Excitations and scattering among the quantized lattice vibrations, i.e., phonons, govern the lattice thermal conductivity (${\kappa }_l$) in crystalline solids. Therefore, effective modulation of ${\kappa }_l$ can be achieved through selective manipulation of phonon modes that strongly participate in the heat transport mechanisms. Here, combining accurate first-principles density-functional theory calculations and Boltzmann transport theory, we report a layered quaternary chalcogenide semiconductor, ${\mathrm{Ba}\mathrm{Cu}\mathrm{Gd}\mathrm{Te}}_3$, which exhibits unusually low ${\kappa }_l$ ($\sim 0.14\mathrm{W}/\mathrm{m}\mathrm{K}$ at room temperature) despite its ordered crystalline structure. Our analysis reveals that the ultralow ${\kappa }_l$ arises mainly from a strong suppression of acoustic phonon modes induced by local distortion, shear vibrations among the layers, and large acousto-optical avoided crossing between phonons, which collectively enhances the phonon scattering rates. Further calculations of the electrical transport properties with explicit consideration of electron-phonon interactions reveal a high thermoelectric figure of merit exceeding unity for this compound at moderate temperature (400–700 K) and carrier concentration $(1–5\times {10}^{19}{\mathrm{cm}}^{-3})$ ranges. Our theoretical predictions warrant experimental investigations of the intriguing phonon dynamics, thermal transport mechanisms, and thermoelectric properties in this compound. Moreover, insights from our analysis can be used to design and engineer compounds with ultralow ${\kappa }_l$.

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• Electrical conductivity
• Electron-phonon coupling
• Lattice thermal conductivity
• Chalcogenides
• Semiconductor compounds
• Thermoelectrics
• Boltzmann theory
• Density functional calculations