Extremely Low Lattice Thermal Conductivity Leading to Superior Thermoelectric Performance in Cu 4 TiSe 4 .

Low thermal conductivity is crucial for obtaining a promising thermoelectric (TE) performance in semiconductors. In this work, the TE properties of Cu 4 TiS 4 and Cu 4 TiSe 4 were theoretically investigated by carrying out first-principles calculations and solving Boltzmann transport equations. The calculated results reveal a lower sound velocity in Cu 4 TiSe 4 compared to that in Cu 4 TiS 4 , which is due to the weaker chemical bonds in the crystal orbital Hamilton population (COHP) and also the larger atomic mass in Cu 4 TiSe 4 . In addition, the strong lattice anharmonicity in Cu 4 TiSe 4 enhances phonon-phonon scattering, which shortens the phonon relaxation time. All of these factors lead to an extremely low lattice thermal conductivity (κ L ) of 0.11 W m -1 K -1 at room temperature in Cu 4 TiSe 4 compared with that of 0.58 W m -1 K -1 in Cu 4 TiS 4 . Owing to the suitable band gaps of Cu 4 TiS 4 and Cu 4 TiSe 4 , they also exhibit great electrical transport properties. As a result, the optimal ZT values for p (n)-type Cu 4 TiSe 4 are up to 2.55 (2.88) and 5.04 (5.68) at 300 and 800 K, respectively. For p (n)-type Cu 4 TiS 4 , due to its low κ L , the ZT can also reach high values over 2 at 800 K. The superior thermoelectric performance in Cu 4 TiSe 4 demonstrates its great potential for applications in thermoelectric conversion.