Thiophosphates Containing Ag and Lone-Pair Cations with Interchiral Double Helix Show Both Ionic Conductivity and Phase Transition.

Quaternary metal thiophosphates containing second-order Jahn-Teller distorted d Ag and lone-pair cations, AgBi(PS) (1), AgSn(PS) (2), and AgPb(PS) (3), were obtained by solid-state synthesis. The structural frameworks of 2 and 3 feature an infinite 1-D interchiral double helix (AgPS), which is rare in inorganic compounds. Compound 3 undergoes a significant first-order structural phase transition from monoclinic to hexagonal at ∼204 °C. This can be ascribed to the significant mismatch in the expansion coefficients between Pb-S (Ag-S) and P-S bonds evaluated by bond valence theory. The three compounds are Ag ionic conductors, and Ag ion migration pathways are proposed by calculating maps of low bond valence mismatch. Moreover, the optical properties of the three compounds were studied, and electronic structure calculations were performed. The combination of second-order Jahn-Teller distorted d cation and lone-pair cation provides a new strategy to explore new metal thiophosphates with interesting structures and promising properties.