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Rhombohedrally Distorted γ-Au5–xZn8+y Phases in the Au–Zn System
Inorganic Chemistry
  • Srinivasa Thimmaiah, Iowa State University
  • Gordon J. Miller, Iowa State University
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The region of the Au–Zn phase diagram encompassing γ-brass-type phases has been studied experimentally from 45 to 85 atom % Zn. The γ phases were obtained directly from the pure elements by heating to 680 °C in evacuated silica tubes, followed by annealing at 300 °C. Powder X-ray and single-crystal diffraction studies show that γ-“Au5Zn8” phases adopt a rhombohedrally distorted Cr5Al8 structure type rather than the cubic Cu5Zn8 type. The refined compositions from two single crystals extracted from the Zn- and Au-rich loadings are Au4.27(3)Zn8.26(3)□0.47 (I) and Au4.58(3)Zn8.12(3)□0.3 (II), respectively (□ = 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 γ 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) Å and c = 8.0410(4) and 8.1103(6) Å 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 γ brass.

Reprinted (adapted) with permission from Inorg. Chem., 2013, 52 (3), pp 1328–1337. Copyright 2012 American Chemical Society.

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American Chemical Society
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Srinivasa Thimmaiah and Gordon J. Miller. "Rhombohedrally Distorted γ-Au5–xZn8+y Phases in the Au–Zn System" Inorganic Chemistry Vol. 52 Iss. 3 (2013) p. 1328 - 1337
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