Complex cubic A_6B compounds. II. The crystal structure of Mg_6Pd

Abstract

Mg_6Pd represents the second of the two structure types so far found for complex A_6B compounds of cubic symmetry containing about 400 atoms each per smallest unit cube, arranged according to space group F4{bar]3m. The first type is represented by Na_6Tl. A complete structure determination has been carried out for Mg_6Pd with the use of packing maps and least-squares refinements employing intensity data of 3233 reflections measured with an automated X-ray diffractometer. The length of the cube edge is a_0 = 20.108 ± 0.002 Å (λ Cu Kα_1 = 1.54050 Å, λ Cu Kα_2 = 1.54434 Å). The calculated density is Q_c = 2.905 g.cm^(-3), the measured density is Q_m = 2.94 g.cm^(-3) and the final R value is 0-046. This unit cube contains one formula unit of Mg_(340)Pd_(56), and the atoms are distributed among 14 different point sets. Some of the palladium atoms are in substitutional disorder with magnesium, thus giving rise to an extended range of homogeneity. For this reason it seems practical to retain the formula Mg_6Pd as a name for this structure type. The basic building block of the structure consists of a complex of 8 icosahedra, 4 tricapped trigonal prisms and 48 pentagonal prisms. Four such complexes share a Laves-Friauf polyhedron, the center of which is occupied in this case but was empty in Na_6TI (first type).