Extremely Low Lattice Thermal Conductivity Leading to Superior Thermoelectric Performance in Cu4TiSe4

Low thermal conductivity is crucial for obtaining a promising thermoelectric (TE) performance in semiconductors. In this work, the TE properties of Cu4TiS4 and Cu4TiSe4 were theoretically investigated by carrying out first-principles calculations and solving Boltzmann transport equations. The calculated results reveal a lower sound velocity in Cu4TiSe4 compared to that in Cu4TiS4, which is due to the weaker chemical bonds in the crystal orbital Hamilton population (COHP) and also the larger atomic mass in Cu4TiSe4. In addition, the strong lattice anharmonicity in Cu4TiSe4 enhances phonon-phonon scattering, which shortens the phonon relaxation time. All of these factors lead to an extremely low lattice thermal conductivity (κL) of 0.11 W m-1 K-1 at room temperature in Cu4TiSe4 compared with that of 0.58 W m-1 K-1 in Cu4TiS4. Owing to the suitable band gaps of Cu4TiS4 and Cu4TiSe4, they also exhibit great electrical transport properties. As a result, the optimal ZT values for p (n)-type Cu4TiSe4 are up to 2.55 (2.88) and 5.04 (5.68) at 300 and 800 K, respectively. For p (n)-type Cu4TiS4, due to its low κL, the ZT can also reach high values over 2 at 800 K. The superior thermoelectric performance in Cu4TiSe4 demonstrates its great potential for applications in thermoelectric conversion.