Secondary phase effect on the thermoelectricity by doping Ag in SnSe

Partial replacement of Sn with Ag is an effective method to optimize the thermoelectric performance of SnSe compounds. However, this approach can easily introduce the AgSnSe2 secondary phase due to the limited solubility of Ag in SnSe. This secondary phase is considered to play positive effects on thermoelectric performance all the way in the previous studies. However, its exact influences on the sample's properties remain ambiguous. Herein, we report a comprehensive study on the crystal structures and the effects of Ag dopant in SnSe polycrystals. Firstly, neutron powder diffraction measurements demonstrate a sudden phase transition nature at 775 K. In addition, we observe a strong competition between the influences from Ag dopant and AgSnSe2 secondary phase. It finds that the precipitation of AgSnSe2 would deplete the carrier concentration and scatter the mobile carriers. Its appearance in the SnSe matrix also suppresses the lattice thermal conductivity, although lower Ag doping content raises the thermal transport properties. This study sheds new light on the understanding of the thermoelectric properties in SnSe polycrystals and is expected to guide further optimization of transport properties.