Cation tuning in anionic framework hydroborates: tailored deep-ultraviolet optical crystals with large birefringence†

Abstract

Birefringent crystals are important optical crystals that have the ability to modulate the polarization of light. It is difficult to achieve the development of modern deep-ultraviolet (DUV) optics. It is urgent to develop DUV birefringent crystals with excellent performance due to the fact that DUV birefringent crystals are relatively rare. Boron–oxygen group systems are the first choice for birefringent materials due to their excellent micro-optical properties and structural diversity. However, eliminating the dangling bonds of terminal oxygen atom and promoting the cutoff edge of borate materials to the DUV region is still a challenge. It has been found that the introduction of hydroxyl groups can effectively eliminate the dangling bonds of terminal oxygen atom and promote the cutoff edge to the DUV region. Here, four DUV metal hydroxyborates with suitable birefringence, LiB₅O₇(OH)₂(NH₄)₂B₁₀O₁₄(OH)₄·H₂O, (NH₄)_0.5K_1.5B₁₀O₁₄(OH)₄·H₂O and (NH₄)_0.7Cs_1.3B₁₀O₁₄(OH)₄·H₂O, were successfully synthesized via a high-temperature solution method. LiB₅O₇(OH)₂ and (NH₄)₂B₁₀O₁₄(OH)₄·H₂O exhibit short UV cutoff edges (<200 nm) and suitable birefringence (0.077/0.054@546 nm). Theoretical results indicate that the optical properties of these compounds are mainly determined by the [B₅O₇(OH)₂] or [B₁₀O₁₄(OH)₄] anionic groups and their arrangement in the structure. The introduction of hydroxyborate groups is an effective way to design and synthesize crystals for DUV optical crystals.