Design of a high-performance infrared birefringent crystal Ba₄HgP₂Se₁₀<i>via</i> 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.