Pd2.28(1)Zn10.37(1)Al0.35(1), a ternary γ-brass-type structure

Palladium zinc aluminium (2.28/10.37/0.35), Pd2.28(1)Zn10.37(1)Al0.35(1), represents the upper limit of Al substitution into the parent cubic γ-brass Pd2+xZn11−x. The structure can be described in terms of a 26-atom cluster consisting of an inner tetrahedron (IT), an outer tetrahedron (OT), an octahedron (OH) and a cuboctahedron (CO), with the substituted Al atoms partially occupying the IT (.3m) and CO (..m) sites.

In terms of the 26-atom clusters (in bcc arrangement) commonly used to describe the structure of γ-brass, the inner tetrahedron (IT) and cuboctahedron (CO) are occupied by mixtures of Zn and Al atoms, the outer tetrahedron (OT) is fully occupied by Pd atoms, and the octahedron (OH) is occupied by a mixture of Zn and Pd atoms (Fig. 1a). Similar mixing of Zn and Pd atoms on the OH sites is observed in binary Pd 2 + x Zn 11 -x (Gourdon & Miller, 2006;Edström & Westman, 1969). An alternative description involves four interpenetrating icosahedra, which are constructed around each OT atom and encapsulate a tetrahedron formed by IT atoms (Fig. 1 b). The IT and OT sites are each surrounded by 12 nearest neighbours [at distances of 2.666 (1)-2.789 (2) Å and 2.624 (1)-2.794 (1) Å, respectively] forming distorted icosahedra.
On the other hand, the coordination numbers are 13 around the OH site [2.591 (2)-2.945 (1) Å] and 11 around the CO site

Experimental
The title compound was prepared from 0.5 -g mixtures of the elements (Pd foil, MPC, Ames Laboratory, 99.999%; Zn ingot, MPC, Ames Laboratory, 99.999%; Al tear drop, MPC, Ames Laboratory, 99.999%) loaded into cleaned Ta tubes, which were placed in evacuated (10 -5 torr) and sealed silica tubes. The tubes were heated at 30 °C h -1 to 850 °C, kept there for 12 h, cooled to 550 °C over 12 h, equilibrated there for 3 d, and then cooled to room temperature by shutting off the furnace.

Refinement
Refinement of a starting model (Gourdon & Miller, 2006) led to a mixture of 0.09 (3) Pd and 0.91 (3) Zn in the OH sites. However, the IT and CO sites, initially assumed to be fully occupied by Zn atoms, exhibited elevated isotropic displacement parameters. Modeling these sites with a mixture of Zn and Al resulted in the refined composition Pd 2.28 (1) Zn 10.37 (1) Al 0.35 (1) . Analysis of multiple crystals obtained from the same and other batches gave the same site occupancies. Within the limitation of the technique, semiquantitative energy-dispersive X-ray analysis corroborate this chemical composition. The structure was standardized by means of the program STRUCTURE TIDY (Gelato & Parthé, 1987).
The highest peak and the deepest hole are located 1.26 Å and 1.18 Å, respectively, from Pd1. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.