beta-Tl2B4O7:Compound Containing a New Three-Dimensional Borate Anion

Single crystals of synthetic beta-Tl2B4O7 or Tl6B12O21 were obtained by heating its hydrated precursor Tl2[B4O6(OH)2]·2H2O. The compound is triclinic, space group P-1; the unit cell parameters are a=6.742(1) ?, b=13.225(2) ?, c=13.389(2) ?, alpha=119.093(3)°, beta= 92.288(3)°, gamma=91.012(3)°; Z=6. The crystal structure was solved from 2893 reflections until R=0.0312. It exhibits a three-dimensional framework containing a new complex borate anion (B12O21)6- formed by six BO3 triangles (delta) and six BO4 tetrahedra (T), which are disposed on two and three B3O3 rings; this leads to an anion with the shorthand notation: 12: 3infinity[(5: 2delta+3T)+(7: 4delta+3T)]. This new anion is different from those described in alkaline tetraborates M2B4O7, and especially K2B4O7 and Rb2B4O7 where the M+ cations have a size close to that of Tl+. This difference may be due to the stereochemical role of the 6s2 lone pairs of Tl+ cations, which has been pointed out. Upon heating at around 500°C, beta-Tl2B4O7 transforms into a second form of thallium tetraborate, alpha-Tl2B4O7.