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Metal chalcohalides, owing to their higher stability over halides and greater tunability of electronic features over chalcogenides, open new avenues for investigating properties of materials. Complex metal chalcohalides can be a good choice for thermoelectric studies for their halide-like low thermal conductivity and chalcogenide-like high electrical conductivity. Here, we have investigated the thermoelectric properties of n -type Bi 13 S 18 Br 2 and utilized the concept of Fajans’ polarization to describe the formation of a dimer Bi 2 4 + and explained how it can help achieve high thermoelectric figure of merit (zT) of ∼1.0 at 748 K. This zT value is so far the highest-reported value for pristine metal chalcohalides. The existence of Bi 2 4 + subunit in Bi 13 S 18 Br 2 is experimentally verified by synchrotron X-ray pair distribution function (X-PDF) analysis. The complex structure of Bi 13 S 18 Br 2 having a large unit cell exhibits simultaneous dimer-cation rattler (i.e., “twin-rattler”), which decreases the lattice thermal conductivity drastically. We observed evidence of such low-energy rattling vibrations from DFT-calculated eigen mode visualizations of the phonon dispersion. The subvalent nature of Bi 2 4 + accommodates an extra electron in Bi ( 6p z ) orbital, which helps form a weakly dispersed donor band just below the Fermi energy ( E F ), leading to a significant reduction in band gap (0.77 eV), which is favorable for high thermoelectric performance. Consequently, we obtained a semiconducting nature of n -type Bi 13 S 18 Br 2 with moderate electrical conductivity, as well as a high Seebeck coefficient. Our investigation presents the importance of fundamental chemistry in thermoelectrics and demonstrates the influence of subvalent twin-rattler in triggering high thermoelectric performance in metal chalcohalides.
Das et al. (Fri,) studied this question.