Wide band gap thiophosphates ASrPS4 (A = Li, Na, K, Rb, Cs): cation size effect induced successive structural transformation

Metal thiophosphates have aroused much research interest due to their structural chemistry and possible applications as infrared functional materials. In this study, five quaternary Sr-based alkali metal thiophosphates ASrPS4 (A = Li, Na, K, Rb, Cs) were obtained. Their structural comparison shows that their symmetry undergoes transformation from tetragonal (I41/acd) to monoclinic (P21/c) to orthorhombic (Pnma) system, which is induced by the cation size effects and coordination features of different alkali metal cations. The experimental and theoretical results demonstrate that the band gaps of all title compounds are large, namely 3.6–3.9 eV (experimental results) and 3.78–4.12 eV (HSE06). Theoretical analyses indicate that the [PS4] group could be regarded as a good unit for designing wide band gap compounds, and the birefringence of NaSrPS4 is 0.08 at the fundemental 1064 nm wavelength, which shows that it may be a potential infrared birefringent material.