THE CRYSTAL STRUCTURE OF PB8O5(OH)2CL4, A SYNTHETIC ANALOGUE OF BLIXITE?

Abstract

The crystal structure of hydrothermally synthesized Pb8O5(OH)2Cl4 [monoclinic, C2/c, a 26.069(5), b 5.8354(11), c 22.736(4) Å, β 102.612(6)°, V 3375.3(11) Å3] has been determined and refined to R1 = 0.047 with data collected from a crystal twinned on (100). There are eight symmetrically independent Pb2+ cations in the structure, with each having a strongly distorted coordination polyhedron due to the presence of stereochemically active pairs of s2 lone electrons on the Pb2+ cations. The structure is based upon [O5Pb8] sheets parallel to (100) formed by edge-sharing (OPb4) oxocentered tetrahedra. Hydroxyl groups form two short (OH)-Pb bonds that result in (OH)Pb2 dimers attached to the [O5Pb8] sheets. The chlorine anions are located between the {[O5Pb8](OH)2} sheets, providing three-dimensional linkage of the structure. The structure is closely related to other structures based on PbO-type defect sheets. On the basis of chemical composition and powder X-ray-diffraction data, we suggest that Pb8O5(OH)2Cl4 is a synthetic analogue of blixite.