First-principles study of the vibrational characteristics of the heavy element substitution on Cu₃SbSe₃

Understanding the thermal transport in solids has been an emerging topic since its importance in many fields, such as thermoelectrics. Some typical thermoelectric compounds which has unique vibrational characteristics have been investigated, such as Cu₃SbSe₃. Experiments have shown that Cu₃SbSe₃ exhibits abnormally low and nearly temperature-independent lattice thermal conductivity. In this work, we theoretically studied the vibrational characteristics of Ag₃SbSe₃, Cu₃BiSe₃ and Cu₃SbTe₃, which are the substitution of prototype Cu₃SbSe₃ with heavier elements on each atomic sites. These compounds all exhibit low bulk moduli and group velocities; Ag₃SbSe₃ and Cu₃BiSe₃ show larger atomic displacement parameters than Cu₃SbSe₃ and thus lower Debye temperatures. Both results lead to abnormally low lattice thermal conductivities of these compounds. Furthermore, heavier element substitutions Cu₃BiSe₃ and Cu₃SbTe₃ reduce the band gaps, which are expected to be beneficial to higher electrical conductivities than that of Cu₃SbSe₃. Therefore, due to the lower lattice thermal conductivities and favourable electrical transport properties, Ag₃SbSe₃ and Cu₃BiSe₃ are promising thermoelectric candidates.