High Thermoelectric Performance of Ag9GaSe6 Enabled by Low Cutoff Frequency of Acoustic Phonons

When the number of atoms in a primitive cell (N) increases, the first Brillouin zone shrinks and folds back the high-frequency portion of acoustic vibrations as optical ones with reduced velocities near zone boundaries. Soft bonds lead to flattened phonon dispersion and thus a low sound velocity. This work shows the cutoff frequency of acoustic phonons (ωm) as an important parameter to include both effects on the lattice thermal conductivity...