First-principles investigations on the thermal transport and thermoelectric properties of anti-perovskite M3OI and M4OI2 (M = K, Rb)

The thermal transport and thermoelectric properties of anti-perovskite M 3 IO and M 4 I 2 O (M = K, Rb) were investigated using first-principles calculations combined with solution of the Boltzmann transport equation. The two-phonon scattering channel was also considered. These structures formed M 6 O octahedra, accompanied by a rattling motion of the O atoms. They exhibit ultra-low lattice thermal conductivity, ranging from 0.30 to 0.89 W m -1 K −1 at room temperature. M 4 I 2 O demonstrates strong anisotropic thermal transport due to weaker bonding interactions along the zz direction, while M 3 IO shows isotropic thermal conductivity. Specifically, Rb 4 OI 2 has the lowest lattice thermal conductivity of 0.47 W m -1 K −1 along the xx direction and 0.30 W m -1 K −1 along the zz direction. Additionally, M 3 IO possesses low lattice thermal conductivity of 0.52 W m -1 K −1 , attributed to the softening behavior of the TA branch at the M and R points. The electronic structure of M 3 IO and M 4 OI 2 reveals a multi-valley phenomenon in the valence band, resulting in a large Seebeck coefficient under p-type doping. Our results indicate maximum thermoelectric figure of merit (ZT) values of 1.91 for p-type Rb 3 OI, and 1.41 for p-type Rb 4 OI 2 along the zz direction at 900 K. Rb 3 OI and Rb 4 OI 2 were proposed as potential p-type thermoelectric materials.