Na6B13O22.5, a new noncentrosymmetric sodium borate

✍ Scribed by N. Penin; M. Touboul; G. Nowogrocki

• Publisher: Elsevier Science
• Year: 2005
• Tongue: English
• Weight: 367 KB
• Volume: 178
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1016/j.jssc.2004.12.015

✦ Synopsis

Na 6 B 13 O 22.5 (B/Na ¼ 2.17) single crystals were obtained by heating, melting and appropriately cooling borax, Na 2 [B 4 O 5 (OH) 4 ] Á 8H 2 O. 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 Iba2 space group, with the following unit cell parameters:

The crystal structure was solved from 3226 reflections until R 1 ¼ 0.0385. It exhibits a three-dimensional framework built up from BO 3 triangles (D) and BO 4 tetrahedra (T). Two kinds of borate groups can be considered forming two different double B 3 O 3 rings: two B 4 O 9 (linkage by two boron atoms) and one B 5 O 11 (linkage by one boron atom); the shorthand notation of the new fundamental building block (FBB) existing in this compound is: 13: 1 3 [(5: 3D+2T)+2(4: 2D+2T)]. The discovery of this new borate questions the real number of Na 2 B 4 O 7 varieties. The existence of Na 6 B 13 O 22.5 (B/Na ¼ 2.17) and of another recently discovered borate, Na 3 B 7 O 12 (B/Na ¼ 2.33; FBB 7: 1 3 [(3: 2D+T)+(3: D+2T)+(1: D)], with a composition close to the long-known borate a-Na 2 B 4 O 7 (B/Na ¼ 2; FBB 8: 1 3 [(5: 3D+2T)+(3: 2D+T)], may explain the very complex equilibria reported in the Na 2 O-B 2 O 3 phase diagram, especially in this range of composition.