Optimal three-dimensional [Ag4P2S6]-framework assembly directed by a threefold roto-inversion axis for enhanced birefringence

Abstract

The birefringence of thiophosphates is often limited by the intrinsically low polarizability anisotropy of tetrahedral building units. Here, guided by a structure-directed strategy, we synthesized a new thiophosphate, named γ-Ag₄P₂S₆. In the structure, the alignment of [P₂S₆]⁴⁻ groups along the c axis, governed by the inherited threefold inversion symmetry from the lattice, contributes to an enhanced birefringence. As a result, the compound exhibits uniaxial optical behavior with large birefringence (0.21@1064 nm), together with a wide band gap (2.50 eV), a high LIDT (1.89 × AGS) and high thermal stability (>750 °C). These findings establish a viable structural approach for enhancing birefringence in thiophosphate systems and provide insight into the design of advanced infrared birefringent materials.