Rattling-Induced\nUltralow Lattice Thermal Conductivity\nLeads to High Thermoelectric Performance in GaAgSnSe<sub>4</sub> and\nInAgGeSe<sub>4</sub>

The thermal and electronic transport properties of Ag-based\nquaternary\ncompounds, GaAgSnSe<sub>4</sub> and InAgGeSe<sub>4</sub>, have been\nexplored by using density functional theory and the Boltzmann transport\nequation. Both the compounds exhibit ultralow lattice thermal conductivities\n(κ<sub>l</sub>) that originate from the anharmonicity induced\nby the rattling effects of the loosely bound Ag atoms in their crystals.\nThe lattice thermal conductivities (κ<sub>l,<i>xx</i>(<i>yy</i>)</sub>, κ<sub>l,<i>zz</i></sub>) at 300 and 800 K are (0.19, 0.23) and (0.07, 0.08) W m<sup>–1</sup> K<sup>–1</sup>, respectively, for GaAgSnSe<sub>4</sub>, and\nthose for InAgGeSe<sub>4</sub> are (1.07, 0.97) and (0.40, 0.36) W\nm<sup>–1</sup> K<sup>–1</sup>, respectively. Due to\nthe huge and steep total density of states (TDOS) at the band edges\nin the vicinity of the Fermi level, both direct band gap semiconductors\nexhibit high Seebeck coefficients (<i>S</i>) with optimum\nelectrical (σ) and electronic thermal conductivities (κ<sub>e</sub>). We have projected an outstanding figure of merit (<i>ZT</i>) for both the p-type and n-type of the two compounds.\nFor the p-type and n-type GaAgSnSe<sub>4</sub>, the maximum <i>ZT</i> estimated at 800 K along the (<i>x</i>(<i>y</i>), <i>z</i>)-directions are (2.74, 2.35) and\n(2.51, 1.84), respectively; for the p-type and n-type InAgGeSe<sub>4</sub>, the values are (1.31, 1.20) and (0.94, 0.90), respectively.\nOur study suggests both GaAgSnSe<sub>4</sub> and InAgGeSe<sub>4</sub> as prospective thermoelectric materials.