Complex cubic A6B compounds. II. The crystal structure of Mg6Pd

Abstract

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).* 51"0% Mg; 49"0+0"4% Pd. t 97+2% Mg.

A case study of complex metallic alloy phases: structure and disorder phenomena of Mg–Pd compounds

“…In order to achieve a deeper insight into the relation between the homogeneity range, crystal structure and disorder phenomena of complex metallic alloy phases (CMAs) we have re-determined the Mg-Pd phase diagram in the range 60 to 100 at-% Mg. 2 (θ) and Mg 2 Pd (ι) has been reconfirmed. The first five of them are CMA's.…”

“…Mg 6 Pd crystallizes [2] with a large face-centred cubic unit cell in the space group F-43m. The unit cell parameter a decreases linearly from 20.1985(6) Å at the Mg-rich limit to 20.045(5) Å at the Pd-rich limit.…”

The Archimedean truncated octahedron. II. Crystal structures with geometric units of symmetry 43m

“…Each unit has a unique atom distribution on the same geometric arrangement. Na6TI and Mg6Pd structures are similar (Samson, 1972;Samson & Hansen, 1972). There are two 26-atom ~,-brass-like units, B and C, in these and two other compounds, Ir7Mg44 and Rh7Mg44 (Westin & Edshammar, 1972).…”

An alternative description of the structures of Rh7Mg44 and Mg6Pd

“…In Samson's (1972)description of the same structure type, he uses complexes of 14 icosahedra and 42 centred pentagonal prisms, and a Friauf polyhedron is shared between four such complexes.…”

“…The octahedral capping of the pyrochlore unit (blue in Fig. 2) also contains many pentagonal prisms, a polyhedron used by Samson (1972) in his description of Na6TI and Mg6Pd.…”