Stereochemically active lone pair-induced large optical anisotropy in a novel infrared birefringent crystal Ba2La2Sb4S10(S2)

Abstract

Birefringent crystals, capable of modulating the polarization state of light, play a critical role in laser technologies such as communication and quantum optics. Despite remarkable progress in commercial birefringent crystals across the ultraviolet to near-infrared spectrum, the design of novel mid- and far-IR (MFIR) birefringent crystals faces fundamental challenges from inherent infrared absorption and poor optical anisotropy in these wavelength ranges. To address this, the first compound Ba₂La₂Sb₄S₁₀(S₂) in the Ba–La–Sb–S system was successfully synthesized, driven by the stereochemically active lone pair (SCALP) effect. Ba₂La₂Sb₄S₁₀(S₂) has a medium band gap (1.86 eV) and exhibits large birefringence (0.53 at 1064 nm). IR and Raman spectra confirm its broad transmission range (1–14 μm) with negligible absorption, demonstrating its suitability for MFIR optical devices. Furthermore, detailed analysis of the structure–property relationship, derived from first-principles calculations, reveals that the strong SCALP effect of Sb³⁺ in the structure significantly enhances the birefringence. This study not only provides a highly promising MFIR birefringent crystal but also validates the effectiveness of the SCALP-driven design strategy in the design of high-performance infrared birefringent materials.