Crystal structure of two new sodium borates Na3B7O12 and Na2Tl2B10O17

The crystal structure of Na3B7O12 and Na2Tl2B10O17 was determined by the single crystal X-ray diffraction method. The first one crystallizes in the triclinic system, space group P-1, with the following unit cell parameters: a = 6.638(2) Angstrom, b = 8.249(3) Angstrom, c = 8.836(3) Angstrom, alpha = 95.875(5)degrees, beta = 100.680(4)degrees, gamma = 99.688(4)degrees; V = 464.2(2) Angstrom(3); Z = 2. The crystal structure was solved from 1982 reflections until R-1 = 0.0293. It exhibits a three-dimensional framework built up from BO3 triangles (Delta) and BO4 tetrahedra (T). Three kinds of borate groups can be considered: B3O7 and B3O8 rings and BO3 triangle; the shorthand notation of the fundamental building block (FBB) existing in this compound is: 7: infinity(3) [(3: 2Delta + T) + (3: Delta + 2T) + (1: Delta)]. The second borate Na2Tl2B10O17 crystallizes in the monoclinic system, space group C2/c, with a = 21.711(10) Angstrom, b = 6.446(3) Angstrom, c = 10.952(5) Angstrom, beta = 105.897(8)degrees; V = 1474.0(1) Angstrom(3); Z = 4. The crystal structure was solved from 1884 reflections until R-1 = 0.0387. It consists of a two-dimensional network of sharing comers BO3 triangles and BO4 tetrahedra forming B5O11 double rings, leading to the FBB: 10: infinity(2) 2[(5: 3Delta + 2T)]. These two new borates were compared with the compounds M3B7O12 (M=Li, Rb, Cs; B/M = 2.33) and AgSrB7O12, and CS2Na2B10O17 and CS2K2B10O17 (M2M'2B10O17, M and M' monovalent elements; B/(M + M') = 2.5). The latter are isotype and isostructural of Na2Tl2B10O17. Na3B7O12 is isotype and isostructural of Li3B7O12; the three-dimensional structure of these two heptaborates is different from the two-dimensional structures of AgSrB7O12, Rb3B7O12 and CS3B7O12, which contain different FBBs with 7, 14 and 63 boron atoms, respectively. (C) 2003 Elsevier B.V. All rights reserved.