Crystal structure of a new form of sodium octoborate β-Na2B8O13

Single crystals of a new form of sodium octoborate, beta-Na2B8O13, were obtained fortuitously from a complex Na2OB2O3-P2O5 mixture, and studied. The compound is monoclinic, space group P2(1)/c; the unit cell parameters are a = 11.731(4) Angstrom, b = 7.880(3) Angstrom, c = 10.410(4) Angstrom, beta = 99.883(3)degrees; Z = 4. The crystal structure was solved from 1653 reflections until R, = 0.0444; it consists of two infinite, independent, and interleaved boron-oxygen networks containing a complex borate anion (B8O13)(2-) formed by six BO3 triangles (A) and two BO4 tetrahedra (T), which can be viewed as a B5O10 group linked to a B3O7 group; this leads to a Fundamental Building Block (FBB) with the shorthand notation: 8: infinity(3) [(5 : 4Delta + T) + (3: 2Delta + T)]. This FBB is identical to that described in other octoborates such as alpha-Na2B8O13 and Ag2B8O13. However, some subtle differences exist in the interlinking of the octoborate anions found in these three compounds, which explains their different structure and unit cell parameters. (C) 2002 Elsevier Science (USA).