Ba(BO2OH) – A Monoprotonated Monoborate from Hydroflux Showing Intense Second Harmonic Generation <jats:title>Abstract</jats:title><jats:p>Pure samples of colorless, air‐stable Ba(BO<jats:sub>2</jats:sub>OH) crystals were obtained from Ba(NO<jats:sub>3</jats:sub>)<jats:sub>2</jats:sub> and H<jats:sub>3</jats:sub>BO<jats:sub>3</jats:sub> under the ultra‐alkaline conditions of a KOH hydroflux at about 250 °C. The product formation depends on the water‐base molar ratio and the molar ratio of the starting materials. B(OH)<jats:sub>3</jats:sub> acts as a proton donor (Brønsted acid) rather than a hydroxide acceptor (Lewis acid). Ba(BO<jats:sub>2</jats:sub>OH) crystallizes in the non‐centrosymmetric orthorhombic space group <jats:italic>P</jats:italic>2<jats:sub>1</jats:sub>2<jats:sub>1</jats:sub>2<jats:sub>1</jats:sub>. Hydrogen bonds connect the almost planar (BO<jats:sub>2</jats:sub>OH)<jats:sup>2−</jats:sup> anions, which are isostructural to HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>, into a syndiotactic chain. IR and Raman spectroscopy confirm the presence of hydroxide groups, which are involved in weak hydrogen bonds. Upon heating in air to about 450 °C, Ba(BO<jats:sub>2</jats:sub>OH) dehydrates to Ba<jats:sub>2</jats:sub>B<jats:sub>2</jats:sub>O<jats:sub>5</jats:sub>. Moreover, the non‐centrosymmetric structure of Ba(BO<jats:sub>2</jats:sub>OH) crystals was verified with power‐dependent confocal Second Harmonic Generation (SHG) microscopy indicating large conversion efficiencies in ambient atmosphere.</jats:p> 648 21 Wiley