CsB(OH) 4 (H 2 O) 2 and Na(NH 4 ) 2 B 10 O 16 OH(H 2 O): a hydroxylation modification strategy regulates the borate structures and optical properties

This study focuses on hydroxyborates and employs a hydroxylation modification strategy to regulate the anionic framework of borates, leading to the successful synthesis of two novel hydroxyborates, namely, CsB(OH)₄(H₂O)₂ and Na(NH₄)₂B₁₀O₁₆OH(H₂O). With an ultraviolet (UV) cutoff edge below 200 nm, CsB(OH)₄(H₂O)₂ possesses considerable potential for deep-ultraviolet (DUV) transmission applications. Notably, Na(NH₄)₂B₁₀O₁₆OH(H₂O) represents a rare case of a hydroxyborate that contains two distinct fundamental building blocks (FBBs), namely [B₅O₁₀]^5- and [B₅O₁₀(OH)]^6-. In its structure, pseudo-layers constructed from 1∞[B₅O₉] chains alternate with those formed from 1∞[B₅O₉(OH)] chains, resulting in a structurally unique arrangement. Furthermore, Na(NH₄)₂B₁₀O₁₆OH(H₂O) displays a large birefringence value of 0.074 at 546 nm, manifesting remarkable optical anisotropy-a key property for DUV polarization optical devices. Overall, these results demonstrate that the introduction of hydroxyl groups can effectively modulate the B-O framework and further enrich the structural diversity of borate-based optical materials.