Bismuth-based ternary chalcogenides Pt3Bi4X9 (X = S, Se) as promising thermoelectric materials

We present a theoretical investigation of thermoelectric transport properties of bismuth-based ternary chalcogenides Pt3Bi4X9 (X = S, Se), which has low thermal conductivity and promising zT as discovered in a recent experiment [Wang et al., J. Am. Chem. Soc. 146, 7352 (2024)], using density functional theory combined with the Boltzmann transport equation within rigid band approximation. We find that the high density of states of valence bands near the Fermi level yields high p-type Seebeck coefficient. The lower effective mass of electron in Pt3Bi4S9 leads to high mobility and long relaxation time, and hence the high n-type electrical conductivity. In contrast, the effective mass of electron is much higher than that of hole in Pt3Bi4Se9 due to the flatted conduction band, which in turn gives rise to higher p-type electrical conductivity. As a result, the p-type zT is much higher than n-type in Pt3Bi4Se9, with an optimal value of 0.5 at 300 K. Considering the experimental carrier concentration for Pt3Bi4S9 (−1.4 × 1019 cm−3) and Pt3Bi4Se9 (−0.898 × 1019 cm−3), calculated n-type zT at 773 K are 0.52 and 0.04, respectively, which are consistent well with the experimental values. Our calculations uncover the upper limit thermoelectric zT of Pt3Bi4X9 and also highlight them as promising thermoelectric materials.