Chemical Bonding Engineering-Tailored Extremely Low Lattice Thermal Conductivity in SnI2-Doped Ag8SnSe6 Realizing High Thermoelectric Performance

Abstract

As a typical argyrodite compound, Ag₈SnSe₆ is a very promising candidate for thermoelectric materials due to its inherently ultralow lattice thermal conductivity. However, the main obstacle to further improving its thermoelectric properties is its low intrinsic carrier concentration. Appropriate doping may increase the carrier concentration, but the electronic thermal conductivity will also increase simultaneously, which partially compensates for the optimized power factor. This is particularly prominent for materials with ultralow lattice thermal conductivity. In this study, SnI₂ was introduced into Ag₈SnSe₆, which provides more carriers and thus successfully improves the electrical properties. The maximum power factor at 748 K is enhanced from 4.63 μW cm^-1 K^-2 to 5.83 μW cm^-1 K^-2 after adding 3 at. % of SnI₂. Meanwhile, the substitution of I for Se weakens the chemical bonding strength, resulting in a lowered sound velocity. In addition, doping of I at the Se site also enhances the bond asymmetry, which causes a stronger lattice anharmonicity. As a result, an extremely low lattice thermal conductivity of 0.058 W m^-1 K^-1 is achieved at 748 K. Eventually, a high zT value of 1.36 was obtained through the above synergistic optimization, which is a 39% improvement compared to the pristine sample (0.98). Our study demonstrates that chemical bonding engineering is an effective way to further suppress the lattice thermal conductivity of Ag₈SnSe₆ and also provides guidance for improving the thermoelectric properties of other argyrodite compounds.

Keywords: Ag8SnSe6; argyrodite compounds; bond asymmetry; lattice thermal conductivity; sound velocity; thermoelectric performance.