B-Tl2B4O7: Compound containing a new three-dimensional borate anion

Single crystals of synthetic beta -TL(2)b(4)o(7) or TL6B12O21 were obtained by heating its hydrated precursor Tl-2[B4O6(OH)(2)] . 2H(2)O. The compound is triclinic, space group P-1; the unit cell parameters are a = 6.742(1) A ngstrom, b = 13.225(2)Angstrom, c = 13.389(2) Angstrom, alpha = 119.093(3)d egrees, beta = 92.288(3)degrees, gamma = 91.012(3)degrees; Z = 6. The cryst al structure was solved from 2893 reflections until R = 0.0312. It exhibits a three-dimensional framework containing a new complex borate anion (B12O2 1)(6-) formed by six BO3 triangles (A) and six BO4 tetrahedra (T), which ar e disposed on two and three B3O3 rings; this leads to an anion with the sho rthand notation: 12: (3)(proportional to) [(5: 2 Delta + 3T) + (7: 4 Delta + 3T)]. This new anion is different from those described in alkaline tetrab orates M2B4O7, and especially K2B4O7 and Rb2B4O7 where the M+ cations have a size close to that of TV. This difference may be due to the stereochemica l role of the 6s(2) lone pairs of Tl+ cations, which has been pointed out. Upon heating at around 500 degreesC, beta -Tl2B4O7 transforms into a second form of thallium tetraborate, alpha -Tl2B4O7. (C) 2001 Academic Press.