Aguilarite AgSSe Thermoelectric Material: Natural Mineral with Low Lattice Thermal Conductivity.

AgSSe, named as aguilarite in mineralogy, belongs to the family of liquid-like superionic compounds at middle temperature, but its thermoelectric (TE) properties are rarely investigated so far. In this study, a series of Ag-deficient AgSSe samples have been synthesized by the melting-annealing method. Their crystal structures, phase transition features, and electrical and thermal transport properties have been systematically investigated. It is found that AgSSe allows a small amount of Ag deficiency inside the lattice without altering the initial crystal structure and phase transition feature. The stoichiometric AgSSe has a low TE figure of merit ( zT) of 0.24 at 613 K because of the overhigh carrier concentration. Introducing Ag deficiency in AgSSe significantly lowers the carrier concentration, yielding a greatly enhanced Seebeck coefficient and suppressed carrier thermal conductivity throughout the entire measured temperature range. A maximum TE zT of 0.53 is achieved at 493 K for AgSSe, about two times that for the stoichiometric AgSSe. The present study suggests that aguilarite AgSSe is a promising TE material applied at the middle temperature range.