Transverse Sound Velocities Research Articles

Electronic Band Structures of the Possible Topological Insulator Pb2BiBrO6 and Pb2SeTeO6 Double Perovskite: An Ab Initio Study

Using the frameworks of density functional theory, we found a new class of three-dimensional (3D) topological insulators (TIs) in Pb2BiBrO6 and Pb2SeTeO6 double perovskites. Our ab initio theoretical calculations show that Pb2BiBrO6 and Pb2SeTeO6 are Z2 nontrivial, and their bandgaps are 0.390 eV and 0.181 eV, respectively. The topology comes from two mechanisms. Firstly, the band inversion occurs at Γ point in the absence of spin-orbit interactions and secondly, the bandgap is induced by the SOC. This results in a larger bandgap for this new class of topological insulators than conventional TI. In Pb2BiBrO6 double perovskites, our slab calculations confirm that the topology-protected surface metallic bands come from the BiBrO4 surface which means that one can build a transport device using Pb2BiBrO6 double perovskites with a PbO layer as an outmost protection layer. The mechanical stabilities such as bulk, shear, Young’s moduli, Poisson’s and Pugh’s ratio, longitudinal, transverse, and average sound velocity, together with Debye temperature are also studied. Our results show that these Pb2AA’O6 (A = Sb and Bi; A’ = Br and I) and Pb2SeTeO6 are mechanically stable.

Acoustic phonons and elastic stiffnesses from Brillouin scattering of CdPS3

We report Brillouin light scattering measurements of monoclinic CdPS3, a 2D layered cadmium chalcogenophosphate. By measurement of over 400 spectra in different orientations and scattering geometries, acoustic phonons are measured to determine the 13 elastic stiffness coefficients, along with longitudinal and transverse sound velocities from Brillouin frequency shifts. Because the sample was polycrystalline with a high density of planar defects, it was not possible to assign a crystal orientation to each spectrum. Instead, we determined the anisotropy in the material properties through statistical analysis, treating each spectrum as coming from a randomly sampled crystal orientation. Values of the Voigt–Reuss averages for the bulk modulus, Young’s modulus, the shear modulus, and Poisson’s ratio are calculated from measured stiffnesses. This work demonstrates analysis of elastic properties of a polycrystalline, monoclinic 2D layered material through statistical analysis of a significant number of spectra.

Computation of phonon dispersion curves and thermodynamic properties of Pd80P20 metallic glass

In this paper, the theoretical computation of phonon frequencies for Pd80P20 metallic glass is presented both for the longitudinal and transverse modes. For computing the phonon frequencies, we follow Bhatia and Singh approach. The elastic constants of the Pd80P20 metallic glass are used for determining the force constants for computing the phonon frequencies. The theoretically obtained phonon frequencies of Pd80P20 show relevant behaviour in the low momentum region for both the modes (longitudinal and transverse). Further, we have estimated transverse sound velocity from the computed phonon frequencies in the low momentum region. Similarly, in the low momentum region of the phonon frequency curves, we have computed longitudinal sound velocity and Debye temperature employing different types of dielectric screenings. The theoretical results obtained from the dispersion curves are in good agreement with the available data on Pd80P20 metallic glass.

Phase Stability, Elastic Modulus and Elastic Anisotropy of X Doped (X = Zn, Zr and Ag) Al3Li: Insight from First-Principles Calculations

In present work, the effects of alloying elements X (X = Zn, Zr and Ag) doping on the phase stability, elastic properties, anisotropy and Debye temperature of Al3Li were studied by the first-principles method. Results showed that pure and doped Al3Li can exist and be stable at 0 K. Zn and Ag elements preferentially occupy the Al sites and Zr elements tend to occupy the Li sites. All the Cij obey the mechanical stability criteria, indicating the mechanical stability of these compounds. The overall anisotropy decreases in the following order: Al23Li8Ag > Al3Li > Al23Li8Zn > Al24Li7Zr, which shows that the addition of Zn and Zr has a positive effect on reducing the anisotropy of Al3Li. The shear anisotropic factors for Zn and Zr doped Al3Li are very close to one, meaning that elastic moduli do not strongly depend on different shear planes. For pure and doped Al3Li phase, the transverse sound velocities νt1 and νt2 among the three directions are smaller than the longitudinal sound velocity νl. Moreover, only the addition of Zn is beneficial to increasing the ΘD of Al3Li among the three elements.

Relationship between the elastic properties and structure of BaO-TiO2-GeO2-SiO2 glasses

The macroscopic elastic modulus of a solid-state matter is generally affected by the packing density and average bond strength of the constituent atoms. However, because of the diversity of network connections in oxide glasses, an understanding based on the network structure is also required for precise structural categorisation. Here, we examine the relationship between the elastic modulus and structural parameters, viz., the correlation length of the Boson peak and cavity size calculated from the positron annihilation measurement, of BaO-TiO2-GeO2-SiO2 glasses. The Poisson's ratio correlates with the ratio of the longitudinal and transversal sound velocities, and thermal stability parameter ΔT. The Boson peak frequencies of these glasses at the GeO2-rich region exhibit an apparent plateau behavior, indicating that the rate of change in the sound velocity is similar to that in the correlation length. Although the numerical calculation of the cavity in this glass by positron annihilation spectroscopy is unavailable because of the d-electron of Ti cations, the τ1 value shows a similar compositional dependence as the correlation length of the Boson peak. It is expected that a combination of elastic modulus and inelastic light scattering of glasses will be helpful in understanding glass-forming melts.

Influence of elastic anisotropy on measured sound velocities and elastic moduli of polycrystalline cubic solids

Cubic solids such as NaCl, crystalline argon, or H2O-ice VII exhibit significant elastic anisotropy strongly increasing upon compression. As earlier recognized for solid argon and H2O-ice (both exhibiting Zener ratio A > 1), longitudinal sound velocities of their polycrystals, VLav, measured using Brillouin light scattering (BLS) or pulse-echo ultrasonics are much closer to VL⟨111⟩ than to VL⟨100⟩, the VL-extremes in any cubic single crystal. Here, we experimentally confirm, using the technique of time-domain Brillouin scattering, the same tendency for NaCl exhibiting the opposite anisotropy type, A < 1. To understand this tendency, we modelled orientational distribution and the frequency of occurrence of VL values in texture-free polycrystalline samples of NaCl and solid argon. We found a remarkable and predictable asymmetry of VL distributions with maxima at VL⟨110⟩ that is always much closer to VL⟨111⟩. This asymmetry persists in BLS peaks but can be obscured in experiments. In the case of solid argon at 49 GPa, the asymmetry can lead to a moderate deviation of experimental VLav from VLH (obtained from elastic-stiffness constants Cij applying the Hill approximation) by ∼7%. The latter can cause, however, a significant overestimation of the aggregate shear modulus by δG/G ∼ 50% or of the bulk modulus by δB/B ∼ 20% if just one BLS peak of longitudinal modes is detectable. A similar analysis, performed for transverse sound velocities, VT and VTav, has shown that by the use of a BLS spectrum showing peaks of both longitudinal and transverse modes, overestimation of B is similarly high but that of G is much less dramatic.

Charged compact star in f(R,\xa0T) gravity in Tolman\u2013Kuchowicz spacetime

In this current study, our main focus is on modeling the specific charged compact star SAX J 1808.4-3658 (M = 0.88 M_{odot }, R = 8.9 km) within the framework of f(R,,T) modified gravity theory using the metric potentials proposed by Tolman–Kuchowicz (Tolman in Phys Rev 55:364, 1939; Kuchowicz in Acta Phys Pol 33:541, 1968) and the interior spacetime is matched to the exterior Reissner–Nordström line element at the surface of the star. Tolman–Kuchowicz metric potentials provide a singularity-free solution which satisfies the stability criteria. Here we have used the simplified phenomenological MIT bag model equation of state (EoS) to solve the Einstein–Maxwell field equations where the density profile (rho ) is related to the radial pressure (p_{mathrm{r}}) as p_{mathrm{r}}(r) = (rho - 4B_{mathrm{g}})/3. Furthermore, to derive the values of the unknown constants a,, b,, B,, C and the bag constant B_{mathrm{g}}, we match our interior spacetime to the exterior Reissner–Nordström line element at the surface of stellar system. In addition, to check the physical validity and stability of our suggested model we evaluate some important properties, such as effective energy density, effective pressures, radial and transverse sound velocities, relativistic adiabatic index, all energy conditions, compactness factor and surface redshift. It is depicted from our current study that all our derived results lie within the physically accepted regime which shows the viability of our present model in the context of f(R,,T) modified gravity.

Influence Factors and Prediction Model of Bond Strength of Tunnel Fireproof Coating under Freeze-Thaw Cycles.

The freeze–thaw resistant performance of a tunnel fireproof coating (TFC) has an important impact on bonding property and durability. The influence of redispersible emulsion powder, polypropylene fiber and air-entraining agent on TFCs was studied. Transverse fundamental frequency and ultrasonic sound velocity were used to evaluate the damage degree of TFC, and the mechanism was revealed by SEM and pore structure. The results show that the most beneficial effect on bond strength of TFC is redispersible emulsion powder, followed by air-entraining agent, and then polypropylene fiber. After freeze–thaw cycles, the cumulative pore volume of micropores in the TFC increases obviously, while the porosity of macropores does not change significantly. A prediction model was proposed, which can calculate the bond strength from the damage degree of TFC under freeze–thaw cycles. The achievement can promote the application of TFC in cold regions.

Sound Velocities of Generalized Lennard-Jones (n - 6) Fluids Near Freezing.

In a recent paper [S. Khrapak, Molecules 25, 3498 (2020)], the longitudinal and transverse sound velocities of a conventional Lennard–Jones system at the liquid–solid coexistence were calculated. It was shown that the sound velocities remain almost invariant along the liquid–solid coexistence boundary lines and that their magnitudes are comparable with those of repulsive soft-sphere and hard-sphere models at the fluid–solid phase transition. This implies that attraction does not considerably affect the magnitude of the sound velocities at the fluid–solid phase transition. This paper provides further evidence to this by examining the generalized Lennard–Jones (n − 6) fluids with n ranging from 12 to 7 and demonstrating that the steepness of the repulsive term has only a minor effect on the magnitude of the sound velocities. Nevertheless, these minor trends are identified and discussed.

Construction of primitive cells of the cubic syngony on the basis of the molecular acoustics method

Topicality of the present article is connected with introduction of new verifying condition, which is required for estimation of structures of cores of the “elementary” lattices of the body-centred cell and face-centred cell (these lattices were proposed in accordance with the Bravais theory), on the basis of theoretical calculations of the ultrasonic velocities, which are directed along the edges of the selected cells and which are verified with the help of the experimental data that were obtained in accordance with the molecular analysis method. The purpose of the present investigation was as follows: to carry out evaluation of the core structures of the elementary and primitive cells of the body-centred and face-centred cell of the crystal lattice of the cubic syngony (for alpha iron and cuprum) in respect of their conformity with methods of the molecular acoustics. The method of the present investigation is based on comparison of values of the ultrasonic velocities (longitudinal velocity, transversal velocity, and velocity within the rod), which were calculated on the basis of the relevant theory and which are transmitted over edges of the primitive cell that is constructed (for alpha iron and cuprum), with the values of these velocities of the ultrasound, which were obtained experimentally with the help of the molecular acoustics method for these materials. Introduction of the additional criterion (requirement) in respect of selection of the least cell on the basis of the molecular analysis has made it possible to draw the conclusion that cores of the “elementary” cells of the body-centred and face-centred crystal lattices, which are determined in accordance with the Bravais theory and which are assumed as the basis of the analysis with the help of the analytic geometry method, are inconsistent with actually existing mechanism of transmission of the ultrasound over the interatomic bonds (edges of the cell). In addition, this conclusion has made it possible to reveal new type (that is, a new syngony), which is the basis of the core structure of these “elementary” cells. The theoretically calculated ultrasound velocities, which are directed along the edges of the primitive cell of the new type (for alpha iron and for cuprum), have been coincident with high accuracy with the values of the longitudinal and transversal sound velocity, as well as with the sound velocity within the rod, which were experimentally established with the help of the molecular acoustics method.

Phonon Dynamics and Acoustic and Thermodynamic Properties of Pt<sub>57.5</sub>Cu<sub>14.7</sub>Ni<sub>5.3</sub>P<sub>22.5</sub> Bulk Metallic Glass

The phonon dispersion curves for Pt57.5Cu14.7Ni5.3P22.5 bulk metallic glass (BMG) are computed employing various dielectric screenings using the simple model given by Bhatia and Singh. The force constants β and δ for computing the dispersion curves are calculated from the elastic constants i.e. bulk modulus (B) and shear modulus (G) along with the calculated value of force constant κe of the material of the glass for the first time. The results of the phonon dispersion curves show appropriate behavior in the long wavelength region in detail for both the longitudinal and transverse modes and give insight regarding the acoustic and thermal properties of the BMG. The transverse sound velocity and the longitudinal velocities with various dielectric screening are calculated from the dispersion curves in the long wavelength region. The corresponding thermodynamic property (Debye temperature) is calculated for different dielectric screenings. The theoretical results predicted are in a good agreement with the reported data in the literature for the Pt57.5Cu14.7Ni5.3P22.5 BMG and may be used for correlating other properties.

Thermal conduction in two-dimensional complex plasma layers

A simple vibrational model of heat transfer in two-dimensional (2D) fluids relates the heat conductivity coefficient to the longitudinal and transverse sound velocities, specific heat, and mean interatomic separation. This model is demonstrated not to contradict the available experimental and numerical data on heat transfer in 2D complex plasma layers. Additionally, the heat conductivity coefficient of a 2D one-component plasma with a logarithmic interaction is evaluated.

First principle studies on lattice thermal conductivity and thermoelectric properties of ScCu(S,Se,Te)2

This study is aimed at evaluating the structural stability, lattice thermal conductivity, transport properties and the thermoelectric figure of merit of scandium copper chalcogenides (denoted by ScCuX2) based on first principles. At the apex of this work, we explored the transport properties by solving Boltzmann transport equations in a relaxation time approximation to obtain transport coefficients of interest which were then integrated with lattice thermal conductivity in order to establish the values of the dimensionless figure of merit (ZT), which dictate not only the possibility of using a given material as a major component in the design of thermoelectric system but also indicates how efficient it is in converting thermal to electrical energy, where values of ZT around unity are preferred.At room temperature (300 K), the values of the averaged lattice thermal conductivity obtained are 1.58, 1.28 and 1.47 W/mK for ScCuS2, ScCuSe2 and ScCuTe2 respectively which are promising for thermoelectric applications where minimal values of lattice thermal conductivity are desirable. We also evaluated the Debye temperatures as well as transverse and longitudinal sound velocity in ScCuX2 compounds. In addition, group velocities, phonon lifetimes and mean free paths were explored in order to uncover the mechanisms of thermal transport in these ternaries. Our study provides important new insights into heat transport and phonon scattering mechanisms in ScCuX2 ternary compounds. From our findings, among the three ternary compounds, ScCuSe2 has the highest value of dimensionless figure of merit of 0.65 at high temperatures (1000 K) when the majority charge carriers are holes having a carrier concentration of 1019 cm−3.

Elastic Properties and Glass Forming Ability of the Zr<sub>50</sub>Cu<sub>40</sub>Ag<sub>10</sub> Metallic Alloy

The elastic properties of the Zr50Cu40Ag10 metallic alloy, such as the bulk modulus B, the shear modulus G, the Young’s modulus E and the Poisson’s ratio σ, are investigated by molecular dynamics simulation in the temperature range T=250–2000 K and at an external pressure of p=1.0 bar. It is shown that the liquid–glass transition is accompanied by a considerable increase in the shear modulus G and the Young’s modulus E (by more than 50%). The temperature dependence of the Poisson’s ratio exhibits a sharp fall from typical values for metals of approximately 0.32–0.33 to low values (close to zero), which are characteristic for brittle bulk metallic glasses. Non-monotonic temperature dependence of the longitudinal and transverse sound velocity near the liquid-glass transition is also observed. The glass forming ability of the alloy is evaluated in terms of the fragility index m. Its value is m≈64 for the Zr50Cu40Ag10 metallic glass, which is in a good agreement with the experimental data for the Zr-based metallic glasses.

Alumina ceramics doped with manganese titanate via applying Mn–Ti–O coatings to corundum micropowder

The aim of this work was (i) to develop an untraditional method of alumina doping that ensures low sintering temperature and (ii) to obtain dense engineering ceramics with good mechanical properties by conventional sintering. A nanostructured coating of manganese–titanium oxides was applied to α-Al2O3 micropowder particles using liquid-phase synthesis of Mn–Ti–O–C–H xerogel and its annealing. The alumina modified in this way was sintered at 1300–1400 °C in air. The sintering process was accompanied by the formation of manganese metatitanate MnTiO3. The sintered samples have the relative density of about 97%, the bending strength of ≥ 260 MPa, and the Vickers microhardness of ≥ 16 GPa. The longitudinal and transverse sound velocities were measured, from which the Poisson’s ratio of 0.24 and the Young’s modulus of ≥ 360 GPa were calculated. Modification with MnTiO3 precursors yielded higher mechanical properties of ceramics than modification with a ready MnTiO3 coating. The results are compared with the properties of Al2O3 doped with Mn–Ti–O formulations in other studies, and of industrial alumina ceramics.

Sound Velocities of Lennard-Jones Systems Near the Liquid-Solid Phase Transition.

Longitudinal and transverse sound velocities of Lennard-Jones systems are calculated at the liquid–solid coexistence using the additivity principle. The results are shown to agree well with the “exact” values obtained from their relations to excess energy and pressure. Some consequences, in particular in the context of the Lindemann’s melting rule and Stokes–Einstein relation between the self-diffusion and viscosity coefficients, are discussed. Comparison with available experimental data on the sound velocities of solid argon at melting conditions is provided.

Structural, elasto-mechanical and phonon-related properties of MnCrP: A DFT Study

The Half Heusler alloy (HHA) MnCrP has been studied theoretically for structural, elasto-mechanical and phonon properties. The structure is optimized and the calculated structural parameters are close to the literature. This optimized data is used to estimate three independent second-order cubic elastic constants [Formula: see text], [Formula: see text] and [Formula: see text]. The mechanical stability criteria are explored by these constants and further used to estimate the elastic moduli; Young’s, bulk and shear modulus. The mechanical parameters like Poisson’s ratio, Pugh’s ratio, anisotropic factor, Cauchy pressure, shear constant, Lame’s constants, Kleinman parameter are also calculated and discussed. Discussions reveal the ductile nature, ionic behavior, anisotropic nature and mechanical stability of MnCrP. The metallic nature, compressibility, stiffness and interatomic forces of material are also described. Furthermore, the Debye temperature, where the collective vibrations shifts to an independent thermal vibrations, is also calculated. Longitudinal and transverse sound velocities are also obtained to investigate the phonon modes of oscillation. These phonon modes confirm the stability of the alloy as no negative phonon frequencies in the phonon-dispersion curves. These curves are used to estimate the reststrahlen band where light reflects 100% and the suitability of material is checked for Far Infrared (FIR), photographic, optoelectronic devices and sensors.

Structure, stress, and mechanical properties of Mo-Al-N thin films deposited by dc reactive magnetron cosputtering: Role of point defects

In this work, the structural and mechanical properties of ternary Mo-Al-N alloys are investigated by combining thin film growth experiments and density functional theory (DFT) calculations. Mo1−xAlxNy thin films (∼300 nm thick), with various Al fractions ranging from x = 0 to 0.5 and nitrogen-to-metal (Al + Mo) ratio ranging from y = 0.78 to 1.38, were deposited by direct-current reactive magnetron cosputtering technique from elemental Mo and Al targets under Ar + N2 plasma discharges. The Al content was varied by changing the respective Mo and Al target powers, at a fixed N2 (20 SCCM) and Ar (25 SCCM) flow rate, and using two different substrate temperatures Ts = 350 and 500 °C. The elemental composition, mass density, crystal structure, residual stress state, and intrinsic (growth) stress were examined by wavelength dispersive x-ray spectroscopy, x-ray reflectivity, x-ray diffraction, including pole figure and sin2ψ measurements, and real-time in situ wafer curvature. Nanoindentation tests were carried out to determine film hardness H and elastic modulus EIT, while the shear elastic constant C44 was measured selectively by surface Brillouin light spectroscopy. All deposited Mo1−xAlxNy films have a cubic rock-salt crystal structure and exhibit a fiber-texture with a [001] preferred orientation. The incorporation of Al is accompanied by a rise in nitrogen content from 44 to 58 at. %, resulting in a significant increase (2%) in the lattice parameter when x increases from 0 to 0.27. This trend is opposite to what DFT calculations predict for cubic defect-free stoichiometric Mo1−xAlxN compounds and is attributed to variation in point defect concentration (nitrogen and metal vacancies) when Al substitutes for Mo. Increasing Ts from 350 to 500 °C has a minimal effect on the structural properties and phase composition of the ternary alloys but concurs to an appreciable reduction of the compressive stress from −5 to −4 GPa. A continuous increase and decrease in transverse sound velocity and mass density, respectively, lead to a moderate stiffening of the shear elastic constant from 130 to 144 GPa with increasing Al fraction up to x = 0.50, and a complex and nonmonotonous variation of H and EIT is observed. The maximum hardness of ∼33 GPa is found for the Mo0.81Al0.19N1.13 film, with nitrogen content close to the stoichiometric composition. The experimental findings are explained based on structural and elastic constant values computed from DFT for defect-free and metal- or nitrogen-deficient rock-salt MoAlN compounds.

Role of lattice trapping for sliding friction

Using molecular dynamics we study the dependence of the friction force on the sliding speed when an elastic slab (block) is sliding on a rigid substrate with a surface height profile. The friction force is nearly velocity independent due to phonon emission at the closing and opening crack tips, where rapid atomic snap-in and -out events occur during sliding. The rapid events result from lattice trapping and are closely related to the velocity gap and hysteresis effects observed in model studies of crack propagation in solids. This indicates that the friction force is dominated by processes occurring at the edges of the contact area, which is confirmed by calculations showing that the friction force is independent of the normal force. The friction force increases drastically when the sliding velocity approaches the solid transverse sound velocity, as expected from the theory of cracks.

Nanometric Fluctuations of Sound Velocity in Alkali Borate Glasses and Fragility of Respective Melts

The network structure of borate glass is remarkably modified by the addition of alkali metals. The alkali metal dependences of the first sharp diffraction peak (FSDP) and the boson peak (BP) are investigated in alkali borate glasses by elastic and inelastic neutron scattering experiments. With an increase in ionic radius of alkali ions, both the width ΔQ of FSDP and the BP energy decrease. The comparison of the characteristic length of medium‐range order (MRO) predicted by the FSDP and the position of the BP allows us to find the mean‐square fluctuations of the transverse sound velocity within series of alkali borate glasses. It is shown that the fragility of the respective alkali borate melts inversely correlates with the mean‐square fluctuations of the transverse sound velocity in glassy state.