We present an ab initio theory of transport quantities of metallic ferromagnets developed in the framework of the fully relativistic tight-binding linear muffin-tin orbital method. The approach is based on the Kubo-Streda formula for the conductivity tensor, on the coherent potential approximation for random alloys, and on the concept of interatomic electron transport. The developed formalism is applied to pure 3d transition metals (Fe, Co, Ni) and to random Ni-based ferromagnetic alloys (Ni-Fe, Ni-Co, Ni-Mn). High values of the anisotropic magnetoresistance (AMR), found for Ni-rich alloys, are explained by a negligible disorder in the majority spin channel while a change of the sign of the anomalous Hall effect (AHE) on alloying is interpreted as a band-filling effect without a direct relation to the high AMR. The influence of disorder on the AHE in concentrated alloys is investigated as well.
Parameters of the energy gap and, consequently, electronic, magnetic and galvanomagnetic properties in different X$_2$YZ Heusler alloys can vary quite strongly. In particular, half-metallic ferromagnets (HMFs) and spin gapless semiconductors (SGSs) with almost 100% spin polarization of charge carriers are promising materials for spintronics. The changes in the electrical, magnetic and galvanomagnetic properties of the Co$_2$YSi (Y = Ti, V, Cr, Mn, Fe) and Co$_2$MnZ Heusler alloys (Z = Al, Si, Ga, Ge) in possible HMF and/or SGS states were followed and their interconnection was established. Sig…
Investigating the grain boundary energies of pure fcc metals and their surface energies obtained from ab initio modeling, we introduce a robust method to estimate the grain boundary energies for complex multicomponent alloys. The input parameter is the surface energy of the alloy, which can easily be accessed by modern ab initio calculations based on density functional theory. The method is demonstrated in the case of paramagnetic Fe-Cr-Ni alloys for which reliable grain boundary data is available.
A density-functional-theory based approach to efficiently compute numerically exact vibrational free energies - including anharmonicity - for chemically complex multicomponent alloys is developed. It is based on a combination of thermodynamic integration and a machine-learning potential. We demonstrate the performance of the approach by computing the anharmonic free energy of the prototypical five-component VNbMoTaW refractory high entropy alloy.
We present a new modeling scheme for ion self-diffusion coefficient, which broadens the applicable scope of ab initio approach. The essential concepts of the scheme are `domain division' and `coarse graining' of the diffusion network based on the barrier energies predicted by the ab initio calculation. The scheme was applied to evaluate Cu ion self-diffusion coefficient in ε-Cu3Sn phase of Cu-Sn alloy, which is a typical system having long-range periodicity. The model constructed with the scheme successfully reproduces the experimental values in a wide temperature range.
We present the ab initio results of the structural and magnetic properties of the Pd host matrix implanted with Fe solute atoms at various concentrations. By means of density functional theory we confirm that iron impurities are able to initialize significant magnetization of the Pd atoms, when the impurity consentation exceeds 3 at.%. Besides, we demonstrate that the imposed magnetization depends on impurity positions in the host matrix, in particular, there is a maximum of magnetization for a uniform distribution of the iron impurity.
Using density functional theory based calculations, we have carried out in-depth studies of effect of Co substitution on the magnetic properties of Ni and Pt-based shape memory alloys. We show the systematic variation of the total magnetic moment, as a function of Co doping. A detailed analysis of evolution of Heisenberg exchange coupling parameters as a function of Co doping has been presented here. The strength of RKKY type of exchange interaction is found to decay with the increase of Co doping.
Neutron diffraction and ab initio studies were carried out on Mn2V1-xCoxGa (x=0, 0.25, 0.5, 0.75, 1) Heusler alloys which exhibits high TC fully compensated ferrimagnetic characteristic for x=0.5. A combined analysis of neutron diffraction and ab initio calculations revealed the crystal structure and magnetic configuration which could not be determined from the X-ray diffraction and magnetic measurements. As reported earlier, Rietveld refinement of neutron diffraction data confirmed L21 structure for Mn2VGa and Xa structure for Mn2CoGa. The alloys with x=0.25 and 0.5 possess L21 structure with…
Based on ab-initio simulations, we report on the nature of principally different mechanisms for interaction of Mg and Zn atoms with grain boundaries in Al alloys leading to different morphology of segregation. The Mg atoms segregate in relatively wide GB region with heterogeneous agglomerations due to the deformation mechanism of solute-GB interaction. In contrast, in the case of Zn atoms an electronic mechanism associated with the formation of directional bonding is dominating in the solute-GB interaction. As a result, for Zn atoms it is energetically beneficial to occupy interstitial positio…
We present a comprehensive first-principles investigation of optical, transport, and thermoelectric properties of pure and doped hexagonal Si$_x$Ge$_{1-x}$ alloys based on density-functional theory calculations, the Boltzmann transport equation, and the generalized quasi-chemical approximation to obtain alloy averages of electronic properties. At low temperature, phase decomposition into the hexagonal elementary crystals is thermodynamically favored, but around and above room temperature random alloys are predicted to be stable. While hexagonal Si has an indirect band gap, the gap of hexagonal…
In this work we developed a practical and general modeling approach for thermal conductivity of metals and metal alloys that integrates ab initio and semiempirical physics-based models to maximize the strengths of both techniques. The approach supports creation of highly accurate, mechanistic, and extensible thermal conductivity modeling of alloys. The model was demonstrated on α-U and U-rich U-Zr and U-Mo alloys, which are potential fuels for advanced nuclear reactors. The safe use of U-based fuels requires quantitative understanding of thermal transport characteristics of the fuel. The model…
Using an ab initio approach, we explore the stability of small vacancy and vacancy-chromium clusters in dilute body-centred cubic Fe-Cr alloys. To explain experimental observations described in C.D. Hardie et al., J. Nucl. Mater. 439, 33 (2013) and showing the occurrence of Cr segregation in low-Cr alloys, we investigate if chromium can form stable bound configurations with vacancies in alloys with chromium concentration below the low-temperature chromium solubility limit of 10-11 at. %. We find that a single vacancy can attract up to four Cr atoms in the most energetically favourable cluster …
Since the pioneering efforts of Duwez and coworkers in 1965, when a solid amorphous phase of Pd-Si ($a$-PdSi) was obtained in the vicinity of the eutectic concentration, much work has been done. However, some points related to the atomic structures remain to be systematized. In this work, 8 amorphous Pd$_{100-c}$Si$_{c}$ alloys (c = 2.5, 5, 10, 13.34, 15, 17.5, 20, and 22 at %) were generated by molecular dynamics $ab$ $initio$ simulations; the short-range structure is analyzed using several correlation functions, like Pair Distribution Functions, reduced Pair Distribution Functions, Plane Ang…
In the framework of density functional theory, the structural and magnetic properties of Fe$_{100-x}$Al$_x$ alloys (${5 \leq x \leq 25}$~at.\%) with the different structural order are investigated. Using the Korringa-Kohn-Rostoker Green's function method with a coherent potential approximation, the equilibrium lattice parameters, ground-state energy, and shear moduli for D0$_3$, B2, and A2 structures are calculated. For all structures, the optimized lattice constant increases while the shear modulus demonstrates a decreasing behavior with increasing Al content. The tetragonal magnetostriction …
This paper presents a detailed investigation of FeCr-based quaternary Heusler alloys. By using ultrasoft pseudopotential, electronic and magnetic properties of the compounds are studied within the framework of Density Functional Theory (DFT) by using the Quantum Espresso package. The thermodynamic, mechanical, and dynamical stability of the compounds is established through the comprehensive study of different mechanical parameters and phonon dispersion curves. The meticulous study of elastic parameters such as bulk, Young's, shear moduli, etc. is done to understand different mechanical propert…
Using first-principles calculations based on density functional theory, we probe the electronic and magnetic properties of X$_{2}$PtGa (X being Cr, Mn, Fe, Co) Heusler alloys. Our calculations predict that all these systems possess inverse Heusler alloy structure in their respective ground states. Application of tetragonal distortion leads to lowering of energy with respect to the cubic phase for all the materials. The equilibrium volumes of both the phases are nearly the same. These results of our calculations indicate that all these materials are prone to undergo martensite transition, as ha…
Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behaviour from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d-band of Mn. We propose effective interaction models for these alloys, which contain both atomic spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of non-collinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fit…
The cubic $Ia\bar{3}$ (BC8) and tetragonal $P4_32_12$ (ST12) high pressure modifications of Si and Ge are attractive candidates for applications in optoelectronic, thermoelectric or plasmonic devices. Si$_x$Ge$_{1-x}$ alloys in BC8/ST12 modifications could help overcome the indirect and narrow band gaps of the pure phases and enable tailoring for specific use-cases. Such alloys have experimentally been found to be stable at ambient conditions after release from high pressure synthesis, however their fundamental properties are not known. In this work, we employ {\it ab initio} calculations base…
Ab initio calculation of structural and electronic properties of Al$_x$Ga$_{1-x}$N and In$_x$Ga$_{1-x}$N alloys
采用密度泛函理论（DFT）和广义梯度近似（GGA）方法，计算了纤锌矿结构的AlN、GAN、InN及其相关合金Al $_x$ Ga $_{1-x}$ N和In $_x$ Ga $_{1-x}$ N的结构和电子性质.我们用全势线性缀加平面波（FP-LAPW）方法进行了精确的{ \it ab initio}总能量计算，研究了结构和电子性质.在这两种合金中，我们发现基本参数不遵循Vegard定律。发现Al $_x$ Ga $_{1-x}$ N合金的晶格参数$a, c,$和$u$表现出向下弯曲，而In $_x$ Ga $_{1-x}$ N的$a$和$c$参数存在向上弯曲，内部参数$u$存在向下弯曲。此外，我们发现，对于这两种合金，带隙值不遵循Vegard定律。作为电子能带计算的副产品，计算了二元化合物及其相关合金的有效质量。我们表明，计算的二元化合物，以及所研究的合金的性能，显示出良好的协议与大多数以前报道的结果。最后，利用冻结声子方法计算了不同体系的A $_1(TO)$模。我们的计算结果显示出良好的协议与实验值报告的二元化合物。对于三元合金，我们的计算再现实验值为Al $_x$ Ga $_{1-x}$ N以及理论预测为In $_x$ Ga $_{1-x}$ N。
Effect of 3d Transition Metal Doping (Mn, Fe, Co, Ni) on the Electronic and Magnetic Properties of Pd Alloys at Low Impurity Concentrations: An Ab initio Study
In traditional body-centered cubic (bcc) metals, the core properties of screw dislocations play a critical role in plastic deformation at low temperatures. Recently, much attention has been focused on refractory high-entropy alloys (RHEAs), which also possess bcc crystal structures. However, unlike face-centered cubic high-entropy alloys (HEAs), there have been far fewer investigations on bcc HEAs, specifically on the possible effects of chemical short-range order (SRO) in these multiple principal element alloys on dislocation mobility. Here, using density functional theory, we investigate the…
In the present work, the aspects of magnetic and structural properties of Ni-Mn-Ga alloys are described in the framework of fist-principles approach and mapped into ternary composition diagrams. The stable atomic arrangement and magnetic alignment for compositions with cubic austenite and tetragonal martensite structures across phase diagrams are predicted. It is shown that Ni- and Ga-rich compositions possess the regular Heusler structure in contrast to Mn-rich compositions with inverse Heusler structure as favorable one. Compositions with unstable austenite structure are concentrated in the …
The different contributions to the entropy change linked to the austenite-martensitic transition in a Ni-Mn-Sn metamagnetic shape memory alloy have been determined by combining different experimental techniques. The vibrational contribution has been inferred from the vibrational density of states of both the martensitic and austenite phases. This has been accomplished by combining time-of-flight neutron scattering measurements and ab-initio calculations. Further, the electronic part of the entropy change has also been calculated. Since the martensitic transformation takes place between two par…
Structural and electronic properties of superconducting Heusler alloy Ni$_{2}$Nb$_{1+x}$Sn$_{1-x}$: \textit{Ab initio} approach
采用\textit{ab initio}计算方法，系统地研究了NiPDMATHABMN Nb $_{1+x}$ Sn $_{1-x}$（$x$ = 0，0.25，0.50）的结构和电子性质.在这里，投影仪增广波方法（PAW）中实现的维也纳\textit{ab initio}模拟包（VASP）内的广义梯度近似（GGA）的交换相关功能已被使用。本文报道了Ni $_{2}$ Nb Sn和Ni $_{2}$ Nb $_{1.25}$ Sn $_{0.75}$虽然没有结构转变，但Ni $_{2}$ Nb $_{1.5}$ Sn $_{0.5}$可以从立方L2 $_{1}$相转变为四方结构。在非化学计量比合金中，立方晶格参数随着Nb在Sn位点处的掺杂而减小。合金在所有结构中均处于顺磁相。Ni和Nb的3d态之间的杂化引发了四重态畸变。由于立方L2 $_{1}$相中的Nb掺杂，在费米能（EPDMATHABMW）（N（EPDMATHABMW））处的总态密度（DOS）存在显著变化。N（EPDMATHABMW）随着Nb掺杂的增加而增加。但在Ni-$_{2}$-$_{1.5}$的结构转变过程中，N（E-$_{F}$）降低.超导临界温度（TPDMATHABMX）也随Nb在立方相中的掺杂和四角畸变而变化，因为TPDMATHABMX非常依赖于N（EPDMATHABMW）。
While the ongoing search to discover new high-entropy systems is slowly expanding beyond metals, a rational and effective method for predicting "in silico" the solid solution forming ability of multi-component systems remains yet to be developed. In this article, we propose a novel high-throughput approach, called "LTVC", for estimating the transition temperature of a solid solution: ab-initio energies are incorporated into a mean field statistical mechanical model where an order parameter follows the evolution of disorder. The LTVC method is corroborated by Monte Carlo simulations and the res…
We investigated six heavy element bismuth-based 18-VEC half-Heusler alloys CoTiBi, CoZrBi, CoHfBi, FeVBi, FeNbBi, and FeTaBi by first principles approach, in search of better thermoelectric prospects. The motivation is driven by expected lower thermal conductivity and the recent discovery of CoZrBi-based materials. Significantly, our calculated power factor values of all the systems show an increment of $\sim$40\% in comparison to the reported \textit{p}-type CoTiSb. We propose that doping at Bi-site, on account of electronic features, will be helpful in achieving the proposed power factor val…
The (Cd, Be)Te semiconductor alloy that exhibits a dramatic mismatch in bond covalency and stiffness clarifying its vibrational and mechanical properties is used as a benchmark to test the limits of the percolation model (PM) worked out to explain the complex Raman spectra of the related but less contrasted (Zn, Be) chalcogenides. The test is done by way of experiment (x smaller than 0.11) combining Raman scattering with X ray diffraction at high pressure and ab initio calculations (x around 0, 0.5 and 1). The (macroscopic) bulk modulus B drops below the CdTe value on minor Be incorporation, a…
Structural, electronic, and transport properties of ScNiSb, ScPdSb, and ScPtSb were investigated from first principles. Electronic band structures derived within the fully relativistic MBJLDA approach were compared with those obtained from the standard GGA calculations. All the compounds studied exhibit indirect narrow band gaps (0.24-0.63 eV). The effective masses of hole-like carriers are relatively small (0.27-0.36), and decrease with an increasing atomic number of the transition metal component. The carrier relaxation time, required for realistic calculations of the electrical conductivity…
Machine Learning-based estimation and explainable artificial intelligence-supported interpretation of the critical temperature from magnetic ab initio Heusler alloys data
