Physical insights on the ultralow thermal conductivity of Ag8XSe6 (X = Si, Ge, and Sn)

We have investigated the origins of ultralow thermal conductivity behavior in Ag8XSe6 (X = Si, Ge, and Sn) argyrodites compounds in orthorhombic Pmn21 structure by exploring mechanical and lattice dynamical properties using first principles calculations. The mechanical and dynamical stability were evaluated by means of elastic constants and phonon spectra. The present soft mechanical properties indicate the low Debye temperature, sound velocity, large Grüneisen parameter, and strong anharmonicity. From the phonon dispersion, it is clear that the acoustic and low optical modes are overlapped largely. This work demonstrates that Ag8SiSe6, Ag8GeSe6, Ag8SnSe6 have ultralow thermal conductivity. They are promising potential thermoelectric materials with the softening of the transverse acoustic (TA) phonon modes caused by the heavy element (Ag).