The study of Bi3B5O12: synthesis, crystal structure and thermal expansion of oxoborate Bi3B5O12

The paper presents a new data on the crystal structure, thermal expansion and IR spectra of Bi3B5O12. The Bi3B5O12 single crystals were grown from the melt of the same stoichiometry by Czochralski technique. The crystal structure of Bi3B5O12 was refined in anisotropic approximation using single-crystal X-ray diffraction data. It is orthorhombic, Pnma, a=6.530(4), b=7.726(5), c=18.578(5) A, V=937.2(5) A3, Z=4, R=3.45%. Bi3+ atoms have irregular coordination polyhedra, Bi(1)O6 (d(B-O)=2.09-2.75 A) and Bi(2)O7 (d(B-O)=2.108-2.804 A). Taking into account the shortest bonds only, these polyhedra are considered here as trigonal Bi(1)O3 (2.09-2.20 A) and tetragonal Bi(2)O4 (2.108-2.331 A) irregular pyramids with Bi atoms in the tops of both pyramids. The BiO4 polyhedra form zigzag chains along b-axis. These chains alternate with isolated anions [B2IVB3IIIO11]7- through the common oxygen atoms to form thick layers extended in ab plane. A perfect cleavage of the compound corresponds to these layers and an imperfect one is parallel to the Bi-O chains. The Bi3B5O12 thermal expansion is sharply anisotropic (α11∼α22=12, α33=3x10-6 deg. C-1) likely due to a straightening of the flexible zigzag chains along b-axis and decreasing of their zigzag along c-axis. Thus the properties like cleavage and thermal expansion correlate to these chains