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.

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 BC 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 massdifference model of impurity scattering, according to which the effects of BC and NC would be similar and much smaller than that of the C vacancy. The strikingly different behavior of the BC defect arises from a unique pattern of resonant phonon scattering caused by the broken structural symmetry around the B impurity.Silicon carbide (SiC) plays a fundamental role in many emerging technologies, ranging from biomedical sensors to optoelectronics, power electronics and photovoltaics [1][2][3][4][5][6][7][8][9]. Most notably, this material has been termed the "linchpin to green energy" that may replace Si-based technology in power electronics [1], owing partly to its large lattice thermal conductivity (κ). From the many stable polytypes of SiC [10], two of the hexagonal ones, 6H-SiC and 4H-SiC, have been extensively studied and widely used [10][11][12]. In contrast, the structurally less complex cubic polytype of SiC with zinc-blende structure (3C-SiC) is much less well understood, despite presumably having the best electronic properties [13], and, as we will see, possibly a higher κ than the other polytypes. This is partly due to the difficulty in synthesizing high quality crystals, although recent improvements in 3C-SiC growth techniques have prompted a renewed interest in it [13].Surprisingly, the reference measurements of κ on pure undoped 3C-SiC are over 20 years old and little detail is known about the quality of the samples [10,14]. The reference value of κ for 3C phase is perplexingly lower than that for the structurally more complex 6H phase, raising doubts about whether this is truly an intrinsic property or just a consequence of the defective, polycrystalline quality of the 3C-SiC samples. It is then clear that to understand the conduction properties of 3C-SiC, and to harness its full potential, one must first comprehend the way defects affect it. As we show here, by comparing predictive ab-initio calculations with experiments on defective samples, a richer physical picture emerges, unveiling the striking differences in the way different dopants affect κ. This also indirectly suggests that the intrinsic κ of defect-free 3C-SiC should be much higher than previously reported and surpass that of the 6H phase.In this paper, we compare our results to the κ(T ) curves for doped samples of 3C-SiC [15]. We use an abinitio approach to quantify the phonon scattering rates of N C substitutional defects. The predicted κ is in excellent agreement with the experimental results. This then allows us to explain the effect of codoping with N and B, and shows that B impurities scatter phonons two orders of magnitude more strongly overall...

Coupling of the Peierls-Boltzmann equation with density functional theory paved the way for predictive thermal materials discovery and a variety of new physical insights into vibrational transport behaviors. Rapid theoretical and numerical developments have generated a wealth of thermal conductivity data and understanding of a wide variety of materials—1D, 2D, and bulk—for thermoelectric and thermal management applications. Nonetheless, modern ab initio descriptions of phonon thermal transport face challenges regarding the effects of defects, disorder, structural complexity, strong anharmonicity, quasiparticle couplings, and time and spatially varying perturbations. Highlighting recent research on these issues, this perspective explores opportunities to expand current ab initio phonon transport techniques beyond the paradigm of weakly perturbed crystals, to the wider variety of materials possible. Recent developments in phonon-defect interactions, complexity, disorder and anharmonicity, hydrodynamic transport, and the rising roles of molecular dynamics simulations, high throughput, and machine learning tools are included in this perspective. As more sophisticated theoretical and computational methods continue to advance thermal transport predictions, novel vibrational physics and thermally functional materials will be discovered for improved energy technologies.