Rhombohedrally Distorted gamma-Au5-xZn8+y Phases in the Au-Zn System

TitleRhombohedrally Distorted gamma-Au5-xZn8+y Phases in the Au-Zn System
Publication TypeJournal Article
Year of Publication2013
AuthorsThimmaiah S, Miller GJ
Journal TitleInorganic Chemistry
Date Published02
Type of ArticleArticle
ISBN Number0020-1669
Accession NumberWOS:000314627700022
Keywordsband calculations, brillouin-zone integrations, cu-zn, electron-concentration rule, gamma-brasses, gold-zinc, initio, molecular-dynamics, total-energy calculations, wave basis-set, zinc system

The region of the Au-Zn phase diagram encompassing gamma-brass-type phases has been studied experimentally from 45 to 85 atom % Zn. The gamma phases were obtained directly from the pure elements by heating to 680 degrees C in evacuated silica tubes, followed by annealing at 300 degrees C. Powder X-ray and single-crystal diffraction studies show that gamma-AuSZn8" phases adopt a rhombohedrally distorted Cr5Al8 structure type rather than the cubic Cu5Zn8 type. The refined compositions from two single crystals extracted from the Znand Au-rich loadings are Au4.27(3)Zn8.26(3)square(0.47) (I) and Au4.58(3)Zn8.12(3)square(0.3). (II), respectively (square = vacancy). These (I and II) refinements indicated both nonstatistical mixing of Au and Zn atoms as well as partially ordered vacancy distributions. The structures of these gamma phases were solved in the acentric space group R3m (No. 160, Z = 6), and the observed lattice parameters from powder patterns were found to be a = 13.1029(6) and 13.1345(8) angstrom and c = 8.0410(4) and 8.1103(6) angstrom for crystals I and II, respectively. According to single-crystal refinements, the vacancies were found on the outer tetrahedron (OT) and octahedron (OH) of the 26-atom cluster. Single-crystal structural refinement clearly showed that the vacancy content per unit cell increases with increasing Zn, or valence-electron concentration. Electronic structure calculations, using the tight-binding linear muffin-tin orbital method with the atomic-sphere approximation (TB-LMTO-ASA) method, indicated the presence of a well-pronounced pseudogap at the Fermi level for "Au5Zn8" as the representative composition, an outcome that is consistent with the Hume-Rothery interpretation of gamma brass.

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