Marco Fornari

BoltzWann: A code for the evaluation of thermoelectric and electronic transport properties with a maximally-localized Wannier functions basis ☆

We present a new code to evaluate thermoelectric and electronic transport properties of extended systems with a maximally-localized Wannier function basis set. The semiclassical Boltzmann transport equations for the homogeneous infinite system are solved in the constant relaxation-time approximation and band energies and band derivatives are obtained via Wannier interpolations. Thanks to the exponential localization of the Wannier functions obtained, very high accuracy in the Brillouin zone integrals can be achieved with very moderate computational costs. Moreover, the analytical expression for the band derivatives in the Wannier basis resolves any issues that may occur when evaluating derivatives near band crossings. The code is tested on binary and ternary skutterudites CoSb3 and CoGe3/2 S-3/2. Program summary Program title: BoltzWann Catalogue identifier: AEQX_v1_0 Program summaiy URL: http://cpc.cs.qub.ac.uk/summaries/AEQX_v1_0.html Program obtainable from: CPC Program Library, Queens University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 710 810 No. of bytes in distributed program, including test data, etc.: 8 337 000 Distribution format: tar.gz Programming language: Fortran 90. Computer: Any architecture with a Fortran 90 compiler. Operating system: Linux, Windows, Solaris, AIX, Tru64 Unix, OSX. Has the code been vectorized or parallelized?: Yes. RAM: The example requires approximately 10 MB. Classification: 7.3, 7.9. External routines: BLAS and LAPACK (available on http://www.netlib.org/); MPI libraries (optional) for parallel execution Nature of problem: Obtain electronic and thermoelectric transport properties for crystals. Solution method: The Boltzmann transport equations in the constant relaxation-time approximation are used. These equations require the integration of the band velocities over all the Brillouin zone; this is done numerically on a sufficiently dense k grid. Band energies and band derivatives are obtained by interpolation using the maximally-localized Wannier functions basis obtained with a preliminary run of the Wannier90 code. Unusual features: The maximally-localized Wannier functions interpolation scheme allows the use of analytical formulas (instead of finite-difference methods) to obtain the band derivatives. Additional comments: This is a package that is tightly integrated with the Wannier90 code (http://www.wannier.org). The Wannier90 code is included in the distribution package. Running time: The example runs (in its serial version) in less than 2 min

ELECTRONIC STRUCTURE AND THERMOELECTRIC PROSPECTS OF PHOSPHIDE SKUTTERUDITES

The prospects for high thermoelectric performance in phosphide skutterudites are investigated based on first principles calculations. We find that stoichiometric CoP_3 differs from the corresponding arsenide and antimonide in that it is metallic. As such the band structure must be modified if high thermopowers are to be achieved. In analogy to the antimonides it is expected that this may be done by filling with La. Calculations for LaFe_4P_12 show that a gap can in fact be opened by La filling, but that the valence band is too light to yield reasonable p-type thermopowers at appropriate carrier densities; n-type La filled material may be more favorable.

High-throughput screening of perovskite alloys for piezoelectric performance and thermodynamic stability

We screen a large chemical space of perovskite alloys for systems with optimal properties to accommodate a morphotropic phase boundary (MPB) in their composition-temperature phase diagram, a crucial feature for high piezoelectric performance. We start from alloy end points previously identified in a high-throughput computational search. An interpolation scheme is used to estimate the relative energies between different perovskite distortions for alloy compositions with a minimum of computational effort. Suggested alloys are further screened for thermodynamic stability. The screening identifies alloy systems already known to host an MPB and suggests a few others that may be promising candidates for future experiments. Our method of investigation may be extended to other perovskite systems, e.g., (oxy-)nitrides, and provides a useful methodology for any application of high-throughput screening of isovalent alloy systems.

Role of A-Site and B-Site Ions in Perovskite Ferroelectricity

The interplay between the various patterns of structural instability that are related to the electromechanical coupling in ferroelectric perovskites is discussed using results of density functional based first principles calculations. We focus on two main issues: (1) the competition between tetragonal and rhombohedral ferroelectric states in A-site driven (tolerance factor, t < 1) perovskites, and (2) the competition of ferroelectricity and antiferroelectric octahedral rotation. The role of Pb is reviewed in terms of cross gap hybridization of Pb 6p states with O 2p orbitals, and an alternate mechanism for reducing the tendency towards tilt instabilities is discussed.

Improved predictions of the physical properties of Zn- and Cd-based wide band-gap semiconductors: a validation of the ACBN0 functional

We study the physical properties of Zn

Lattice instabilities in (Pb,Cd)TiO3 alloys

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Prediction of high thermoelectric potential in AMN2 layered nitrides: electronic structure, phonons, and anharmonic effects

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Accurate tight-binding Hamiltonians for two-dimensional and layered materials

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Accurate ab initio tight-binding Hamiltonians: Effective tools for electronic transport and optical spectroscopy from first principles

=O, S, Se, Te) and Cd

Accurate tight-binding Hamiltonian matrices from ab-initio calculations: Minimal basis sets

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