Designing High-Performance Short-Wave UV Tellurium-Based Mixed Halides Birefringent Crystals via Halogen Substitution Engineering

The development of birefringent crystals with large optical anisotropy, short ultraviolet (UV) cutoff edges, and scalable crystal growth is essential for advancing UV polarized optical devices. In this study, we successfully synthesized two new tellurium-based mixed halide crystals, (NH4)2TeF5Cl and (NH4)2TeF5Br, using a hydrothermal method. Both compounds exhibit large birefringence (0.21 and 0.27 at 546 nm, respectively) and short UV cutoff edges (244 and 257 nm, respectively). The band gap of the crystals was optimized through energy band engineering via halogen substitution, preventing red-shifting of the cutoff edges and maintaining high UV transparency. In addition, the successful growth of centimeter-sized (NH4)2TeF5Cl crystals is beneficial for further applications. These properties make (NH4)2TeF5Cl and (NH4)2TeF5Br promising candidates for UV polarized optical applications, providing a new design strategy for high-performance birefringent crystals.