Na6B13O22.5, a new noncentrosymmetric sodium borate

Na6B13O22.5 (B/Na=2.17) single crystals were obtained by heating, melting and appropriately cooling borax, Na2[B4O5(OH)4]·8H2O. Its formula has been determined by the resolution of the structure from single-crystal X-ray diffraction data. The compound crystallizes in the noncentrosymmetric orthorhombic image space group, with the following unit cell parameters: a=33.359(11) Å, b=9.554(3) Å, c=10.644(4) Å; V=3392.4(19) Å3; Z=8. The crystal structure was solved from 3226 reflections until R1=0.0385. It exhibits a three-dimensional framework built up from BO3 triangles (Δ) and BO4 tetrahedra (T). Two kinds of borate groups can be considered forming two different double B3O3 rings: two B4O9 (linkage by two boron atoms) and one B5O11 (linkage by one boron atom); the shorthand notation of the new fundamental building block (FBB) existing in this compound is: 13: image [(5: 3Δ+2T)+2(4: 2Δ+2T)]. The discovery of this new borate questions the real number of Na2B4O7 varieties. The existence of Na6B13O22.5 (B/Na=2.17) and of another recently discovered borate, Na3B7O12 (B/Na=2.33; FBB 7: image [(3: 2Δ+T)+(3: Δ+2T)+(1: Δ)], with a composition close to the long-known borate α-Na2B4O7 (B/Na=2; FBB 8: image [(5: 3Δ+2T)+(3: 2Δ+T)], may explain the very complex equilibria reported in the Na2O–B2O3 phase diagram, especially in this range of composition.