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BoltzTraP2 is a software package for calculating a smoothed Fourier expression of periodic functions and the Onsager transport coefficients for extended systems using the linearized Boltzmann transport equation. It uses only the band and k-dependent quasiparticle energies, as well as the intra-band optical matrix elements and scattering rates, as input. The code can be used via a command-line interface and/or as a Python module. It is tested and illustrated on a simple parabolic band example as well as silicon. The positive Seebeck coefficient of lithium is reproduced in an example of going beyond the constant relaxation time approximation.

The lattice thermal conductivity (κ ω ) is a key property for many potential applications of compounds. Discovery of materials with very low or high κ ω remains an experimental challenge due to high costs and time-consuming synthesis procedures. High-throughput computational prescreening is a valuable approach for significantly reducing the set of candidate compounds. In this article, we introduce efficient methods for reliably estimating the bulk κ ω for a large number of compounds. The algorithms are based on a combination of machine-learning algorithms, physical insights, and automatic ab initio calculations. We scanned approximately 79,000 half-Heusler entries in the AFLOWLIB.org database. Among the 450 mechanically stable ordered semiconductors identified, we find that κ ω spans more than 2 orders of magnitude-a much larger range than that previously thought. κ ω is lowest for compounds whose elements in equivalent positions have large atomic radii. We then perform a thorough screening of thermodynamical stability that allows us to reduce the list to 75 systems. We then provide a quantitative estimate of κ ω for this selected range of systems. Three semiconductors having κ ω < 5 Wm −1 K −1 are proposed for further experimental study.

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Using ab initio calculations, we have investigated the phonon linewidths and the thermal conductivity (κ) of monolayer MoS2. κ for a typical sample size of 1 μm is 83 W/m K at room temperature in the completely rough edge limit, suggesting κ is not a limiting factor for the electronic application of monolayer MoS2. κ can be further increased by 30% in 10 μm sized samples. Due to strong anharmonicity, isotope enhancement of room temperature κ is only 10% for 1 μm sized samples. However, linewidths can be significantly reduced, for instance, for Raman active modes A1g and E2g1, in isotopically pure samples.

In this theoretical study, we investigate the origins of the very low thermal conductivity of tin selenide (SnSe) using ab-initio calculations. We obtained high-temperature lattice thermal conductivity values that are close to those of amorphous compounds. We also found a strong anisotropy between the three crystallographic axes: one of the in-plane directions conducts heat much more easily than the other. Our results are compatible with most of the experimental literature on SnSe, and differ markedly from the more isotropic values reported by a recent study.The ongoing quest for improved thermoelectric materials has spanned the past six decades. 1,2 Good thermoelectrics are characterized by their high figures of merit ZT = P T /κ. This requires a low thermal conductivity κ, and a high power factor P = σS 2 (where σ is the electrical conductivity, and S the thermopower) across their intended range of working temperatures. Nanotechnology has been able to greatly improve on earlier single-crystal thermoelectric materials 3 via hindering phonon flow while keeping favorable electric properties, even when starting from a poor performer such as bulk silicon. 4 Thus, it was somewhat surprising when single-crystal SnSe recently emerged as the best thermoelectric material ever measured. Experimental measurements on monocrystalline samples 5 point to a record figure of merit of ZT = 2.6 at T = 923 K. This is due mainly to its very low lattice thermal conductivity κ . Intense interest in SnSe was spurred by these results, and to date two independent sets of measurements on polycrystals have been published. 6,7 The two series are compatible with each other, yet they both display higher values of κ than those reported in Ref. 5 for the single crystal. This contrasts with the expectation that grain boundaries should decrease the thermal conductivity below that of the single crystal. Remarkably, preexisting studies of single crystals 8 reported even higher values of κ, with a directional maximum of around 1.8 W m −1 K −1 at room temperature.To address this controversy, we studied phonon transport in SnSe from first principles based on the Boltzmann transport equation (BTE) formalism. Such modeling studies offer substantially more detailed information than is readily available from the experiments, and have demonstrated both their wide range of applicability and their predictive power. 9-13 For the present work, we focused on the Pnma-symmetric phase of SnSe. This is the stable phase from room temperature up to ∼ 750 − 800 K. 5 Hence, it is the pertinent phase for most of the aforementioned measurements. We started with an atomistic description of Pnma SnSe extracted from the AFLOWLIB.org consortium repository [auid=aflow:d188eb9b8df60e58]. 14,15 We relaxed the structure without any geometrical constraint using the density functional theory (DFT) software package VASP. 16 All technical details are presented in the supplementary material. We obtained unit cell lengths of a = 11.72Å, b = 4.20Å and c = 4.55Å, which are in reasonab...

By building physically sound interatomic force constants, we offer evidence of the universal presence of a quadratic phonon branch in all unstrained 2D materials, thus contradicting much of the existing literature. Through a reformulation of the interatomic force constants (IFCs) in terms of internal coordinates, we find that a delicate balance between the IFCs is responsible for this quadraticity. We use this approach to predict the thermal conductivity of Pmmn borophene, which is comparable to that of MoS 2 , and displays a remarkable in-plane anisotropy. These qualities may enable the efficient heat management of borophene devices in potential nanoelectronic applications. IMPACT STATEMENTThe newly found universality of quadratic dispersion will change the way 2D-material phonons are calculated. Predicted results for borophene shall become a fundamental reference for future research on this material. ARTICLE HISTORY

almaBTE is a software package that solves the space-and time-dependent Boltzmann transport equation for phonons, using only abinitio calculated quantities as inputs. The program can predictively tackle phonon transport in bulk crystals and alloys, thin films, superlattices, and multiscale structures with size features in the nm-µm range. Among many other quantities, the program can output thermal conductances and effective thermal conductivities, space-resolved average temperature profiles, and heat-current distributions resolved in frequency and space. Its first-principles character makes almaBTE especially well suited to investigate novel materials and structures. This article gives an overview of the program structure and presents illustrative examples for some of its uses.