Crystal Growth and Optical Properties of the Polar Hydrated Pentaborates Rb[B5O6(OH)4] . 2H2O and NH4[B5O6(OH)4] . 2H2O and Structure Redetermination of the Ammonium Compound

Large single crystals of the polar, hydrated pentaborates Me[B5O6(OH)4]�2H2O (Me = Rb, NH4) were grown from aqueous solutions. The structure of NH4[B5O6(OH)4] . 2H2O was redetermined and proved to be orthorhombic (space group Aba2 (No. 41) with a = 11.3192(7) Å, b = 11.0261(8) Å, c = 9.2316(5) Å, V = 1152.2(1) Å3, Dx = 1.569 Mgm-3, Z = 4, M = 272.15 gmol-1, R(F) = 0.0369 for 1266 unique reflections (I >= 4 σ (I)), isomorphous with the potassium and rubidium compounds. Refractive indices between 365 nm and 1100 nm as well as unpolarized absorption spectra of [B5O6(OH)4] . 2H2O and NH4[B5O6(OH)4] . 2H2O were determined and phase-matching curves for second-harmonic generation were calculated. Both compounds allow type I and type II phase-matching at wavelengths from about 450 nm to the near infrared region.