CRYSTAL CHEMISTRY OF BASIC LEAD CARBONATES. II. CRYSTAL STRUCTURE OF SYNTHETIC 'PLUMBONACRITE'

Abstract

The crystal structure of synthetic 'plumbonacrite', Pb5O(OH)(2)(CO3)(3), hexagonal, P6(3)cm, a = 9.0921(7), c = 24.923(3) Angstrom, V = 1784.3(3) Angstrom (3), Z = 6, has been solved by direct methods and refined to R = 0.049. There are six symmetrically independent Pb2+ positions in the structure, the coordination polyhedra of which are strongly distorted due to the effect of s(2) lone-electron pairs on the Pb2+ cations. The Pb(6) position is disordered with a Pb-Pb distance of 1.24 Angstrom. The structure of 'plumbonacrite' is built from complex Pb-O layers parallel to the (001) plane similar to those observed in polymorphs of Pb-4(OH)(2)(SO4)(CO3)(2). The structure of these layers can be described in terms of Pb-O,OH motifs and CO3 groups. The smallest structural subunit is the oxocentred [OPb4] tetrahedron that shares three of its Pb-Pb edges with three [(OH)Pb-3] distorted triangles to form [O(OH)(3)Pb-7] clusters. These clusters are surrounded by CO3 groups, forming more complex clusters. Due to the disorder in the Pb(6) site, the clusters comprising this site are linked with each other via OH(10) groups to give continuous sheets of Pb-O,OH bonds which are parallel to the (001) plane. The [O(OH)(3)Pb-7] clusters in 'plumbonacrite' may be important in more complex Pb carbonate oxo/hydroxo complexes that may exist in aqueous environments.