Muffin-tin Orbitals Research Articles

Ab-initio prédiction of the structural ,electro-magnetic and thermodynamic properties of Co-based Full Heusler alloy

The structural, thermodynamic, electronic, magnetic and elasticproperties of the Full-Heusler Co2MnTi compoundon the cobalt (Co) are determined byusing first-principles calculations based on density functional theory (DFT). We have used the linear Muffin-Tin Orbitals method (FP-LMTO) combined with the generalized gradient approximation (GGA) with spin. The obtained results demonstrate that our material is ferromagnetic and exhibits metallic behavior around the Fermi level .The study of the elastic properties show that Co2MnTi is mechanically stable and is also classified as a ductile material. The GIBBS program is used to comput the thermodynamic properties such as the modulus of compressibility, the heat capacity volume and pressur, the thermal expansion, the Debye temperature and the volume variation at constants. This type of material is considered as promising for spintronic applications.

First-principles study of the phase stability and elastic properties of TiVNbMoM (M = Al, Sc, Ni and Cu) high entropy alloys

The ab initio exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) were used to study the influence of alloying elements M = Al, Sc, Ni and Cu on the phase stability, lattice constants, elastic constants, polycrystalline elastic moduli and electronic structure of equiatomic and non-equiatomic TiVNbMoM refractory high entropy alloys. The agreement between our results and the available experimental and theoretical data is quite good. It was found that the equiatomic systems are stable in the body-centered cubic (bcc) structure. Alloying elements decrease the stability of the bcc against the face-centered cubic (fcc) and the hexagonal close-packed (hcp) structures. Scandium enlarges the lattice constants of equiatomic and non-equiatomic systems significantly. According to the calculated bulk modulus to shear modulus, Poisson’s ratio and Vickers hardness, all studied equiatomic and non-equiaomic systems are found to be ductile. However, alloying elements Al, Ni and Cu reduce the ductility and improve the hardness of equiatomic and non-equiatomic TiVNbMoM systems, while the ductility (hardness) of non-equiatomic systems enhances (reduces) by substitution with Sc element. The present theoretical results provide insight for the design and improvement of high entropy alloys and complete information on the alloying effects.

Magnetization Dynamics in FexCo1-x in Presence of Chemical Disorder

In this paper, we present a theoretical formulation of magnetization dynamics in disordered binary alloys, based on the Kubo linear response theory, interfaced with a seamless combination of three approaches: density functional-based tight-binding linear muffin-tin orbitals, generalized recursion and augmented space formalism. We applied this method to study the magnetization dynamics in chemically disordered FexCo1−x (x = 0.2, 0.5, 0.8) alloys. We found that the magnon energies decreased with an increase in Co concentration. Significant magnon softening was observed in Fe20Co80 at the Brillouin zone boundary. Magnon–electron scattering increased with increasing Co content, which in turn modified the hybridization between the Fe and Co atoms. This reduced the exchange energy between the atoms and softened down the magnon energy. The lowest magnon lifetime was found in Fe50Co50, where disorder was at a maximum. This clearly indicated that the damping of magnon energies in FexCo1−x was governed by hybridization between Fe and Co, whereas the magnon lifetime was controlled by disorder configuration. Our atomistic spin dynamics simulations show reasonable agreement with our theoretical approach in magnon dispersion for different alloy compositions.

Elastic Properties of B2-NiAl with W addition: A first-principles study

The effect of tungsten alloying on the elastic properties of B2-NiAl at low-temperature and high-temperature distribution of W atoms on sublattices. By means of the exact muffin-tin orbitals method in conjunction with the coherent potential approximation, the constants C11,C12,C44, Young's modulus E, shear modulus G, Cauchy pressure values, G/B ratios are calculated. Using phenomenological criteria of the correlation between ductility and elastic properties of solution phases, it has been shown that the addition of tungsten could yield improved ductility for B2-NiAl in both types of alloys. It is established that with the high-temperature distribution of W atoms on the Al sublattice, there is a loss of mechanical stability and a decrease in mechanical properties with increasing W concentration. In alloys with a low-temperature distribution of tungsten atoms between Al and Ni sites a unique combination of properties occurs: in addition to the ductility enhancement, a simultaneous increase of the elastic constants C44 and C11 and C11, shear modulus G, Young's modulus E with increasing W content is observed. The differences in the behavior of elastic constants in alloys with different types of tungsten distribution on NiAl sublattices are analyzed by calculating the density of electronic states. Keywords: NiAl, alloying elements, tungsten, elastic properties, ductility, first-principles calculations.

Упругие свойства B2-NiAl с добавлением W: исследование из первых принципов

The effect of tungsten alloying on the elastic properties of B2 NiAl at low-temperature and high-temperature distribution of W atoms on sublattices. By means of the exact muffin-tin orbitals method in conjunction with the coherent potential approximation, the constants C11, C12, C44, Young's modulus E, shear modulus G, Cauchy pressure values, G/B ratios are calculated. Using phenomenological criteria of the correlation between ductility and elastic properties of solution phases, it has been shown that the addition of tungsten could yield improved ductility for B2 NiAl in both types of alloys. It is established that with the high-temperature distribution of W atoms on the Al sublattice, there is a loss of mechanical stability and a decrease in mechanical properties with increasing W concentration. In alloys with a low–temperature distribution of tungsten atoms between Al and Ni sites a unique combination of properties occurs: in addition to the ductility enhancement, a simultaneous increase of the elastic constants C44 and C11, shear modulus G, Young modulus E with increasing W content is observed. The differences in the behavior of elastic constants in alloys with different types of tungsten distribution on NiAl sublattices are analyzed by calculating the density of electronic states.

First-principles calculations of the cleavage energy in random solid solutions: A case study for TiZrNbHf high-entropy alloy

The {100} and (110) cleavage energies of body-centered cubic TiZrNbHf high-entropy alloy are calculated using two alloy models: special quasi-random structures (SQSs) and the coherent potential approximation (CPA). The projector augmented wave method, as implemented in the Vienna ab initio simulation package (VASP), in combination with SQSs is adopted to evaluate the impact of local lattice distortions, whereas the exact muffin-tin orbitals (EMTO) method is used in combination with both SQSs and CPA to study the effect of chemical disorder using rigid underlying lattices. The variations of the cleavage energy as a function of surface chemistry and structure from the EMTO and VASP calculations are consistent with each other. Furthermore, the cleavage energies from CPA are in good agreement with those from SQSs, confirming that an averaged supercell approach reproduces well the mean-field CPA results. The alloy’s cleavage energies estimated by the rule of mixtures compare well with those from the direct calculations, and the surface chemistry dependence of the cleavage energies is mainly controlled by the number of Nb atoms in the surface terminal layers owing to the large cleavage energy of Nb metal.

Real-space representation of the quasiparticle self-consistent GW self-energy and its application to defect calculations

The quasiparticle self-consistent QS$GW$ approach incorporates the corrections of the quasiparticle energies from their Kohn-Sham density functional theory (DFT) eigenvalues by means of an energy independent and Hermitian self-energy matrix usually given in the basis set of the DFT eigenstates. By expanding these into an atom-centered basis set (specifically here the linearized muffin-tin orbitals) a real space representation of the self-energy corrections becomes possible. We show that this representation is relatively short-ranged. This offers new opportunities to construct the self-energy of a complex system from parts of the system by a cut-and-paste method. Specifically for a point defect, represented in a large supercell, the self-eneregy can be constructed from those of the host and a smaller defect containing cell. The self-energy of the periodic host can be constructed simply from a $GW$ calculation for the primitive cell. We show for the case of the As$_\mathrm{Ga}$ in GaAs that the defect part can already be well represented by a minimal 8 atom cell and allows us to construct the self-energy for a 64 cell in good agreement with direct QS$GW$ calculations for the large cell. Using this approach to an even larger 216 atom cell shows the defect band approaches an isolated defect level. The calculations also allow to identify a second defect band which appears as a resonance near the conduction band minimum. The results on the extracted defect levels agree well with Green's function calculations for an isolated defect and with experimental data.

High-throughput prediction of intrinsic properties of L12-(Nix1,Crx2,Cox3)3(Aly1,Tiy2) precipitates

The structural and mechanical properties, e.g., lattice constants, elastic constants, elastic moduli, Poisson’s ratios, and Vickers hardness, of L12-(Nix1,Crx2,Cox3)3(Aly1,Tiy2) precipitates are predicted by high-throughput first-principles calculations combined with exact muffin-tin orbitals and coherent potential approximation methods. It turned out that the elastic constants C11 and C44, shear modulus, Young’s modulus, Vickers hardness, and yield strength raise with increasing Ni, Cr, Co, or Al content as a whole, the elastic constant C12, Poisson’s ratios, and Pugh’s ratio decrease gradually, reflecting that the content change of Ni, Cr, Co elements on the left has basically similar effects on the physical properties of L12 precipitates, while the influences of Al and Ti on the right are opposite. Moreover, increasing Ni and Al, Cr and Al, or Co and Al contents at the same time is more conducive to improve the deformation resistance of alloys, but rapidly decrease the plastic properties. Meanwhile, the crystal structures of L12 precipitates may be mechanically unstable due to negative C11−C12 values when Ni, Cr, or Co concentration is less than 30 at%, and the ductile-brittle transition will occur for the L12 particles with high Cr and Al contents or Co and Al contents due to B/G<1.75. By comparison, Co3Al and Cr3Al precipitates exhibit excellent deformation resistance, (Cr,Co)3Ti and (Ni,Co)3Ti particles demonstrate outstanding plastic properties in the [001] and [111] directions, respectively. This work provides valuable insights for the intrinsic properties of L12-(Nix1,Crx2,Cox3)3(Aly1,Tiy2) precipitates.

Atomistic study of stability and new phase transitions of zinc selenide using FP-LMTO-PLW method

By full potential linear muffin-tin orbitals (FP-LMTO-PLW) method, we have studied the phase transitions of ZnSe under high pressures. The local density approximation (LDA) was used for the exchange and correlation energy. The most important result is the prediction of the possibility of four phase transitions from the cubic zincblende (ZnS-B3) to the B81 (NiAs) structure at 8.00 GPa, the second from B3 to the B32 (NaTi) and Bh (WC) structures at above 45 GPa for each one of them, and from B3 to B2 (CsCl) phase at around 31GPa. The first one (ZnS–NiAs) occurring at a lower pressure than the well knownZnS to NaCl transition (found here to be 11.5 GPa).

OPTIMIZED QUASIPARTICLE DENSITY FUNCTIONAL AND GREEN’S FUNCTIONS METHOD TO COMPUTING BOND ENERGIES OF DIATOMIC MOLECULES

It is presented an advanced approach to computing the energy and spectral parameters of the diatomic molecules, which is based on the hybrid combined density functional theory (DFT) and the Green’s-functions (GF) approach. The Fermi-liquid quasiparticle version of the density functional theory is modified and used. The density of states, which describe the vibrational structure in photoelectron spectra, is defined with the use of combined DFT-GF approach and is well approximated by using only the first order coupling constants in the optimized one-quasiparticle approximation. Using the combined DFT-GF approach to computing the spectroscopic factors of diatomic molecules leads to significant simplification of the calculation procedure and increasing an accuracy of theoretical prediction. As illustration, the results of computing the bond energies in a number of known diatomic molecules are presented and compared with alternative theoretical results, obtained within discrete-variational , muffin-tin orbitals and other methods.

Alloying and magnetic disordering effects on phase stability of Co2 Y Ga (Y = Cr, V, and Ni) alloys: A first-principles study

The alloying and magnetic disordering effects on site occupation, elastic property, and phase stability of Co2 YGa (Y = Cr, V, and Ni) shape memory alloys are systematically investigated using the first-principles exact muffin-tin orbitals method. It is shown that with the increasing magnetic disordering degree y, their tetragonal shear elastic constant C′ (i.e., (C 11 – C 12)/2) of the L21 phase decreases whereas the elastic anisotropy A increases, and upon tetragonal distortions the cubic phase gets more and more unstable. Co2CrGa and Co2VGa alloys with y ≥ 0.2 thus can show the martensitic transformation (MT) from L21 to D022 as well as Co2NiGa. In off-stoichiometric alloys, the site preference is controlled by both the alloying and magnetic effects. At the ferromagnetism state, the excessive Ga atoms always tend to take the Y sublattices, whereas the excessive Co atom favor the Y sites when Y = Cr, and the excessive Y atoms prefer the Co sites when Y = Ni. The Ga-deficient Y = V alloys can also occur the MT at the ferromagnetism state by means of Co or V doping, and the MT temperature T M should increase with their addition. In the corresponding ferromagnetism Y = Cr alloys, nevertheless, with Co or Cr substituting for Ga, the reentrant MT (RMT) from D022 to L21 is promoted and then T M for the RMT should decrease. The alloying effect on the MT of these alloys is finally well explained by means of the Jahn–Teller effect at the paramagnetic state. At the ferromagnetism state, it may originate from the competition between the austenite and martensite about their strength of the covalent banding between Co and Ga as well as Y and Ga.

Alloying effect on the order\u2013disorder transformation in tetragonal FeNi

Tetragonal ({hbox{L1}}_{0}) FeNi is a promising material for high-performance rare-earth-free permanent magnets. Pure tetragonal FeNi is very difficult to synthesize due to its low chemical order–disorder transition temperature (approx {593} K), and thus one must consider alternative non-equilibrium processing routes and alloy design strategies that make the formation of tetragonal FeNi feasible. In this paper, we investigate by density functional theory as implemented in the exact muffin-tin orbitals method whether alloying FeNi with a suitable element can have a positive impact on the phase formation and ordering properties while largely maintaining its attractive intrinsic magnetic properties. We find that small amount of non-magnetic (Al and Ti) or magnetic (Cr and Co) elements increase the order–disorder transition temperature. Adding Mo to the Co-doped system further enhances the ordering temperature while the Curie temperature is decreased only by a few degrees. Our results show that alloying is a viable route to stabilizing the ordered tetragonal phase of FeNi.

First-principles study of the anomalous Hall effect based on exact muffin-tin orbitals

Based on the exact muffin-tin orbitals (EMTOs), we developed a first-principles method to calculate the current operators and investigated the anomalous Hall effect in bcc Fe as an example, with which we successfully separated the skew scattering contribution from the side jump and intrinsic contributions by fitting the scaling law with the introduction of sparse impurities. By investigating the temperature dependence of the anomalous Hall effect in bulk Fe, we predicted a fluctuated anomalous Hall angle as a function of temperature when considering only phonons, which, in the future, can be measured in experiments by suppressing magnon excitation, e.g., by applying a high external magnetic field.

Parameters of crystal and electronic structure and magnetic properties of mechanically alloyed Ni3C cubic carbide

The cubic Ni3.3C carbide has been fabricated by mechanical alloying of elemental Ni powder and the multiwalled carbon nanotubes in a high energy planetary ball mill. Crystal structure of carbide obtained belongs to the defective structure of ZnS sphalerite type according to x-ray diffraction data. Parameters of the electronic structure of Ni3.3C were calculated by linearized muffin-tin orbitals method within the plane-wave approximation using as an input the defined parameters of crystal structure. Magnetic properties, such as temperature and field dependences of the magnetic susceptibility of Ni3.3C have been studied. Based on experimental data obtained by studying the crystal structure and magnetic properties of Ni3.3C, as well as on the basis of calculations of electronic structure parameters, a preferred displacement of the carbon atoms in tetrahedral voids of Ni crystal lattice has revealed.

Electron localization function implementation in the exact muffin-tin orbitals method

We report implementation of the electron localization function (ELF) within the exact muffin-tin orbitals (EMTO) formalism. The ELF is often used to study the nature of electronic bonding in different types of materials, and it is also an important ingredient in meta-generalized gradient approximations, which are one of the classes of exchange-correlation functionals. The correctness of the ELF implementation is verified with test calculations and comparison with previous literature results. The implementation supports not only regular ordered systems but also disordered systems that have been calculated using the coherent potential approximation method.

First principles calculation of structural, electronic and optical properties of (001) and (110) growth axis (InN)/(GaN)n superlattices

Based on the full potential linear muffin-tin orbitals (FPLMTO) calculation within density functional theory, we systematically investigate the electronic and optical properties of (100) and (110)-oriented (InN)/(GaN)n zinc-blende superlattice with one InN monolayer and with different numbers of GaN monolayers. Specifically, the electronic band structure calculations and their related features, like the absorption coefficient and refractive index of these systems are computed over a wide photon energy scale up to 20 eV. The effect of periodicity layer numbers n on the band gaps and the optical activity of (InN)/(GaN)n SLs in the both growth axis (001) and (110) are examined and compared. Because of prospective optical aspects of (InN)/(GaN)n such as light-emitting applications, this theoretical study can help the experimental measurements.

Magnetic Phase Transition, Elastic and Thermodynamic Properties of L12-(Ni,Cu)3(Al,Fe,Cr) in 3d High-Entropy Alloys

The phase stability and elastic properties of paramagnetic (PM), ferromagnetic (FM) and antiferromagnetic (AFM) phases in L12-(Ni,Cu)3(Al,Fe,Cr) alloy are first investigated using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA). The result shows the AFM structure phase of the three is the most stable in the ground state. Calculated elastic constants show that all the phases are mechanically stable, and have uncovered that L12-(Ni,Cu)3(Al,Fe,Cr) can achieve good strength and ductility simultaneously. Then, crucial thermal properties are described satisfactorily using the Debye–Grüneisen model, showing heat capacity, Gibbs free energy G, the competitive contribution of entropy −TS and enthalpy H exhibiting significant temperature dependences. Moreover, the magnetic phase transition thermodynamics was studied, which suggests that −TS has a primary contribution to Gibbs free energy and may play a key role in the phase transition. The present results can benefit the understanding of the mechanical, thermodynamic and magnetic properties of the L12 structure phase in 3d high-entropy alloys.

Ground state parameters, electronic properties and elastic constants of CaMg3: DFT study

The present study aims to investigate the equation of state (EOS) parameters of CaMg 3 in αReO 3 (D0 9 ), AlFe 3 (D0 3 ), Cu 3 Au (L1 2 ) and CuTi 3 (L6 0 ) structures, using full potential linear muffin-tin orbitals (FP-LMTO) approach based on the density functional theory (DFT). The local density approximation (LDA) and the generalized gradient approximation (GGA) were both applied for the exchange-correlation potential term. The calculated equation of state parameters at equilibrium, in general, agreed well with the available data of the literature. The calculations showed that under compression CaMg 3 transforms from D0 3 to D0 9 at about 29.96 GPa, and 25.1 GPa using LDA and GGA, respectively. The elastic constants C ij , aggregate moduli, Vickers hardness, sound velocity, and Debye temperature of CaMg 3 in D0 3 structure were also reported, discussed and analyzed. Using LDA (GGA), the calculated values of H V and θ D were found at around 5.80 GPa (5.93 GPa) and 393.44 K (389.91 K), respectively. Electronic band structure, total density of states (TDOS) as well as the partial density of states (PDOS) have been also obtained. The electronic band structure confirms the metallic behavior of CaMg 3 in D0 3 phase, the valence bands are dominated by the maximum contribution of ‘ d ’ like states of Ca in the energy ranging from 2 to 3 eV for GGA, and from 4.5 to 5 eV for LDA, respectively.

Generalized stacking fault energies and critical resolved shear stresses of random α-Ti-Al alloys from first-principles calculations

The critical resolved shear stress (CRSS) and its related plastic deformation of titanium with hexagonal close packed structure are highly anisotropic, leading to low ductility of the material. Understanding the alloying effect on the CRSS is crucial for the improvement of the mechanical properties through rational composition design. Accurate prediction of the CRSS is not straightforward due to the atomic randomness of the alloy. The generalized stacking fault energies (GSFEs) for the basal and prismatic plane 〈a〉 slips of random α-Ti1−xAlx alloys (0≤x≤0.1875) are calculated by using first-principles methods including exact muffin-tin orbitals (EMTO) and plane-wave psuedopotential (VASP) methods in this work. The random distribution of Al in the alloy is treated by using both coherent potential approximation (CPA) for EMTO and special quasirandom structure (SQS) techniques for VASP. The CRSSs are then evaluated within the frame work of semi-discrete variational Peierls-Nabarro model. The VASP-SQS calculations with atomic relaxation generate reasonably good GSFE and CRSS compared with the EMTO-CPA and VASP-SQS calculations without atomic relaxation. For pure Ti, the unstable stacking fault energy (γusf) and CRSS (τa) for the basal 〈a〉 slip are higher than those for the prismatic 〈a〉 slip. With increasing Al concentration x, γusfb for the basal 〈a〉 slip decreases whereas γusfp for the prismatic 〈a〉 increases. The CRSSs for the basal 〈a〉 slip (τab) and the prismatic 〈a〉 slip (τap) both increase with x while τap increases faster than τab such that τab approaches to τap. The calculated CRSSs explain successfully our recently measured mechanical properties (yield strength, fracture toughness, ultimate strength, and elongation) of the α-Ti1−xAlx alloy against x.

Investigation on vibrational, electronic excitation entropy and magnetic moment contributions to phase stability of off-stoichiometric Ni50MnxIn50-x alloys at finite temperatures by first-principle calculations

The effects of Mn concentration and thermally excited contributions (including the vibrational, electronic excitation and magnetic contributions) on phase stability of austenite and martensite from 0 K to finite temperatures in Heusler typed Ni50MnxIn50-x shape memory alloys were studied by the first-principle calculations using exact muffin-tin orbitals with coherent potential approximation. Based on this, the martensitic transformation tendency was explored. Results show that at 0 K, the energy differences between the non-modulated martensite and the austenite become negative when extra Mn is added, indicating that the added Mn stabilizes the martensite and promotes martensitic transformation. The promoting effect increases with the increase of Mn content. At finite temperatures, the three thermal contributions (the vibrational, electronic excitation and magnetic contributions) were further calculated based on the equilibrium structure at 0 K. It was revealed that the vibrational entropies of the two phases increase with the increase of the temperature for all Mn contents. Under the two effects (temperature and Mn-content), the austenite has a larger vibrational entropy than the martensite, which indicates that the vibrational entropy contributes to promoting the martensitic transition. The Mn content and the temperature show a similar influence on the electronic entropies of the two phases. However, compared with the vibrational entropy, the contribution of the electronic entropy is much smaller. Furthermore, the influences of Mn content and temperature on the magnetic moment of both phases were simulated in their ferromagnetic state. The results show that the magnetic moments increase linearly with the Mn content, however, the influence of temperature is relatively small. Above 100 K, the magnetic moment of the austenite is higher than that of the martensite in ferromagnetic Ni50Mn29.25In20.75 alloy, suggesting that the magnetic entropy makes a similar contribution to promote the martensitic transformation, like the vibrational and electronic excitation entropies.