Topological insulators generally share commonalities with good thermoelectric (TE) materials because of their narrow band gaps and heavy constituent elements. Here, we propose that a topological crystalline insulator (TCI) could exhibit a high TE performance by breaking its crystalline symmetry and tuning the chemical potential by elemental doping. As a candidate material, we investigate the TE properties of the Cl-doped TCI Pb0.7Sn0.3Se. The infrared absorption spectra reveal that the band gap is increased from 0.055 eV for Pb0.7Sn0.3Se to 0.075 eV for Pb0.7Sn0.3Se0.99Cl0.01, confirming that the Cl doping can break the crystalline mirror symmetry of a TCI Pb0.7Sn0.3Se and thereby enlarge its bulk electronic band gap. The topological band inversion is confirmed by the extended X-ray absorption fine structure spectroscopy, which shows that the TCI state is weakened in a chlorine x = 0.05-doped compound. The small gap opening and partial linear band dispersion with massless and massive bands may have a high power factor (PF) for high electrical conductivity with an enhancement of the Seebeck coefficient. As a result, Pb0.7Sn0.3Se0.99Cl0.01 shows a considerably enhanced ZT of 0.64 at 823 K, which is about 1200% enhancement in ZT compared with that of the undoped Pb0.7Sn0.3Se. This work demonstrates that the optimal n-type Cl doping tunes the chemical potential together with breaking the state of the TCI, suppresses the bipolar conduction at high temperatures, and thereby enables the Seebeck coefficient to increase up to 823 K, resulting in a significantly enhanced PF at high temperatures. In addition, the bipolar contribution to thermal conductivity is effectively suppressed for the Cl-doped samples of Pb0.7Sn0.3Se1- xCl x ( x ≥ 0.01). We propose that breaking the crystalline mirror symmetry in TCIs could be a new research direction for exploring high-performance TE materials.
Keywords: chemical potential; power factor; thermoelectric; topological crystal insulator; topological semimetal.