Peierls-Boltzmann transport equation, coupled with third-order anharmonic lattice dynamics calculations, has been widely used to model lattice thermal conductivity ($kappa_{l}$) in bulk crystals. However, its application to materials with structural phase transition at relatively high temperature is fundamentally challenged by the presence of lattice instabilities (imaginary phonon modes). Additionally, its accuracy suffers from the absence of higher-than-third-order phonon scattering processes, which are important near/above the Debye temperature. In this letter, we present an effective scheme that combines temperature-induced anharmonic phonon renormalization and four-phonon scattering to resolve these two theoretical challenges. We apply this scheme to investigate the lattice dynamics and thermal transport properties of GeTe, which undergoes a second-order ferroelectric phase transition from rhombohedral $alpha$-GeTe to rocksalt $beta$-GeTe at about 700~K. Our results on the high-temperature phase $beta$-GeTe at 800~K confirm the stabilization of $beta$-GeTe by temperature effects. We find that considering only three-phonon scattering leads to significantly overestimated $kappa_{l}$ of 3.8~W/mK at 800~K, whereas including four-phonon scattering reduces $kappa_{l}$ to 1.7~W/mK, a value comparable with experiments. To explore the possibility to further suppress $kappa_{l}$, we show that alloying $beta$-GeTe with heavy cations such as Pb and Bi can effectively reduce $kappa_{l}$ to about 1.0~W/mK, whereas sample size needs to be around 10nm through nanostructuring to achieve a comparable reduction of $kappa_{l}$.
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