Aguilarite Ag₄SSe Thermoelectric Material: Natural Mineral with Low Lattice Thermal Conductivity

Ag4SSe, 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 Ag4SSe 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 Ag4SSe allows a small amount of Ag deficiency inside the lattice without altering the initial crystal structure and phase transition feature. The stoichiometric Ag4SSe has a low TE figure of merit (zT) of 0.24 at 613 K because of the overhigh carrier concentration. Introducing Ag deficiency in Ag4SSe 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 Ag3.96SSe, about two times that for the stoichiometric Ag4SSe. The present study suggests that aguilarite Ag4SSe is a promising TE material applied at the middle temperature range.