Influence of Subvalent\nTwin-Rattler for High <i>n</i>‑Type Thermoelectric\nPerformance in Bi<sub>13</sub>S<sub>18</sub>Br<sub>2</sub> Chalcohalide

Metal\nchalcohalides, owing to their higher stability\nover halides\nand greater tunability of electronic features over chalcogenides,\nopen new avenues for investigating properties of materials. Complex\nmetal chalcohalides can be a good choice for thermoelectric studies\nfor their halide-like low thermal conductivity and chalcogenide-like\nhigh electrical conductivity. Here, we have investigated the thermoelectric\nproperties of <i>n</i>-type Bi<sub>13</sub>S<sub>18</sub>Br<sub>2</sub> and utilized the concept of Fajans’ polarization\nto describe the formation of a dimer Bi24+ and explained how\nit can help achieve high\nthermoelectric figure of merit (zT) of ∼1.0 at 748 K. This\nzT value is so far the highest-reported value for pristine metal chalcohalides.\nThe existence of Bi24+ subunit\nin Bi<sub>13</sub>S<sub>18</sub>Br<sub>2</sub> is experimentally verified\nby synchrotron X-ray pair\ndistribution function (X-PDF) analysis. The complex structure of Bi<sub>13</sub>S<sub>18</sub>Br<sub>2</sub> having a large unit cell exhibits\nsimultaneous dimer-cation rattler (i.e., “twin-rattler”),\nwhich decreases the lattice thermal conductivity drastically. We observed\nevidence of such low-energy rattling vibrations from DFT-calculated\neigen mode visualizations of the phonon dispersion. The subvalent\nnature of Bi24+accommodates\nan extra electron in <i>Bi</i>(<i>6p</i><sub><i>z</i></sub>) orbital,\nwhich helps form a weakly dispersed donor band just below the Fermi\nenergy (<i>E</i><sub>F</sub>), leading to a significant\nreduction in band gap (0.77 eV), which is favorable for high thermoelectric\nperformance. Consequently, we obtained a semiconducting nature of <i>n</i>-type Bi<sub>13</sub>S<sub>18</sub>Br<sub>2</sub> with\nmoderate electrical conductivity, as well as a high Seebeck coefficient.\nOur investigation presents the importance of fundamental chemistry\nin thermoelectrics and demonstrates the influence of subvalent twin-rattler\nin triggering high thermoelectric performance in metal chalcohalides.