AEAg7(PS4)3 (AE = Ba, Sr): The homovalent substitution of lead by alkaline earth metals achieves the separation of the two phases and enlarges the band gap

Abstract

Two alkaline earth metal-containing thiophosphate compounds, BaAg7(PS4)3 and SrAg7(PS4)3, were synthesized via the homovalent substitution of Pb2+ in PbAg7(PS4)3 by Ba2+ and Sr2+, respectively. The space group of BaAg7(PS4)3 (P21/c) and SrAg7(PS4)3 (P63/m) is the same as the low- and high-temperature phases of PbAg7(PS4)3, respectively. The two compounds exhibit structural similarity and possess three-dimensional honeycomb-shaped architectures. These honeycomb structures are arranged in a layered fashion via hexagonal rings, which arise from the sharing of edges or vertices between [AgS4] and [PS4] tetrahedral, accommodating discrete entities of Ba2+ and Sr2+ cations. BaAg7(PS4)3 melts congruently with a melting point of 760 °C and a solidification point of 675 °C, while the melting behaviour of SrAg7(PS4)3 was incongruently and thermally stable up to 689 °C under vacuum of 1 × 10−3 Pa. Moreover, the band gaps of BaAg7(PS4)3 (2.69 eV) and SrAg7(PS4)3 (2.85 eV) have a significant enlargement compared to PbAg7(PS4)3 (2.09 eV). The combination of d10 cation Ag+ with second-order Jahn-Teller distortion and alkaline earth metals cation enriches the system of new metal thiophosphates and provides ideas for exploring other functional materials.