Functional form of the generalized gradient approximation for exchange: The PBEα\alpha functional

A new functional form for the exchange enhancement in the generalized gradient approximation within density functional theory is given. The functional form satisfies the constraints used to construct the Perdew-Burke-Ernzerhof (PBE) functional but can be systematically varied using one parameter. This gives the possibility to estimate the reliability of a computational result or to fit the parameter for a certain problem. Compared to other semi-empirical functionals, the present has the advantage that only one physically transparent parameter is used and that the fitted functional will obey the same exact conditions as PBE functional. Furthermore the simple form of the exchange enhancement means that oscillating terms in the exchange potential are avoided

Comparing the performance of LDA and GGA functionals in predicting the lattice thermal conductivity of semiconductor materials: the case of AlAs

In this contribution we assess the performance of two different exchange-correlation functionals in the first-principle prediction of the lattice thermal conductivity of bulk semiconductors, namely the local density approximation (LDA) and the Perdew-Burke-Ernzerhof implementation of the generalized gradient approximation (GGA). Both functionals are shown to give results in good agreement with experimental measurements. Such a consistency between the two functionals may seem a bit surprising, as the LDA is known to overbind and the GGA to soften the interatomic bonds. Such features ought to greatly affect the value of the system interatomic force constants (IFCs) which are necessary for the first-principle prediction of the lattice thermal conductivity. In this study we show that the errors introduced by such approximations tend to cancel themselves. In the case of LDA, the overbinding generates larger absolute third-order IFCs, which tend to increase the three-phonon scattering rates. On the other hand, larger absolute second-order IFCs lead to a a larger acoustic-optical phonon band gap which in turns decrease the available phase space for three-phonon scattering, compensating the increase in the scattering rates due to stiffer IFCs

High-throughput exploration of alloying as design strategy for thermoelectrics

We explore a material design strategy to optimize the thermoelectric power factor. The approach is based on screening the band structure changes upon a controlled volume change. The methodology is applied to the binary silicides and germanides. We first confirm the effect in antifluorite Mg2Si and Mg2Ge where an increased power factor by alloying with Mg2Sn is experimentally established. Within a high-throughput formalism we identify six previously unreported binaries that exhibit an improvement in their transport properties with volume. Among these, hexagonal MoSi2 and orthorhombic Ca2Si and Ca2Ge have the highest increment in zT with volume. We then perform super-cell calculations on special quasi-random structures to investigate the possibility of obtaining thermodynamically stable alloy systems which would produce the necessary volume changes. We find that for Ca2Si and Ca2Ge the solid solutions with the isostructural Ca2Sn readily forms even at low temperatures.Comment: 9 Pages, 13 Figure

Comparison of the Green-Kubo and homogeneous non-equilibrium molecular dynamics methods for calculating thermal conductivity

Different molecular dynamics methods like the direct method, the Green-Kubo (GK) method and homogeneous non-equilibrium molecular dynamics (HNEMD) method have been widely used to calculate lattice thermal conductivity ($\kappa_\ell$). While the first two methods have been used and compared quite extensively, there is a lack of comparison of these methods with the HNEMD method. Focusing on the underlying computational parameters, we present a detailed comparison of the GK and HNEMD methods for both bulk and vacancy Si using the Stillinger-Weber potential. For the bulk calculations, we find both methods to perform well and yield $\kappa_\ell$ within acceptable uncertainties. In case of the vacancy calculations, HNEMD method has a slight advantage over the GK method as it becomes computationally cheaper for lower $\kappa_\ell$ values. This study could promote the application of HNEMD method in $\kappa_\ell$ calculations involving other lattice defects like nanovoids, dislocations, interfaces

Optimized Orthogonal Basis Tight Binding. Application to Iron

The formal link between the linear combination of atomic orbitals approach to density functional theory and two-center Slater-Koster tight-binding models is used to derive an orthogonal $d$-band tight-binding model for iron with only two fitting parameters. The resulting tight-binding model correctly predicts the energetic ordering of the low energy iron-phases, including the ferromagnetic BCC, antiferromagnetic FCC, HCP and topologically close-packed structures. The energetics of test structures that were not included in the fit are equally well reproduced as those included, thus demonstrating the transferability of the model. The simple model also gives a good description of the vacancy formation energy in the nonmagnetic FCC and ferromagnetic BCC iron lattices

First-principles quantitative prediction of the lattice thermal conductivity in random semiconductor alloys: the role of force-constant disorder

The standard theoretical understanding of the lattice thermal conductivity, $\kappa_{\ell}$, of semiconductor alloys assumes that mass disorder is the most important source of phonon scattering. In contrast, we show that the hitherto neglected contribution of force-constant (IFC) disorder is essential to accurately predict the $\kappa_{\ell}$ of those polar compounds characterized by a complex atomic-scale structure. We have developed an \emph{ab initio} method based on special quasirandom structures and Green's functions, and including the role of IFC disorder, and applied it in order to calculate the $\kappa_{\ell}$ of $\mathrm{In_{1-x}Ga_xAs}$ and $\mathrm{Si_{1-x}Ge_x}$ alloys. We show that, while for $\mathrm{Si_{1-x}Ge_x}$, phonon-alloy scattering is dominated by mass disorder, for $\mathrm{In_{1-x}Ga_xAs}$, the inclusion of IFC disorder is fundamental to accurately reproduce the experimentally observed $\kappa_{\ell}$. As the presence of a complex atomic-scale structure is common to most III-V and II-VI random semiconductor alloys, we expect our method to be suitable for a wide class of materials

Low thermal conductivities and excellent thermoelectric performances of the pyrite-type IIB-VIA2 dichalcogenides: ZnS2, CdS2 and CdSe2

By solving the phonon and electron Boltzmann transport equations, we calculate the thermoelectric properties of three pyrite-type IIB-VIA2 dichalcogenides (ZnS2, CdS2 and CdSe2). The results show that they both have low lattice thermal conductivities and promising electrical transport properties. Comparing their detailed phonon properties with that of FeS2, we find that their low lattice thermal conductivities come from their soft phonon modes and strong anharmonicity resulted by their weak bonds between the metal atoms with the nonmetal atoms. Analysis of their electronic band structures indicates that their promising electrical transport properties are contributed by the non-spherical isoenergy Fermi surface in the valence bands and the large energy valley degeneracies and light carrier effective masses in the conduction bands. Additionally, due to their calculated carrier relaxation times of p-type carriers are larger than that of n-type carriers, their thermopower factors of p-type doping are higher than that of n-type doping. As a result, their figure of merit, ZT values, can reach 1.45, 1.37 and 2.29 for p-type doping, and 1.01, 0.57 and 1.16 for n-type doping, respectively. This means that three IIB-VIA2 dichalcogenides (ZnS2, CdS2 and CdSe2) exhibit both excellent thermoelectric properties for p-type and n-type doping, and their thermoelectric properties of p-type doping are better than n-type doping

Spinney: post-processing of first-principles calculations of point defects in semiconductors with Python

Understanding and predicting the thermodynamic properties of point defects in semiconductors and insulators would greatly aid in the design of novel materials and allow tuning the properties of existing ones. As a matter of fact, first-principles calculations based on density functional theory (DFT) and the supercell approach have become a standard tool for the study of point defects in solids. However, in the dilute limit, of most interest for the design of novel semiconductor materials, the raw DFT calculations require an extensive post-processing. Spinney is an open-source Python package developed with the aim of processing first-principles calculations to obtain several quantities of interest, such as the chemical potential limits that assure the thermodynamic stability of the defectladen system, defect charge transition levels, defect formation energies, including electrostatic corrections for finite-size effects, and defect and carrier concentrations. In this paper we demonstrate the capabilities of the Spinney code using c-BN, Mg-doped GaN, TiO2 and ZnO as examples

Phonon transport unveils the prevalent point defects in GaN

Determining the types and concentrations of vacancies present in intentionally doped GaN is a notoriously difficult and long-debated problem. Here we use an unconventional approach, based on thermal transport modeling, to determine the prevalence of vacancies in previous measurements. This allows us to provide conclusive evidence of the recent hypothesis that gallium vacancies in ammonothermally grown samples can be complexed with hydrogen. Our calculations for O-doped and Mg-O co-doped samples yield a consistent picture interlinking dopant and vacancy concentration, carrier density, and thermal conductivity, in excellent agreement with experimental measurements. These results also highlight the predictive power of ab initio phonon transport modeling, and its value for understanding and quantifying defects in semiconductors.Comment: 5 pages, 4 figures, 1 supplementary material fil

Exceptionally strong phonon scattering by B substitution in cubic SiC

We use ab-initio calculations to predict the thermal conductivity of cubic SiC with different types of defects. An excellent quantitative agreement with previous experimental measurements is found. The results unveil that B$_\mathrm{C}$ substitution has a much stronger effect than any of the other defect types in 3C-SiC, including vacancies. This finding contradicts the prediction of the classical mass-difference model of impurity scattering, according to which the effects of B$_\mathrm{C}$ and N$_\mathrm{C}$ would be similar and much smaller than that of the C vacancy. The strikingly different behavior of the B$_\mathrm{C}$ defect arises from a unique pattern of resonant phonon scattering caused by the broken structural symmetry around the B impurity.Comment: 5 pages, 5 figure