Design of high-performance infrared birefringent crystal Ba4HgP2Se10 via a low-dimensional motif intercalation strategy

The development of birefringent materials, especially infrared (IR) birefringent crystals, has been limited by conflicting microstructural requirements, such as broad transmission range, wide band gap, and large birefringence. In this work, we propose a "low-dimensional motif intercalation" strategy for designing high-performance birefringent materials. Utilizing the layered selenophosphorus Ba3P2Se8 as a prototype compound, a novel IR birefringent crystal Ba4HgP2Se10 with enhanced birefringence was successfully synthesized by the intercalation of the linear [HgSe2]2- moieties, while the outstanding optical properties such as wide band gap and long IR cutoff were maintained. Notably, the Ba4HgP2Se10 crystal exhibits the largest band gap among known selenophosphorus compounds, accompanied by a wide IR transmittance range and substantial birefringence. Theoretical calculations reveal that the outstanding optical properties of Ba4HgP2Se10 arise from the synergistic interaction between Ba2+ cations, [HgSe2]2- anions, and the layered selenophosphorus framework.