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Metal halides have emerged as a new generation of semiconductors with applications ranging from solar cells to chemical sensors. We assess the thermoelectric potential of Cs₃Cu₂I₅, which has a crystal structure formed of zero-dimensional [Cu₂I₅]³⁻ anionic clusters that are separated by Cs⁺ counter cations. We find the compound exhibits the characteristics of a phonon-glass electron-crystal with a large imbalance in the conduction of heat and electrons predicted from first-principles transport theory. Strong anharmonic phonon–phonon scattering results in short-lived acoustic vibrations and an ultra-low lattice thermal conductivity (<0.1 W m⁻¹ K⁻¹). The dispersive conduction band leads to a high electron mobility (>10 cm² V⁻¹s⁻¹). For an n-type crystal at 600 K, a thermoelectric figure-of-merit ZT of 2.6 is found to be accessible, which for a cold source of 300 K corresponds to a thermodynamic heat-to …