Crystal chemistry of two monoclinic modifications of Bi₂Cu(SeO₃)₄

Two modifications of hydrothermally synthesized Bi2Cu(SeO3)4 crystallize in space group P21/c: a 4.440(1), b 15.852(4), c 8.094(2) .ANG., beta 94.95(3)° and a 10.513(2), b 7.224(3), c 8.160(3) .ANG., beta 110.60(2)°; Z = 2. The crystal structures were refined from 2067 obsd. x-ray reflections to R = 0.029, Rw = 0.033 and from 2100 obsd. reflections to R = 0.046, Rw = 0.037. At. coordinates are given. The irregular coordination around the BiIII atoms is caused by the lone-pair electrons, the CuII atoms are planar [4] coordinated; the SeIVO3 groups form the usual trigonal pyramids. Distinguishing features are Bi2(SeO3)2 layers, which are interconnected in the two structures by Cu(SeO3)2 chains and Cu(SeO3)2 layers, resp.