Abstract
Engineering lattice thermal conductivity requires to control the heat carried by atomic vibration waves, the phonons. The key parameter for quantifying it is the phonon lifetime, limiting the travelling distance, whose determination is however at the limits of instrumental capabilities. Here, we show the achievement of a direct quantitative measurement of phonon lifetimes in a single crystal of the clathrate Ba7.81Ge40.67Au5.33, renowned for its puzzling ‘glass-like’ thermal conductivity. Surprisingly, thermal transport is dominated by acoustic phonons with long lifetimes, travelling over distances of 10 to 100 nm as their wave-vector goes from 0.3 to 0.1 Å−1. Considering only low-energy acoustic phonons, and their observed lifetime, leads to a calculated thermal conductivity very close to the experimental one. Our results challenge the current picture of thermal transport in clathrates, underlining the inability of state-of-the-art simulations to reproduce the experimental data, thus representing a crucial experimental input for theoretical developments.
Highlights
Engineering lattice thermal conductivity requires to control the heat carried by atomic vibration waves, the phonons
As a rule of thumb, low thermal conductivity can be achieved by lowering phonon lifetimes, a strategy which has led to an intensive research activity in the ‘phonon engineering’ of TE materials
Low thermal conductivities have been observed in structurally complex crystals with a large number of atoms in the unit cell (> 50 atoms), such as guest–host structures, where phonon lifetimes result beyond the standard resolution limit
Summary
Engineering lattice thermal conductivity requires to control the heat carried by atomic vibration waves, the phonons. The effect of mass disorder has been calculated only in the case of simple element alloys[11, 13,14,15], such as Si0.5Ge0.5, where the relevant mean free path was found to be 0.1–5 μm Face to these recent computational advances, it is necessary to validate the theoretical predictions by actual measurements of the lifetimes of individual phonon states. Low thermal conductivities have been observed in structurally complex crystals with a large number of atoms in the unit cell (> 50 atoms), such as guest–host structures (clathrates and skutterudites), where phonon lifetimes result beyond the standard resolution limit These materials, known as cage a c
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