Hydroxyl-induced structural reconstruction: two new potassium hepta-borates with deep-UV transparency and enhanced birefringence

Abstract

As a critical component of polarization devices and optical devices, birefringent crystals are widely used in high-tech fields such as laser communication, laser polarization engineering, and high-precision scientific research instruments. In this study, two new hydroxyl borates, KB₇O₉(OH)₄ and KB₇O₁₀(OH)₂, were synthesized successfully via the hydrothermal method. The two hepta-borates feature different fundamental building units: [B₇O₁₀(OH)₄] in KB₇O₉(OH)₄ and [B₇O₁₂(OH)₂] in KB₇O₁₀(OH)₂. These units polymerize through oxygen atoms to form a one-dimensional (1D) infinite [B₇O₁₀(OH)₄] chain and a two-dimensional (2D) [B₇O₁₂(OH)₂] layer, respectively, demonstrating a hydroxyl-induced structural reconstruction. Notably, the 1D [B₇O₉(OH)₄]_∞ infinite chain in KB₇O₉(OH)₄ represents the first such structure identified in inorganic hydrated hepta-borates, as confirmed by the ICSD database. Performance characterization reveals that both compounds exhibit deep-UV transparency (with cut-off wavelengths shorter than 200 nm) and moderate birefringence values ranging from 0.041 to 0.078 at 546 nm, highlighting their potential as deep-UV birefringent crystal candidates.