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The lattice thermal conductivity of CsX (X = Cl, Br, and I) and its pressure dependence are investigated using first-principles third-order anharmonic force constants. Contrary to the expectation that compounds with heavier atoms usually exhibit lower lattice thermal conductivity (

We present our latest advancements of machine-learned potentials (MLPs) based on the neuroevolution potential (NEP) framework introduced in [Fan et al., Phys. Rev. B 104, 104309 (2021)] and their implementation in the open-source package GPUMD. We increase the accuracy of NEP models both by improving the radial functions in the atomic-environment descriptor using a linear combination of Chebyshev basis functions and by extending the angular descriptor with some four-body and five-body contributions as in the atomic cluster expansion approach. We also detail our efficient implementation of the NEP approach in graphics processing units as well as our workflow for the construction of NEP models, and we demonstrate their application in large-scale atomistic simulations. By comparing to state-of-the-art MLPs, we show that the NEP approach not only achieves above-average accuracy but also is far more computationally efficient. These results demonstrate that the GPUMD package is a promising tool for solving challenging problems requiring highly accurate, large-scale atomistic simulations. To enable the construction of MLPs using a minimal training set, we propose an active-learning scheme based on the latent space of a pre-trained NEP model. Finally, we introduce three separate Python packages, GPYUMD, CALORINE, and PYNEP, which enable the integration of GPUMD into Python workflows., 29 pages, 15 figures, code and data available

GeTe that exhibits a strong anharmonicity and a ferroelectric phase transition between the rhombohedral and cubic structures has emerged as one of the leading thermoelectric materials. Herein, combining molecular dynamics simulations and inelastic neutron scattering measurements, the lattice dynamics in GeTe have been investigated to reveal the soft-mode mechanisms across the phase transition. We have constructed a first-principles-based machine-learning interatomic potential, which successfully captures the dynamical ferroelectric phase transition of GeTe by adopting the neural network technique. Although the low-energy acoustic phonons remain relatively unaffected at elevated temperatures, the high-energy optical, and longitudinal acoustic phonons demonstrate strong renormalizations as evidenced from the vibrational phonon spectra, which are attributed to the large anharmonicity accompanying the phase transition. Furthermore, our results reveal a nonmonotonic temperature dependence of the soft-modes beyond the perturbative regime. The insight provided by this work into the soft-modes may pave the way for further phonon engineering of GeTe and the related thermoelectrics.

Toward high-performance thermoelectric energy conversion, the electrons and holes must work jointly like two wheels of a cart: if not longitudinally, then transversely. The bipolar effect — the main performance restriction in the traditional longitudinal thermoelectricity, can be manipulated to be a performance enhancer in the transverse thermoelectricity. Here, we demonstrate this idea in semimetal Mg2Pb. At 30 K, a giant transverse thermoelectric power factor as high as 400 μWcm−1K−2 is achieved, a 3 orders-of-magnitude enhancement than the longitudinal configuration. The resultant specific heat pumping power is ~ 1 Wg−1, higher than those of existing techniques at 10~100 K. A large number of semimetals and narrow-gap semiconductors making poor longitudinal thermoelectrics due to severe bipolar effect are thus revived to fill the conspicuous gap of thermoelectric materials for solid-state applications. Heat pumping is in high demand at cryogenic temperature, but whether thermoelectricity can take on cryogenic heat pumping is an open question. Here, the authors answer this question by leveraging bipolar effect to enhance transverse thermoelectricity in semimetal Mg2Pb for cryogenic heat pumping.

Funding Information: This work is supported by the National Natural Science Foundation of China (11874313), the Zhejiang Provincial Natural Science Foundation (LR19A040001), the Research Grants Council of Hong Kong (17201019 and 17300018), and the National Key Research and Development Program of China (2019YFA0209904). Y.X. and C.W. acknowledge financial supports from the Department of Energy, Office of Science, Basic Energy Sciences under Grant DE-SC0014520 (theory of anharmonic phonons). The authors are grateful for the research computing facilities offered by ITS, HKU. Publisher Copyright: © 2021 American Physical Society. We study the role of nonperturbative phonon scattering in strongly anharmonic materials having ultralow lattice thermal conductivity with unusual temperature dependence. We take Tl3VSe4 as an example and investigate its lattice dynamics using perturbation theory (PT) up to the fourth order and molecular dynamics (MD) with a machine-learning potential. We find distinct differences of phonon linewidth between PT and MD in the whole Brillouin zone. The comparison between the theoretical phonon linewidths and experiments suggests that PT severely underestimates the phonon scatterings, even when the fourth-order anharmonicity is included. Moreover, we extend our calculations to higher temperatures and evaluate the two-channel thermal conductivity based on the unified theory developed by Simoncelli et al. [Nat. Phys. 15, 809 (2019)1745-247310.1038/s41567-019-0520-x]. We find a crucial coherence contribution to the total thermal conductivity at high temperatures. Our results pave the path for future studies of phonon properties and lattice thermal conductivities of strongly anharmonic crystals beyond the conventional PT realm.

In‐grain dislocation‐induced lattice strain fluctuations are recently revealed as an effective avenue for minimizing the lattice thermal conductivity. This effect could be integratable with electronic enhancements such as by band convergence, for a great advancement in thermoelectric performance. This motivates the current work to focus on the thermoelectric enhancements of p‐type PbTe alloys, where monotelluride‐alloying and Na‐doping are used for a simultaneous manipulation on both dislocation and band structures. As confirmed by synchrotron X‐ray diffractions and Raman measurements, the resultant dense in‐grain dislocations induce lattice strain fluctuations for broadening the phonon dispersion, leading to an exceptionally low lattice thermal conductivity of ≈0. 4 W m‐K−1. Band structure calculations reveal the convergence of valence bands due to monotelluride‐alloying. Eventually, the integration of both electronic and thermal improvements lead to a realization of an extraordinary figure of merit zT of ≈2.5 in Na0.03Eu0.03Cd0.03Pb0.91Te alloy at 850 K., Simultaneous manipulation of in‐grain dislocations for lattice thermal conductivity reduction and band convergence for electronic enhancement leads to a realization of an extraordinary thermoelectric figure of merit in p‐PbTe alloys.

Utilizing molecular dynamics (MD) simulations based on the highly precise force-fields, we find that phonon scattering strengths induced by the cubic anharmonicity can be significantly underestimated by the perturbation theory (PT) approach in materials with sizable frequency gaps or branch bunching. We trace this result to the additional three-phonon scatterings in MD enabled by the fluctuating phonon energy and the continuous energy exchange between modes. These channels are essential to accurately evaluate the zone-center phonon linewidth in boron arsenide as compared to the experiment and could noticeably lower the lattice thermal conductivity of beryllium telluride and tungsten carbide. Accordingly, due to the stronger three-phonon scatterings, four-phonon scatterings would become less important than previously believed in this type of material. Moreover, our work emphasizes the different phonon scattering processes in MD and PT simulations, offering new insights for an improved description of anharmonic properties.

Understanding the ultralow and glass-like lattice thermal conductivity (κ) of complex thermoelectric (TE) materials is challenging due to the intrinsic complexity that bridges glasses and crystals. Herein, we study the ultralow and glass-like κ of BaAg2Te2, a promising high-performance thermoelectric material with complex crystal structure. With a first-principles-based machine-learning potential, we thoroughly investigate the thermal transport mechanisms of BaAg2Te2 using the perturbation theory up to the fourth-order anharmonicity and molecular dynamics simulations. We find the strong four-phonon processes are crucial to determine the ultralow κ. We demonstrate the breakdown of conventional Peierls-Boltzmann theory and the dominance of diffusive thermal transport in BaAg2Te2. This two channel thermal transport behavior directly unearths the nature of the nearly temperature-independent κ of BaAg2Te2. The origin of the dominant diffusive thermal transport is further revealed using the unified theory. Our work paves an avenue to better understand the ultralow and glass-like κ of complex crystals.

SnS that exhibits strong lattice anharmonicity and a structural phase transition between the Pnma and Cmcm phases has emerged as a high-performance thermoelectric material. Herein, the lattice dynamics of SnS have been investigated by molecular dynamics to reveal the soft mode mechanisms across the phase transition. We construct a first-principles-based machine-learning potential, which is capable of reproducing the dynamical nature of the structural phase transition of SnS. We reproduce an explicit softening of the zone-center phonon mode and unveil a similar behavior at the zone boundary U = (0.5, 0.0, 0.5) of SnS, which are attributed to the large anharmonicity induced by the phase transition. Our results reveal a non-monotonic temperature dependence of the linewidth of the soft modes beyond the perturbative regime. The zone-center optical soft mode collapses across the structural phase transition, corresponding to a second-order nature. These results pave the way for further phonon engineering of SnS and the related IV–VI thermoelectrics.

Two-dimensional (2D) InSe, which exhibits high electron mobility and a wide band gap has emerged as a promising material for photoelectric and thermoelectric applications. The inadequate understanding of the lattice thermal conductivity (κ), however, hampers the advancement of 2D InSe. Herein, by taking into account anharmonicity up to the fourth order and introducing the equibiaxial tensile strain (ϵ), we have performed an in-depth study on the lattice dynamics of 2D InSe. Interestingly, the κ exhibits a non-monotonic behaviour as a function of equibiaxial tensile strain, which can be attributed to the changes in acoustic phonon lifetimes. At the Γ point, a blue-shift of the lowest optical mode and a red-shift of the uppermost optical mode are reported for the first time. More strikingly, the blue-shift can be largely suppressed by equibiaxial tensile strain. Further study indicates that the unique transition of the potential energy surface is responsible for the disappearance of the blue-shift. Our work may enlighten the future research on phonon engineering and management of the lattice thermal conductivity of 2D InSe.