Zinc diborate crystallizes in the orth0rhombic system, space group Pbca, with eight formula units in a cell of dimensions a= 13"714_+0"005, b=8"091 _+0.005 and c=8.631 _+0.005 A. The calculated density is 3"07 g.cm-3, and all atoms are in general positions. The structure of ZnB407 has been solved by three-dimensional Fourier syntheses. The positional and isotropic thermal parameters were refined by the least-squares method based on 530 observed reflexions. The final conventional R index is 0.067. The structure consists of BO3 triangles and BO4 tetrahedra sharing a common vertex. Each zinc atom is surrounded by four close oxygen atoms, arranged in an irregular tetrahedron.
Nickel orthoborate, Nia(BO3)2, crystallizes in the orthorhombic system, space group Pnnm, with two formula units in a cell of dimensions a = 5.396 + 0"001, b = 4.459 + 0"001 and e = 8.297 + 0"002/~. The structure of Nia(BOa)2 has been determined by three-dimensional Patterson synthesis. The positional and isotropic thermal parameters were refined by least-squares analysis, yielding a final R value of 0.086. All boron atoms are triangularly coordinated. There are two types of Ni atoms, both octahedrally coordinated and linked together by oxygen sharing to form a three-dimensional network. Each oxygen atom is surrounded by three nickel atoms and one boron atom arranged in an irregular tetrahedron.
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