Revised Bismuth Chloroselenite System: Evidence of a Noncentrosymmmetric Structure with a Giant Unit Cell

The reactions between PbO, Bi<sub>2</sub>O<sub>3</sub> (or BiOCl), and SeO<sub>2</sub> by the chemical vapor transport method using HCl as a transporting agent afforded three novel bismuth/lead chloroselenites, namely, β-BiSeO<sub>3</sub>Cl (<b>1</b>), Bi<sub>6</sub>(SeO<sub>3</sub>)<sub>4</sub>Cl<sub>10</sub> (<b>2</b>), and PbBi<sub>10</sub>(SeO<sub>3</sub>)<sub>12</sub>Cl<sub>8</sub> (<b>3</b>). Compound <b>1</b> is noncentrosymmetric (space group <i>Cc</i>, SHG active) and has a giant unit cell (<i>V</i> = 19792(2) Å<sup>3</sup>). In the context of the complex BiSeO<sub>3</sub>Cl phase diagram reported by Oppermann et al., it was assigned to the undescribed β-form on the basis of its IR spectra and powder X-ray diffraction pattern. The comparison between the α-, β-, and γ-forms suggests their formation via the condensation of volatile Bi­(SeO<sub>3</sub>)Cl molecules. Analysis of the structures of the α-, β-, and γ-forms indicates that the α → β → γ phase transitions are associated with a dramatic fluctuation of structural complexity together with the transitional character of the β phase. Compounds <b>1</b> and <b>3</b> are layered compounds with identical ([M<sub>8</sub>Cl<sub>16</sub>]<sup>8+</sup> and [M<sub>14</sub>(SeO<sub>3</sub>)<sub>24</sub>]<sup>6−</sup>) layers, where M stands for Bi in <b>1</b> and Pb/Bi in <b>3</b>. There are additional [Bi<sub>12</sub>Cl<sub>32</sub>]<sup>4+</sup> layered subunits in <b>1</b>. The crystal structure of <b>2</b> consists of the [Bi<sub>6</sub>(SeO<sub>3</sub>)<sub>4</sub>Cl<sub>10</sub>] building blocks forming an open framework with six-membered-ring channels. These three compounds complete the poorly known bismuth selenium oxochloride panorama.