Thermoelectric p-Type Ag₉GaTe₆ with an Intrinsically Low Lattice Thermal Conductivity

Ag-based argyrodites with a general formula of Ag(12–n)Bn+X6 (B = Ga/Al/Si/Ge/Sn and X = S/Se/Te) have recently attracted much attention as promising thermoelectrics because of their ultralow lattice thermal conductivity and richness in the composition. Most of these Ag-based argyrodites are n-type conduction semiconductors and show a promising thermoelectric figure of merit (zT). However, p-type argyrodites have been much less reported, and this motivates the current work to focus on a novel p-type argyrodite Ag9GaTe6 for thermoelectric applications. Although various doping (Cd, Zn, Mg, Nb, Se, and Cu) were found to insufficiently increase the hole concentration for a degenerate conduction, a peak zT of 0.65 at 600 K can still be realized in p-type Ag9Ga0.95Cd0.05Te6. This largely relies on a lattice thermal conductivity of only ∼0.2 W/m-K (nearly independent with temperature and doping), which results from the low sound velocity, strong lattice anharmonicity, and complex crystal structure. This work demonstrates the potential of Ag9GaTe6 as a promising p-type thermoelectric material, particularly with a further increase in hole concentration.