Thermoelectric study of Zn-doped n-type AgIn₅Se₈: Hopping and band electrical conduction along with low lattice thermal conduction in diamond-like structure

The diamond-like structured compounds with intrinsically low lattice thermal conductivity form a class of promising thermoelectric materials. Despite the stride of developing high performance p-type diamond-like materials, the report of thermoelectric study of n-type diamond-like materials is scarce. Here, we report thermoelectric study of Zn-doped n-type AgIn₅Se₈ with the emphases on the underlying mechanism of electrical conduction and low lattice thermal conductivity. Specifically, we found that 1) the low lattice thermal conductivity was related to the low speed of sound, low-energy Einstein modes arising from the weak Ag1-Se2 and In2-Se1 bonds, and anharmonicity arising from the distorted Se1-In2-Se1 tetrahedra at high temperatures; 2) a temperature-driven transition occurred in the electrical conduction from variable range hopping (VRH) to band conduction; 3) Zn doping regulated the carrier concentration and enhanced the electronic density of states, thereby yielding a zT value of 0.60 at 880 K in the 2% Zn-doped AgIn₅Se₈ sample. The derived zT value is promising among the n-type diamond-like structured compounds.