Strain-Induced Medium-Temperature Thermoelectric Performance of Cu4TiSe4 : The Role of Four-Phonon Scattering

Motivated by the synthesized ${\mathrm{Cu}}_{4}{\mathrm{Ti}\mathrm{Se}}_{4}$ with ultralow room-temperature thermal conductivity [Angew. Chem., Int. Ed. 60, 9106 (2021)], we systematically investigate its thermoelectric (TE) properties via combining Boltzmann transport equations and first-principles calculations. The results show that the thermal conductivity of ${\mathrm{Cu}}_{4}{\mathrm{Ti}\mathrm{Se}}_{4}$ determined considering only three-phonon scattering is reduced by about 40% after including four-phonon scattering at room temperature, indicating the importance of four-phonon scattering in phonon transport. On the other hand, the coexistence of high dispersion and valley degeneracy at the top valence band in the electronic structure causes a high power factor. Consequently, the isotropic figure of merit (ZT) value of 1.5 at 500 K is captured in the p-type doped ${\mathrm{Cu}}_{4}{\mathrm{Ti}\mathrm{Se}}_{4}$. In addition, the value of ZT can be further enhanced to 2.2 by applying 2.25% triaxial tensile strain, which is ascribed to the remarkably enhanced four-phonon scattering processes induced by the tensile strain. Meanwhile, the significant suppression of thermal conductivity allows the optimal carrier concentration for the ZT peak to be reduced, which is of important practical significance for the experimental preparation of ${\mathrm{Cu}}_{4}{\mathrm{Ti}\mathrm{Se}}_{4}$-based TE devices. Our results pave a way for the design of adjustable medium-temperature TE devices.