Influence of Sn<sub>Bi</sub> Antisite Defects on the\nElectronic Band Structure and Transport Properties of the Layered\nChalcogenide Semiconductor SnBi<sub>2</sub>Te<sub>4</sub>

Narrow-band\ngap, layered chalcogenide semiconductors\nprovide a\nrich area of research for designing highly efficient thermoelectric\nmaterials for near-room-temperature applications due to their intrinsically\nlow lattice thermal conductivity and multivalley electronic band structure.\nHere, we report on a comprehensive theoretical and experimental investigation\nof the thermoelectric properties of one member of this class, SnBi<sub>2</sub>Te<sub>4</sub>, in the temperature range of 5–700 K.\nPolycrystalline samples crystallize with a rhombohedral crystal structure\ndescribed in the <i>R</i>3̅<i>m</i> space\ngroup (No. 166) with room-temperature lattice parameters <i>a</i> = 4.398(1) and <i>c</i> = 41.615(1) Å. To probe the\nelectronic tunability of SnBi<sub>2</sub>Te<sub>4</sub>, the synthesis\nof a Sn-deficient sample Sn<sub>0.95</sub>Bi<sub>2</sub>Te<sub>4</sub> was attempted but led to an intergrowth of SnBi<sub>2</sub>Te<sub>4</sub> and SnBi<sub>4</sub>Te<sub>7</sub>, indicating that only\na very limited range of Sn-deficiency can exist. Transport property\nmeasurements combined with electronic band structure calculations\nshow that SnBi<sub>2</sub>Te<sub>4</sub> is a p-type, narrow-band\ngap semiconductor with a multivalley valence band structure and a\nstrong asymmetry between the valence and conduction bands. The presence\nof Sn<sub>Bi</sub> antisite defects, supported by Mössbauer\nspectroscopy, plays an important role in shaping the electronic transport\nproperties through a decrease in the electronic band gap and a reduction\nin the hole mobility, which causes the onset of bipolar conduction\nnear 350 K. Some degree of anisotropy in the electrical resistivity\nand thermal conductivity measured parallel and perpendicular to the\npressing direction exists, while the thermopower remains nearly isotropic.\nThe complex crystal structure of SnBi<sub>2</sub>Te<sub>4</sub> contributes\nto the very low lattice thermal conductivity of the order of 0.5 W\nm<sup>–1</sup> K<sup>–1</sup> at 300 K, leading to peak <i>ZT</i> values of 0.32 at 350 and 400 K for Sn<sub>0.95</sub>Bi<sub>2</sub>Te<sub>4</sub> and SnBi<sub>2</sub>Te<sub>4</sub>,\nrespectively.