Synthesis, Characterization And Crystal Structure Of Novel Hydrated Alkali Metals Borates

Boron atoms coordinate with oxygen not only in four-fold coordination (tetrahedral, BO₄) but also in three-fold coordination (triangular, BO3). Boron can form a large variety of compounds because of the complexity of the structures involved. Some of these compounds have useful physical properties, such as non-linearity due to it's noncentrosymmetric structure. So it is important to investigate the synthesis and crystal structures of borates. In the paper, five new hydrated borates were synthesized by solid reaction and solvothermal technology and further characterized by FT-IR, Raman spectra and TG - DTA.Three new hydrated borates with formula Na₄[B₁₀O₁₆(OH)₂]·4H₂O, K₂[B₅O₈(OH)]·2H₂O and Cs₂B₇O₉(OH)₅ were prepared by solid reaction, respectively. The crystal structures of these borates were determined by single-crystal x-ray diffraction data. The compound Na₄[B₁₀O₁₆(OH)₂]·4H₂O is monoclinic, space group Pc with unit cell dimensions a=11.323(2)A, b=6.5621(14)A, c=12.244(3)A, β=91.050(3)°, V=909.7(3)A³, Z=2. There are four Na⁺ ions in the molecule, two displays seven coordination while the others display eight coordination. The compound K₂[B₅O₈(OH)]·2H₂O belongs to the Pna21 Orthorhombic space group, with the following unit cell parameter: a=12.528(3)A, b=6.6731(17)A, c=11.533(3)A, V=909.7(3)A³, Z=4. The crystal structure of K₂[B₅O₈(OH)]·2H₂O consists of [B₅O₈(OH)]^2- polyborate anions, two K-O polyhedrons. The crystal structure of Cs₂B₇O₉(OH)₅ is monoclinic, space group P21/n with unit cell dimensions a=8.365(2) A, b=12.033(3) A, c=14.164(4) A, β=103.52(2)°, V=1386.13(61) A³, Z = 4. Three are two cesium ions and one isolated heptaborate anion [B₇O₉(OH)₅]^2- in the molecular, which is the first example of the isolated chain heptaborate anion.A new hydrated rubidium heptaborate Rb₂B₇O₉(OH)₅ was synthesized by solvothermal reaction. The crystal structure of Rb₂B₇O₉(OH)₅ is similar to Cs₂B₇O₉(OH)₅. It is belongs to monoclinic, space group P21/n with unit cell dimensions a=8.047(3))A, b=11.785(3)A, c=13.952(4)A, β= 102.974(4)°, V=1289.46(3)A³, Z=4.. consists of two rubidium ions and one isolated heptaborate.