Achieving giant optical anisotropy in the mid-far infrared region via ideal alignment of dual functional motifs

Abstract

Birefringent crystals serve as crucial optical materials for polarization modulation, yet designing and synthesizing crystals with both giant birefringence and wide band gaps in the mid-far infrared (MFIR) region remains a formidable challenge. Herein, we report the synthesis of two antimony-based chalcohalide birefringent crystals, La₂SbS₃(S₂)X (X = Cl, Br; LSSC, LSSB), via high-temperature solid-state reactions. These crystals feature a unique 3D structure composed of [La(S₂)S_nX]_∞ layers (n = 2, 3) with neatly aligned linear [S₂]²⁻ units and infinite [SbS₃]_∞³⁻ chains along the b-axis constructed from [SbS₄]⁵⁻ motifs, indicating an optimal superposition of their large polarizability anisotropy. Therefore, LSSC and LSSB exhibit record-high birefringence (Δn = 1.049 and 1.057 @ 546 nm) among all reported chalcogenide birefringent crystals, with band gaps exceeding 2.0 eV. Meanwhile, they also have a wide IR transparency range (2.5–25 μm) along with excellent thermal stability (>600 °C). The mechanistic investigation reveals that the giant optical anisotropy of LSSC and LSSB originates from the efficient synergy between [SbS₄]⁵⁻ and [S₂]²⁻ units. This work not only introduces two promising MFIR birefringent materials, but also provides new insights for the rational design of crystals with extremely large birefringence in the future.