Electronic structure,phase stability and elastic properties of ruthenium based four intermetallic compounds: Ab-initio study

The structural, electronic and elastic properties of four RuX (X = Sc, Ti, V and Zr) intermetallic compounds have been investigated by using density functional theory within full potential linearized augmented plane wave method and using generalized gradient approximations in the scheme of Perdew, Burke and Ernzrhof (PBE), Wu and Cohen (WC) and Perdew et al. (PBEsol) for the exchange correlation potential. The relative phase stability in terms of volume-energy and enthalpy-pressure for these compounds is presented for the first time in three different (B₁, B₂ and B₃) structures. The total energy is computed as a function of volume and fitted to Birch equation of states to find the ground state properties such as lattice constant (a₀), bulk modulus (B) and its pressure derivative (B′). It is found that the lattice parameters in B₂-phase agree well with the existing experimental and previous theoretical results. The second order elastic constants (SOECs) are also predicted for the above compounds. All the four compounds show ductile behavior. The ductility of these compounds has been analyzed using Pugh's rule. From the plots of electronic density of states (DOS), it can be concluded that these intermetallic compounds are metallic in nature.     

Electronic,optical and thermoelectric properties of Ce3PdIn11 and Ce5Pd2In19: An ab initio study

Density functional calculations with Engel and Vosko generalized gradient approximation are applied to investigate the electronic, optical and thermoelectric properties of Ce₃PdIn₁₁ and Ce₅Pd₂In₁₉ compounds. Analysis of the calculated band structure of Ce₃PdIn₁₁ and Ce₅Pd₂In₁₉ demonstrates their metallic character. The calculated densities of states N(E_F) of Ce₃PdIn₁₁ and Ce₅Pd₂In₁₉ at the Fermi level are 19.60 states/eV and 33.50 states/eV, respectively. The bonding nature in these compounds is discussed via the calculated contour map of the charge density in (1 1 0) crystallographic plane. Imaginary parts of the complex dielectric function show considerable isotropy between 3 and 14 eV Ce₃PdIn₁₁ have large dielectric constant. Thermoelectric properties results reveal that both compounds possess high Seebeck coefficient and electrical conductivity at high temperature. This is the first quantitative theoretical prediction of the theremoelectric properties for these investigated compounds and still awaits experimental confirmations.     

Structural,elastic, electronic,and thermodynamic properties of intermetallic Zr2Cu: A first-principles study

Three competing structures (C11_b, C16 and E9₃) of intermetallic Zr₂Cu have been systematically investigated by first-principles calculations and quasi-harmonic Debye model. Both the calculated equation of states (EOS) and pressure–enthalpy results indicate a structural phase transition from C11_b to C16 phase at around 11–14 GPa. The calculated equilibrium crystal parameters and elastic constants are in consistence with available experimental or theoretical data. All three phases are mechanically stable according to the elastic stability criteria, and ductile according to Pugh's ratio, while the ambient-stable C11_b phase shows a higher elastic anisotropy. Furthermore, differences in the nature of bonding between three competing structures are uncovered by electron density topological analysis. C11_b Zr₂Cu possesses an intriguing pseudo BaFe₂As₂-type structure with the charge density maxima at Zr tetrahedral interstices serving as Fe-position pseudoatoms; C16 Zr₂Cu contains Zr-pair configurations bonded through bifurcated Zr–Zr bonding paths; while the E9₃ phase has only conventional straight bonding. Additionally, through quasi-harmonic Debye model, the pressure and temperature dependences of the bulk modulus, specific heat, Debye temperature, Grüneisen parameter and thermal expansion coefficient for three phases are obtained and discussed.     

Theoretical prediction of the electronic structure,bonding behavior and elastic moduli of scandium intermetallics

We report the structural, electronic, bonding, elastic and mechanical properties of nine scandium intermetallic compounds, ScTM (TM = Co, Rh, Ir, Ni, Pd, Pt, Zn, Cd and Hg), using ab initio density functional theory with the generalized gradient approximation for exchange and correlation potentials. The calculated structural parameters, such as the lattice constant (a₀), bulk modulus (B) and its pressure derivative (B₀) and elastic constants, are calculated using the CsCl-(B₂ phase) structure. The electronic and bonding properties of the ScX compounds are quantitatively analyzed using band structures, DOS, Fermi surfaces and contour plots. The mechanical properties and ductile behaviors of these compounds are also predicted based on the calculated elastic constants.     

Physical properties of the InPd intermetallic catalyst

The intermetallic phase InPd is a candidate material for the use as a catalyst in the methanol steam reforming process. To study the connection between the catalytic properties of the surface and the structural and electronic properties of the bulk, we have grown single crystals of the InPd phase by the Czochralski method and determined their electronic, thermal, magnetic and hydrogen-absorption properties. By growing crystals from a high-temperature solution, we could crystallize a slightly off-stoichiometric In-rich composition In_1.04Pd_0.96, which contained a significant amount of constitutional defects in the lattice (Pd vacancies on the Pd sublattice) to retain the CsCl-type structure. The strongly inhomogeneously broadened ¹¹⁵In NMR spectrum and the high residual (T → 0) electrical resistivity confirmed the presence of constitutional defects. Single crystals of InPd do not absorb hydrogen, as requested for a good hydrogenation catalyst material. Calculated electronic density of states (DOS) shows large contribution of Pd(d) states at the Fermi level. Application of the electron localizability indicator reveals ionic and multi-centre In–Pd interactions stabilizing the crystal structure. The electrical and thermal conductivities of InPd show metallic character, whereas the thermoelectric power and the Hall coefficient both show positive sign, revealing that InPd is a predominant hole-type conductor. The calculated electronic DOS at the Fermi energy is in a good agreement with the experimental value determined from the low-temperature specific heat. Magnetic measurements have shown that InPd is a diamagnet. All results are compared to the chemically related intermetallic compound GaPd. The active–site-isolation concept for increased catalytic selectivity is discussed in relation to the InPd and GaPd structures.     

Ab initio based study of finite-temperature structural,elastic and thermodynamic properties of FeTi

We employ density functional theory (DFT) to calculate pressure dependences of selected thermodynamic, structural and elastic properties as well as electronic structure characteristics of equiatomic B2 FeTi. We predict ground-state single-crystalline Young's modulus and its two-dimensional counterpart, the area modulus, together with homogenized polycrystalline elastic parameters. Regarding the electronic structure of FeTi, we analyze the band structure and electronic density of states. Employing (i) an analytical dynamical matrix parametrized in terms of elastic constants and lattice parameters in combination with (ii) the quasiharmonic approximation we then obtained free energies, the thermal expansion coefficient, heat capacities at constant pressure and volume, as well as isothermal bulk moduli at finite temperatures. Experimental measurements of thermal expansion coefficient complement our theoretical investigation and confirm our theoretical predictions. It is worth mentioning that, as often detected in other intermetallics, some materials properties of FeTi strongly differ from the average of the corresponding values found in elemental Fe and Ti. These findings can have important implications for future materials design of new intermetallic materials.     

Intrinsic brittleness of Mo5SiB2 and alloying effect on ductility studied by first-principles calculations

The ordered intermetallic Mo₅SiB₂ displays a ceramic-like brittleness at the ambient temperature. The state density, charge distribution and elastic parameters were calculated by first-principles, based on the density functional method. The results indicated that the two different kinds of covalent bonds were intricately woven into the refractory phase. The improved Peierls–Nabarro stress which is caused by this kind of distribution mode makes dislocations move difficultly, resulting in intrinsic brittleness. The effects of substitutional alloying on the ductility of Mo₅SiB₂ were also assessed by the calculations on the elastic properties and dislocation line energy. It was shown that the metal (Nb, Tc) alloying was not to enhance effectively its toughness, but to improve the brittle-to-ductile transition temperature.     

Effects of Ni vacancy,Ni antisite,Cr and Pt on the third-order elastic constants and mechanical properties of NiAl

Effects of Ni vacancy, Ni antisite in Al sublattice, Cr in Al sublattice, Pt in Ni sublattice on the second-order elastic constants (SOECs) and third-order elastic constants (TOECs) of the B2 NiAl have been investigated using the first-principles methods. Lattice constant and the SOECs of NiAl are in good agreement with the previous results. The brittle/ductile transition map based on Pugh ratio G/B and Cauchy pressure P_c shows that Ni antisite, Cr, Pt and pressure can improve the ductility of NiAl, respectively. Ni vacancy and lower pressure can enhance the Vickers hardness H_v of NiAl. The density of states (DOS) and the charge density difference are also used to analysis the effects of vacancy, Ni antisite, Cr and Pt on the mechanical properties of NiAl, and the results are in consistent with the transition map.     

Point defects and diffusion in ordered alloys: An ab initio study of the effect of vibrations

We present a detailed investigation of the influence of atomic vibrations on the point defect and diffusion properties of ordered metallic alloys, by means of ab initio calculations with density-functional theory. Considering the case of Ni₂Al₃ which provides a rich panel of defect-related properties, our study reveals that the behaviour of this compound is largely monitored by self-interstitials, whereas such defects are usually ignored in metallic compounds. The vibration free energies are obtained for the full set of point defects of Ni₂Al₃, showing that these quantities are strongly defect-dependent, and significantly modify the free energy of the compound in an intricate composition-dependent manner. The second key-issue is the first ab initio full analysis of attempt frequencies, via the coupling of vibration analysis and saddle-point search for significant atomic jumps. This analysis indicates that attempt frequencies range over several orders of magnitude and exponentially increase with migration energies. We show the importance of these factors in reaching realistic composition-dependent diffusion coefficients.     

A first principles study of structural and mechanical properties of Ti2AlZr intermetallic

The present work describes the structural and mechanical behaviour of three phases namely B2, D0₁₉ and O phases of Ti₂AlZr intermetallic using first principles density functional theory (DFT) within generalized gradient approximation (GGA). The equilibrium lattice constant values of B2, D0₁₉ and O phases are in good agreement with the experimental and theoretical data, respectively. Formation energy of O phase is minimum followed by D0₁₉ and B2. Bonding characteristics of these phases have been explained based on electronic density of states and charge density. All the three phases satisfy the Born stability criteria in terms of elastic constants and are associated with ductile behaviour based on G/B ratios. The B2 phase exhibits very high anisotropy in comparison to those of the D0₁₉ and O.     

First-principles study of the bonding characteristics of TiAl(111)/Al2O3(0001) interface

First principles calculations were carried out for α-Al₂O₃(0001) surface and γ-TiAl(111)/α-Al₂O₃(0001) interface to study the adhesion properties of the interface and to clarify the mechanisms that govern the adhesion of TiAl and its oxidation product Al₂O₃. Two type interface models, the TiAl(111) with Al- and O-terminated α-Al₂O₃(0001) surfaces denoted as T(A₁)-type and T(O_T)-type interface, respectively, were considered. The Universal Binding Energy Relation (UBER) was used to determine the separation between TiAl and Al₂O₃ and the work of adhesion of the γ-TiAl(111)/α-Al₂O₃(0001) interface. Optimization was then performed for all interfaces considered here using the separation obtained with UBER. The lowest work of adhesion is −1.05 J/m² for the T(A₁)-type interface and is −4.04 J/m² for the T(O_T)-type interface. There exists competition between O–Ti and O–Al (on the TiAl side) interactions; however O–Al bond is stronger than O–Ti bond because the main body of the Al valences is involved in the hybriding with O p electrons, while only part of the Ti d valence is involved in the O–Ti bonding. Thus the O–Al interaction dominates the adhesion between TiAl(111) and Al₂O₃(0001) surfaces, and it can be inferred that an Al-rich TiAl surface will favor the adhesion between TiAl/Al₂O₃.     

Structural,electronic, elastic,mechanical and thermal behavior of RESn3(RE = Y,La and Ce) compounds: A first principles study

The structural, electronic, elastic, mechanical and thermal properties of the isostructural and isoelectronic nonmagnetic RESn₃ (RE = Y, La and Ce) compounds, which crystallize in AuCu₃-type structure, are studied using first principles density functional theory based on full potential linearized augmented plane wave (FP-LAPW) method. The calculations are carried out within PBE-GGA, WC-GGA and PBE-sol GGA for the exchange correlation potential. Our calculated ground state properties such as lattice constant (a₀), bulk modulus (B) and its pressure derivative (B′) are in good agreement with the experimental and other available theoretical results. We first time predict the elastic constants for these compounds using different approximations of GGA. All these RESn₃ compounds are found to be ductile in nature in accordance with Pugh's criteria. The computed electronic band structures and density of states show metallic character of these compounds. The elastic properties including Poisson's ratio (σ), Young's modulus (E), shear modulus (G_H) and anisotropy factor (A) are also determined using the Voigt–Reuss–Hill (VRH) averaging scheme. The average sound velocities (v_m), density (ρ) and Debye temperature (θ_D) of these RESn₃ compounds are also estimated from the elastic constants. We first time report the variation of elastic constants, elastic moduli, Cauchy's pressure, sound velocities and Debye temperatures of these compounds as a function of pressure.     

Synergistic effect of co-alloying elements on site preferences and elastic properties of Ni3Al: A first-principles study

The site preferences of co-alloying elements (Mo–Ta, Mo–Re, Mo–Cr) in Ni₃Al are studied using first-principles calculations, and the effects of these alloying elements on the elastic properties of Ni₃Al are evaluated by elastic property calculations. The results show that the Mo–Ta, Mo–Re and Mo–Cr atom pairs all prefer Al–Al sites and the spatial neighbor relation of substitution sites almost has no influence on the site preference results. Furthermore, the Young's modulus of Ni₃Al increases much higher by substituting Al–Al sites with co-alloying atoms, among which Mo–Re has the best strengthening effect. The enhanced chemical bondings between alloying atoms and their neighbor host atoms are considered to be the main strengthening mechanism of the alloying elements in Ni₃Al.     

Debye temperature, self-diffusion and elastic constants of intermetallic compounds

The Debye temperature of a material is a suitable parameter to describe phenomena of solid-state physics which are associated with lattice vibrations. It basically depends on the elastic constants. In recent work a simple method was put forward that allows one to derive precise Debye temperatures of crystals with cubic, hexagonal and tetragonal symmetry from the elastic constants. The type of chemical binding does not play any role. It is one aim of the present work to apply this method to various intermetallic compounds, i.e. to critically analyse published Debye temperatures and to calculate hitherto unknown values. It is a further aim to show that the activation energy of self-diffusion is also connected with the elastic constants by a simple law at least for the cubic B2 and L1₂ intermetallics, as it was recently found for face-centred cubic metals. Some consequences for high-temperature plasticity are discussed.     

Solubility and ordering of Ti,Ta, Mo and W on the Al sublattice in L12-Co3Al

Co–Al–W-based alloys are promising new materials for high-temperature applications. They owe their high-temperature strength to hardening by ternary L1₂-Co₃(Al_1−xW_x) precipitates, which may form even though binary Co₃Al is not stable. In the current work, density functional theory calculations are performed to study the solubility and ordering of the transition metals W, Mo, Ti, and Ta at the Al sublattice in L1₂-Co₃Al. The sublattice disorder is modelled with a newly parametrised cluster expansion and compared to results using special quasi-random structures. Our results for W and Mo show that the mixing energy exhibits a minimum at approximately x = 0.7. However, the computed small values of the mixing energies indicate that W and Mo atoms are fully disordered with the Al atoms already at low temperatures. For Ti and Ta we find no sizeable driving force for ordering with the Al atoms. The computed solubilities on the Al sublattice obtained are in the range of 40–80 meV/atom for W and Mo and less than 25 meV/atom for Ti and Ta.     

First-principles investigations on structural,elastic, thermodynamic and electronic properties of Ni3X (X = Al,Ga and Ge) under pressure

The structural, elastic, thermodynamic and electronic properties of L1₂-ordered intermetallic compounds Ni₃X (X = Al, Ga and Ge) under pressure range from 0 to 50 GPa with a step of 10 GPa have been investigated using first-principles method based on density functional theory (DFT). The calculated structural parameters of Ni₃X at zero pressure and zero temperature are consistent with the experimental data. The results of bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio v, anisotropy index A^U and Debye temperature Θ_D increase with the increase of external pressure. In addition, the Debye temperature of these compounds gradually reduce as the order of Ni₃Al > Ni₃Ga > Ni₃Ge. The ratio of shear modulus to bulk modulus G/B shows that the three binary compounds are ductile materials, and the ductility of Ni₃Al and Ni₃Ga can be improved with pressure going up, while Ni₃Ge is opposite. Finally, the pressure-dependent behavior of density of states, Mulliken charge and bond length are analyzed to explore the physical origin of the pressure effect on the structural, elastic and thermodynamic properties of Ni₃X.     

Site preference and alloying effect on elastic properties of ternary B2 RuAl-based alloys

The structural and elastic properties of ternary B2 RuAl-based alloys are studied using first-principles calculations. Single-crystal elastic constants, atomic volumes, transfer energies, and electronic densities for RuAl-TM are computed, considering all possible transition-metal solute species TM. Calculated elastic constants are used to compute values of some commonly considered elasticity parameters, such as bulk modulus, shear modulus, Yong's modulus, Pugh ratio, and Cauchy pressure. The present results suggest that the bulk modulus of RuAl-TM increase approximately linearly with increasing electron density. Calculated elastic properties are in favorable accord with available experimental and theoretical data.     

Enthalpies of formation of magnesium compounds from first-principles calculations

An energetics database of binary magnesium compounds has been developed from first-principles calculations. The systems investigated include Mg–X (X = As, Ba, Ca, Cd, Cu, Dy, Ga, Ge, La, Lu, Ni, Pb, Sb, Si, Sn and Y). The calculated lattice parameters and enthalpies of formation of binary compounds in these systems are compared with both experimental data and thermodynamic databases.     

In-situ synthesis and mechanical properties of Zr-based bulk metallic glass matrix composites manipulated by nitrogen additions

Our study investigates in-situ synthesis and mechanical properties of Zr-based bulk metallic glass (BMG) matrix composites via arc plasma-induced accelerated displacement reaction (APADR) process. The aluminum nitride precursor under arc plasma-induced ultra-high temperature results in higher contents of dissolved nitrogen as well as precipitation of zirconium nitride (ZrN) particles in a Zr-based amorphous matrix. The nitrogen in the matrix results in a decrease of crystallization resistance (lower T_x and reduced glass-forming ability), but an increase of mechanical stability (a decrease of strain burst sizes). In particular, in-situ formed ZrN, which exhibits a homogeneous distribution and strong interfacial bonding with the matrix, causes an increase in compressive fracture strength and significant plastic deformation in the composite compared with the monolithic BMG. The formation of multiple shear bands and the enhancement of shear band interactions by the dissolved nitrogen as well as the in-situ formed ZrN particles were carefully confirmed by a statistical analysis on serrated flows. These results give us a guideline on how to manipulate nitrogen contents and fabricate in-situ BMG matrix composites with improved mechanical properties via APADR process.     

First-principle calculations of the electronic,elastic and thermoelectric properties of Ag doped CuGaTe2

To improve the performance of a thermoelectric material CuGaTe₂, element Ag is doped to replace element Ga and we investigate the electronic structure, phase stability, elastic and thermoelectric properties of CuGa_1−xAg_xTe₂ (x = 0, 0.25 and 0.5) via first-principles method. The phase stability of CuGa_1−xAg_xTe₂ is discussed by analyzing the formation energy, cohesive energy and elastic constants. The calculated sound velocities decrease with the increase of Ag content, which is favorable for reducing the lattice thermal conductivity. The analysis of band structures shows that the replacement of Ga by Ag makes CuGaTe₂ undergo a direct-indirect semiconductor transition. The Ag doping induces steep density of states in valence band edge, which is beneficial for increasing the carrier concentration and improving thermoelectric performance of CuGaTe₂.