Crystal chemistry of the mendipite-type system Pb₃O₂Cl₂—Pb₃O₂Br₂

Abstract

The crystal structures of the mendipite series Pb₃O₂Cl₂—Pb₃O₂Br₂ have been solved by direct methods. The structures are based upon [O₂Pb₃]²⁺ double chains of edge-sharing OPb₄ tetrahedra. There are three symmetrically independent Pb²⁺ cations. The number of nonequivalent halogen sites is two (X1, X2). Short Pb—O bonds are located on one side of the Pb²⁺ cations and weak Pb-X bonds are located on the other side of the Pb²⁺ coordination sphere. The evident strong distortion of the Pb²⁺ coordination polyhedra is due to the stereoactivity of the 6s² lone electron pairs of the Pb²⁺ cations. Pb1-X2 and Pb2-X2 bonds are the most sensitive to the X site occupancy, which is in agreement with the non-linear behavior of the a and c parameters. Determination of unit-cell parameters by single crystal studies showed strong deviation from Vegard's rule. Nonlinearity of the lattice parameters is caused by selective ordering of the halide anions over X1 and X2 sites. Br atoms prefer the X2 position, whereas Cl prefers the X1 site. The angle between two adjacent OPb₄ tetrahedra was determined to analyze the influence of halogen atoms on the structure of the [O₂Pb₃]²⁺ chain. Different occupancy of the X1 site by Cl and Br atoms leads to most pronounced angular changes. These observations may be interpreted as adaptation of the [O₂Pb₃]²⁺ double chains to the large halide ions in the crystal structures of the mendipite series compounds.