Rattling-Induced Ultralow Lattice Thermal Conductivity Leads to High Thermoelectric Performance in GaAgSnSe4 and InAgGeSe4

The thermal and electronic transport properties of Ag-based quaternary compounds, GaAgSnSe₄ and InAgGeSe₄, have been explored by using density functional theory and the Boltzmann transport equation. Both the compounds exhibit ultralow lattice thermal conductivities (κ_l) that originate from the anharmonicity induced by the rattling effects of the loosely bound Ag atoms in their crystals. The lattice thermal conductivities (κ_l,xx(yy), κ_l,zz) at 300 and 800 K are (0.19, 0.23) and (0.07, 0.08) W m^–1 K^–1, respectively, for GaAgSnSe₄, and those for InAgGeSe₄ are (1.07, 0.97) and (0.40, 0.36) W m^–1 K^–1, respectively. Due to the huge and steep total density of states (TDOS) at the band edges in the vicinity of the Fermi level, both direct band gap semiconductors exhibit high Seebeck coefficients (S) with optimum electrical (σ) and electronic thermal conductivities (κ_e). We have projected an outstanding figure of merit (ZT) for both the p-type and n-type of the two compounds. For the p-type and n-type GaAgSnSe₄, the maximum ZT estimated at 800 K along the (x(y), z)-directions are (2.74, 2.35) and (2.51, 1.84), respectively; for the p-type and n-type InAgGeSe₄, the values are (1.31, 1.20) and (0.94, 0.90), respectively. Our study suggests both GaAgSnSe₄ and InAgGeSe₄ as prospective thermoelectric materials.