In this study we discuss the performance of approximate SQS supercell models in describing the cubic elastic properties of B1 (rocksalt) Ti$_{0.5}$Al$_{0.5}$N alloy by using a symmetry based projection technique. We show on the example of Ti$_{0.5}$Al$_{0.5}$N alloy, that this projection technique can be used to align the differently shaped and sized SQS structures for a comparison in modeling elasticity. Moreover, we focus to accurately determine the cubic elastic constants and Zener's type elastic anisotropy of Ti$_{0.5}$Al$_{0.5}$N. Our best supercell model, that captures accurately both the randomness and cubic elastic symmetry, results in $C_{11}=447$ GPa, $C_{12}=158$ GPa and $C_{44}=203$ GPa with 3% of error and $A=1.40$ for Zener's elastic anisotropy with 6% of error. In addition, we establish the general importance of selecting proper approximate SQS supercells with symmetry arguments to reliably model elasticity of alloys. In general, we suggest the calculation of nine elastic tensor elements - $C_{11}$, $C_{22}$, $C_{33}$, $C_{12}$, $C_{13}$, $C_{23}$, $C_{44}$, $C_{55}$ and $C_{66}$, to evaluate and analyze the performance of SQS supercells in predicting elasticity of cubic alloys via projecting out the closest cubic approximate of the elastic tensor. The here described methodology is general enough to be applied in discussing elasticity of substitutional alloys with any symmetry and at arbitrary composition.
Compound semiconductor alloys of the type ABC find widespread applications as their electronic bulk band gap varies continuously with x, and therefore a tayloring of the energy gap is possible by variation of the concentration. We model the electronic properties of such semiconductor alloys by a multiband tight-binding model on a finite ensemble of supercells and determine the band gap of the alloy. This treatment allows for an intrinsic reproduction of band bowing effects as a function of the concentration x and is exact in the alloy-induced disorder. In the present paper, we concentrate on b…
We present a systematic methodology for the accurate calculation of defect structures in supercells which we illustrate with a study of the neutral vacancy in silicon. This is a prototypical defect which has been studied extensively using ab initio methods, yet remarkably there is still no consensus about the energy or structure of this defect, or even whether the nearest neighbour atoms relax inwards or outwards. In this paper we show that the differences between previous calculations can be attributed to supercell convergence errors, and we demonstrate how to systematically reduce each such …
We present an efficient and accurate method for calculating electronic structure and related properties of random alloys with a proper treatment of local environment effects. The method is a generalization of the locally self-consistent Green's function (LSGF) technique for the exact muffin-tin orbital (EMTO) method. An alloy system in the calculations is represented by a supercell with a certain set of atomic distribution correlation functions. The Green's function for each atom in the supercell is obtained by embedding the cluster of neighboring atoms lying within a local interaction zone (L…
We present a comparative study of different modeling approaches to the electronic properties of the $\textrm{Hf}_{0.05}\textrm{Nb}_{0.05}\textrm{Ta}_{0.8}\textrm{Ti}_{0.05}\textrm{Zr}_{0.05}$ high entropy alloy. Common to our modeling is the methodology to compute the one-particle Green's function in the framework of density functional theory. We demonstrate that the special quasi-random structures modeling and the supercell, i.e. the locally self-consistent multiple-scatering methods provide very similar results for the ground state properties such as the spectral function (density of states)…
Incorporation of bismuth (Bi) in dilute quantities in (In)GaAs has been shown to lead to unique electronic properties that can in principle be exploited for the design of high efficiency telecomm lasers. This motivates the development of simple models of the electronic structure of these dilute bismide alloys, which can be used to evaluate their potential as a candidate material system for optical applications. Here, we begin by using detailed calculations based on an $sp^{3}s^{*}$ tight-binding model of (In)GaBi$_{x}$As$_{1-x}$ to verify the presence of a valence band-anticrossing interaction…
The nitrogen-vacancy center in diamond is a promising candidate for realizing the spin qubits concept in quantum information. Even though this defect is known for a long time, its electronic structure and other properties have not yet been explored in detail. We study the properties of the nitrogen-vacancy center in diamond through density functional theory within the local spin density approximation, using supercell calculations. While this theory is strictly applicable for ground state properties, we are able to give an estimate for the energy sequence of the excited states of this defect. W…
We present a first-principles approach for the computation of the magnetic Gibbs free energy of materials using magnetically constrained supercell calculations. Our approach is based on an adiabatic approximation of slowly varying local moment orientations, the so-called finite-temperature disordered local moment picture. It describes magnetic phase transitions and how electronic and/or magnetostructural mechanisms generate a discontinuous (first-order) character. We demonstrate that the statistical mechanics of the local moment orientations can be described by an affordable number of supercel…
Many important ultrafast phenomena take place in the liquid phase. However, there is no practical theory to predict how liquids respond to intense light. Here, we propose an $ab~initio$ accurate method to study the non-perturbative interaction of intense pulses with a liquid target to investigate its high-harmonic emission. We consider the case of liquid water, but the method can be applied to any other liquid or amorphous system. The liquid water structure is reproduced using Car-Parrinello molecular dynamics simulations in a periodic supercell. Then, we employ real-time time-dependent densit…
Scandium fluoride (ScF3) belongs to a class of negative thermal expansion (NTE) materials. It shows a strong lattice contraction up to about 1000 K switching to expansion at higher temperatures. Here the NTE effect in ScF3 is studied in the temperature range from 300 K to 1600 K using ab initio molecular dynamics (AIMD) simulations in the isothermal-isobaric (NpT) ensemble. The temperature dependence of the lattice constant, inter-atomic Sc-F-Sc bond angle distributions and the Sc-F and Sc-Sc radial distribution functions is obtained as a function of supercell size from 2a x 2a x 2a to 5a x 5a…
The lattice dynamics of AgPd, Ni55Pd45, Ni95Pt05, and Cu0.715Pd0.285 intermetallic have been investigated using the DFT calculation. The phonon dispersions and phonon densities of states along for two symmetry directions are calculated by Weighted Dynamical Matrix (WDM) and compared with virtual crystal approximation (VCA), supercell approach, and inelastic neutron scattering experimental results. The impact of mass, force-constant fluctuation, and Ag concentration on lattice dynamics of AgPd are discussed, and a comparison between WDM and Supercell approach is performed. The averaged first Ne…
Supercells are often used in ab initio calculations to model compound alloys, surfaces and defects. One of the main challenges of supercell electronic structure calculations is to recover the Bloch character of electronic eigenstates perturbed by disorder. Here we apply the spectral weight approach to unfolding the electronic structure of group III-V and II-VI semiconductor solid solutions. The illustrative examples include: formation of donor-like states in dilute Ga(PN) and associated enhancement of its optical activity, direct observation of the valence band anticrossing in dilute GaAs:Bi, …
We propose a simple and effective approach to construct the empirical tight-binding parameters of ternary alloys in the virtual crystal approximation. This combines a new, compact formulation of the strain parameters and a linear interpolation of the hamiltonians of binary materials strained to the alloy equilibrium lattice parameter. We show that it is possible to obtain a perfect description of the bandgap bowing of ternary alloys in the InGaAsSb family of materials. Furthermore, this approach is in a good agreement with supercell calculations using the same set of parameters. This scheme op…
We present ab initio calculations for single-atom vacancies in graphene. In agreement with earlier work, we find that vacancies are responsible for the magnetism recently observed experimentally, creating a center with net spin. For small supercells, there is a strong symmetry breaking in the supercell with respect to the perfect graphene structure, and this symmetry breaking is much smaller for larger supercells. The influence of the adsorption of H, O and N atoms on the spin center of the bare single-atom vacancy and its influence on the magnetism induced in these samples is investigated. A …
Si$_{y}$Ge$_{1-x-y}$(C,Sn,Pb)$_{x}$ alloys have attracted significant attention as a route to achieve a direct-gap group-IV semiconductor. Using density functional theory (DFT) - employing local density approximation and hybrid Heyd-Scuzeria-Ernzerhof exchange-correlation functionals - we compute the lattice parameters, relaxed and inner elastic constants, and internal strain (Kleinman) parameters for elemental (diamond) group-IV materials and zinc blende IV-IV compounds. Our DFT calculations support a little-known experimental re-evaluation of the $α$-Sn elastic constants, and contradict a re…
The lattice dynamics of Cu3Au, Ni70Pt30, Pd90Fe10, and Pd96Fe04 intermetallic is studied using the DFT calculations. We calculated the phonon dispersions and phonon densities of states along two high symmetry paths of the Brillouin zone by Weighted Dynamical Matrix (WDM) approach. We also compared the results with the supercell approach and inelastic neutron scattering. Furthermore, we calculated the impact of mass and force-constant fluctuations on the Cu3Au and made a comparison with both WDM and supercell approaches results. The averaged first Nearest Neighbor (1NN) force constants between …
We present a theoretical analysis of electronic structure evolution in the group-IV alloy Ge$_{1-x}$Pb$_{x}$ based on density functional theory. For ordered alloy supercells we demonstrate the emergence of a singlet conduction band (CB) edge state, suggesting the emergence of a direct band gap for Pb compositions as low as $x \approx 1$%. However, application of hydrostatic pressure reveals Pb-induced hybridisation, with the CB edge state in a Ge$_{63}$Pb$_{1}$ ($x = 1.56$%) supercell retaining primarily indirect (Ge L$_{6c}$) character. For an ordered Ge$_{15}$Pb$_{1}$ ($x = 6.25$%) supercell…
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.
We perform a systematic theoretical analysis of the nature and importance of alloy disorder effects on the electronic and optical properties of GaN$_{y}$As$_{1-x-y}$Bi$_{x}$ alloys and quantum wells (QWs), using large-scale atomistic supercell electronic structure calculations based on the tight-binding method. Using ordered alloy supercell calculations we also derive and parametrise an extended basis 14-band \textbf{k}$\cdot$\textbf{p} Hamiltonian for GaN$_{y}$As$_{1-x-y}$Bi$_{x}$. Comparison of the results of these models highlights the role played by short-range alloy disorder -- associated…
Predicting material properties of disordered systems remains a long-standing and formidable challenge in rational materials design. To address this issue, we introduce an automated software framework capable of modeling partial occupation within disordered materials using a high-throughput (HT) first principles approach. At the heart of the approach is the construction of supercells containing a virtually equivalent stoichiometry to the disordered material. All unique supercell permutations are enumerated and material properties of each are determined via HT electronic structure calculations. …
An archetypical spin-glass metallic alloy, Cu0.83Mn0.17, is studied by means of an ab-initio based approach. First-principles calculations are employed to obtain effective chemical, strain-induced and magnetic exchange interactions, as well as static atomic displacements, and the interactions are subsequently used in thermodynamic simulations. It is shown that the calculated atomic and magnetic short-range order accurately reproduces the results of neutron-scattering experiments. In particular, it is confirmed that the alloy exhibits a tendency toward ordering and the corresponding ordered pha…
The magnetic and optical properties of titanium dichalcogenide compounds, TiX$_2$ (X = S, Se and Te) have been calculated by first principles calculations using density functional theory (DFT) as implemented in SIESTA code. A 3x3 supercell of the compounds is taken in this study to be able to tune the properties obtained from the unit cell of these compounds. With magnetic states (ferromagnetic nature) as the attained ground state, spin polarized calculations are performed to obtain the mentioned properties. Further, with spin polarization into effect, external electric field (along z-directio…
We demonstrate how to overcome serious problems in understanding and classification of vibration spectra in semiconductor alloys, following from traditional use of the virtual crystal approximation (VCA). We show that such different systems as InGaAs (1-bond->1-mode behavior), InGaP (modified 2-mode) and ZnTeSe (2-bond->1-mode) obey in fact the same phonon mode behavior - hence probably a universal one - of a percolation-type (1-bond->2-mode). The change of paradigm from the `VCA insight' (an averaged microscopic one) to the `percolation insight' (a mesoscopic one) offers a promising link towa…
Core-level shifts and core-hole screening effects in alloy formation are studied ``ab initio'' by constrained-density-functional total-energy calculations. For our case study, the ordered intermetallic alloy MgAu, final-state effects are essential to account for the experimental Mg 1s shift, while they are negligible for Au 4f. We explain the differences in the screening by analyzing the calculated charge density response to the core hole perturbation.
Raman spectroscopy is a widely used, powerful, and nondestructive tool for studying the vibrational properties of bulk and low-dimensional materials. Raman spectra can be simulated using first-principles methods, but due to the high computational cost calculations are usually limited only to fairly small unit cells, which makes it difficult to carry out simulations for alloys and defects. Here, we develop an efficient method for simulating Raman spectra of alloys, benchmark it against full density-functional theory calculations, and apply it to several alloys of two-dimensional transition meta…
By fitting to a database of ab-initio forces and energies, we can extract pair potentials for alloys, with a simple six-parameter analytic form including Friedel oscillations, which give a remarkably faithful account of many complex intermetallic compounds. As examples we show results for (crystal or quasicrystal) structure prediction and phonon spectrum for three systems: Fe--B, Al--Mg--Zn, and Al--Cu--Fe.
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.
Ab-initio calculations for the structural properties of Zr-Nb alloys at different values of the niobium concentration are done at zero temperature. Different cases for Zr-Nb alloys with unit cells having BCC and HCP structures are considered. Optimal values of the lattice constants are obtained. Critical value for the niobium concentration corresponding to the structural transformation HCP \rightarrow BCC at zero temperature is determined. Electronic densities of states for two different structures with niobium concentrations 12.5% and 25% having HCP and BCC structures, accordingly, are studie…
The use of a special quasirandom structure (SQS) is a rational and efficient way to approximate random alloys. A wide variety of physical properties of metallic and semiconductor random alloys have been successfully estimated by a combination of an SQS and density functional theory (DFT) calculation. Here, we investigate the application of an SQS to the ionic multicomponent systems with configurations of heterovalent ions, including point-charge lattices, MgAl$_2$O$_4$ and ZnSnP$_2$. It is found that the physical properties do not converge with the supercell size of the SQS. This is ascribed t…
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 study the electronic states of the magnetically ordered Mn3Pt alloy within the density functional theory. Mn3Pt has been believed that one third of Mn atoms have no magnetic moment in an antiferromagnetic phase (so-called the F-phase) realized in the temperature range of 400 K < T < 475 K. We show that this experimentally suggested spin configuration is energetically so much unfavorable that it would be irrelevant to the F-phase. We discuss the possibility that the spin moments on the one third of Mn atoms are not paramagnetic but thermally fluctuating in the F-phase. The present results ha…
A Kubo-Greenwood-like equation for the Gilbert damping parameter $α$ is presented that is based on the linear response formalism. Its implementation using the fully relativistic Korringa-Kohn-Rostoker (KKR) band structure method in combination with Coherent Potential Approximation (CPA) alloy theory allows it to be applied to a wide range of situations. This is demonstrated with results obtained for the bcc alloy system Fe$_x$Co$_{1-x}$ as well as for a series of alloys of permalloy with 5d transition metals. To account for the thermal displacements of atoms as a scattering mechanism, an alloy…
Lattice parameter, bulk modulus, formation energy and magnetism equilibrium structure in all-d transition metals $Ni_2XMn$ (X=Co,Cr,Fe) for Full and Inverse Heusler structure are studied by first-principles calculations. Tetragonal distortion is calculated for the equilibrium volume of each alloy and possible new equilibrium structures are reported. Density of electronic states are analyzed and the results lead to half-metallicity for this type of alloys.
In this progress report we present a new approach to the ab-initio guided bottom up design of beta-Ti alloys for biomedical applications using a quantum mechanical simulation method in conjunction with experiments. Parameter-free density functional theory calculations are used to provide theoretical guidance in selecting and optimizing Ti-based alloys with respect to three constraints: (i) the use of non-toxic alloy elements; (ii) the stabilization of the body centered cubic beta phase at room temperature; (iii) the reduction of the elastic stiffness compared to existing Ti-based alloys. Follo…
First-principles alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to study the mechanical properties of ferromagnetic body-centered cubic (bcc) Fe$_{1-x}$M$_x$ alloys (M=Mn or Ni, $0\le x \le 0.1$). We consider several physical parameters accessible from \emph{ab initio} calculations and their combinations in various phenomenological models to compare the effect of Mn and Ni on the properties of Fe. Alloying is found to slightly alter the lattice parameters and produce noticeable influence on elastic moduli. …
By fitting to a database of ab-initio forces and energies, we can extract pair potentials for alloys, with a simple six-parameter analytic form including Friedel oscillations, which give a remarkably faithful account of many complex intermetallic compounds. Furthermore, such potentials are combined with a method of discovering complex zero-temperature structures with hundreds of atoms per cell, given only the composition and the con- straint of known lattice parameters, using molecular-dynamics quenches. We apply this approach to structure prediction in the Al-Cu-Sc quasicrystal-related system…
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, ar…
The structural, electronic, mechanical and thermal properties of Ir$_{1-x}$Rh$_{x}$ alloys were studied systematically using ab initio density functional theory at different concentrations (x = 0.00, 0.25, 0.50, 0.75, 1.00). A Special Quasirandom Structure method was used to make alloys having FCC structure with four atoms per unit cell. The ground state properties such as lattice constant and bulk modulus were calculated to find the equilibrium atomic position for stable alloys. The calculated ground state properties are in good agreement with the experimental and previously presented other t…
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.
Comparison of periodic B dopants with a random alloy of substitional boron in diamond is carried out using several supercells and the coherent potential approximation (CPA) for the random alloy case. The main peak in the B local density of states is shifted to lower binding energy compared to the corresponding C peak in intrinsic diamond. In supercells, this shows up as strongly B-character bands split from bulk C bands away from the zone center,in an energy region around -1 eV. Even for a 4*4*4 supercell (BC$_{127}$), effects of the dopant order are evident in the form of primarily B-characte…
Compositionally complex alloys or concentrated solid solutions are the latest frontier in catalyst design, but mixing different elements in one catalyst may result in surface segregation. Atomistic simulations can predict segregation patterns, but standard approaches based on mean-field models, cluster expansion, or classical interatomic potentials are often limited for the description of multicomponent alloys. We present machine learning potentials that can describe surface segregation with near DFT accuracy. The method is used to study a complex Co-Cu-Fe-Mo-Ni quinary alloy. For this alloy, …
A first-principles derived approach is combined with the inverse Monte Carlo technique to determine the atomic orderings leading to prefixed properties in Pb(Sc_{0.5}Nb_{0.5})O_{3} perovskite alloy. We find that some arrangements between Sc and Nb atoms result in drastic changes with respect to the disordered material, including ground states of new symmetries, large enhancement of electromechanical responses, and considerable shift of the Curie temperature. We discuss the microscopic mechanisms responsible for these unusual effects.
A reliable prediction of interatomic force constants in disordered alloys is an outstanding problem. This is due to the need for a proper treatment of multisite (atleast pair) correlation within a random environment. The situation becomes even more challenging for systems with large difference in atomic size and mass. We propose a systematic density functional theory (DFT) based study to predict the ab-initio force constants in random alloys. The method is based on a marriage between special quasirandom structures (SQS) and the augmented space recursion (ASR) to calculate phonon spectra, densi…
We calculate the electronic structure of germanium-tin (Ge$_{1-x}$Sn$_{x}$) binary alloys for $0 \leq x \leq 1$ using density functional theory (DFT). Relaxed alloys with semiconducting or semimetallic behaviour as a function of Sn composition $x$ are identified, and the impact of epitaxial strain is included by constraining supercell lattice constants perpendicular to the [001] growth direction to the lattice constants of Ge, zinc telluride (ZnTe), or cadmium telluride (CdTe) substrates. It is found that application of 1% tensile strain reduces the Sn composition required to bring the (positi…
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…
Accelerated design of novel hard coating materials requires state-of-the-art computational tools, which include data-driven techniques, building databases, and training machine learning models against the databases. In this work, we present a development of a heavily automated high-throughput workflow to build a database of industrially relevant hard coating materials, such as binary and ternary nitrides. We use Vienna Ab initio Simulation package as the density functional theory calculator and the high-throughput toolkit to automate the calculation workflow. We calculate and present results, …
We present results of extensive theoretical studies of Co$_2$FeAl$_{1-x}$Si$_x$ Heusler alloys, which have been performed in the framework of density functional theory employing the all-electron full-potential linearized augmented plane-wave scheme. It is shown that the Si-rich alloys are more resistive to structural disorder and as a consequence Si stabilizes the $L2_1$ structure. Si alloying changes position of the Fermi level, pushing it into the gap of the minority spin-band. It is also shown that the hyperfine field on Co nuclei increases with the Si concentration, and this increase origi…
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…
A scheme for making ab-initio calculations of the dynamic paramagnetic spin susceptibilities of solids at finite temperatures is described. It is based on Time-Dependent Density Functional Theory and employs an electronic multiple scattering formalism. Incommensurate and commensurate anti-ferromagnetic spin fluctuations in paramagnetic Cr and compositionally disordered Cr95V5 and Cr95Re5 alloys are studied together with the connection with the nesting of their Fermi surfaces. We find that the spin fluctuations can be described rather simply in terms of an overdamped oscillator model. Good agre…
Comparison of free energies between different phases and different compositions underlies the prediction of alloy phase diagrams. To allow direct comparison, consistent reference points for the energies or enthalpies are required, and the entropy must be placed on an absolute scale, yielding absolute free energies. Here we derive absolute free energies of liquids from ab-initio molecular dynamics (AIMD) by combining the directly simulated enthalpies with an entropy derived from simulated densities and pair correlation functions. As an example of the power of this method we calculate the phase …
We present results of systematic fully relativistic first-principles calculations of the uniaxial magnetic anisotropy energy (MAE) of a disordered and partially ordered tetragonal Fe-Co alloy using the coherent potential approximation (CPA). This alloy has recently become a promising system for thin ferromagnetic films with a perpendicular magnetic anisotropy. We find that existing theoretical approaches to homogeneous random bulk Fe-Co alloys, based on a simple virtual crystal approximation (VCA), overestimate the maximum MAE values obtained in the CPA by a factor of four. This pronounced dif…
Lattice dynamical methods used to predict phase-transformations in crystals typically evaluate the harmonic phonon spectra and therefore do not work in frequent and important situations where the crystal structure is unstable in the harmonic approximation, such as the $β$ structure when it appears as a high-temperature phase of the shape memory alloy (SMA) NiTi. Here it is shown by self consistent {\it ab initio} lattice dynamical calculations (SCAILD) that the critical temperature for the pre-martensitic $R$ to $β$ phase-transformation in NiTi can be effectively calculated with good accuracy,…
