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.