On the Structural Chemistry of [gamma]-Brasses: Two Different Interpenetrating Networks in Ternary F-Cell Pd-Zn-Al Phases

• Published in: Chemistry : a European journal 2010-05, Vol.16 (18), p.5461
• Main Authors: Thimmaiah, Srinivasa, Miller, GordonJ
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200903300

Novel ternary phases, (Pd1-xZnx)18(Zn1-yAly)86-[delta] (0≤x≤0.162, 0.056≤y≤0.088, 0≤[delta]≤4), which adopt a superstructure of the [gamma]-brass type (called [gamma]'-brass), have been synthesized from the elements at 1120K. Single-crystal X-ray structural analysis reveals a phase width (F$\bar 4$3m, a=18.0700(3)-18.1600(2)Å, Pearson symbols cF400-cF416), which is associated with structural disorder based on both vacancies as well as mixed site occupancies. These structures are constructed of four independent 26-atom [gamma]-clusters per primitive unit cells and centered at the four special positions A (0, 0, 0), B (1/4, 1/4, 1/4), C (1/2, 1/2, 1/2) and D (3/4, 3/4, 3/4). Two of these, centered at B and C, are completely ordered Pd4Zn22 clusters, whereas the other two, centered at A and D, contain all structural disorder in the system. According to our single-crystal X-ray results, Al substitutions are restricted to the A- and D-centered clusters. Moreover, the outer tetrahedron (OT) site of the 26-atom cluster at D is completely vacant at the Al-rich boundary of these phases. Electronic structure calculations, using the tight-binding linear muffin-tin orbital atomic-spheres approximation (TB-LMTO-ASA) method, on models of these new, ternary [gamma]'-brass phases indicate that the observed chemical compositions and atomic distributions lead to the presence of a pseudogap at the Fermi level in the electronic density of states curves, which is consistent with the Hume-Rothery interpretation of [gamma]-brasses, in general.