Halogen-Mediated Dimensionality Reduction in Bismuth Oxychlorides: Trade-off between Birefringence and Band Gap

Two new bismuth oxychlorides provide a direct 2D-versus-3D structural comparison. • Layered LiBi 2 O 3 Cl combines Δ n  = 0.18 at 1064 nm with a 298 nm UV cutoff edge. • Halogen regulation of framework dimensionality tunes optical anisotropy in Bi 3+ crystals. Birefringent crystals are indispensable for manipulating light polarization in advanced optical communication and laser systems. The quest for materials that combine a large birefringence (Δ n ) with a suitable ultraviolet (UV) cutoff edge remains a central challenge. Herein, two new bismuth oxychloride birefringent crystals, LiBi 2 O 3 Cl and Li 2 BiO 2 Cl, were successfully synthesized via a high-temperature melt method. Single-crystal structure determination reveals that LiBi 2 O 3 Cl crystallizes in a two-dimensional (2D) layered motif, whereas Li 2 BiO 2 Cl adopts a three-dimensional (3D) network. The 2D layered compound exhibits a larger calculated birefringence of 0.18 at 546 nm, a short UV cutoff edge of 298 nm, and a remarkable thermal stability up to 746 °C without decomposition, surpassing the corresponding properties of many reported bismuthates. First-principles density functional theory calculations attribute the large optical anisotropy to the cooperative effects of the stereochemically active lone pairs of Bi 3+ cations and the preferential alignment of distorted [BiO 4 ] units within the 2D layers. This work provides a promising UV birefringent candidate and demonstrates halogen regulation of framework dimensionality as an effective design approach for lone-pair-containing optical crystals. Download: Download high-res image (211KB) Download: Download full-size image