Crystal growth, crystal structure of new polymorphic modification, β-Bi2B8O15 and thermal expansion of α-Bi2B8O15

Single crystals of α- and β-polymorphs of Bi2B8O15 were grown by Czochralski method from a charge of the stoichiometric composition. The crystal structure of β-Bi2B8O15 was solved by direct methods from a twinned crystal and refined to R 1=0.081 (wR =0.198) on the basis of 1584 unique observed reflections (I >2σ (I )). The compound is triclinic, space group View the MathML sourceP1¯, a=4.3159(8), b=6. 4604(12), c=22.485(4) Å, α=87.094(15)°, β=86.538(15)°, γ=74.420(14)°, V=602.40(19) Å3, Z=2. The B–O layered anion of β-Bi2B8O15 is topologically identical to the anion of α-Bi2B8O15 but the orientation of neighboring layers is different. Thermal expansion of α-Bi2B8O15 has been investigated by X-ray powder diffraction in air in temperature range from 20 to 700 °C. It is strongly anisotropic, which can be explained by the hinge mechanism applied to chains of Bi–O polyhedra. While the anisotropy of thermal expansion is rather high, the volume thermal expansion coefficient αV=40×106 °C−1 for α-Bi2B8O15 is close to those of other bismuth borates.