Preparation, crystal structure and thermal expansion of a new bismuth barium borate, BaBi2B4O10

Single crystals of a new compound, BaBi2B4O10 Were grown by cooling a melt with the stoichiometric composition. The crystal structure of the compound has been solved by direct methods and refined to R-1 = 0.049 (wR = 0.113) on the basis of 1813 unique observed reflections (vertical bar F-o broken vertical bar > 4 sigma vertical bar F-o vertical bar). It is monoclinic, space group P2(1)/c, a = 10.150(2), b = 6.362(1), c = 12.485(2) angstrom, beta = 102.87(1)degrees, V = 786.0(2) angstrom(3), Z = 4. The structure is based upon anionic thick layers that are parallel to (001). The layers can be described as built from alternating novel borate [B4O10](infinity)(8-) chains and bismuthate [Bi2O5](infinity)(4-). chains extended along b-axis. The borate chains are composed of [1330 8](7-) triborate groups of three tetrahedra and single triangles with a [BO2](-) radical. The borate chains are interleaved along the c-axis with rows of the Ba2+ cations so that the Ba atoms are located within the layers. The layers are connected by two nonequivalent Ba-O bonds as well as by two equivalent Bi-O bonds with bond valences in the range of 0.2-0.3 v.u. Thermal expansion of BaBi2B4O10 studied by high-temperature X-ray powder diffraction in the temperature range of 20-700 degrees C (temperature step 30-35 degrees C) is highly anisotropic. While the b and c unit-cell parameters increase almost linearly on heating, temperature dependencies of a parameter and beta monoclinic angle show nonlinear behavior. As a result, on heating orientation of thermal expansion tensor changes, and bulk thermal expansion increases from 20 x 10(-6) degrees C-1 at the first heating stage up to 57 x 10(-6)degrees C-1 at 700 degrees C that can be attributed to the increase of thermal mobility of heavy Bi3+ and Ba2+ cations. (c) 2006 Elsevier Inc. All rights reserved.