La4.27Mg2.89Zn30, a new structure type with strong positional disorder.

Ternary tetralanthanum trimagnesium tricontazinc, La(4.27)Mg(2.89)Zn(30), crystallizes as a new structure type. It belongs to the structural family that may be derived from the hexagonal CaCu(5) and Th(2)Ni(17) structure types by combination of internal deformation and multiple substitution. The triangular 3(6) and hexagonal 6(3) nets are alternately stacked with Kagomé 3636 nets. The atoms with the largest radius (La) are enclosed in 18-vertex polyhedra (distorted pseudo-Frank-Kasper polyhedra). The coordination polyhedra of the two Mg atoms are bicapped and monocapped hexagonal antiprisms, with coordination numbers of 14 and 13, respectively. For all the Zn atoms, the typical icosahedral coordination is observed. In the direction of the six- and threefold axes, strong positional disorder is observed as a result of partial substitutions of La atoms by pairs of Mg atoms.