Preparation, crystal structure and thermal expansion of a new bismuth barium borate, BaBi 2B 4O 10

Abstract

Single crystals of a new compound, BaBi 2B 4O 10 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 (w R=0.113) on the basis of 1813 unique observed reflections (| F o|>4 σ| F o|). It is monoclinic, space group P2 1/ c, a=10.150(2), b=6. 362(1), c=12.485(2) Å, β=102.87(1) o, V=786.0(2) Å 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 [B 4O 10] 8− ∞ chains and bismuthate [Bi 2O 5] 4− ∞ chains extended along b-axis. The borate chains are composed of [B 3O 8] 7− triborate groups of three tetrahedra and single triangles with a [BO 2] − radical. The borate chains are interleaved along the c-axis with rows of the Ba 2+ 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 BaBi 2B 4O 10 studied by high-temperature X-ray powder diffraction in the temperature range of 20–700 °C (temperature step 30–35 °C) is highly anisotropic. While the b and c unit-cell parameters increase almost linearly on heating, temperature dependencies of a parameter and β monoclinic angle show nonlinear behavior. As a result, on heating orientation of thermal expansion tensor changes, and bulk thermal expansion increases from 20×10 −6 °C −1 at the first heating stage up to 57×10 −6 °C −1 at 700 °C that can be attributed to the increase of thermal mobility of heavy Bi 3+ and Ba 2+ cations.