Complex Cubic A 6 B Compounds . II . The Crystal Structure of Mg 6 Pd

Mg6Pd represents the second of the two structure types so far found for complex A6B compounds of cubic symmetry containing about 400 atoms each per smallest unit cube, arranged according to space group F-43m. The first type is represented by Na6T1. A complete structure determination has been carried out for Mg6Pd with the use of packing maps and least-squares refinements employing intensity data of 3233 reflecuons measured with an automated X-ray diffractometer. The length of the cube edge is ao = 20.108+0.002/~ (2 Cu K0q= 1.54050 A, 2 Cu Ke2= 1.54434 A). The calculated density is oc=2"905 g.cm -3, the measured density is Q,, = 2.94 g.cm -3 and the final R value is 0-046. This unit cube contains one formula unit of Mg340Pd56, 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 Mg6Pd 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 Na6TI (first type).