Origin of ultra-low lattice thermal conductivity in Cs2BiAgX6 (X=Cl, Br) and its impact on thermoelectric performance

We have explored electronic and thermoelectric properties of bismuth-based double-perovskite halides Cs2BiAgX6 (X = Cl, Br) by using first principles calculations. The calculated indirect bandgaps 2.85 eV and 1.99 eV for Cs2BiAgCl6 and Cs2BiAgBr6, respectively well agree with the measured values (2.77 eV of Cs2BiAgCl6 and 2.19 eV of Cs2BiAgBr6). We have calculated the relaxation time and lattice thermal conductivity by using relaxation time approximation (RTA) within the supercell approach. The lattice thermal conductivities for both compounds are remarkably low and the obtained values at 300 K for Cs2BiAgCl6 and Cs2BiAgBr6 are 0.078 and 0.065 Wm-1K-1, respectively. Such quite low lattice thermal conductivity arises due to low phonon group velocity in the large weighted phase space and large phonon scattering. The large Seebeck coefficient obtained for both halides at 400 K. We have obtained the maximum power factors at 700 K and the corresponding thermoelectric figure of merit for Cs2BiAgCl6 and Cs2BiAgBr6 are 0.775 and 0.774, respectively. The calculated results reveal that both halides are potential thermoelectric materials.