Crystal chemistry of the mendipite-type system Pb₃O₂Cl₂—Pb₃O₂Br₂

Abstract

Abstract The crystal structures of the mendipite series Pb 3 O 2 Cl 2 —Pb 3 O 2 Br 2 have been solved by direct methods. The structures are based upon [O 2 Pb 3 ] 2+ double chains of edge-sharing OPb 4 tetrahedra. There are three symmetrically independent Pb 2+ cations. The number of nonequivalent halogen sites is two (X1, X2). Short Pb—O bonds are located on one side of the Pb 2+ cations and weak Pb-X bonds are located on the other side of the Pb 2+ coordination sphere. The evident strong distortion of the Pb 2+ coordination polyhedra is due to the stereoactivity of the 6 s 2 lone electron pairs of the Pb 2+ 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 OPb 4 tetrahedra was determined to analyze the influence of halogen atoms on the structure of the [O 2 Pb 3 ] 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 [O 2 Pb 3 ] 2+ double chains to the large halide ions in the crystal structures of the mendipite series compounds.