Structure of bismuth oxoborate Bi4B2O9 at 20, 200, and 450 degrees C

Single crystals of bismuth oxoborate Bi4B2O9 have been grown by slowly cooling the melt of a stoichiometric Bi2O3 + H3BO3 mixture. The structure of the borate (monoclinic space group P2(1)/c, a = 11.107 angstrom, b = 6.629 angstrom, c = 11.044 angstrom, beta = 91.04 degrees, Z = 4) has been studied at 20, 200, and 450 degrees C. The structure is described not only in terms of full BiO6- and BiO7 polyhedra but also in terms of truncated BiO3- and BiO4-polyhedra and BO3 triangles, as well as oxo-centered OBi3 triangles and OBi4 tetrahedra. It is shown that both the B-O and Bi-O bond lengths are practically unaffected by temperature. Only the angles between polyhedra change with temperature, being responsible for the strong anisotropy of Bi4B2O6 thermal expansion, which was measured by high-temperature powder X-ray diffraction: alpha(11) = 20, alpha(22) = 15, alpha(33) = 6 x 10(-6)degrees C-1, and mu = (c, alpha(33)) = -19 degrees.