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

The reactions between PbO, Bi₂O₃ (or BiOCl), and SeO₂ by the chemical vapor transport method using HCl as a transporting agent afforded three novel bismuth/lead chloroselenites, namely, β-BiSeO₃Cl (1), Bi₆(SeO₃)₄Cl₁₀ (2), and PbBi₁₀(SeO₃)₁₂Cl₈ (3). Compound 1 is noncentrosymmetric (space group Cc, SHG active) and has a giant unit cell (V = 19792(2) ų). In the context of the complex BiSeO₃Cl 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(SeO₃)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 1 and 3 are layered compounds with identical ([M₈Cl₁₆]⁸⁺ and [M₁₄(SeO₃)₂₄]⁶⁻) layers, where M stands for Bi in 1 and Pb/Bi in 3. There are additional [Bi₁₂Cl₃₂]⁴⁺ layered subunits in 1. The crystal structure of 2 consists of the [Bi₆(SeO₃)₄Cl₁₀] building blocks forming an open framework with six-membered-ring channels. These three compounds complete the poorly known bismuth selenium oxochloride panorama.