Crystal growth and optical properties of the polar hydrated pentaborates Rb[B5O6(OH)(4)] center dot 2H(2)O and NH4[B5O6(OH)(2)] center dot 2H(2)O andstructure redetermination of the ammonium compound

Large single crystals of the polar, hydrated pentaborates Me[B5O6(OH)(4)]. 2H(2)O (Me Rb, NH4) were grown from aqueous solutions. The structure of NH4 [B5O6(OH)(4)]. 2H(2)O was redetermined and proved to be orthorhombic (space group Aba2 (No. 41) with a = 11.3192(7) Angstrom, b = 11.0261(8) Angstrom, c = 9.2316(5) Angstrom, V = 1152.2(1) Angstrom', D, = 1.569 Mgm(-3), Z = 4 , M = 272.15 gmol(-1), R(F) 0.0369 for 1266 unique reflections(I greater th an or equal to 4 sigma (I)), isomorphous with the potassium and rubidium co mpounds. Refractive indices between 365 nm and 1100 nm as well as unpolariz ed absorption spectra of NH4[B5O6(OH)(4)]. 2H(2)O and Rb[B5O6(OH)(4)]. 2H(2 )O 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.