Extending Unique 1D Double-Chains to 2D Layers with Birefringent Gain by Introducing a Hydroxyl Group

It remains a significant hurdle for discovering birefringent materials in the deep ultraviolet (DUV, λ < 200 nm). It is well-known that the OH anions are recognized for their capability to eliminate the dangling bonds from terminal oxygen atoms, promoting the ultraviolet (UV) cutoff edge blueshift and regulating the crystal structure. Here, two new barium hydroxyborates, Ba3B11O18(OH)3(H2O) (BaBOH) and Na2BaB10O16(OH)2(H2O)2 (NaBaBOH), were designed and synthesized while displaying different dimensions. Remarkably, BaBOH presents novel one-dimensional (1D) [B22O37(OH)6]∞ double-chains formed by a new fundamental building block (FBB)[B11O21(OH)3]. NaBaBOH possesses a 2D [B10O16(OH)2]∞ layer with a less common FBB [B10O19(OH)2]. They enrich the structural diversity of hydroxyborates. Moreover, NaBaBOH exhibits a broad transparent window within the DUV spectral range (<190 nm) and possesses a favorable birefringence of 0.064. Furthermore, detailed summaries and structural comparisons have been implemented for all hydroxyborates containing alkali and alkaline-earth metals. This reveals that the OH group modulation strategy can be appropriately employed for the structural design.