Polarity and Chirality in Uranyl Borates: Insights into Understanding the Vitrification of Nuclear Waste and the Development of Nonlinear Optical Materials

Abstract

Four new sodium uranyl borates, α-Na[(UO2)2B10O15(OH)5)] (NaUBO-1), β-Na[(UO2)2B10O15(OH)5] (NaUBO-2), Na[(UO2)2B10O15(OH)5]·3H2O (NaUBO-3), and Na[(UO2)B6O10(OH)]·2H2O (NaUBO-4), and four new thallium uranyl borates, α-Tl2[(UO2)2B11O18(OH)3] (TlUBO-1), β-Tl2[(UO2)2B11O18(OH)3] (TlUBO-2), Tl[(UO2)2B10O16(OH)3] (TlUBO-3), and Tl2[(UO2)2B11O19(OH)] (TlUBO-4), have been prepared via the reaction of sodium nitrate or thallium nitrate, uranyl nitrate, and excess boric acid at 190 °C. These compounds share a common structural motif consisting of a linear uranyl, UO22+, cation surrounded by BO3 triangles and BO4 tetrahedra to create a UO8 hexagonal bipyramidal environment around uranium. The borate anions bridge between uranyl units to create sheets. Additional BO3 triangles extend from the polyborate layers and are directed approximately perpendicular to the sheets. In some compounds, these units can link the layers together to yield three-dimensional networks with large pores to house the Na+ or Tl+ cations and water molecules. The structures are all noncentrosymmetric and are either polar or chiral. While the uranyl borate layers are noncentrosymmetric in and of themselves, there is also twisting of the interlayer BO3 groups to reduce the interlayer spacing, producing helical features in some structures. Na[(UO2)B6O10(OH)]·2H2O and β-Tl2[(UO2)2B11O18(OH)3], which can be obtained as pure phases, display second-harmonic generation of 532 nm light from 1064 nm light.