Syntheses and crystal structures of the homoeotypic selenite bromides Bi-8(SeO3)(9)Br-6 and CsSm21(SeO3)(24)Br-16 [German]

摘要

On the basis of thermodynamic calculations, Bi-8(SeO3)(9)Br-6 was synthesized from BiOBr and SeO2 via an autotransport reaction over a temperature gradient of 670 to 660 K. Crystals of CsSm21(SeO3)(24)Br-16 were obtained from the reaction of an equimolar mixture of SmOBr and SeO2 in a CsBr flux at 1070 K. The two selenite bromides crystallize in the orthorhombic space group B 2/b 2/a 2/b (Nr. 68) with pseudo-tetragonal lattices (Bi-8(SeO3)(9)Br-6: a = b = 1588.4(5), c = 1830.5(5) pm, Z = 5.333; CsSm21(SeO3)(24)Br-16: a = b = 1579.7(2), c = 1796.3(2) pm, Z = 2). In the crystal structures, the heavy atoms form square nets. The O atoms of the pyramidal selenite groups connect them into slabs of composition [Bi-21.33(III)(SeIVO3)(24)Br-16] or [CsSm21III((SeO3)-O-IV)(24)Br-16], which are stacked along [001]. In CsSm21(SeO3)(24)Br-16, electroneutrality is achieved through the substitution of one Sm-III cation per formula unit by a Cs-I cation. In the case of Bi-8(SeO3)(9)Br-6 = Bi-21.33(III)((SeO3)-O-IV)(24)Br-16, the partial depopulation of Bi positions maintains charge balance. The intrinsic symmetry of the slabs, which corresponds to layer group P (4) 2/b 2/m, results in pseudomerohedral twins with twin axis [110]. [References: 61]