Crystal structure of two new sodium borates Na3B 7O12 and Na2Tl2B10O 17

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) Å, b=8.249(3) Å, c=8.836(3) Å, α=95.875(5)°, β=100.680(4)°, γ=99.688(4)°; V=464.2(2) Å3; Z=2. The crystal structure was solved from 1982 reflections until R1=0.0293. It exhibits a three-dimensional framework built up from BO3 triangles () and BO4 tetrahedra (T). Three kinds of borate groups can be considered: B 3O7 and B3O8 rings and BO 3 triangle; the shorthand notation of the fundamental building block (FBB) existing in this compound is: 7: ∞3 [(3: 2+T)+(3: +2T)+(1: )]. The second borate Na2Tl2B 10O17 crystallizes in the monoclinic system, space group C2/c, with a=21.711(10) Å, b=6.446(3) Å, c=10.952(5) Å, β=105.897(8)°; V=1474.0(1) Å3; Z=4. The crystal structure was solved from 1884 reflections until R1=0.0387. It consists of a two-dimensional network of sharing corners BO3 triangles and BO4 tetrahedra forming B5O11 double rings, leading to the FBB: 10: ∞2 2[(5: 3+2T)]. These two new borates were compared with the compounds M3B 7O12 (M=Li, Rb, Cs; B/M=2.33) and AgSrB7O 12, and Cs2Na2B10O17 and Cs2K2B10O17 (M 2M′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. © 2003 Elsevier B.V. All rights reserved.