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) Å3). 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.

以氯化氢（HCl）为输运剂，通过化学气相输运法使氧化铅（PbO）、三氧化二铋（Bi₂O₃，或氯氧化铋BiOCl）与二氧化硒（SeO₂）发生反应，成功得到三种新型铋/铅氯亚硒酸盐：分别为β-亚硒酸氯铋（β-BiSeO₃Cl，编号1）、六铋四亚硒酸十氯盐（Bi₆(SeO₃)₄Cl₁₀，编号2）以及铅铋十二亚硒酸八氯盐（PbBi₁₀(SeO₃)₁₂Cl₈，编号3）。化合物1属于非中心对称结构（空间群为Cc，具备二次谐波发生（Second Harmonic Generation，SHG）活性），且拥有超大晶胞（晶胞体积V=19792(2) ų）。结合Oppermann等人报道的复杂BiSeO₃Cl相图，通过红外光谱与粉末X射线衍射图谱分析，将该化合物归属于尚未见报道的β晶型。对α、β、γ三种晶型的对比分析表明，三者均通过挥发性Bi(SeO₃)Cl分子的缩合反应生成。对α、β、γ三种晶型的结构分析显示，α→β→γ的晶型转变过程伴随着结构复杂度的显著变化，且β相具有过渡相特征。化合物1与3均为层状结构，二者拥有相同的([M₈Cl₁₆]⁸⁺与[M₁₄(SeO₃)₂₄]⁶⁻)层结构：其中化合物1中的M为铋（Bi），化合物3中的M为铅/铋（Pb/Bi）。化合物1中还存在额外的[Bi₁₂Cl₃₂]⁴⁺层状亚结构单元。化合物2的晶体结构由[Bi₆(SeO₃)₄Cl₁₀]结构基元构成，形成带有六元环孔道的开放骨架结构。这三种化合物填补了目前研究尚不完善的铋硒氧氯化合物体系的空白。