Temperature Dependence Of Thermodynamic Properties Research Articles

The structural, mechanical, electronic, lattice dynamics and thermodynamic properties of TaTSi (T = Rh, Os, Ir) compounds by first-principles calculations

We have performed a theoretical calculation, based on density functional theory, to investigate the structural, mechanical, electronic, lattice dynamics, and thermodynamic properties of TaTSi (T = Rh, Os, Ir) within the full potential linearized augmented plane wave method. The calculated values of the lattice constant and unit cell volume are in excellent agreement with the available experimental data. The mechanical stability is substantiated by our calculated elastic constants, while the mechanical properties are subsequently derived. The results indicate that the compounds are ductile materials, except for TaRhSi which exhibits quasi-ductile behavior; however, all of them exhibit elastic anisotropy. In addition, the electronic density of states of the TaTSi (T = Rh, Os, Ir) compounds reveal that all these materials have a metallic character. Moreover, the dynamical stability is confirmed by our calculated phonon spectra. Finally, we present temperature-dependent thermodynamic properties including free energy, internal energy, entropy, and heat capacity.

Structural, electronic, magnetic and thermodynamic properties of ZrCoTiX (X = Ga and Sn) alloys

Structural, electronic, magnetic, elastic, thermodynamic and optical properties of ZrCoTiX (X = Ga and Sn) alloys have been investigated using density functional theory (DFT) within the full-potential linearized augmented plane wave (FP-LAPW) method. Half-metallic nature of ZrCoTiGa1-ySny (y = 0, 0.25, 0.5, 0.75 and 1) alloys is predicted which justifies their applicability in spintronic devices and the respective magnetic moments are obtained as 1.99, 2.25, 2.51, 2.76 and 3.02 μB/f. u. in accordance with the Slater-Pauling rule. Elastic parameters validated the mechanical and thermodynamic stabilities of these alloys. The Curie temperature (TC) is determined using mean field approximation theory as 516.04 and 941.17 K for ZrCoTiGa and ZrCoTiSn, respectively. The optical properties of these alloys demonstrated their applicability in optoelectronic devices. Moreover, the temperature dependent thermodynamic properties like entropy, heat capacity, thermal expansion, etc. are reported for ZrCoTiGa and ZrCoTiSn in the pressure range 0-40 GPa.

Generating Predictive Chemical Kinetic Models for Quaternary Ammonium Monomers Chemical Degradation in Anion Exchange Membrane

It is widely accepted that the high cost of Proton Exchange Membrane (PEM) fuel cells (FC) is a major inhibitor of the transition to extensive commercialization of hydrogen-fueled FCs. One known alternative to this technology is Anion Exchange Membrane (AEM), greatly reducing the need for precious metals, e.g., platinum, and allowing a wider choice of lower cost polymeric materials1. Although research in the field of AEM has greatly evolved recently, AEM-FCs still lack sufficient durability and cell longevity. The current major challenge revolves around the chemical and structural stability of polymers constructing the membrane, which undergo different degradation reactions under cell operating conditions. A widely investigated AEM monomer is Benzyl trimethylammonium (BTMA), a quaternary ammonium (QA) cation, in which the positively charged nitrogen atom enables the selective transfer of hydroxide ions through the membrane2. BTMA degradation occurs in several reaction paths, the main one involving a nucleophilic attack of the hydroxide at the benzylic carbon atom, resulting in complete displacement of the functional group (Fig. 1a). Recently, it has also been shown that low hydration levels of hydroxide, evidently more relevant to the FC’s environment, have an immense impact on the reduced stability of BTMA3,4. Despite the elaborate research done on BTMA degradation, as well as on other QA monomers, complete understanding of the chemical mechanisms affecting the stability of AEM-FCs is still lacking. Modeling these mechanisms can assist in facilitating practical implementation of this promising technology, discovering novel chemically-stable ionomers, while saving time and resources in trial-and-error efforts.The goal of this research is to develop an automated workflow for generating and refining predictive chemical kinetic models describing chemical degradation of AEM monomers, by combining several computational tools, and tailoring them to suit our specific needs. The first is the Reaction Mechanism Generator (RMG) software, which automatically generates chemical kinetic models by utilizing a basic comprehension of chemical reactions5. RMG derives its data from “libraries” of kinetic and thermodynamic parameters based on the literature, and from estimations schemes based on group or decision trees. Since RMG currently does not support ion chemistry, it is required to add this feature to the software, along with solvation corrections, while extending its estimation schemes. To achieve this, accurate kinetic and thermodynamic parameters of the species and reactions of interest are necessary.The present work focuses on describing BTMA degradation. Preliminary results for the temperature-dependent rate coefficient and thermodynamic properties were obtained using the Automated Rate Calculator (ARC) software6. This tool facilitates electronic structure calculations relevant for chemical kinetic modeling, by automatically spawning, tracking, and analyzing relevant computations. We computed the BTMA + OH– reaction rate coefficient at a low hydration level (λ=1) at the ωB97xD/Def2-TZVP level of theory, and compared it to the experimental results obtained at three different temperatures (Fig. 1b). The calculations were performed using the SMD solvation model with DMSO as the solvent, in accordance with the experimental conditions. Our preliminary results show some agreement with experimental measurements, and we expect this agreement to improve by calculating single-point energies using the more accurate coupled-cluster (CC) theory.The methods developed to generate a chemical kinetic model for BTMA, can then be applied for other potential QA monomers. Furthermore, an additional software called The Tandem Tool (T3), can be used to perform model refinement, through a process of iterations between RMG and ARC, creating the desired automated model generation workflow. This process is designed to obtain quantum mechanics-based calculated parameters for significant species and reactions in the model, chosen using sensitivity analysis. Altogether, this work can create a powerful tool, providing insights on known AEM ionomers, as well as the flexibility to investigate additional polymeric materials, with minimal experimental resources.

Modelling of the Corrosion-Induced Gas Impact on Hydraulic and Radionuclide Transport Properties of Geological Repository Barriers

The geological disposal of high-level radioactive waste is the final step in the nuclear fuel cycle. It is realized via isolating the high-level radioactive waste in the geological environment with an appropriate system of engineered barriers. Radionuclides-containing materials must be isolated from the biosphere until the radioactivity contained in them has diminished to a safe level. In the case of high-level radioactive waste, it could take hundreds of thousands of years. Within such a long timescale, a number of physical and chemical processes will take part in the geological repository. For the assessment of radionuclide migration from a geological repository, it is necessary to predict the repository’s behavior once placed in the host rock as well as the host-rock response to disturbances due to construction. In this study, the analysis of repository barriers (backfill, concrete, inner excavation disturbed zone (EDZ), outer EDZ, host rock) thermo–hydraulic–mechanical (THM) evolution was performed, and the scope of gas-induced desaturation was analyzed with COMSOL Multiphysics. The analysis was based on modelling of a two-phase flow of miscible fluid (water and H2) considering important phenomena such as gas dissolution and diffusion, advective–diffusive transport in the gaseous phase, and mechanical deformations due to thermal expansion of water and porous media. The importance of proper consideration of temperature-dependent thermodynamic properties of water and THM couplings in the analysis of near-field processes was also discussed. The modelling demonstrated that such activities as 50 years’ ventilation of the waste disposal tunnel in initially saturated porous media, and such processes as gas generation due to corrosion of waste package or heat load from the waste, also led to desaturation of barriers. H2 gas generation led to the desaturation in engineered barriers and in a part of the EDZ close to the gas generation place vanishing soon after finish of gas generation, while the host rock remained saturated during the gas generation phase (50–100,000 years). Radionuclide transport properties in porous media such as effective diffusivity are highly dependent on the water content in the barriers determined by their porosity and saturation.

First principle insights into the physical properties of Ti-based 211-MAX phase nitrides Ti2AN (A = Tl and Pb)

We have used the theoretical ab initio approach to scrutinize the electronic and other physical properties of Ti2AN (A = Tl and Pb). Geometrical optimization has been carried out to obtain accurate lattice constants and internal coordinates. The formation energies of Ti2TlN and Ti2PbN are found to be negative, which confirms their stability. The aforementioned compounds are found to be metallic because of their zero-band gaps. The metallicity f m (x 10−3) of Ti2TlN and Ti2PbN phases were determined to be 1.77 and 2.11, respectively. In addition, we evaluate the elastic constant C ij , which obeys the Born-Huang mechanical stability criterion. We used the Voigt-Reuss-Hill approximation for the analysis of Young’s modulus, shear modulus, and bulk modulus successfully. Furthermore, Ti2TlN is found to be brittle, but Ti2PbN is close to the brittle-ductile boundary line according to Pugh’s and Poisson’s ratios. The Debye temperature, melting temperature, and minimum thermal conductivity have all been rigorously studied to examine the potential scenarios of genuine high-temperature applications. Lower Young’s modulus, the minimum thermal conductivity (Ti2TlN and Ti2PbN), and Debye temperature values reveal that Ti2PbN might be used as a thermal barrier coating application. A study of elastic anisotropy demonstrates that Ti2PbN has a higher degree of anisotropy than Ti2TlN, according to the universal anisotropy index. We confirmed the dynamic stability (i.e., no negative frequencies at the gamma point) of predicted compounds by performing phonon DOS and phonon band structures. Finally, the temperature-dependent thermodynamic properties of Ti2TlN and Ti2PbN have been thoroughly analyzed, where the entropy (S), free energy, and internal energy (E) vary with respect to temperature. Moreover, the convergence of specific heat capacity is observed at constant volume to the Dulong-Petit limit at higher temperatures.

Development of a flexible quasi-harmonic-based approach for fast generation of self-consistent thermodynamic properties used in computational thermochemistry

We present in this work a new formalism for the rapid and accurate evaluation of the thermodynamic properties of compounds and solid solution end-members which are required in the high-throughput computational thermochemistry discovery of new materials. The method is based on the Debye approximation and is fully thermodynamically self-consistent. It considers several energetic contributions, such as the vibrational free energy (including both implicit and explicit anharmonicity), the defect-free energy (via the introduction of thermal vacancies), the energetic effect of thermal excitation of electrons (for metallic solids) as well as other excess terms to precisely reproduce experimental data. Sets of fully self-consistent pressure- and temperature-dependent thermodynamic properties are evaluated from the exact differentiation of this complete free energy formalism. This strategy allows the formalism to be fully integrated in the CALPHAD framework for the development of large thermodynamic databases of both stable and metastable compounds which may form in multicomponent/high-order systems. An emphasis is put in this work on the analysis and precise description of anharmonic vibrational contributions. Finally, sets of self-consistent thermodynamic and thermo-physical properties have been predicted as a function of temperature and compared with both experiments and classical QHA for pure elements (i.e. Ag, Li, V, Al, and C) and stoichiometric compounds of different chemical nature (i.e. MgO, CaO, Al2O3, NaCl, LiFePO4 and InP). The impact of various input parameters of the formalism on the free energy and the derived thermodynamic properties for these solids is analyzed. A better predictive capability and the enhanced flexibility of the proposed formalism are demonstrated.

Effects of solutes on thermodynamic properties of (TMZrU)C (TM = Ta, Y) medium-entropy carbides: a first-principles study

High entropy carbide ceramics have garnered significant interest as a novel class of ultra-high temperature and superhard metallic materials. In the present work, a comparative investigation was conducted for the first time on the stability, mechanical, and thermodynamic properties of two medium entropy carbides (MECs), (TaZrU)C and (YZrU)C, using high-throughput first-principles calculations. Additionally, data from groups IV and V transition metal monocarbides were employed for comparison. The temperature-dependent thermodynamic properties, including bulk modulus (B), constant volume/constant pressure heat capacity (Cv/Cp), Gibbs free energy, volume, entropy, and thermal conductivity, were evaluated using the Debye-Gruneisen model. The results demonstrate that (TaZrU)C and (YZrU)C exhibit similar trends in their thermodynamic properties, with (YZrU)C displaying slightly superior performance as the temperature rises. This work provides valuable insights into the design of innovative high entropy fuels, holding significant implications for the advancement of MEC ceramic fuels in advanced nuclear power systems and nuclear thermal propulsion systems.

Assessment of the thermodynamics of Na,K-shlykovite as potential alkali-silica reaction products in the (Na,K)2O-CaO–SiO2-H2O system

The thermodynamic description of the alkali-silica reaction is incomplete, due to the lack of thermodynamic data for the alkali-silica reaction products. Here, we estimate the temperature-dependent thermodynamic properties of Na- or K-shlykovite phases which are found to form during alkali-silica reaction in concrete. The thermodynamic properties are then used to produce a series of binary phase diagrams considering different concentrations of alkalis. The results show that these two products start to form at 0.01 mol/kg of alkalis in the CaO–SiO2 binary phase diagrams. Both Na- and K-shlykovite can co-exist with C-S-H at an increased concentration of CaO, consistent with findings that these alkali-silica reaction products may form in real cement systems. However, there is a maximum alkali concentration in the binary phase diagram beyond which shlykovite cannot form, due to limited calcium solubility. This work can help understand and improve alkali-silica testing protocols proposed in standards.

Binding, brightness, or noise? Extracting temperature-dependent properties of dye bound to DNA

We present a method for extracting temperature-dependent thermodynamic and photophysical properties of SYTO-13 dye bound to DNA from fluorescence measurements. Together, mathematical modeling, control experiments, and numerical optimization enable dye binding strength, dye brightness, and experimental noise (or error) to be discriminated from one another. By focusing on the low-dye-coverage regime, the model avoids bias and can simplify quantification. Utilizing the temperature-cycling capabilities and multi-reaction chambers of a real-time PCR machine increases throughput. Significant well-to-well and plate-to-plate variation is quantified by using total least squares to account for error in both fluorescence and nominal dye concentration. Properties computed independently for single-stranded DNA and double-stranded DNA by numerical optimization are consistent with intuition and explain the advantageous performance of SYTO-13 in high-resolution melting and real-time PCR assays. Distinguishing between binding, brightness, and noise also clarifies the mechanism for the increased fluorescence of dye in a solution of double-stranded DNA compared to single-stranded DNA; in fact, the explanation changes with temperature.

Modeling sensible thermal energy storage in solid blocks for concentrating solar power

In concentrating solar power plants, the mismatch between solar energy availability and energy demand requires the development of thermal storage systems. This study analyses a solid thermal energy storage module made of alumina. The block has a honeycomb pattern where the air flows through hexagonal channels. A transient 1D model, based on the finite difference method, is implemented and verified against CFD simulations. Such a simplified model considers the temperature dependence of thermodynamic properties for air and alumina. With the same boundary conditions, a 3D RNG k-ε turbulence CFD model is built, whose mesh and time step are assessed by means of sensitivity analyses. The average temperature difference between both models is as low as 14.8 K, even though the simplified one is around 300 times faster than the detailed CFD model.

Characterization, Kinetic Using Deconvolution Techniques and Thermodynamic Study of Synthetic MgHPO4·3H2O

MgHPO4·3H2O was synthesized via a simple wet reaction and transformed to its final decomposed product, Mg2P2O7 at 873 K. The thermal properties, vibrational modes of block units, and solid phases of the synthetic compounds have been investigated by TG/DTG/DTA, FTIR, and XRD techniques. The temperature dependence of thermodynamic properties estimated from the DSC data occurred in two regions (before and after the dehydration reaction) indicating a characteristic of the prepared MgHPO4·3H2O sample. The dehydration reactions of 3.5 mol of water molecules in the structure overlapped in the temperature of 363–823 K and showed the phase transition at 921 K as revealed by TG/DTG/DTA. The deconvolution technique using the newly modified co-mathematical functions was used to separate the overlapped dehydration steps in the DTG curve, which obtained the three steps. Only the first and second steps were estimated using kinetic triplet parameters (Ea, A, and model), which indicate the single dehydration mechanism with the A1.25 model and the multi-step of a parallel mechanism with the A1.6 model, respectively. The data of kinetics and thermodynamics play key points in theoretical study, application development, and industrial production of magnesium phosphates as a theoretical basis.

Thermodynamic re-modeling of the Yb-Sb system aided by first-principles calculations

The thermodynamic description of the Yb-Sb binary system is developed by means of the CALculations of PHAse Diagrams (CALPHAD) method by combining experimental data in the literature and predictions from first-principles calculations based on density functional theory (DFT) in the literature and the present work. Two pseudopotentials of Yb are compared in the present DFT-based calculations with 14 and 13 f-electrons frozen in the core, i.e., 5p66s2 and 5p66s25 d1 electrons as valence electrons, termed Yb_2 and Yb_3, respectively. It is shown that the phonon spectrum of the YbSb phase calculated using the Yb_3 pseudopotential does not have imaginary phonon modes and is subsequently used to predict its temperature dependent thermodynamic properties by the DFT-based quasiharmonic phonon calculations. The present thermodynamic database includes the Yb16Sb11 phase in addition to five intermetallic phases that were considered in previous modeling studies, i.e., YbSb2, YbSb, Yb11Sb10, Yb4Sb3, and Yb5Sb3. The high temperature orthorhombic structure of the Yb5Sb3 phase is not considered in the present work as it was stabilized by hydrogen. The associate solution model is used to describe the short-range ordering behavior in the liquid phase. The calculations from the present thermodynamic model show good agreement with thermochemical and phase equilibrium data from both the present work and the literature.

Studies towards investigation of Naphthoquinone-based scaffold with crystal structure as lead for SARS-CoV-19 management

In this work, 1-(4-bromophenyl)-2a,8a-dihydrocyclobuta[b]naphthalene-3,8‑dione (1-(4-BP)DHCBN-3,8-D) has been characterized by single crystal X-ray to get it's crystal structure with R(all data) - R1 = 0.0569, wR2 = 0.0824, 13C and 1HNMR, as well as UV–Vis and IR spectroscopy. Quantum chemical calculations via DFT were used to predict the compound structural, electronic, and vibrational properties. The molecular geometry of 1-(4-BP)DHCBN-3,8-Dwas optimized utilizing the B3LYP functional at the 6–311++G(d,p) level of theory. The Infrared spectrum has been recorded in the range of 4000–550 cm−1. The Potential Energy Distribution (PED) assignments of the vibrational modes were used to determine the geometrical dimensions, energies, and wavenumbers, and to assign basic vibrations. The UV–Vis spectra of the titled compound were recorded in the range of 200-800 nm in ACN and DMSO solvents. Additionally, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy gap and electronic transitions were determined using TD-DFT calculations, which also simulate the UV–Vis absorption spectrum. Natural Bond Orbital (NBO) analysis can be used to investigate electronic interactions and transfer reactions between donor and acceptor molecules. Temperature-dependent thermodynamic properties were also calculated. To identify the interactions in the crystal structure, Hirshfeld Surface Analysis was also assessed. The Molecular Electrostatic Potential (MEP) and Fukui functions were used to determine the nucleophilic and electrophilic sites. Additionally, the biological activities of 1-(4-BP)DHCBN-3,8-D were done using molecular docking. These results demonstrate a significant therapeutic potential for 1-(4-BP)DHCBN-3,8-D in the management of Covid-19 disorders. Molecular Dynamics Simulation was used to look at the stability of biomolecules.

Temperature-dependent structural, mechanical, and thermodynamic properties of B2-phase Ti2AlNb for aerospace applications

Temperature-dependent structural, mechanical, and thermodynamic properties of B2-phase Ti2AlNb for aerospace applications

The mechanical and thermodynamic properties of the tetragonal Mo5SiB2 and Mo5Si3 phase under high pressure: a first-principles study

The mechanical and thermodynamics properties of Mo5Si3 and Mo5SiB2 have been investigated by using the first-principle calculations. The cell structure parameters with pressure are briefly discussed. The values of micro-hardness and melting point of both Mo5SiB2 and Mo5Si3 are studied. In addition, both composite alloys’ elastic constants Cij and elastic modulus (B, G, E) are close to increase linearly with pressure. The B/G ratios and Cauchy pressure (C12–C44) were also investigated by this method, and the results suggest that pressure can improve the ductility properties of both alloys. The Poisson’s ratio indicates that the two alloys are bonded mainly metallic. Finally, the Debye temperature with pressure concluded from elastic constants was studied, and the temperature dependence of thermodynamic properties, including the entropy, constant volume heat capacity (CV), enthalpy, and Gibbs free energy, are also studied and discussed.

Accurate thermochemistry prediction of extensive Polycyclic aromatic hydrocarbons (PAHs) and relevant radicals

Polycyclic aromatic hydrocarbons (PAHs) are important intermediates to soot formation in combustion. A reliable database of their thermochemistry is required for the development of chemical kinetic models describing the gas-phase chemistry of hydrocarbon fuels. In this study, temperature-dependent thermodynamic properties are consistently determined using high-accuracy quantum chemistry calculations for an extensive set of PAH compounds. The developed database comprehensively consists of 125 C6-C18 PAH molecules and radicals, which are important and commonly included in chemical mechanisms studies. At the M06-2X/6-311++G(d,p) level of theory, geometry optimizations, vibrational frequency calculations, and dihedral angle scans are performed for all PAH species. The G3 method, together with the atomization reaction approach, is selected to derive the average atomization formation enthalpy. This method produces the most accurate thermochemistry quantities for PAHs, as demonstrated in a previous study. The entropy and heat capacity values are calculated using statistical thermodynamics in MultiWell. These results exhibit good agreement with the databases in literature. To examine the application of the group additivity (GA) method for PAHs, the Bland−Altman plot, a statistical analysis approach, is employed to visualize the agreement between the results from the quantum chemical calculations and GA methods. Two GA methods are examined and significant differences are found, which indicates that GA values of relevant groups need to be further updated. The database of thermodynamic quantities developed in this study are of particular value in modeling studies and important for exploring mechanisms of the PAH growth.

Toward accurate and efficient dynamic computational strategy for heterogeneous catalysis: Temperature-dependent thermodynamics and kinetics for the chemisorbed on-surface CO

Computational tools on top of first principle calculations have played an indispensable role in revealing the molecular details, thermodynamics, and kinetics in catalytic reactions. Here we proposed a highly efficient dynamic strategy for the calculation of thermodynamic and kinetic properties in heterogeneous catalysis on the basis of efficient potential energy surface (PES) and MD simulations. Taking CO adsorbate on Ru(0001) surface as the illustrative model system, we demonstrated the PES-based MD can efficiently generate reliable two-dimensional potential-of-mean-force (PMF) surfaces in a wide range of temperatures, and thus temperature-dependent thermodynamic properties can be obtained in a comprehensive investigation on the whole PMF surface. Moreover, MD offers an effective way to describe the surface kinetics such as adsorbate on-surface movement, which goes beyond the most popular static approach based on free energy barrier and transition state theory (TST). We further revealed that the dynamic strategy significantly improves the predictions of both thermodynamic and kinetic properties as compared to the popular ideal statistic mechanics approaches such as harmonic analysis and TST. It is expected that this accurate yet efficient dynamic strategy can be powerful in understanding mechanisms and reactivity of a catalytic surface system, and further guides the rational design of heterogeneous catalysts.

Aviation Turbine Fuel Thermal Conductivity: A Predictive Approach Using Entropy Scaling-Guided Machine Learning with Experimental Validation.

Although typical aircraft fuel thermal management analysis relies upon temperature-dependent thermodynamic and transport properties of aviation turbine fuel, the variation in properties associated with compositional variation in fuels and the subsequent impacts on system performance are not well established. With this in mind, the present work aimed to develop a predictive model of aviation turbine fuel thermal conductivity which utilized only compositional (hydrocarbon) and state (temperature and pressure) inputs and had errors within the bounds of typical uncertainty of the associated test data (3%). A novel modeling approach was developed to predict thermal conductivity using pseudo-component entropy scaling techniques with a machine learning-developed intermediate step in the overall model. Simple hyper-parameter optimization techniques were developed to promote model stability, computational efficiency, and long-term repeatability of the novel architecture. Validation data were gathered which included four fuel samples (3 JP-5 and 1 F-24), which underwent two-dimensional gas chromatography compositional testing and temperature-dependent density, viscosity, thermal conductivity, and specific heat testing. Model performance on the validation data set assembled from the literature data and present efforts showed an average deviation of 1% and an absolute average deviation of 2.5%. Model outputs outside the validation range are well-behaved and are expected to perform well on a large range of liquid hydrocarbon mixtures with the overall process expected to be well suited to prediction of other properties.

Effect of hydrostatic compression on physical properties of Li2TmSi3 (Tm = Ir, Pt, Rh, Os) with ground-state optical features

Using density functional theory (DFT) the first-principles calculations are employed to systematically investigate the effect of hydrostatic compression on the structural, elastic, electronic, thermal and thermodynamic properties of ternary silicides Li2TmSi3 (Tm = Ir, Pt, Rh, Os) with the ground-state optical features. The gradual decrease of bond length, unit cell volume and a smooth change of c/a with pressure deny the possibility of pressure induced phase transition up to 50 GPa. All the compounds studied here are mechanically stable within the considered pressure range. The constants C11, C12, C13 and C33 exhibit an almost linear increase trend with increase of pressure, while C44 shows anomalous behavior. The bond-stretching and bond-bending force constants are calculated based on the data of elastic constants and their pressure dependent behavior are discussed. Hydrostatic pressure enhances the inherent elastic anisotropy of Li2TmSi3. The ternary silicides studied here cannot be easily compressed under uniaxial stress. The atomic bonds between the nearest neighbors along [001] direction are stronger than those along [100] direction in the whole range of pressure. All ternary silicides except Li2PtSi3 are brittle in nature at ambient pressure. Under elevated pressure all studied compounds show brittle to ductile transition except Li2PtSi3 that exhibits brittle nature under 10 GPa. The shear modulus of Li2OsSi3 shows anomalous change under pressure, which may be attributed from its anomalous change of θD. The higher melting point of all silicides implies that they might have possible uses in harsh environments. The TDOS at EF in Li2TmSi3 decreases almost linearly with pressure. Pressure and temperature dependent thermodynamic properties and ground-state optical functions are described with implications, which may be supportive for further investigation of these compounds through experiments and theories.

First-Principles Study of Mechanical and Thermodynamic Properties of Binary and Ternary CoX (X = W and Mo) Intermetallic Compounds.

In this study, the structural, elastic, and thermodynamic properties of DO19 and L12 structured Co3X (X = W, Mo or both W and Mo) and μ structured Co7X6 were investigated using the density functional theory implemented in the pseudo-potential plane wave. The obtained lattice constants were observed to be in good agreement with the available experimental data. With respect to the calculated mechanical properties and Poisson’s ratio, the DO19-Co3X, L12-Co3X, and μ-Co7X6 compounds were noted to be mechanically stable and possessed an optimal ductile behavior; however, L12-Co3X exhibited higher strength and brittleness than DO19-Co3X. Moreover, the quasi-harmonic Debye–Grüneisen approach was confirmed to be valid in describing the temperature-dependent thermodynamic properties of the Co3X and Co7X6 compounds, including heat capacity, vibrational entropy, and Gibbs free energy. Based on the calculated Gibbs free energy of DO19-Co3X and L12-Co7X6, the phase transformation temperatures for DO19-Co3X to L12-Co7X6 were determined and obtained values were noted to match well with the experiment results.