NHBO(OH) and CsRbBO(OH)·HO: Two Hydroxyborates Feature Significant Optical Anisotropy and Contain [BO(OH)] Chains.

Two hydroxyborates, NHBO(OH) and CsRbBO(OH)·HO, featuring hydroxylated fundamental building blocks (FBBs) [BO(OH)], were synthesized via the high-temperature melting method and hydrothermal synthesis method. Both compounds contain [BO(OH)] chains that assemble into pseudo-2D layers via O-H···O/N-H···O hydrogen bonds and further interconnect into 3D frameworks. Despite slight structural differences between the BO rings in the [BO(OH)] FBB of the two compounds, they exhibit comparable band gaps (6.68 and 6.61 eV, respectively) and calculated birefringence (0.076 and 0.069 at 546 nm, respectively). First-principles calculations reveal the structure-property relationships, highlighting the influence of anionic group arrangement on optical performance.