Stereochemically Active Lone Pair-Induced Large Optical Anisotropy in Novel Infrared Birefringent Crystal Ba 2La2Sb4S10(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 transparency (1-14 μm) with negligible absorption, demonstrating 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 FMIR birefringent crystal but also validates the effectiveness of the SCALP-driven design strategy in the design of high-performance infrared birefringent materials.