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AlCoCrFeMo0.05Ni2 high-entropy alloy (HEA) has great potential for high-temperature applications. However, its FCC matrix will decompose during annealing, which seriously limits its high-temperature applications. Here, the lattice distortion, Gibbs free energy change, phonon spectra, density of states, and bond length distribution of the FCC matrix were calculated by first-principles calculations. The findings indicate that the primary barrier to the formation of a single homogeneous solid solution in the alloy is more closely associated with Gibbs free energy rather than the degree of lattice distortion. The vibration frequency of Al is not in sync with the overall structure, making it prone to desolvation at elevated temperatures. Furthermore, an analysis of bond lengths reveals that the Al–Ni bond length is significantly shorter than the theoretical value, facilitating the formation of a (Ni, Al)-rich phase. In contrast, the longer bond lengths of Al–Cr and Al–Mo facilitate the detachment of Cr and Mo from the (Ni, Al)-rich phase, resulting in the formation of (Cr, Mo)-rich phase. This in-depth investigation sheds light on the phase decomposition mechanism of high-entropy alloys, offering valuable insights for future research in this field.

Abstract 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.

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]

Using the microscopic s-f model and Green's function theory, we study the temperature dependence of the band gap energy E g and the phonon energy ω and damping γ of ferro- and antiferromagnetic semiconductors, i.e., with different signs of the s-f interaction constant I. The band gap is a fundamental quantity which affects various optical, electronic and energy applications of the materials. In the temperature dependence of E g and the phonon spectrum, there is a kink at the phase transition temperature T C or T N due to the anharmonic spin–phonon interaction (SPI) R. Moreover, the effect of the SPI R and electron–phonon interaction (EPI) A on these properties is discussed. For I > 0 , R > 0 , E g decreases with increasing SPI and EPI, whereas for I < 0 , R > 0 , there is a competition; E g increases with raising the EPI and decreases for enhanced SPI. For R < 0 , in both cases, the SPI and EPI reduce E g . The magnetic field dependence of E g for the two signs of I and R is discussed. The SPI and EPI lead to reducing the energy of the phonon mode ω = 445 cm−1 in EuO ( I > 0 , R < 0 ), whereas ω = 151 cm−1 in EuSe ( I > 0 , R > 0 ) is enhanced with increasing EPI and reduced with SPI. Both the SPI and EPI lead to an increasing of the phonon damping in EuO and EuSe. The results are compared with the existing experimental data. [ABSTRACT FROM AUTHOR]

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.

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]

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]

We used the first - principles method based on density functional theory to calculate the crystal structures, electronic structures, optical properties and phonon spectra of α - Si3N4 and β - Si3N4• The obtained results have been theoretically analyzed. The calculated results show that both phases contain strong covalent bonds and are insulators. The band gap of α - Si3N4 is slightly larger than that of β - Si3N4• The calculated optical absorption coefficients show that α - Si3N4 and β - Si3N4 mainly absorb ultraviolet light, and their energy loss peaks are around 24. 4 eV. In the phonon spectra, α - Si3N4 and β - Si3N4 have no imaginary frequency, indicating that they are stable. In the range of 0 - 1000 K, the heat capacity of α - Si3N4 is about twice that of β - Si3N4 . The calculated results in this paper will provide a theoretical basis for the related applications of Si3N4. [ABSTRACT FROM AUTHOR]

Using the microscopic s-f model and Green’s function theory, we study the temperature dependence of the band gap energy Eg and the phonon energy ω and damping γ of ferro- and antiferromagnetic semiconductors, i.e., with different signs of the s-f interaction constant I. The band gap is a fundamental quantity which affects various optical, electronic and energy applications of the materials. In the temperature dependence of Eg and the phonon spectrum, there is a kink at the phase transition temperature TC or TN due to the anharmonic spin–phonon interaction (SPI) R. Moreover, the effect of the SPI R and electron–phonon interaction (EPI) A on these properties is discussed. For I>0,R>0, Eg decreases with increasing SPI and EPI, whereas for I<0,R>0, there is a competition; Eg increases with raising the EPI and decreases for enhanced SPI. For R<0, in both cases, the SPI and EPI reduce Eg. The magnetic field dependence of Eg for the two signs of I and R is discussed. The SPI and EPI lead to reducing the energy of the phonon mode ω = 445 cm−1 in EuO (I>0, R<0), whereas ω = 151 cm−1 in EuSe (I>0, R>0) is enhanced with increasing EPI and reduced with SPI. Both the SPI and EPI lead to an increasing of the phonon damping in EuO and EuSe. The results are compared with the existing experimental data.

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It is of great significance to study the nucleation mechanism of Si and P elements in the Fe 2 Mo-Laves phase to control their precipitation and improve the strength of steel. Based on the first-principles calculations, the effects of Si and P elements on the stability of Fe 2 Mo were studied. The addition of Si and P greatly increases the possibility of its formation in steel. When the Si and P concentrations are 37.5 %, the formation energy is the lowest. Si and P elements improve the stress deformation resistance and the stiffness of Laves phase, which makes Laves phase more stable in high-temperature environment. The addition of Si and P significantly improves the kinetic stability of Fe 2 Mo, but the improvement effect of P is not as good as that of Si. This study provides theoretical guidance for controlling the precipitation of the Fe 2 Mo-Laves phase in practical production. The addition of Si and P greatly increases the possibility of its formation in steel. The addition of Si and P significantly improves the kinetic stability of Fe 2 Mo, but the improvement effect of P is not as good as that of Si. [Display omitted] • Si and P elements have stronger interaction with Mo by replacing Fe in Fe 2 Mo phase. • When the Si and P concentrations are 37.5 %, the formation energy is the lowest. • The doping of Si and P elements improves the mechanical properties and thermal stability of Fe 2 Mo phase. • The improvement effect of Si doping on the kinetic stability of Fe 2 Mo is better than that of P. [ABSTRACT FROM AUTHOR]

The vibrational, electronic and Raman properties of monolayer Ti2C under planar symmetrical strain excited by two commonly used laser lines (532 and 633 nm) are investigated by first‐principles calculations. By analyzing the phonon spectrum, the stability of strained monolayer Ti2C is obtained. The strained Ti2C is found to have the same point group symmetry and thereby the same Raman‐active modes as unstrained Ti2C, but their Raman peaks exhibit notable red (blue) shift characteristic with symmetrical tensile (compressive) strain. We also discussed the polarization angle dependent Raman intensity in parallel and perpendicular configuration as a guide for experimental characterization in terms of the locations and magnitudes of maxima and found that the locations of maxima for the Eg (A1g) mode are dependent (independent) of the laser wavelength and the strain. The variation of the polarization direction averaged Raman spectra versus strain can be further explained by the characteristics of light absorbance. [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]

In this paper, the phonon spectra of ZrSe 2 , HfS 2 , and HfS 2 /ZrSe 2 heterojunctions are calculated based on the density-functional perturbation method for biaxial tensile strains to study their stability and thermal properties. It was found that monolayers of ZrSe 2 and HfS 2 should not be subjected to > 8% biaxial tensile strain, and HfS 2 and HfS 2 /ZrSe 2 heterojunctions should not be subjected to > 6%. By looking at the graphs of enthalpy, entropy, free energy, and heat capacity under strain for the three systems, it is found that the effect on heat capacity is greater after destabilization in the monolayer system and after destabilization in the heterojunction structure, it is greater for the free energy. Enthalpy, entropy and free energy all increase continuously with increasing temperature, and heat capacity increases continuously with the rising temperature at temperatures < 200 K and tends to remain constant when the temperature is >200 K. Fig. 1 (a and b) Geometric structures of monolayers of HfS 2 and ZrSe 2. (c) The structure of the HfS 2 /ZrSe 2 heterojunction. [Display omitted] In this paper, the phonon spectra of ZrSe 2 , HfS 2 , and HfS 2 /ZrSe 2 heterojunctions are calculated based on the density-functional perturbation method for biaxial tensile strains to study their stability and thermal properties. It was found that monolayers of ZrSe 2 and HfS 2 should not be subjected to >8 % biaxial tensile strain, and HfS 2 /ZrSe 2 heterojunctions should not be subjected to >6 %. By looking at the graphs of enthalpy, entropy, free energy, and heat capacity under strain for the three systems, it is found that the effect on heat capacity is greater after destabilization in the monolayer system and after destabilization in the heterojunction structure, it is greater for the free energy. Enthalpy, entropy and free energy all increase continuously with increasing temperature, and heat capacity increases continuously with the rising temperature at temperatures <200 K and tends to remain constant when the temperature is >200 K. [ABSTRACT FROM AUTHOR]

This study reveals unexpected strain-dependent friction, when applied strain to one of two layers of relatively sliding black phosphorus, friction is likely to exhibit characteristic rise and subsequent fall, forming a distinct friction peak. Existence and location of such peak are closely related to magnitude of normal loads. Interfacial pinning is weak at lower normal load, matched dissipation channels are easy to be destroyed, resulting in a monotonic decrease of friction. At higher normal load, tip is also pulled apart once applying a small strain to substrate, stored deformation energy leads to greater energy dissipation than at strain of zero, causing friction to decrease non-monotonically. Moreover, ability of sliding layers to withstand strain decreases with increasing normal loads or temperatures. [Display omitted] • When strain is applied to one of the two sliding layers, the friction increases and then decreases to form a distinct peak. • The ability of sliding layers to withstand strain is influenced by normal load and temperature. • A method for controlling two-dimensional materials interlayer phononic friction is investigated using phonon spectrum. [ABSTRACT FROM AUTHOR]

First‐principles calculation and Boltzmann transport theory have been combined to comparatively investigate the integration of Crystal Orbital Hamilton Populations (−ICOHP), band structures, phonon spectrums, lattice thermal conductivities, electronic transport properties, Seebeck coefficients, and thermoelectric figure of merits of β‐VA monolayers (Sb, As, and P). Calculations reveal that the thermoelectric properties increase with the decrease of the bond strength, which should be mainly due to the lower lattice thermal conductivity, suitable band gap, and weakened coupling of electrons and phonons. It is also found that the ZT value of the β‐Sb monolayer for the electronic along the x direction is the best among the β‐VA monolayers. Furthermore, the origin of the lowest lattice thermal conductivity of the β‐Sb monolayer in the β‐VA monolayers (Sb, As, and P) may be attributed to the more phonon scattering channels (P3) and the lower phonon velocity (v).The derived results are in good agreement with other theoretical results in the literature, and could provide a deep understanding of thermoelectric properties of the β‐VA monolayer materials. [ABSTRACT FROM AUTHOR]

According to the experimental synthesis of ZnTiO3 ferroelectric compound with LiNbO3 ( LN) structure, the characteristics of the LN-ZnTiS3 compound were designed and studied based on the first-principles method. The calculated results show that the LN-ZnTiS3 compound meets the conditions of mechanical stability. According to the analysis of the chemical potential equilibrium phase diagram, LN-ZnTiS3 will not form a stable structure under normal pressure, but can form a stable structure under external pressure. The calculated density of states and energy band indicate that the valence band maximum (VBM) of LN-ZnTiS3 is mainly composed of S-p orbitals, while the conduction band minimum (CBM) is composed of Ti-d orbitals. The substitution of sulfur atoms can promote the electron state above Fermi energy level of the system to drop significantly to the lower energy level, thus reducing the band gap of LN-ZnTiS3 . The calculated band gap of LN-ZnTiS3 is 1. 04 eV, which can promote the absorption of visible light. It shows that LN-ZnTiS3 is a potential high-efficiency photovoltaic material. [ABSTRACT FROM AUTHOR]

The paper is devoted to model phonon spectra calculations of argyrodite family crystals, namely: Ag7GeSe5I and Cu7GeSe5I (Ag7SiS5I and Cu7SiS5I). The specificity of the crystalline structure of these crystals is realization of variable partial populating different crystallographic orbits with Cu, I and Ag atoms (in 24 (g) and 48 (h) Positions Multiplicity, Wyckoff letter, Site symmetry of the space group F 43m ), which are associated with high superionic conductivity in crystals of this family. Compared in this paper are model phonon spectra calculated with account of three different configurations of orbital populations with Cu and Ag atoms (6 + 1, 5 + 2 and 4 + 3), by choosing the same lattice model for the unit cell and the set of force constants of each crystal. The calculations were performed by finding the eigenvalues of the generalized dynamic matrix perturbed by the modulation function of the mass defect obtained in the concept over spatial symmetry. It is shown that the phonon spectra do not undergo significant qualitative changes, which can serve as indirect evidence of the possibility to realize the jumping mechanism of super-ionic conductivity in these crystals in the high-temperature phase. [ABSTRACT FROM AUTHOR]

Motivated to understand phonon spectrum renormalization in the ground state of the half-filled Su–Schrieffer–Heeger model, we use the Born–Oppenheimer approximation together with the harmonic approximation to evaluate semi-analytically the all-to-all real-space ionic force constants generated through both linear and quadratic electron-phonon coupling. We then compute the renormalized phonon spectrum and the corresponding lattice zero-point energy (ZPE) as a function of the lattice dimerization. Crucially, the latter is included in the system’s total energy, and thus has a direct effect on the equilibrium dimerization. We find that inclusion of a small quadratic coupling leads to very significant changes in the predicted equilibrium dimerization, calling into question the use of the linear approximation for this model. We also argue that inclusion of the ZPE is key for systems with comparable lattice and electronic energies, and/or for finite size chains. Our method can be straightforwardly generalized to study similar problems in higher dimensions.

The multi-body system’s elasticity and vibrational behavior is an old topic. The conventional method to study this issue is to transform a multi-body system to an abstracted network model consisting of nodes connected by central-force springs. In recent years, abnormal mechanical responses, including negative Poisson ratio, vanishing shear modulus, non-trivial topological characteristics, are discovered in such network systems making this topic be with lasting charms. In this paper, we investigate the vibrational modes of a series of distorted square lattice networks. The distorted degree for this network is only characterized by one parameter, the twisting angle θ. By studying their phonon spectrums, we find a band gap between the two lowest modes of the distorted square lattice. The gap width is linearly correlated to θ, and the gap will be closed when θ reaches a specific value. Our results may find applications in device protection and energy harvesting.

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]

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]

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]

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 Cu6PS5Br argyrodite crystal has been developed. Zone structures have been calculated and presented for a series of model Сu6PS5Br phases. The eigenvalues of the generalized dynamic matrix have been found as well as the dispersion dependences have been built for the directions –X–M–R––M of the Brillouin zone.

• Physical properties of lead free Cs Cu 2 X 3 (X = C l , B r , I) perovskites are studied. • The studied structures of Cs Cu 2 X 3 compounds are mechanically and dynamically stable confirmed by elastic constants and phonon dispersion. • Cu-based lead-free perovskites are a greener alternative to standard lead-based perovskite materials. • Cs Cu 2 X 3 possesses excellent optical properties highly effective as solar reflector for visible light, and ultraviolet absorber. This study effectively explores in detail the structural, optical, electronic, elastic, mechanical, lattice vibrational and thermodynamic features of orthorhombic CsCu 2 X 3 (X = Cl, Br, I) perovskites using first-principles calculations. Formation energy, Born stability criterion, and phonon dispersion validated our compounds thermodynamic, mechanical, and dynamical stability. All the studied compounds possess significant elastic anisotropy and are fairly machinable with Cs Cu 2 I 3 are relatively hard than the other two. UV light can be efficiently absorbed by Cs Cu 2 X 3 (X = Cl, Br, I) compounds. Due to their strong UV reflectivity, all the compounds have the potential to be utilized as shielding to reduce solar heating by up to ∼ 20 eV with Cs Cu 2 Cl 3 being more efficient. The compounds under study have low Debye temperatures, low melting points, and low minimum phonon thermal conductivities. Relatively low values of Debye temperature and minimum thermal conductivity for Cs Cu 2 X 3 making them potential as thermal barrier coatings (TBC) materials. [ABSTRACT FROM AUTHOR]

• Performed double differential scattering cross section measurements for lucite. • Measured dynamic structure factor for lucite at VISION spectrometer. • Developed a new methodology for creation of thermal scattering libraries. • Produced a new improved thermal scattering library for lucite. With the advancements in technology (both experimental and computational) the determination of the"true" experimental phonon spectrum became more accessible. In this work a methodology for producing thermal scattering libraries from the experimental data (namely the DFT + oClimax method) for lucite (C 5 O 2 H 8) 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 the phonon spectrum for (C 5 O 2 H 8) n ,were created using oClimax and NJOY2016 codes. In order to compare and asses the performance of the newly created library, the experimental setup was simulated using MCNP6.1. Compared to the current ENDF/B-VIII.0, the resulting RPI (C 5 O 2 H 8) n library improved the calculation of both double differential scattering and total scattering cross sections. A set of criticality benchmarks containing (C 5 O 2 H 8) n from HEU-MET-THERM resulted in an overall improved calculation of K eff . The DFT + oClimax method is shown to be the most comprehensive method for analysis of moderator materials, due to the fact that it can be verified against all data measured at VISION, ARCS and SEQUOIA neutron spectrometers at SNS ORNL, and experimental total scattering cross section measurements. This method also provides a new technique for calculating any phonon spectrum-related quantities such as scattering law kernel, specific heat capacity, thermal conductivity, etc. for any solid state material. [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]

The stability and magnetic properties of the R E 6 Fe 13 X compounds, as well as their preferential localization in Nd-Fe-B magnets, have been investigated by first-principles calculations. We find that the R E 6 Fe 13 X (X = Al, Si, Zn, and Ga) have negative formation energies and are more likely to form in the triple junctions, while Nd 6 Fe 13 Cu with the formation energy of 0.73 eV tends to localize in the thin intergranular phase. These compounds contribute to the exchange decoupling of Nd 2 Fe 14 B grains and thus enhance the coercivity of Nd-Fe-B magnets. Such decoupling can be more pronounced in the cases of Nd 6 Fe 13 Cu and Nd 6 Fe 13 Zn , which exhibit antiferromagnetic ordering in the Fe sublattice according to our calculations. We propose that the partial substitution of Nd atoms with Pr can gradually reduce the formation energy of these phases and maintain their original antiferromagnetic states. [Display omitted] • Clarify the optimal growing conditions for Nd 6 Fe 13 X. • Nd 6 Fe 13 Cu and Nd 6 Fe 13 Zn have antiferromagnetic states, which effectively avoid coupling between the Nd 2 Fe 14 B phases. • Proposing an improved approach for partial replacement of Nd atoms with Pr atoms, which reduces formation energy while preserving magnetic properties. [ABSTRACT FROM AUTHOR]

We report an ab initio investigation of a hydrogen-based high- T _c superconductor candidate—crystalized C _4 H _4 in the cubic-gauche ( cg -C _4 H _4 ) structure, with the symmetry of space group I _213 . We find the cg -C _4 H _4 structure to be stable under ambient environment; and the evaluation of the electron-phonon coupling strength indicates that the heavily doped cg -C _4 H _4 can generate superconductivity with a T _c ∼ 72 K. The high frequency vibrational modes of hydrogen atoms are found to play an important role in the total electron-phonon interaction strength, and the reduction of structural symmetry compared with graphene further enhances the electron–phonon coupling of the carbon framework. Our investigation illustrates a BCS route to realizing the hydrogen-based high T _c superconductivity.

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