Mechanical Stability Criteria Research Articles

Electronic topological transitions and vibrational properties of A-15 type X3Y (X= V, Cr and Mo; Y= Os, Ir and Pt) compounds: A first-principles study

Ab-initio studies of A-15 type X3Y (X = V, Cr and Mo; Y= Os, Ir and Pt) compounds are presented at ambient and high pressures. All the studied compounds satisfy the mechanical stability criteria and are also dynamically stable as evidenced from the positive phonon dispersion curves. Camels back type band structure features are observed at Fermi level in some of the investigated compounds. Electronic topological transitions (ETTs) are the main highlights in all the compounds under pressure at different compressed volumes. The electron-phonon coupling constants λ are calculated on the basis of the Eliashberg theory for the Mo3Y (Y= Os, Ir and Pt) series that, according to the experiments, have the largest transition temperature among the studied compounds. Mo3Os superconductivity is fully understood on the basis of the Eliashberg theory of the phonon-mediated electron pairing and the experimental Tc is recovered using the well known Mc Millan formula. On the contrary, for the other two compounds of the same series Mo3Y (Ir and Pt) the calculated mass enhancement parameters show an unexpected behavior and for Mo3Ir, the experimental transition temperature values cannot be easily recovered through the Mc Millan formula.

Ruthenium-based half Heusler alloys RuTiX (X = Si, Ge, Sn): An FP-LAPW-based analytical study of structural, electronic, elastic, mechanical and transport properties

An full potential linearized augmented plane wave (FP-LAPW)-based analytical study of structural, electronic, mechanical and thermoelectric properties has been done for the Ruthenium-based half heusler RuTiX (X = Si, Ge and Sn) compounds. An efficient method to develop Half Heusler (HH) alloys is by examining their stability of structure in various phases, by plotting electronic band structures, computing elastic constants and also by studying the presence of magnetic moments. In this study, we have used DFT-based calculations to scrutinise the paramagnetic/ferromagnetic (FM) as well as metallic/semiconducting behavior of these HH compounds. The predicted phase stability using the energy versus volume curves reveals that they are stable in Type C phase. RuTiSi and RuTiGe are found to be stable in the paramagnetic phase whereas RuTiSn is stable in the FM phase with a finite value of magnetic moment. The electronic band structures and density of states (DOS) plots predict that the studied compounds belong to [Formula: see text]-type degenerate semiconductors as the Fermi Level lies within the valence band. Due to the existence of finite DOS at the Fermi level, they show an enhanced metallic behavior. A small value indirect gap is found between valence band maximum (VBM) and conduction band minimum (CBM) in all these studied RuTiX HH alloys depicting their semiconducting nature. The elastic constants of cubic phase are computed for the first time and they obey the mechanical stability criteria. The positive value of [Formula: see text] and value of [Formula: see text]/[Formula: see text] ratio of these HH compounds exhibit their ductile nature. The thermoelectric properties of these compounds are investigated, and a comparatively higher figure of merit reveals their scope of application in thermoelectric devices.

Pressure‐dependent elasto‐mechanical stability and thermoelectric properties of MYbF 3 (M = Rb, Cs) materials for renewable energy

Pressure-dependent elasto-mechanical, thermoelectric and thermodynamic properties of two direct band gap halide perovskites MYbF3 (M = Rb, Cs) have been investigated using density functional theory calculations. These calculations were carried out within Wein2k simulation code aided by generalized gradient approximation, Charpin method, BoltzTrap package and quasi-harmonic Debye model. The computed elastic constants (C11, C12 and C44) are all positive and follow the mechanical stability criteria within the applied pressure range (0-15 GPa). The obtained mechanical properties show that both the compounds are stiffer and oppose changes in the shape and volume up to a great extent. Moreover, MYbF3 compounds are ductile and bonded with central forces whereas exhibiting unique properties in different crystallographic positions as shown by Zener anisotropic factor values. Thermoelectric parameters in the temperature range 200 to 800 K suggest these compounds for commercial thermoelectric device engineering due to low thermal conductivity, high electronic conductivity and moderate values of the figure of merit. The overall effect of thermodynamic parameters shows these compounds can withstand high temperature and pressure. The calculations specified these materials ideal for renewable energy generation.

The "isothermal" compressibility of active matter.

We demonstrate that the mechanically defined "isothermal" compressibility behaves as a thermodynamic-like response function for suspensions of active Brownian particles. The compressibility computed from the active pressure-a combination of the collision and unique swim pressures-is capable of predicting the critical point for motility induced phase separation, as expected from the mechanical stability criterion. We relate this mechanical definition to the static structure factor via an active form of the thermodynamic compressibility equation and find the two to be equivalent, as would be the case for equilibrium systems. This equivalence indicates that compressibility behaves like a thermodynamic response function, even when activity is large. Finally, we discuss the importance of the phase interface when defining an active chemical potential. Previous definitions of the active chemical potential are shown to be accurate above the critical point but breakdown in the coexistence region. Inclusion of the swim pressure in the mechanical compressibility definition suggests that the interface is essential for determining phase behavior.

Structural and Physical Properties of DyCu, NdAg, LaCd, YIn, ErCu, ErAg, and ErAu Rare-Earth Intermetallic Compounds: Ab initio Investigations Analyzed by Data Mining Technique

In this work, the structural, elastic, mechanical, thermodynamic, and electronic properties of ErCu, ErAg, ErAu, DyCu, NdAg, LaCd, and YIn compounds have been investigated systematically using full-potential linearized augmented plane wave (FP-LAPW) within the density functional theory (DFT). In this approach, the exchange–correlation energy was calculated with generalized gradient approximation (GGA), by Perdew–Burke–Ernzerhof (PBE). We have firstly calculated lattice constants, bulk modulus, elastic constants, and mechanical properties such as Young’s and shear moduli, Pugh factor (B/G), anisotropy constant, and Cauchy pressure, as well as Poisson coefficient. The elastic constants and their related properties verify the criteria of mechanical stability, signifying that these compounds are stable in B2 structure. The ductility of the intermetallics investigated here is evaluated. The obtained values show that ErCu is the most rigid while NdAg the most ductile. The total and partial electron density of states (DOS) were evaluated to offer a detailed explanation of the contribution of atomic orbitals in the energy bands. The thermodynamic properties of ErCu, ErAg, ErAu, NdAg, LaCd, YIn, and DyCu are calculated via the quasi-harmonic Debye model, in which the lattice vibrations are considered. So temperature-dependent parameters such as lattice constant, thermal expansion, and specific heat at constant volume are analyzed. Furthermore, the relationship between several thermo-physical and mechanical properties was discussed and analyzed with data mining techniques. The obtained results confirm that this B2-type of rare-earth intermetallics has very interesting mechanical and thermal properties for structural applications.

Electronic Structure and Magnetic Properties of OsCrMnSb and IrCrMnSb from Ab Initio Calculations

The structural, elastic, electronic and magnetic properties of quaternary OsCrMnSb and IrCrMnSb Heusler alloys are performed employing ab initio electronic structure calculations. It has been identified that the YI type is the most stable structure among the three configurations for both OsCrMnSb and IrCrMnSb alloys in the magnetic state. The calculated cubic elastic constants show that these alloys fulfill the mechanical stability criteria. The band structures and density of state calculations reveal the half-metallic (HM) behavior of these alloys with a direct gap, and the half-metallicity is rather originated from the Cr-d states. Results on magnetic properties suggest that OsCrMnSb and IrCrMnSb are HM antiferromagnets. However, the inclusion of spin–orbit coupling affects strongly the IrCrMnSb alloy, losing its HM nature.

High-pressure structural, lattice dynamics, and electronic properties of beryllium aluminate studied from first-principles theory

The present work reports the complete study of structural, vibrational, mechanical, and electronic properties of BeAl2O4 (known as Chrysoberyl) using first-principles computing methods. The calculated ground-state properties agree quite well with previous experiments. The computed phonon dispersion curves do not show imaginary frequencies confirming the dynamical stability. In addition, the calculated elastic constants also ensure the mechanical stability through fulfillment of mechanical stability criteria. Apart from that, the theoretically determined phonon frequencies agree quite well with previous Raman and infrared experiments at ambient conditions. Various thermodynamic properties are also being calculated as a function of temperature. The thermal expansion computed within the quasi-harmonic approximation is found to be positive in agreement with the experiments. From the electronic structure we can see that Chrysoberyl possess a direct band gap of 8.3 eV with the oxygen- 2p states dominating close to fermi level in the valence band. The influence of pressure on different properties is discussed. The highlight of the present work is the presence of optical isotropy in Chrysoberyl although the crystal structure is highly anisotropic.

Metric-driven search for structurally stable inorganic compounds

We report a facile `metric’ for the identification of structurally and dynamically (positive definite phonon structure) stable inorganic compounds. The metric considers charge-imbalance within the local substructures in crystalline compounds calculated using first-principles density-functional theory. To exemplify, we chose carbon-based nitrides as it provides a large pool of structurally stable and unstable phases. We showcase how local structural information related to Wyckoff symmetry uniquely identifies four new carbon-nitride phases. The metric predicts three new structurally stable phases of 4 (C):3 (N) stoichiometry, i.e., C4N3, which is further confirmed by direct phonon calculations. The structurally stable phases also satisfy the thermodynamic stability and mechanical stability criteria. New phases possess extraordinary mechanical properties, similar to diamond, and show insulating bandgap ranging from optimal, 1.45 eV (optimal) to 5.5 eV (large gap). The structural, electronic, and optical properties of Pm(1)−C4N3, discussed in detail, indicate possible application in optoelectronic and photovoltaic technologies. The way metric is designed, it can be used to predict stability of any material with three-dimensional bonding network. The metric was also able to predict structural stability of other nitride polymorphs with 100% accuracy. We believe that the proposed ‘Metric’ will accelerate the search of unknown and unexplored inorganic compounds by quick filtering of dynamically stable phases without expensive density-functional theory calculations.

Phase transitions and elastic anisotropies of SiC polymorphs under high pressure

The structures, equations of state, phase transitions, mechanical stability, and elastic anisotropies of five SiC polymorphs (wurtzite, 4H, 6H, zinc blende, and rocksalt phases) under pressure were investigated based on first-principles calculations. The optimal structural parameters, densities, and volumes of the five SiC polymorphs under zero and high pressure were obtained and analyzed. The pressures of the phase and stability–instability transitions were predicted using the minimum energy principle and improved Born mechanical stability criteria, respectively, and a new phase transition criterion was developed. The elastic anisotropies of SiC under zero and high pressure were systematically discussed, and the negative Poisson's ratio of zinc blende SiC was verified.

Theoretical predictions of two new chiral solid carbon oxides

Two carbon oxides, termed as C6O9-154-6c6c3a and C6O9-154-6c6c3b, are theoretically predicted by first-principles calculations. Both structures belong to space group P3221, indicating that both are chiral structures. Both phonon spectra have no negative branches, certifying their dynamic stability. Both calculated elastic constants satisfy the criteria of mechanical stability. The ab initio molecular dynamics simulations reveal that both structures are not only room-temperature stable but also thermally stable even at 1000 K. The band structures demonstrate that both structures are indirect semiconductors. The C6O9-154-6c6c3a has a bandgap of 7.23 eV, while the bandgap of C6O9-154-6c6c3b is 8.82 eV. Simulated XRD features, Raman spectra, and vibration patterns are provided to help identify the future experimental samples. These two new chiral stable carbon oxides can not only provide more new chemical and physical properties about solid carbon oxides but also offer potential applications for carbon capture and utilization in the future.

Structural, mechanical and thermodynamic properties study on Mg–Y alloys from first-principles calculations

The structures with different compositions of the binary Mg–Y alloys have been predicted by first-principles calculations combined with an unbiased Crystal structure Analysis by Particle Swarm Optimization (CALYPSO) structure searching method. The two already known stoichiometries alloys of Mg1Y1 with Pm-[Formula: see text] symmetry and Mg3Y1 with Fm-[Formula: see text] are confirmed, and a new stoichiometry alloy of Mg1Y3 with [Formula: see text] symmetry is proposed. The dynamical and mechanical stabilities for the three alloys at different pressures are investigated by phonon spectra and mechanical stability criteria, respectively. Subsequently, the bulk modulus, shear modulus, Young’s modulus, the brittleness/ductile behavior, the elastic anisotropy as well as Vickers hardness for the three alloys at 0 GPa are discussed in detail. The results show that the Mg1Y1, Mg3Y1 and Mg1Y3 alloys improve the hardness and stiffness compared with pure Mg, and Mg1Y3 alloy is of the best ductility in the three alloys. Meanwhile, the three alloys exhibit anisotropic. Moreover, the thermodynamic properties, such as Debye temperature, heat capacity at constant volume, entropy and Helmholtz free energy for the three stable alloys, are predicted and discussed.

DFT – GGA calculations of magnetic, elastic, thermodynamic and thermoelectric properties of Co2YAl (Y = Ti, V) full Heusler alloy systems

This study used first principles calculations within the generalized gradient approximation (GGA) via plane-wave pseudo-potential scheme to extensively study the magnetic, elastic, thermodynamic and thermoelectric properties of Co2TiAl and Co2VAl Heusler alloys at their optimized lattice constants. Elastic properties of these alloys were obtained by fitting the stress-strain relationship via Voigt-Reuss-Hill approximations. Thermodynamic properties were calculated within the implementation of thermo_pw package while thermoelectric properties were estimated from non-spin polarized calculations via Boltzmann semi-classical approach as implemented in BoltzTraP package at temperatures between 0 and 800 K. Calculations reaffirmed the stable electronic and magnetic states for the alloys to be half metallic ferromagnetic (HMF) states. The estimated band gaps in the minority spin channels were 0.431 and 0.399 eV for Co2TiAl and Co2VAl alloys respectively and these were justified by their corresponding density of states. The calculated magnetic moments for the respective alloys were 0.949 and 1.978 μB, which agreed with experimental, theoretical and Slater-Pauling rule determined magnetic moments. Elastic constants (C11, C12 and C44) obtained for these cubic systems fulfilled Born-Huang mechanical stability criteria, from which other elastic related quantities were extracted. Elastic calculations predicted these alloys to be metallic bonded, extremely incompressible, highly anisotropic and ductile in nature. Thermodynamic calculations showed that Debye vibrational energies, entropies and constant volume heat capacities of the alloys increased remarkably as temperature increased whereas vibrational free energies decreased steadily with increased temperature. Thermoelectric properties were also determined, with Co2TiAl having higher figure of merit (ZT) than Co2VAl within the investigative temperatures.

First principles investigation of thermoelectric and mechanical properties of VScO3 semiconductor perovskite for sustainable and renewable energy

Semiconductor perovskite material has given important interest as a potential thermoelectric material (TE) due to its large thermopower (Seebeck coefficient, S). VScO3 Semiconductor Perovskite is investigated by the Density Functional Theory (DFT) using full potential linearized augmented plane wave (FP-LAPW) method with mBJ approximation. The energy band structure obtained from WIEN2k calculations is used to calculate the transport coefficients via the semi-classical Boltzmann transport theory with constant relaxation time (τ) as employed in the BoltzTraP package. VScO3 semiconductor perovskite is considered as a potential candidate for thermoelectric application. The calculated elastic constants show that VScO3 perovskite suitably verify Born's mechanical stability criteria and characterized by ductile manner.

First Principles Investigations of Ta4AlX3 (X= B, C, N) MAX Phase Ceramics

Ta4AlX3 (X=B, C, N) MAX phase ceramics have been examined using first principles calculations in this study. Ta4AlX3 MAX phase ceramics have hexagonal crystal structure and the formation energies have been determined for the optimized crystal structures. The elastic constants of Ta4AlX3 MAX phase ceramics have been determined and these constants satisfy the mechanical stability criteria. In addition, the mechanical properties such as bulk modulus, shear modulus, etc. have been obtained to reveal the detailed properties of these compounds. The anisotropic elastic properties have been visualized in both 3D and 2D. Moreover, the thermal properties of Ta4AlX3 MAX phase ceramics such as thermal expansion coefficient, heat capacity etc. have been studied in 0 to 1000 K temperature range and 0 to 40 GPa pressure range. In this study, Ta4AlB3 has been considered for the first time along with Ta4AlC3 and Ta4AlN3 compounds and the effect of X atom to the properties of these compounds have been discussed in detail.

Pressure-induced the formation of Mg(CH3)2 and Ca(CH3)2 studied by the first principles

Hydrogen-rich materials have always attracted the interest for the crystal structures and electronic properties under high pressures. The crystal structures of alkaline earth compounds Mg(CH3)2 and Ca(CH3)2 have been predicted by employing a particle swarm optimization algorithm at high pressures. The P-3c1 phases of the both compounds were synthesized at about 17.5 GPa and 3 GPa, respectively. The current thermodynamical calculations reveal that the P-3c1 phases are the most stable up to 50 GPa. No imaginary phonon frequencies in the whole Brillouin zone indicate the P-3c1stuctures of Mg(CH3)2 and Ca(CH3)2 are dynamically stable. The computations of the elastic constants indicate P-3c1 structures satisfy the mechanical stability criteria under high pressures. The strong covalent bonds between C atoms and H atoms have been proved by the electronic properties and population analysis, which confirms that the CH3 groups can exist stable.

Theoretical prediction, synthesis, and crystal structure determination of new MAX phase compound V2SnC

Guided by the theoretical prediction, a new MAX phase V2SnC was synthesized experimentally for the first time by reaction of V, Sn, and C mixtures at 1000 °C. The chemical composition and crystal structure of this new compound were identified by the cross-check combination of first-principles calculations, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and high resolution scanning transmission electron microscopy (HR-STEM). The stacking sequence of V2C and Sn layers results in a crystal structure of space group P63/mmc. The a- and c-lattice parameters, which were determined by the Rietveld analysis of powder XRD pattern, are 0.2981(0) nm and 1.3470(6) nm, respectively. The atomic positions are V at 4f (1/3, 2/3, 0.0776(5)), Sn at 2d (2/3, 1/3, 1/4), and C at 2a (0, 0, 0). A new set of XRD data of V2SnC was also obtained. Theoretical calculations suggest that this new compound is stable with negative formation energy and formation enthalpy, satisfied Born-Huang criteria of mechanical stability, and positive phonon branches over the Brillouin zone. It also has low shear deformation resistance c44 (second-order elastic constant, cij) and shear modulus (G), positive Cauchy pressure, and low Pugh’s ratio (G/B = 0.500 < 0.571), which is regarded as a quasi-ductile MAX phase. The mechanism underpinning the quasi-ductility is associated with the presence of a metallic bond.

Structural, elastic, vibrational, thermophysical properties and pressure-induced phase transitions of ThN2, Th2N3, and Th3N4: An ab initio investigation

The structural, electronic, elastic, lattice dynamical properties and pressure-induced phase transitions in ThN2, Th2N3, and Th3N4 have been investigated through density functional theory based electronic band structure calculations. Our theoretical calculations on ThN2 reveal the monoclinic structure (C2/m space group) at 0 GPa instead of the previously reported cubic (Fm3¯m spatial crystal symmetry) phase [K. O. Obodo and N. Chetty, J. Nucl. Mater. 440, 229 (2013)]. More refined calculations on enthalpy of formation reveal that this ground state C2/m phase of ThN2 transforms to an orthorhombic structure (Pnma symmetry) at a pressure of ∼7 GPa. In agreement with experimental observations, we predict the La2O3-type trigonal structure (P3¯ml symmetry) in Th2N3 at ambient conditions, which is further predicted to transform to an initial monoclinic structure again at ∼62 GPa. Our theoretical results also agree with the experiment regarding the rhombohedral structure (R3¯m symmetry) of Th3N4 revealed at 0 GPa, which, at ∼37 GPa, is predicted to transform to an another rhombohedral structure with reduced space group symmetry of R3¯. The predicted structural phases are further substantiated with the mechanical and dynamical stability criteria in the pressure regime of their structural stability. Furthermore, the electronic band structure calculations at zero pressure suggest that with limited density of states above Fermi energy, ThN2 and Th2N3 exhibit semi-metallic characteristics, whereas a bandgap of ∼1.44 eV in Th3N4 makes it a semiconductor. The semiconducting nature of Th3N4 ceases at a transition pressure of ∼62 GPa.

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.

Spinel-type Na2MoO4 and Na2WO4 as promising optoelectronic materials: First-principle DFT calculations

The mechanical, thermodynamic, electronic, and optical properties of Na2MoO4 (NMO) and Na2WO4 (NWO) spinels are elaborated by density functional theory (DFT) based full potential augmented plane wave method (FP-LAPW + lo). Our optimized lattice constants for the studied spinels are in good agreement with that obtained experimentally. The enthalpy of formation ensures the thermodynamic stability of NMO and NWO in the cubic phase. The Born mechanical stability criteria guarantees their mechanical stability, while Poisson ratio (ν) and Pugh's ratio (B/G) infer their brittle behavior. The Debye temperature (θD) is significant for NMO than NWO. The wide bandgap of 3.5 eV for NMO and 4.4 eV for NWO show the maximum absorption in the ultraviolet region that increases their importance for optoelectronic applications. The optical properties are explained in term of dielectric constant, refractive index, absorption of light, reflection, and optical loss factor.

Ab initio study of the mechanical properties and thermal conductivity of Bi2O2X (X = Se, Te) under pressure

The structural and mechanical properties, and thermal conductivity of Bi2O2X (X = Se, Te) under pressures ranging up to 50 GPa were evaluated by first-principles calculations based on the density functional theory. The calculated lattice parameters and cell volume of Bi2O2Se and Bi2O2Te in the ground state are in good agreement with the available experimental and theoretical data. Moreover, the calculated elastic constants indicate that both Bi2O2Se and Bi2O2Te are mechanically stable at least up to 50 GPa. When the pressure is increased up to 60 GPa, the calculated elastic constants of these two materials fail to satisfy the mechanical stability criteria, indicating that phase transition might have occurred. In addition, further studies on the elastic properties under pressure reveal that both Bi2O2Se and Bi2O2Te show ductility within the pressure range investigated in this study. Meanwhile, the shear modulus, Young's modulus, and sound velocities of these two materials are all anisotropic in the ground state, and their anisotropic characteristics became increasingly significant with an increase in pressure. Finally, the minimum thermal conductivities and the anisotropy of Bi2O2Se and Bi2O2Te under pressure are further analyzed using Clarke's and Cahill's models. The results show that both these materials are potential anisotropic thermoelectric materials.