Damping of phonon momentum suppresses the lattice thermal conductivity (${\kappa }_{\mathrm{l}}$) through low-energy acoustic-optical phonon interactions. We studied the thermal transport properties and underlying mechanism of phonon interactions in the large unit cell $\mathrm{C}{\mathrm{u}}_{26}{\mathrm{V}}_2\mathrm{S}{\mathrm{n}}_6\mathrm{S}{\mathrm{e}}_{32}$. 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 ($\mathrm{C}{\mathrm{u}}_{26}{\mathrm{V}}_2\mathrm{S}{\mathrm{n}}_6\mathrm{S}{\mathrm{e}}_{30}{\mathrm{S}}_2$) distort the crystal lattice by offering additional scattering mechanism at the anionic sites, thereby increasing the power factor and decreasing the ${\kappa }_{\mathrm{l}}$. This strategic manipulation of phonon scattering towards ultralow ${\kappa }_{\mathrm{l}}$ 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.

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• Acoustic phonons
• Density of states
• Electrical conductivity
• Electrical properties
• Electronic structure
• First-principles calculations
• Optical phonons
• Phonons
• Thermoelectrics
• Raman spectroscopy
• Resistivity measurements