Materials Today Physics 28 (2022) 100890 Available online 19 October 2022 2542-5293/© 2022 Elsevier Ltd. All rights reserved. Role of alloying in the phonon and thermal transport of SnS–SnSe across the phase transition ☆ Niuchang Ouyang a , Chen Wang a , Yue Chen a, b, * a Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China b HKU Zhejiang Institute of Research and Innovation, 1623 Dayuan Road, Lin An, 311305, China A R T I C L E INFO Keywords: Phase transition Alloying Lattice thermal conductivity Disordering ABSTRACT Atomic-level investigations of the underlying phonon and thermal transports of strongly anharmonic alloy-based thermoelectric materials and their structural phase transitions are yet to be fully explored. Herein, we system- atically investigate the anharmonic lattice dynamics and thermal transport of SnS 0.75 Se 0.25 using perturbation theory up to quartic anharmonicity and molecular dynamics simulations with the first-principles-based machine- learning potential. We find the non-monotonic temperature dependence of the phonon linewidths and fre- quencies of the Γ 4 and Y 1 modes. This work demonstrates an apparent κ L reduction from SnS to SnS 0.75 Se 0.25, mainly attributed to the enhanced scattering rates of the middle-frequency phonons and the decreased group velocities of the high-frequency phonons. We also find that the effects of the quartic anharmonicity on the thermal transport of SnS and SnS 0.75 Se 0.25 are significant, and the phonon coherence contributions are non- negligible in describing the thermal transport. Moreover, we reveal a decrease of κ L in SnS 0.75 Se 0.25 by randomizing Se atoms, which can be ascribed to an additional phonon scattering arising from sublattice mass disorder. 1. Introduction Atomic-level understanding of the thermal conduction in crystalline materials with low lattice thermal conductivity κ L plays a vital role in various science and technology fields, including thermal management [1,2] and thermoelectricity [3,4]. In particular, a high figure of merit (zT) value is achieved by reducing κ L for thermoelectric materials [5]. Although many experimental results [6–8] demonstrate that κ L can be significantly reduced by alloying, the underlying thermal transport mechanism is still not fully revealed due to the lack of suitable phonon-based description. The validity of the phonon quasiparticle picture adopted in the conventional phonon-gas model (PGM) is controversial in complex alloying systems with a structural phase tran- sition mainly due to the presence of lattice instabilities and high-order anharmonicity. Recently, SnSe is considered as a promising thermo- electric material [9–11] with zT ≈ 2.6 at 923 K due to its intrinsic low κ L . Despite sharing similar chemical compositions, the binary chalcogenide SnS, an analogue compound of SnSe, has only demonstrated zT ≈ 0.8 at 875 K [6] due to a relatively high κ L . SnS 0.91 Se 0.09 alloy was shown by He et al. [6] to have a suppressed thermal conductivity, whereas, the thermal transport mechanism of Se-alloyed SnS is yet to be fully inves- tigated. Therefore, it is of great importance to study the Se-alloyed ef- fects on the phonon properties of SnS to better understand thermal transport. In addition, it is challenging to deal with the strong anhar- monicity across the structural phase transition of Se-alloyed SnS within the perturbative framework, which prevents the extraction of the temperature-dependent phonon properties. Thus, a thorough investi- gation of the temperature-dependent phonon spectra with a non- perturbative treatment of the anharmonic effects is of great importance to better understand the phonon transport of Se-alloyed SnS across the phase transition. In recent years, theoretical calculations have demonstrated that temperature-dependent phonon frequency renormalization [12] and high-order phonon scattering [13–15] play indispensable roles in κ L reduction for strongly anharmonic materials. Although the thermal transport properties of SnS [16,17] have been calculated with the perturbation theory (PT) up to the third order by solving the Boltzmann transport equation (BTE) [18], the role of fourth-order anharmonicity in suppressing κ L of SnS and Se-alloyed SnS is not yet ascertained. More- over, besides the particle-like contributions, the coherence effects ☆ N.O. and C.W. contributed equally to this work. * Corresponding author. Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China. E-mail address: yuechen@hku.hk (Y. Chen). Contents lists available at ScienceDirect Materials Today Physics journal homepage: www.journals.elsevier.com/materials-today-physics https://doi.org/10.1016/j.mtphys.2022.100890 Received 5 September 2022; Received in revised form 9 October 2022; Accepted 15 October 2022