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We have explored electronic and thermoelectric properties of bismuth-baseddouble-perovskite halides Cs2BiAgX6 by using first principles calculations. Thecalculated indirect bandgaps 2.85 eV and 1.99 eV for Cs2BiAgCl6 and Cs2BiAgBr6,respectively well agree with the measured value (2.77 eV of Cs2BiAgCl6 and 2.19eV of Cs2BiAgBr6). We have calculated the relaxation time and lattice thermalconductivity by using relaxation time approximation (RTA) within supercellapproach. The lattice thermal conductivities for both compounds are remarkablylow and the obtained values at 300K for Cs2BiAgCl6 and Cs2BiAgBr6 are 0.078 and0.065 Wm-1K-1, respectively. Such quite low lattice thermal conductivity arisesdue to low phonon group velocity in the large weighted phase space and largephonon scattering. The large Seebeck coefficient obtained for both halides at400K. We have obtained the maximum power factors at 700K and the correspondingthermoelectric figure of merit for Cs2BiAgCl6 and Cs2BiAgBr6 are 0.775 and0.774, respectively. The calculated results reveal that both halides arepotential thermoelectric materials.

Ultra-low lattice thermal conductivity in Cs2BiAgX6 (X=Cl, Br): Potential thermoelectric materials