Rhombohedrally Distorted γ-Au5–xZn8+y 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-"Au(5)Zn(8)" phases adopt a rhombohedrally distorted Cr(5)Al(8) structure type rather than the cubic Cu(5)Zn(8) 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) (I) and Au(4.58(3))Zn(8.12(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 "Au(5)Zn(8)" as the representative composition, an outcome that is consistent with the Hume-Rothery interpretation of γ brass.