Ultralow lattice thermal conductivity in complex structure Cu26V2Sn6Se32 due to interaction of low-frequency acoustic-optical phonons

Damping of phonon momentum suppresses the lattice thermal conductivity (Kl) through low-energy acousticoptical 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 rattlerlike vibrations and strong crystalline anharmonicity. The crystal structure complexity disrupts the phonon propagation through weak bonded Cu atoms, bosonlike 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 increasing the power factor and decreasing the Kl. This strategic manipulation of phonon scattering towards ultralow 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.