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 (κ_L) 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 κ_L, 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.