  Damping of phonon momentum suppresses the lattice thermal conductivity (kl)
through low energy acoustic-optical phonon interactions. We studied the thermal
transport properties and underlying mechanism of phonon interactions in the
large unit cell Cu26V2Sn6Se32. The large number of atoms in the unit cell
results in low acoustic phonon cutoff frequency, flat phonon branches, low
frequency Raman active modes, localized rattler-like vibrations and strong
crystalline anharmonicity. The crystal structure complexity disrupts the phonon
propagation through weak bonded Cu atoms, boson peak and poor phonon velocity.
The sulfur at selenium sites (Cu26V2Sn6Se30S2) distort the crystal lattice by
offering additional scattering mechanism at the anionic sites, thereby
increases the power factor and decreases the kl. This strategic manipulation of
phonon scattering towards ultra-low kl not only results in improved
thermoelectric performance but also offers insights into the fundamental
understanding of heat transport in complex structured, large unit cell
compounds.
