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 Ag_3.96SSe, 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.