Heat Transport in Clathrate Hydrates Controlled by Guest Frequency and Host-Guest Interaction.

The underlying mechanism of common limited lattice thermal conductivity (κ) in energy-related host-guest crystalline compounds has been an ongoing topic in recent decades. Here, the guest-triggered intrinsic ultralow κ of the representative xenon clathrate hydrate was investigated using the time domain thermoreflectance technique and theoretical calculations. The localized guest modes were observed to hybridize with acoustic branches and severely limit the acoustic κ contribution. Besides, the strong mode coupling enables the reshaping of the overall lattice dynamics, especially for optical branches. More importantly, we identified that guest fillers prompt great phonon scattering in wide frequencies, which originates from both the guest-frequency-controlled enhancement of phase space and the host-guest-interaction-governed lattice anharmonicity. The extremely low guest frequency and strong host-guest interaction and coupling were thereby underlined to play vital but distinct roles in κ minimization. Our results unveil the dominant factors of guest reduction effects and facilitate the design of efficient thermoelectric or other thermal-related materials.