Rhombohedrally Distorted γ‑Au_5–<i>x</i>Zn_8+<i>y</i> Phases in the Au–Zn System

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 Au_4.27(3)Zn_8.26(3)□_0.47 (<b>I</b>) and Au_4.58(3)Zn_8.12(3)□_0.3 (<b>II</b>), respectively (□ = vacancy). These (<b>I</b> and <b>II</b>) 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 <i>R</i>3<i>m</i> (No. 160, <i>Z</i> = 6), and the observed lattice parameters from powder patterns were found to be <i>a</i> = 13.1029(6) and 13.1345(8) Å and <i>c</i> = 8.0410(4) and 8.1103(6) Å for crystals <b>I</b> and <b>II</b>, 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