Structural properties of ordered high-melting-temperature intermetallic alloys from first-principles total-energy calculations

First-principles study of thermodynamic stability and mechanical properties of fifteen high-entropy quaternary metal disilicides

By combining first-principles density-functional calculations and thermodynamics, we investigated the thermodynamic stability and mechanical properties of 15 quaternary high-entropy metal disilicides composed of silicon and four of the six refractory transition metals Ti, Zr, Hf, V, Nb, and Ta. We constructed a three-dimensional diagram specified by two thermodynamic parameters (the mixing enthalpy and the ratio of the entropy term in the Gibbs free energy to enthalpy) and a structural parameter (the lattice size difference). The obtained diagram allows us to predict that, except for TiZrHfVSi8, the formation of all other fourteen single-phase metal disilicides is thermodynamically favorable. Our calculations show that, for the formation of each of the 14 metal disilicides, the driving force suppresses the resistance at temperatures well below the melting point, suggesting that it is feasible to synthesize these high-entropy materials. One of these (TiHfNbTaSi8) has already been experimentally realized. Furthermore, the values of the mechanical parameters and melting points of the predicted fourteen quaternary high-entropy metal disilicides are all greater than the corresponding average values of the four single-metal disilicides.

Ab initio study of structural, mechanical and electronic properties of 3d transitional metal carbide in cubic rocksalt (rs), zincblende (zb), and cesium chloride (cc) structures by using LDA and GGA Approximation

CONTEXT

This study rigorously investigates three 3d transition metal carbide (TMC) structures via LDA and GGA approximations. It examines cohesive energy (Ecoh), Vickers hardness (Hv), mechanical stability, and electronic properties. Notably, most 3d TMCs exhibit higher cohesive energy than nitrides, and rs-TiC demonstrates a Vickers hardness of 25.66 GPa, outperforming its nitride counterpart. The study employs theoretical calculations to expedite research, revealing mechanical stability in CrC and MnC (GGA) and CrC (LDA in cc structure), while all 3d TMCs in rs and seven in zb structures show stability. Charge transfer and bonding analysis reveal enhanced covalency along the series, influenced by the interplay between p orbitals of carbon and d orbitals of the metal. Most 3d TMCs exhibit metallic properties, excluding zb-TiC and zb-FeC in all phases. An inverse correlation between elastic constant C44 and electronic states near the Fermi level (EF) emerges, guiding applications and design. This study efficiently uncovers 3d TMC properties, offering insights for applications and design.

METHODS

We employed the Vienna ab initio Simulation software (VASP) to perform computations based on density functional theory (DFT). Our approach incorporated both the projector augmented wave (PAW) and PW91 general gradient approximation (GGA) methods within the local density approximation (LDA).

Ab Initio Studies of Structural and Mechanical Properties of NH3, NO, and N2O Hydrates

The investigation on the mechanical properties of clathrate hydrate is closely related to the exploitation of hydrates and gas transportation. In this article, the structural and mechanical properties of some nitride gas hydrates were studied using DFT calculations. First, the equilibrium lattice structure is obtained by geometric structure optimization; then, the complete second-order elastic constant is determined by energy-strain analysis, and the polycrystalline elasticity is predicted. It is found that the NH₃, N₂O, and NO hydrates all have high elastic isotropy but are different in shear characteristics. This work may lay a theoretical foundation for studying the structural evolution of clathrate hydrates under the mechanical field.

First-principles calculations to investigate physical properties of orthorhombic perovskite YBO3 (B = Ti & Fe) for high energy applications

Orthorhombic oxide perovskite compounds are very promising materials for the applications of optoelectronics and thermal barrier coating. This work represents a numerical simulation of YBO₃ compounds through the first-principles ab initio approach. The electronic and magnetic properties are investigated employing the general gradient approximation (GGA) coupled to the integration of the Hubbard U-term which is the GGA + U. The Ti and Fe-based YBO₃ perovskite compounds are found to be actively promising within the ferromagnetic configuration and their lattice parameters are consistent with the previous studies. The calculations of formation energy signify that the compounds YBO₃ are stable thermodynamically. The electronic properties are computed and evaluated by the band structure and density of states for both compounds and the results depict that these materials are ferromagnetic half-metallic. Mechanically these compounds are stable, ductile, anisotropic, and hard to scratch. The thermal properties are evaluated for YBO₃ (B = Ti and Fe) compounds up to a temperature range of 2000 K. This work can open new opportunities for further exploration in this field.

Solids that are also liquids: elastic tensors of superionic materials

Superionics are fascinating materials displaying both solid- and liquid-like characteristics: as solids, they respond elastically to shear stress; as liquids, they display fast-ion diffusion at normal conditions. In addition to such scientific interest, superionics are technologically relevant for energy, electronics, and sensing applications. Characterizing and understanding their elastic properties is, e.g., urgently needed to address their feasibility as solid-state electrolytes in all-solid-state batteries. However, static approaches to elasticity assume well-defined reference positions around which atoms vibrate, in contrast with the quasi-liquid motion of the mobile ions in fast ionic conductors. Here, we derive the elastic tensors of superionics from ensemble fluctuations in the isobaric-isothermal ensemble, exploiting extensive Car-Parrinello simulations. We apply this approach to paradigmatic Li-ion conductors, and complement with a block analysis to compute statistical errors. Static approaches sampled over the trajectories often overestimate the response, highlighting the importance of a dynamical treatment in determining elastic tensors in superionics.

A First-Principles Study of the Cu-Containing β″ Precipitates in Al-Mg-Si-Cu Alloy

The nanostructured β″ precipitates are critical for the strength of Al-Mg-Si-(Cu) aluminum alloys. However, there are still controversial reports about the composition of Cu-containing β″ phases. In this work, first-principles calculations based on density functional theory were used to investigate the composition, mechanical properties, and electronic structure of Cu-containing β″ phases. The results predict that the Cu-containing β″ precipitates with a stoichiometry of Mg4+xAl2-xCuSi4 (x = 0, 1) are energetically favorable. As the concentration of Cu atoms increases, Cu-containing β″ phases with different compositions will appear, such as Mg4AlCu2Si4 and Mg4Cu3Si4. The replacement order of Cu atoms in β″ phases can be summarized as one Si3/Al site → two Si3/Al sites → two Si3/Al sites and one Mg1 site. The calculated elastic constants of the considered β″ phases suggest that they are all mechanically stable, and all β″ phases are ductile. When Cu atoms replace Al atoms at Si3/Al sites in β″ phases, the values of bulk modulus (B), shear modulus (G), and Young's modulus (E) all increase. The calculation of the phonon spectrum shows that Mg4+xAl2-xCuSi4 (x = 0, 1) are also dynamically stable. The electronic structure analysis shows that the bond between the Si atom and the Cu atom has a covalent like property. The incorporation of the Cu atom enhances the electron interaction between the Mg2 and the Si3 atom so that the Mg2 atom also joins the Si network, which may be one of the reasons why Cu atoms increase the structure stability of the β″ phases.

Using first-principles calculations to predict the mechanical properties of transmuting tungsten under first wall fusion power-plant conditions

Tungsten and tungsten alloys are being considered as leading candidates for structural and functional materials in future fusion energy devices. The most attractive properties of tungsten for the design of magnetic and inertial fusion energy reactors are its high melting point, high thermal conductivity, low sputtering yield and low long-term disposal radioactive footprint. Yet, despite these relevant features, tungsten also presents a very low fracture toughness, mostly associated with inter-granular failure and bulk plasticity, that limits its applications. Significant neutron-induced transmutation happens in these tungsten components during nuclear fusion reactions, creating transmutant elements including Re, Os and Ta. Density functional theory (DFT) calculations that allow the calculation of defect and solute energetics are critical to better understand the behavior and evolution of tungsten-based materials in a fusion energy environment. In this study, we present a novel computational approach to perform DFT calculations on transmuting materials. In particular, we predict elastic and plastic mechanical properties (such as bulk modulus, shear modulus, ductility parameter, etc) on a variety of W-X compositions that result when pure tungsten is exposed to the EU-DEMO fusion first wall conditions for ten years.

Intrinsic Thermal Shock Behavior of Common Rutile Oxides

Rutile TiO2, VO2, CrO2, MnO2, NbO2, RuO2, RhO2, TaO2, OsO2, IrO2, SnO2, PbO2, SiO2, and GeO2 (space group P42/mnm) were explored for thermal shock resistance applications using density functional theory in conjunction with acoustic phonon models. Four relevant thermomechanical properties were calculated, namely thermal conductivity, Poisson’s ratio, the linear coefficient of thermal expansion, and elastic modulus. The thermal conductivity exhibited a parabolic relationship with the linear coefficient of thermal expansion and the extremes were delineated by SiO2 (the smallest linear coefficient of thermal expansion and the largest thermal conductivity) and PbO2 (vice versa). It is suggested that stronger bonding in SiO2 than PbO2 is responsible for such behavior. This also gave rise to the largest elastic modulus of SiO2 in this group of rutile oxides. Finally, the intrinsic thermal shock resistance was the largest for SiO2, exceeding some of the competitive phases such as Al2O3 and nanolaminated Ti3SiC2.

Effect of Ta addition on the structural, thermodynamic and mechanical properties of CoCrFeNi high entropy alloys

Ta addition alters the microstructures, thermodynamic and mechanical properties of CoCrFeNi alloys.

Study of the magneto-electronic, optical, thermal and thermoelectric applications of double perovskites Ba2MTaO6 (M = Er, Tm)

The structural, electronic, optical, thermodynamic and thermoelectric properties of double perovskites Ba₂MTaO₆ (M = Er, Tm) have been studied. These alloys stabilize in cubic structure with paramagnetic phases and structural parameters that are in good agreement with experimental results. The elastic parameters reveal both materials as being super hard and brittle in nature. The band profile displays the direct bandgap semiconducting behavior of both compounds in the spin-up channels. The optical coefficients, viz. optical conductivity, reflectivity and electron energy loss, are intended for a photon energy of up to 14 eV to perceive the optical response. The effects of temperature and pressure on the thermodynamic properties have also been evaluated via the quasi-harmonic Debye model. Post-DFT treatment was performed to explore the thermoelectric properties; both perovskites exhibit a colossal power factor of 90 × 10¹¹ W m⁻¹ K⁻² s⁻¹ and 5 × 85 × 10¹¹ W m⁻¹ K⁻² s⁻¹ for Ba₂TmTaO₆ and Ba₂ErTaO₆, respectively, at 800 K.

The structural, electronic, optical, thermodynamic and thermoelectric properties of double perovskites Ba₂MTaO₆ (M = Er, Tm) have been studied.

A near-zero Poisson's ratio of Si with ordered nanopores

Significant reduction in the Poisson's ratio is predicted for Si with cylindrical nanopores through first-principles DFT calculations.

Impact of twin boundaries on bulk elastic constants: Density-functional theory data for Young׳s modulus of Ag

Experimental and theoretical studies on nanowires have reported a size-dependence of the Young׳s modulus in the axial direction, which has been attributed to the increasing influence of surface stresses with decreasing wire diameter. Internal interfaces and their associated interface stresses could lead to similar changes in the elastic properties. In Kobler et al. [1], however, we reported results from atomistic calculations which showed for Ag that twin boundaries have a negligible effect on the Young׳s modulus. Here, we present data of density-functional theory calculations of elastic constants and Young׳s modulus for defect-free bulk Ag as well as for bulk Ag containing dense arrays of twin boundaries. It is shown that rigorous convergence tests are required in order to be able to deduce changes in the elastic properties due to bulk defects in a reliable way.

First-principles calculations, experimental study, and thermodynamic modeling of the Al-Co-Cr system

The phase relations and thermodynamic properties of the condensed Al-Co-Cr ternary alloy system are investigated using first-principles calculations based on density functional theory (DFT) and phase-equilibria experiments that led to X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) measurements. A thermodynamic description is developed by means of the calculations of phase diagrams (CALPHAD) method using experimental and computational data from the present work and the literature. Emphasis is placed on modeling the bcc-A2, B2, fcc-γ, and tetragonal-σ phases in the temperature range of 1173 to 1623 K. Liquid, bcc-A2 and fcc-γ phases are modeled using substitutional solution descriptions. First-principles special quasirandom structures (SQS) calculations predict a large bcc-A2 (disordered)/B2 (ordered) miscibility gap, in agreement with experiments. A partitioning model is then used for the A2/B2 phase to effectively describe the order-disorder transitions. The critically assessed thermodynamic description describes all phase equilibria data well. A2/B2 transitions are also shown to agree well with previous experimental findings.

First-principles study of electronic structures and stability of body-centered cubic Ti-Mo alloys by special quasirandom structures

The electronic structures and structural properties of body-centered cubic Ti-Mo alloys were studied by first-principles calculations. The special quasirandom structures (SQS) model was adopted to emulate the solid solution state of the alloys. The valence band electronic structures of Ti-Mo and Ti-Mo-Fe alloys were measured by hard x-ray photoelectron spectroscopy. The structural parameters and valence band photoelectron spectra were calculated using first-principles calculations. The results obtained with the SQS models showed better agreement with the experimental results than those obtained using the conventional ordered structure models. This indicates that the SQS model is effective for predicting the various properties of solid solution alloys by means of first-principles calculations.

A DFT study on structural, vibrational properties, and quasiparticle band structure of solid nitromethane

We report a detailed theoretical study of the structural and vibrational properties of solid nitromethane using first principles density functional calculations. The ground state properties were calculated using a plane wave pseudopotential code with either the local density approximation, the generalized gradient approximation, or with a correction to include van der Waals interactions. Our calculated equilibrium lattice parameters and volume using a dispersion correction are found to be in reasonable agreement with the experimental results. Also, our calculations reproduce the experimental trends in the structural properties at high pressure. We found a discontinuity in the bond length, bond angles, and also a weakening of hydrogen bond strength in the pressure range from 10 to 12 GPa, picturing the structural transition from phase I to phase II. Moreover, we predict the elastic constants of solid nitromethane and find that the corresponding bulk modulus is in good agreement with experiments. The calculated elastic constants show an order of C11> C22 > C33, indicating that the material is more compressible along the c-axis. We also calculated the zone center vibrational frequencies and discuss the internal and external modes of this material under pressure. From this, we found the softening of lattice modes around 8-11 GPa. We have also attempted the quasiparticle band structure of solid nitromethane with the G0W0 approximation and found that nitromethane is an indirect band gap insulator with a value of the band gap of about 7.8 eV with G0W0 approximation. Finally, the optical properties of this material, namely the absorptive and dispersive part of the dielectric function, and the refractive index and absorption spectra are calculated and the contribution of different transition peaks of the absorption spectra are analyzed. The static dielectric constant and refractive indices along the three inequivalent crystallographic directions indicate that this material has a considerable optical anisotropy.

Thermal, electronic and ductile properties of lead-chalcogenides under pressure

Fully relativistic pseudo-potential ab-initio calculations have been performed to investigate the high pressure phase transition, elastic and electronic properties of lead-chalcogenides including the less known lead polonium. The calculated ground state parameters, for the rock-salt structure show good agreement with the experimental data. PbS, PbSe, PbTe and PbPo undergo a first-order phase transition from rock-salt to CsCl structure at 19.4, 15.5, 11.5 and 7.3 GPa, respectively. The elastic properties have also been calculated. The calculations successfully predicted the location of the band gap at L-point of Brillouin zone and the band gap for each material at ambient pressure. It is observed that unlike other lead-chalcogenides, PbPo is semi-metal at ambient pressure. The pressure variation of the energy gap indicates that these materials metalize under pressure. The electronic structures of these materials have been computed in parent as well as in high pressure B2 phase.

Phase stability and elastic properties of Cr-V alloys

V is the only element in the periodic table that forms a complete solid solution with Cr and thus is particularly important in alloying strategy to ductilize Cr. This study combines first-principles density functional theory calculations and experiments to investigate the phase stability and elastic properties of Cr-V binary alloys. The cluster expansion study reveals the formation of various ordered compounds at low temperatures that were not previously known. These compounds become unstable due to the configurational entropy of bcc solid solution as the temperature is increased. The elastic constants of ordered and disordered compounds are calculated at both T = 0 K and finite temperatures. The overall trends in elastic properties are in agreement with measurements using the resonant ultrasound spectroscopy method. The calculations predict that addition of V to Cr decreases both the bulk modulus and the shear modulus, and enhances the Poisson's ratio, in agreement with experiments. Decrease in the bulk modulus is correlated to decrease in the valence electron density and increase in the lattice constant. An enhanced Poisson's ratio for bcc Cr-V alloys (compared to pure Cr) is associated with an increased density of states at the Fermi level. Furthermore, the difference charge density in the bonding region in the (110) slip plane is highest for pure Cr and decreases gradually as V is added. The present calculation also predicts a negative Cauchy pressure for pure Cr, and it becomes positive upon alloying with V. The intrinsic ductilizing effect from V may contribute, at least partially, to the experimentally observed ductilizing phenomenon in the literature.

Topological partition of the elastic constants of crystals

We present a partitioning of the elastic constants of a crystal into atomic contributions by using the atomic basin concept inherent to Bader's Quantum Theory of Atoms in Molecules. The partition is made by following the evolution of the cell volume and the atomic basin volumes under appropriately defined cell deformations. The method is carefully examined, including internal consistency checks. The transferability of atomic contributions between different crystals is determined by obtaining and comparing the oxygen contribution to the elastic constants of a selection of cubic oxides that includes the rock-salt, perovskite, antifluorite, and cuprite crystal families.

Phase-field-crystal methodology for modeling of structural transformations

We introduce and characterize free-energy functionals for modeling of solids with different crystallographic symmetries within the phase-field-crystal methodology. The excess free energy responsible for the emergence of periodic phases is inspired by classical density-functional theory, but uses only a minimal description for the modes of the direct correlation function to preserve computational efficiency. We provide a detailed prescription for controlling the crystal structure and introduce parameters for changing temperature and surface energies, so that phase transformations between body-centered-cubic (bcc), face-centered-cubic (fcc), hexagonal-close-packed (hcp), and simple-cubic (sc) lattices can be studied. To illustrate the versatility of our free-energy functional, we compute the phase diagram for fcc-bcc-liquid coexistence in the temperature-density plane. We also demonstrate that our model can be extended to include hcp symmetry by dynamically simulating hcp-liquid coexistence from a seeded crystal nucleus. We further quantify the dependence of the elastic constants on the model control parameters in two and three dimensions, showing how the degree of elastic anisotropy can be tuned from the shape of the direct correlation functions.

Pressure induced magnetic, electronic and mechanical properties of SmX (X = Se, Te)

The magnetic, structural, elastic and electronic properties of Sm-chalcogenides in the stable [Formula: see text] and high pressure [Formula: see text] phase have been analyzed using an ab initio pseudo-potential method with a spin-polarized GGA based on exchange-correlation energy optimization, as implemented in SIESTA code. The magnetic phase stability has been determined from the total energy calculations in non-magnetic and magnetic phases, which clearly indicate that at ambient and high pressures, these compounds are ferromagnetically stable. Also, the Sm ion is described in both five and six localized f electrons. Under compression the Sm chalcogenides undergo a first-order transformation from Sm(2+) to a stable valence state (Sm(3+)) with delocalization of the 4f electrons into the 5d states of Sm followed by a structural transition from the B1 to the B2 phase. The structural properties namely, equilibrium lattice constant, bulk modulus, its pressure derivative, transition pressure and volume collapse agree well with the experimental results. We have also computed the electronic structure at different volumes.

Ab initio calculations of the structure and mechanical properties of vanadium oxides

VO, V(2)O(3), VO(2), V(6)O(13), V(4)O(9), V(3)O(7) and V(2)O(5) have been investigated in terms of structure, bulk modulus B and elastic constant C(44) using ab initio calculations. The C(44) values for V(6)O(13), V(4)O(9), V(3)O(7) and V(2)O(5) are significantly lower than those for V(2)O(3) and VO(2). As the V valency is increased from 3 to 5, C(44) decreases by 83%, whereas the bulk modulus decreases by 61%, leading to an increase in the B/C(44) ratio from 1.4 to 3.4. This is consistent with calculated decohesion energies for cleavage in VO(2) and V(2)O(5). When cleaving V(2)O(5), decohesion energies are considerably lower than those of VO(2). This behaviour may be understood based on V valency induced changes in the crystal and electronic structure as well as in the chemical bonding. As the V valency is increased, the bond strength decreases. The phases with a V valency >4 exhibit low C(44) values, large anisotropy and possess weak ionic bonding between the layers. The formation of easily plastically deformable structures is enabled by the screened Coulomb potential. The largest distance and therefore weakest bond strength is observed for V(2)O(5) in the (002) plane.

Fundamental state quantities and high-pressure phase transition in beryllium chalcogenides

In this work we study the structural and electronic properties of Be chalcogenides (BeS, BeSe and BeTe) using two different methods: the full-potential linear augmented-plane wave (FP-LAPW) and the plane-wave pseudopotential (PPsPW). The exchange-correlation effects are treated in the local-density approximation (LDA) and the generalized-gradient approximation (GGA). We have evaluated the ground-state quantities such as equilibrium volume, bulk modulus and its pressure derivative as well as the elastic constants. Various structural phase transitions were considered here in order to confirm the most stable structure and to predict the phase transition under hydrostatic pressure. In addition we have studied the band structure and the density of states, which show a wide indirect band gap for these compounds. These results were in favourable agreement with previous theoretical works and the existing experimental data. To complete the fundamental characteristics of beryllium chalcogenide compounds we have analysed their bonding character in terms of charge transfer and the ionicity parameter. The latter is found to be in agreement with the charge transfer behaviour, which shows an important ionic localization.

High-Pressure Elasticity of Iron and Anisotropy of Earth's Inner Core

A first principles theoretical approach shows that, at the density of the inner core, both hexagonal [hexagonal close-packed (hcp)] and cubic [face-centered-cubic (fcc)] phases of iron are substantially elastically anisotropic. A forward model of the inner core based on the predicted elastic constants and the assumption that the inner core consists of a nearly perfectly aligned aggregate of hcp crystals shows good agreement with seismic travel time anomalies that have been attributed to inner core anisotropy. A cylindrically averaged aggregate of fcc crystals disagrees with the seismic observations.