PbCa2[Al8O15] with a novel three-dimensional aluminate anion

The asymmetric unit of the title compound, lead(II) dicalcium octaaluminate, contains one Pb, one Ca, four Al and eight O atoms, with the Pb atom and one O atom situated on mirror planes. Three Al atoms exhibit slightly distorted tetrahedral coordinations with a mean Al-O bond length of 1.76 Å. The fourth Al atom is in a considerably distorted trigonal-bipyramidal coordination with a mean Al-O bond length of 1.89 Å. One AlO(4) tetrahedron forms infinite chains parallel to [100] via corner-sharing. These chains are linked by parallel chains of edge-sharing AlO(5) trigonal bipyramids into layers A of six-membered double rings extending parallel to (010). The second layer B is made up of the remaining two AlO(4) tetrahedra. These tetrahedra share corners, resulting in likewise six-membered double rings. Finally, the parallel layers A and B are linked into a three-dimensional framework by common corners. Charge compensation is achieved by the Pb(2+) and Ca(2+) cations, which are situated in the cavities of the anionic framework, and which are surrounded by seven and six O atoms, respectively, both within highly irregular coordination polyhedra.