CsB(OH)4(H2O)2 and Na(NH4)2B10O16OH(H2O): a hydroxylation modification strategy regulates the borate structures and optical properties

Abstract

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.