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Room-temperature superconductivity has been attracting increasing attention in recent years. Recent studies have proved the potential of compressed H-rich materials for achieving room-temperature superconductivity. In this paper, we study the phononic thermal transport in the rare earth yttrium hydrides allotropes under 0 GPa, 50 GPa, and 300 GPa by using Boltzmann transport equation. We find that the lattice thermal conductivity of yttrium hydrides increases with the pressure among different allotropes, which is attributed to the increase of bond strength and the decrease of phonon-phonon scattering due to structural compression. Yttrium hydrides structure at high pressure of 300 GPa is the superconducting phase, and has high thermal conductivity around 1360 Wm-1K-1 at room temperature. Comparison of phonon properties with existing high thermal conductivity materials further uncovers the origin for the observed high thermal conductivity. For the zero pressure allotrope, a large number of optical flat bands mix with the low-frequency acoustic phonons, which significantly increases the phonon scattering channel and effectively suppresses the phonon lifetime. As for yttrium hydrides allotropes under 50 GPa and 300 GPa, there are two obvious band gaps in the phonon dispersion relation, and the band gap of the structure at 300 GPa is significantly wider. The occurrence of the band gap effectively inhibits the absorption and emission process of the three-phonon interactions, leading to the decrease of phonon scattering and thus the increase of the phonon lifetime and thermal conductivity at high pressure. Our work reveals the physical mechanism of the thermal transport behaviors in yttrium hydrides structures under different pressures.

Energy conversion from waste heat to electric power is a promising approach for energy harvest, and the clathrates crystals have received lots of attentions in this field from the concept of ‘phonon-glass and electron-crystal’. However, the thermal transport mechanisms and roles of rattlers have yet been clearly revealed in clathrates. By using iterative solution of Peierls-Boltzmann transport equation and first principle calculations, we have systematically revisited the thermal transport properties of a simple binary representative of clathrates, Ba8Si46. Our results confirm that the suppressed phonon lifetime is responsible for the huge reduction of lattice thermal conductivity (κ_l) in clathrates, in addition to the decrease of phonon group velocity. Furthermore, we clarify that phonon scatterings in a wide frequency range and the resonant characteristic scatterings coexist in clathrates, due to the emergence of hybridized modes introduced by the rattlers. We also elucidate that the hybridized modes dramatically suppress the acoustic phonon contribution to κ_l, leading to the non-negligible relative contribution from optical phonon to thermal transport in clathrates. Moreover, the impacts of the hybridized modes on different scattering channels in the phase space are also discussed. Our study provides fundamental physical insights into the impacts of rattlers on thermal conductivity of clathrates, which is valuable towards the design of efficient thermoelectric materials based on the concept of ‘phonon-glass and electron-crystal’.