrum upon the transition to the high-temperature cubic phase is investigated. The calculated temperature dependences of the lattice thermal conductivity are compared with the available experimental data on GeTe and its solid solutions. [ABSTRACT FROM AUTHOR]

Herein, frictional phonon dissipation in monolayer/bilayer graphene was modeled using phonon spectra based on molecular dynamics simulations. The results indicate that the number of excited acoustic phonon modes is the primary reason for increased friction. Specifically, the frequencies of flexural acoustic modes shifted to high levels as thickness increased during the sliding process, resulting in increased friction. The increase in friction with sliding velocity is caused by an increase in the number of in-plane acoustic modes. Higher normal loads can increase both the in-plane and flexural acoustic modes, leading to increased friction. Our observations further suggest that the variation in temperature at the friction interface results from the competition between frictional energy and thermal conductivity. Both high normal loads and thick layers increase the thermal conductivity, ultimately improving the friction dissipation efficiency. Hence, it can be concluded that the increase in thermal conductivity is the reason for the counterintuitive decrease in the interfacial temperature resulting from high friction. [ABSTRACT FROM AUTHOR]

Crystal structure and phonon spectrum of rare-earth stannates R 2 Sn 2 O 7 and zirconates R 2 Zr 2 O 7 (R = La–Lu) with the pyrochlore structure were studied within the framework of DFT and MO LCAO approach. The calculations were performed by using hybrid functional PBE0 that take into account nonlocal exchange at the Hartree-Fock formalism. The calculation of the phonon spectrum at the Γ -point made it possible to supplement the available experimental data on the IR and Raman spectra of stannates and zirconates. In the framework of the ab initio approach, the change of the elastic properties of zirconium oxide t-ZrO 2 by doping the yttrium was studied. Calculations were performed in the CRYSTAL17 program designed to simulate periodic structures. [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]

This article presents the results of inelastic scattering studies of neutrons and copper and silver chalcogenides at a temperature of 300 K in the non-superionic phase. We were the first to obtain dynamic structural factors and generalized the phonon densities of the states of these compounds. Inelastic peaks observed at energies of 3–5 meV presumably correspond to the acoustic vibrations of phonons. The phonon spectra of copper and silver chalcogenides have features characteristic of compounds that are structurally disordered. [ABSTRACT FROM AUTHOR]

In the CeO2, ThO2, and NpO2 crystals, the acoustic branches of the phonon spectra correspond predominantly to the vibrations of metal ions, and the optical branches correspond mainly to the vibrations of oxygen ones, due to a significant difference in the masses of these components of the compounds. An important crystallographic phenomenon is that metal and oxygen are located in sublattices corresponding to different Bravais types, which makes it possible to analyze vibrations in the corresponding Brillouin zones (BZ). This analysis was carried out by unfolding the optical branches of the phonon spectrum from the BZ of the crystal into the BZ of the oxygen sublattice. The number of optical branches in this BZ is half that in the BZ of the crystal. The effect of anharmonicity on the thermal broadening of the spectral lines of the phonon frequencies is studied in the approximation of two-phonon densities of states. For all three compounds, the calculated two-phonon densities of states represent two broad structured peaks. The temperature dependences of the upper peak show thermal broadening of the upper part of the optical branches and the lower peak shows thermal broadening of the low-energy optical and acoustic branches. [ABSTRACT FROM AUTHOR]

The different translational symmetry of the cuprite crystal sublattices is taken into account by unfolding the phonon spectrum from the Brillouin zone of the crystal into the Brillouin zones of the anion and metal. Such a representation of the spectrum is reasonable because 6 optical branches are essentially caused by the vibrations of anions and 12 acoustic branches are due to the vibrations of metal owing to the great value difference between their masses. The volumes of the Brillouin zones of the anion and metal sublattices are two and four times, respectively, as large as those of the crystal resulting in the qualitative differences in the corresponding unfolded spectra because of the metal tetrahedron vibrations of the distortion type in the acoustic part of the spectrum. These vibrations located on the Х′(1,0,0)–W′(1,0,1/2) and Х′(0,1,0)– W′(0,1,1/2) lines of the metal Brillouin zone are responsible for the negative thermal expansion. The unfolded spectra have only three branches in the corresponding Brillouin zones and this simplifies the selection rules for the physical processes with the participation of the sublattice particle vibrations. [ABSTRACT FROM AUTHOR]

The structure and properties of two-dimensional monolayers composed of 1,6-carboranes have been studied by means of density functional theory calculations with periodic boundary conditions. Calculations of the phonon dispersion spectrum and band structure have shown that the 2D monolayers are dynamically stable and refer to indirect band gap semiconductors with a band gap of ~2.5 eV. Calculations of mechanical properties demonstrate that Young's modulus for 2D 1,6-carborane nanosheets is lower than that for the monolayer of hexagonal boron nitride (h-BN) but exceeds that for the MoS2 monolayer. [ABSTRACT FROM AUTHOR]

The article presents the results of investigations of copper selenide Cu1.75Se using the NMR method. The 77Se NMR spectrum for Cu1.75Se is a single line with full width at half maximum (FWHM) of 4 kHz at the temperature of 250 K. Within the temperature range of 200-450 K, the rates of transverse and longitudinal relaxation are equal. This phenomenon is mainly typical of a liquid, where the homonuclear dipole-dipole interaction is effectively averaged due to the Brownian motion of molecules. In this case, the main mechanism that averages the dipole-dipole interaction is the motion of copper ions. Below 200 K, copper ions are partially localized due to the phase transition, and the behavior of the spin-spin and spin-lattice relaxation rates (as well as their values) are different. [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]

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]

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 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]

Existing models for calculation of thermal conductivity of uranium dioxide are analyzed. Anomalous growth of the thermal conductivity coefficient of nuclear fuel in the high-temperature region is explained. Models of specific heat at constant volume and thermal conductivity are proposed. It is demonstrated that the contribution of degrees of freedom of charge carriers to the observed thermal conductivity coefficient of admixture-free stoichiometric UO can be neglected up to the melting point. [ABSTRACT FROM AUTHOR]

The vibrational spectrum, phonon dispersion curve, and phonon density of states (DOS) of β-octahydro-1,3,5,7- tetranitro-1,3,5,7-tetrazocine (β-HMX) crystal were obtained by molecular simulation and calculations. As results, it was found that the peaks at low frequency (0–2.5 THz) are comparable with the experimental Terahertz absorption and the molecular vibrational modes are in agreement with previous reports. Thermodynamic properties including Gibbs free energy, enthalpy, and heat capacity as functions of temperature were obtained based on the calculated phonon spectrum. The heat capacity at normal temperature was calculated using linear fitting method, with a result consistent with experiments. [ABSTRACT FROM AUTHOR]

The possibility of calculating radial and tangential force constants, as well as elastic constants of face-centered cubic (FCC) metals, has been demonstrated for the case study of nickel on the basis of the experimentally measured phonon spectrum, which is theoretically described by the DeLaunay model in terms of the dynamic matrix D( q). The proposed calculation procedure is an alternative to the Born-von Karman model, which is widely applied for the description of the phonon spectrum in terms of force constants for interatomic interactions and in calculations of elastic constants using the phonon spectrum. The reliability of the obtained force constants for phonon spectrum was verified by comparison between the experimental and calculated data for the phonon spectrum and elastic constants for nickel. The obtained values of force constants for nickel have been compared with the theoretically calculated values using the pseudopotential method. [ABSTRACT FROM AUTHOR]

The phonon spectrum of hypothetical superhard cp-BC2N is calculated based on ab initio method of density functional in the center of the Brillouin zone and interpolated over the entire Brillouin zone using the Keating phenomenological model. The interaction parameters are determined by optimization of the IR- and Ramanactive frequencies for a phenomenological model by their comparison with the results of ab initio calculations. Numerical values of short-range interaction constants and charges are in agreement with the characteristics of the chemical bond calculated ab initio. These parameters have transparent physical meaning and chemical nature and can further be used for both qualitative estimations of any physical and physico-chemical quantities and quantitative calculations of the phonon spectra of a number of isostructural compounds. The Keating phenomenological model is used to study the genesis of the phonon spectrum from the spectra of sublattices. [ABSTRACT FROM AUTHOR]

By the example of cesium, a procedure for calculating the radial and tangential force constants of the interatomic interaction of metals with the body-centered cubic (BCC) structure from the experimentally measured phonon spectrum, which can be theoretically described using elements of the dynamic matrix D( q), is proposed. This technique is an alternative to the Born-von Karman model, which is still widely applied in describing the phonon spectrum in terms of the force constants of interatomic interaction and in calculation of the elastic constants of the phonon spectrum. The reliability of the procedure and obtained values of the force constants is verified by calculating the phonon spectrum and elastic constants of cesium and by comparing the results with experimental data. [ABSTRACT FROM AUTHOR]

The phonon spectra, electron-phonon Eliashberg function, and T of the hafnium diboride were investigated from the first principles using the full potential linear muffin-tin orbital method. The calculations of the dynamic matrix were carried out within the framework of the linear response theory. Comparative analysis of different values T, based upon a b i n i t i o calculation of the electronic band structures as well as the phonon dispersion curves, was performed for row: HfB, ScB, and MgB. Different theoretical approaches to the calculation of the phonon spectra were discussed. [ABSTRACT FROM AUTHOR]

The phonon spectra of a series of A 2B 4C 25 and A 1B 3C 26 crystals with the chalcopyrite structure are calculated in the Keating model in the basis set of polarization vectors of their sublattices. The dependences of frequency values and partial contributions of sublattices to the polarization vectors on the chemical composition are studied. The effect of the mass ratio and the strengths of covalent bonds between the compound components on the phonon spectrum formation is found. [ABSTRACT FROM AUTHOR]

In the recent decade, the spatially localized large-amplitude vibrational modes in defect-free crystals, referred to as discrete breathers (DBs), are intensively investigated in the materials science. This review reports the main results on gap DBs in crystals with the NaCl structure. The experimental proof of their existence in a NaI crystal is described. A number of molecular dynamics simulation results are presented, including new so far unpublished data. The properties of crystals potentially affected by the DBs are discussed. [ABSTRACT FROM AUTHOR]

calculation of heat capacity of diamond using reliable data on the diamond spectroscopic and elastic-dynamic characteristics is performed together with an experimental investigation of a low-temperature (50-300 K) heat capacity of synthetic diamond produced by General Electric. Based on the established temperature dependence of heat capacity, the values of the main thermodynamic functions of diamond under standard conditions have been calculated. The results obtained have been tabulated and recommended for practical use. [ABSTRACT FROM AUTHOR]

The phonon spectrum of ordered zincblende Si50Ge50 alloy is calculated by ab initio method. The energy band structure at zero pressure and the pressure dependence of phonon dispersion curves are shown up to 20 GPa. The calculation finds a pressure-induced softening of the transverse acoustic phonon mode and the mode frequency reaching zero at about 14 GPa, which indicate breaking of the symmetry and formation of a new phase under high pressure. [ABSTRACT FROM AUTHOR]

Using Path Integral Monte Carlo and the Maximum Entropy method, we calculate the dynamic structure factor of solid 3He in the bcc phase at a finite temperature of T = 1.6 K and a molar volume of 21.5 cm3. From the single phonon dynamic structure factor, we obtain both the longitudinal and transverse phonon branches along the main crystalline directions, [001], [011] and [111]. Our results are compared with other theoretical predictions and available experimental data. [ABSTRACT FROM AUTHOR]

In the present work the author describes a method for investigating phonon spectra of thin films of semiconductor solid solutions that is based on reflection-absorption spectroscopy in the far IR region. Analysis of spectra is carried out for two radiation polarizations (of the s- and p-type) and for the following experimental conditions: the spectral region 30–250 cm−1 and an angle of incidence of 45°. By comparing theoretical and experimental spectra for quaternary solid solutions of ZnxCdyHg1−x−yTe (ZCMT), the frequencies of LO- and TO-phonons are found and the three-mode behavior of the phonon spectra of ZCMT is established. [ABSTRACT FROM AUTHOR]