The structural,mechanical, and thermodynamic properties of B2-type TMZr (TM = Ru,Mo, Rh,Os, and Re) compounds from first-principles calculations

The B2-type cubic Zr-based compounds are attractive advanced high-temperature materials because of the strong and symmetrical bonds. However, the mechanical and thermodynamic properties of the B2-type cubic Zr-based compounds are not well understood. Here, we use the first-principles calculations to investigate the structural, elastic modulus, ductility, and thermodynamic properties of TMZr (TM = Ru, Mo, Rh, Os, and Re) compounds. Two novel TMZr compounds, MoZr and ReZr, are first predicted by using the phonon dispersion and formation enthalpy, respectively. The results show that the B2-type TMZr compounds not only exhibit high elastic modulus but also show better ductility due to the symmetrical TM-Zr metallic bonds. In particular, the calculated elastic modulus of OsZr is larger than that of the other four TMZr compounds, indicating that the OsZr shows the strongest deformation resistance in five TMZr compounds. The calculated Θ _D of RuZr is 328 K, which is larger than that of the other four TMZr compounds. The calculated phonon density of state shows that the high-temperature thermodynamic properties of TMZr derive from the vibration of Zr atom. Therefore, our work predicts that the B2-type OsZr is an attractive high-temperature structural material.     

Influence of alloying elements on the mechanical and thermodynamic properties of ZrB12 ceramics from first-principles calculations

Although ZrB₁₂ is a promising advanced material because of the boron cuboctahedron cages, the hardness of ZrB₁₂ remains controversy. Here, we apply first-principles calculations to study the influence of transition metals (4d- and 5d-) on the hardness and thermodynamic properties of ZrB₁₂. The calculated hardness of ZrB₁₂ is 32.9 GPa, which is in good agreement with the previous theoretical result. Importantly, the calculated hardness of Re-doped ZrB₁₂ is up to 40.0 GPa, which is a potential superhard material. The essential reason is that the alloying element of Re enhances the localized hybridization of B B and Zr B atoms, and then forms the strong B B covalent bond and Zr B bond. The result is well demonstrated by the chemical bonding and lattice parameter. Here, our work shows that the alloying elements of Nb, Mo, and Re enhance the thermodynamic properties of ZrB₁₂. The Debye temperature of Re-doped ZrB₁₂ is 1225.2 K, which is larger than that of the parent ZrB₁₂ (1213.5 K).     

Exploring the structural,mechanical and thermodynamic properties of Ti-V solid solutions

Although Ti-V based high-temperature alloys are used in aerospace engine, rocket engine and hot sections, the structure and mechanical properties of Ti-V alloys remains controversy. To explore the correlation between structural and mechanical properties, we apply employed the DFT method to study the phases stability, mechanical and thermodynamic properties of Ti-V solid solution. Two Ti-V solid solutions: Ti(V)_ss solid solution and V(Ti)_ss solid solution are discussed. Two Ti-V solid solutions are thermodynamic stability. In particular, the Ti-V solid solution prefers to form V(Ti)_ss solid solution, in while the V(Ti)_ss solid solution remains cubic structure. Furthermore, the Ti(V)_ss solid solution is a mechanical instability. However, the V(Ti)_ss solid solution is a mechanical stability. Here, the bulk modulus, shear modulus and Young's modulus of V(Ti)_ss solid solution are 136.9, 23.5 and 66.7 GPa. In particular, the bulk modulus of V(Ti)_ss solid solution is higher than the bulk modulus of the pure Ti. In addition, the V(Ti)_ss solid solution shows better ductility compared to the pure Ti and V. Naturally, the stability and mechanical properties of V(Ti) solid solution is related to the Ti-V metallic bond because of the localized hybridization between the Ti(3d) and V(3d).     

Investigation on the stability,electronic, optical,and mechanical properties of novel calcium carbonate hydrates via first-principles calculations

Calcium carbonate (CaCO₃) is an inorganic compound which is widely used in industry, chemistry, construction, ocean acidification, and biomineralization due to its rich constituent on earth and excellent performance, in which calcium carbonate hydrates are important systems. In Zou et al's work (Science, 2019, 363, 396-400), they found a novel calcium carbonate hemihydrate phase, but the structural stability, optical, and mechanical properties have not been studied. In this work, the stability, electronic, optical, and mechanical properties of novel calcium carbonate hydrates were investigated by using the first-principles calculations using density functional theory. CaCO₃·xH₂O (x = 1/2, 1 and 6) are determined dynamically stable phases by phonon spectrum, but the Gibbs energy of reaction of CaCO₃·1/2H₂O is higher than other calcium carbonate hydrates. That is why CaCO₃·1/2H₂O is hard to synthesize in the experiments. In addition, the optical and mechanical properties of CaCO₃·xH₂O (x = 1/2, 1 and 6) are expounded in detail. It shows that the CaCO₃·1/2H₂O has the largest bulk modulus, shear modulus, and Young's modulus with the values 60.51 GPa, 36.56 GPa, and 91.28 GPa. This work will provide guidance for experiments and its applications, such as biomineralization, geology, and industrial processes.     

Structural,mechanical, electronic,and thermodynamic properties of pure tungsten metal under different pressures: A first-principles study

The structural, mechanical, electronic, and thermodynamic properties of pure W metal under different pressures have been investigated using the first-principles method. Our calculated structural parameters are in good agreement with experimental and previous theoretical results. The obtained elastic constants show that pure W metal is mechanically stable. Elastic properties such as the bulk modulus (B), shear modulus (G), Young's modulus (E), Poisson's ratio (ν), Cauchy pressure (C′), and anisotropy coefficients (A) are calculated by the Voigt-Reuss-Hill method. The results show that the pressure can improve the strength of pure tungsten and has little effect on the ductility. In addition, the total density of states as a function of pressure is analyzed. Thermodynamic properties such as the Debye temperature, phonon dispersion spectrum, free energy, entropy, enthalpy, and heat capacity are also discussed.     

Theoretical exploration on the vibrational and mechanical properties of M3C2/M3C2T2 MXenes

MXenes have attracted intensive attention in chemistry and material science for their special structures and properties. In order to understand the basic physical properties of the M₃C₂/M₃C₂T₂ (MSc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; TF, O, OH) MXenes, first-principles calculations are carried out to investigate the structural, vibrational, and mechanical properties in this work. Both the metal atoms and surface groups can significantly influence the configurations or mechanical behaviors of the MXenes. The dehydrogenation tendency is calculated to evaluate the possible forms of the M₃C₂(OH)₂ toward M₃C₂O₂. The work functions of MXenes functionalized by different groups are compared, and the lower work functions for the  OH functionalized ones, which can be as low as 1.358 eV for the Sc₃C₂(OH)₂, suggest potential good performance in electron emission. In addition, the stability, mechanical properties, and the Raman and infrared (IR) activity modes of the MXenes are reported. Generally, functionalized MXenes would present smaller lattice parameters, lower free energies, and stronger mechanical strength compared to their counterparts. The data obtained may provide important theoretical ground for the investigations of the applications of MXenes.     

The influence of high pressure on the structural stability,Vickers hardness and mechanical properties of Re and Ru dodecaborides

We apply the first-principles approach to study the structural stability, Vickers hardness, and elastic modulus of ReB₁₂ and RuB₁₂. In particular, we further investigate the influence of high pressure on the structural stability and mechanical properties of ReB₁₂ and RuB₁₂. The calculated results show that ReB₁₂ and RuB₁₂ are thermodynamic stability under high pressure. Here, ReB₁₂ is more thermodynamic stability than that of the RuB₁₂. The calculated Vickers hardness of ReB₁₂ and RuB₁₂ is 16.25 and 16.55 GPa, respectively. It is found that the calculated elastic constants and elastic modulus of ReB₁₂ and RuB₁₂ increase with increasing pressure. In particular, the calculated elastic constants and elastic modulus of ReB₁₂ are larger than that of the RuB₁₂. The calculated electronic structure shows that the high hardness and elastic modulus of ReB₁₂ and RuB₁₂ are attributed to the 3D network B-B covalent bonds.     

The thermodynamic properties of alkali metal compounds at high temperatures

A research program has been in progress to obtain reliable thermodynamic data on various binary and ternary alkali metal compounds in the temperature range of 300 to 1500 K. To date, heat capacity measurements have been made on cesium and rubidium chromates, dichromates, zirconates, molybdates, dimolybdates, and halides in the temperature range of 300 to 800K. In addition, measurements are planned or are currently in progress on cesium and rubidium chalcogenides, aluminates, uranates, silicates, and several other lithium, sodium, and potassium compounds. The status of the research program is discussed.     

Structural and mechanical properties of alkali hydrides investigated by the first-principles calculations and principal component analysis

The structural and mechanical properties of alkali hydrides (LiH, NaH, KH, RbH, and CsH) were investigated via first-principles calculations which cover the optimized structural parameters. The density functional theory in combination with the generalized gradient approximation (GGA) were used in this study. From the present study, one could note that alkali hydrides are brittle materials and mechanically stable. It was found that stiffness and shear resistance are greater in LiH than in other hydrides. It is more brittle in nature, and comparatively harder than the other materials under study; it also presents a high degree of anisotropy. The results were then investigated and analyzed with principal component analysis (PCA), which is one of the most common techniques in multivariate analysis, was used to explore the correlations among material properties of alkali hydrides and to study their trends. The alkali hydrides obtained by the first-principles calculations were also compared with the alkaline-earth metal hydrides (BeH₂, MgH₂, CaH₂, SrH₂, and BaH₂) and discussed in this work.     

Modified calculations of hydrocarbon thermodynamic properties

A test set of 65 hydrocarbons was examined to elucidate theoretically their thermodynamic properties by performing the density-functional theory (DFT) and ab initio calculations. All the calculated data were modified using a three-parameter calibration equation and the least-squares approach, to determine accurately enthalpies of formation (DeltaH(f)), entropies (S), and heat capacities (C(p)). Calculation results demonstrated that the atomization energies of all compounds exhibited an average absolute relative error ranging between 0.11- 0.13%, and an DeltaH(f) of formation with a mean absolute absolute error (M.|A.E.|) ranging from only 5.7-6.8 kJ/mol (1.3-1.6 kcal/mol) (i.e., those results correlated with those of Dr. Herndon's 1.1 kcal/mol). Additionally, the entropy ranged from 3.5-4.2 J/mol K (0.8-1.0 cal/mol K) M.|A.E.|; a heat capacity between 2.3-2.9 J/mol K (0.5-0.7 cal/mol K) M.|A.E.| was obtained as well.     

Structural, thermodynamic and optical properties of MgF2 studied from first-principles theory

A detailed theoretical study of structural, electronic, elastic, thermodynamic and optical properties of rutile type MgF₂ has been carried out by means of first-principles Density Functional Theory (DFT) calculations using plane wave pseudo-potentials within the local density approximation and generalized-gradient approximation for the exchange and correlation functionals. The calculated ground state properties and elastic constants agree quite well with experimental values. From the calculated elastic constants we conclude that MgF₂ is relatively hard when compared to other alkaline-earth fluorides and ductile in nature. The thermodynamic properties such as heat capacity, entropy, free energy, phonon density of states and Debye temperatures are calculated at various temperatures from the lattice dynamical data obtained through the quasi-harmonic Debye model. From free energy and entropy it is found that the system is thermodynamically stable up to 1200 K. The imaginary part of the calculated dielectric function ε₂(ω) could reproduce the six prominent peaks which are observed in experiment. From the calculated ε(ω), other optical properties such as refractive index, reflectivity and electron energy-loss spectrum are obtained up to the photon energy range of 30 eV.     

Calculation of thermodynamic and transport properties of a typical arc furnace plasma

Using a previously developed computer program, thermodynamic and transport properties of a typical arc furnace plasma are calculated in order to single out those species and / or reactions which exert a dominating influence on the properties of such complex mixtures. The results indicate that dissociation of molecular species in the arc furnace atmosphere has a strong effect on the specific heat and on the thermal conductivity of the mixture. The electrical conductivity is strongly affected by metallic vapors from the molten metal pool and the slag cover.     

Interfacial composition, structural parameters and thermodynamic properties of water-in-oil microemulsions

The formation and structural characteristics of water-in-oil microemulsions comprising hexadecylpyridinium chloride (CPC), alkanols (C₄–C₆) and alkanes (C₅, C₈–C₁₀) have been investigated by the method of dilution. The compositions of the surfactant and the cosurfactant in the interfacial region (interphase) of the microemulsion droplets have been determined. The thermodynamics of transfer of the cosurfactants (alkanols) from the continuous oil (alkane) phase to the interface have been evaluated from dilution measurements at different temperatures. The structural parameters, radii of the droplet and the waterpool, aggregation numbers of CPC and the alkanols in the interphase of a droplet, and the nanoparticle density of solution have been estimated assuming monodispersity of the droplets. The thermodynamics and structural parameters have been examined in terms of the chain lengths of the alkanols and alkanes. Received: 12 September 2000 Accepted: 27 October 2000     

First-principles investigation of structural,electronic, and optical properties of transition metal-doped C40 CrSi2

Although CrSi₂ silicide is an attractive advanced functional material, the improvement of electronic and optical properties is still a challenge for its applications. Here, we apply the first-principles calculations to investigate the influence of transition metals (TMs) on the electronic and optical properties of C40 CrSi₂ silicide. Five possible TMs, Ti, V, Pd, Ag, and Pt, are considered in detail. The calculated results show that the additive metals Ti, V, Pd, and Pt are thermodynamically stable in C40 CrSi₂ because the calculated impurity formation energy of TM-doped C40 CrSi₂ is lower than zero. In particular, the V dopant is more thermodynamically stable than that of the other TMs. The calculated electronic structure shows that the band gap of C40 CrSi₂ is 0.391 eV, which is in good agreement with the other results. In particular, the additive TMs improve the electronic properties of C40 CrSi₂ due to the role of the d-state of TMs. Naturally, the additive TMs result in band migration (Cr-3d state and Si-3p state) from the valence band to the conduction band. Interestingly, the additive TMs lead to a red shift for optical adsorption of C40 CrSi₂ silicide.     

Theoretical prediction of the electronic and thermodynamic properties of YN‐ZrN solid solutions

In this study, the results of structural parameters, electronic structure, and thermodynamic properties of the Zr_xY_1–xN solid solutions are presented. The effect of zirconium composition on lattice constant, and bulk modulus shows nonlinear dependence on concentration. Deviations of the lattice constant from Vegard's law and deviations of the bulk modulus from linear concentration dependence were found. Our findings indicate that the Zr_xY_1–xN solid solutions are metallic for x = 0.25, 0.5, 0.75. The calculated excess mixing enthalpy is positive over the entire zirconium composition range. The positive mixing enthalpies for Zr_xY_1–xN alloys indicate the existence of miscibility gaps and spinodal decompositions. The effect of temperature on the volume, bulk modulus, Debye temperature, and the heat capacity for Zr_xY_1–xN alloys were analyzed using the quasi‐harmonic Debye model. Results show that the heat capacity is slightly sensitive to composition as temperature increases. © 2015 Wiley Periodicals, Inc.     

Theoretical assessment of structural,mechanical, and thermodynamic properties of Pd2Al

The physical properties, namely structural, mechanical, and thermodynamic properties, of Pd₂Al intermetallic compound were explored through first-principles calculations within the framework of density functional theory. The calculated lattice constants were consistent with the available experimental data. The calculated elastic constants revealed that Pd₂Al was mechanically stable. By the predicted elastic constants, several related properties, namely Cauchy pressures, shear anisotropy factors, directional Young's modulus, bulk, shear and Young's moduli, the ratio of K/G, Vickers hardness, sound velocity, and minimum thermal conductivity for Pd₂Al were evaluated. According to the calculated results, it was found that Pd₂Al possesses a highly anisotropic feature and behaves in a ductile manner with low stiffness. Finally, temperature-dependence of thermodynamic properties, namely Debye temperature and heat capacity, were also evaluated through the quasi-harmonic Debye model.     

Crystal structures,theoretical calculations,spectroscopic and electrochemical properties of Cr(III) complexes with dipicolinic acid and 1,10-phenantroline

The crystal structures of compounds Na[Cr(dipic)₂] · 2H₂O (1) and [Cr(dipic)(phen)Cl] · 1/2H₂O (2), dipic = dipicolinate, phen = 1,10-phenantroline, were determined. In both complexes, Cr(III) is in a distorted octahedral environment. In complex (1), the metal is coordinated to two nearly perpendicular dipic anions acting as tridentate ligands through one oxygen of each carboxylate group and the pyridinic nitrogen atom. In complex (2), Cr(III) ion is similarly coordinated to a dipic anion, defining a ligand equatorial plane. The phen molecule bridges the remaining equatorial coordination site and one of the axial positions through its N-atoms. The other axial position is occupied by a chloride ion.     

Comparison of thermodynamic stabilities and mechanical properties of CO2, SiO2, and GeO2 polymorphs by first-principles calculations

The recent discovery that molecular CO(2) transforms under compression into carbon four-coordinated, 3-dimensional network solid phases has generated considerable interests on possible new phases in the fourth-main-group elemental oxides. Based on density-functional theory calculations, we have investigated the thermodynamic stability, mechanical properties and electronic structure of proposed guest-free clathrates, quartz and cristobalite phases for CO(2), SiO(2), and GeO(2), and the dry ice phase for CO(2). It was predicted that a GeO(2) clathrate, likely a semiconductor, could be synthesized presumably with some suitable guest molecules. The hypothetical CO(2) guest-free clathrate phase was found hardly to be formed due to the large energy difference with respect to the other polymorphs. This phase is unstable at all pressures, which is also implied by its different electronic structure in comparison with SiO(2) and GeO(2). Finally, the SiO(2) clathrate presents a uniquely high bulk modulus, which is higher than that of quartz and three times of the experimental data, might not be a weak point of ab-initio calculations such as pseudopotentials, correlation functional etc., instead it can be readily understood by the constraint as imposed by the high symmetry. Either temperature or an "exhausted" relaxation (without any symmetry constraint) can remedy this problem.     

Equiatomic binary phases of Copper-Rare Earth Elements. An overview of monocuprides from first-principles calculations

Equiatomic binary phases of copper with rare earth (RE) elements exhibit either primitive cubic ( ) or orthorhombic (Pnma) structures and in some cases both. By using density functional theory (DFT), we calculated the enthalpies of formation along the series of RE elements combined equimolarly with copper. For RE from Sc to Lu, the calculated enthalpies of formation fall in the range −49.8 kJ/mol for LuCu to −9.1 kJ/mol for the least thermodynamically stable CeCu. Except NdCu, all the other cubic or orthorhombic compounds exhibit lattice stability. Either forms of NdCu indicated lattice instability. Along the Sc-group, the hypothetical primitive cubic and orthorhombic forms of LuCu are found thermodynamically and mechanically stable. The overall trend of the formation enthalpies as a function of the Meyer Periodic Number is consistent with the energy trend of the 4 f-orbital filling as moving from Sc to Lu monocuprides. In addition, the calculated Gibbs free energies indicate that the thermodynamic stability is largely due to the entropic contributions. All standard DFT calculations were also repeated with DFT+U to better describe the correlation between the 5d–4f and 3d shells of RECu compounds. It has been found that DFT+U slightly affects the enthalpies of formation of RECu binaries. Moreover, DFT+U shifts up the f-band energies of RECu with light RE elements (such as La, Ce and Pr) and in contrast lowers them in the case of RECu with heavy RE elements from Nd to Lu.