Atomic distributions in the γ-brass structure of the Cu-Zn system:: A structural and theoretical study

The crystal structures, atomic distributions, and theoretical electronic structures of five different Cu5-xZn8+x gamma-brass compounds (x = -0.59(3), -0.31(3), 0.00(3), 0.44(3), and 0.79(3)) are reported with the goal of identifying chemical influences on the observed phase width. These structures have been refined by both neutron and X-ray powder diffraction to obtain accurate crystal chemical parameters. All compounds crystallize in the space group I(4) over bar 3m (No. 217) (Z = 4), and the unit cell parameters are a = 8.8565(4), 8.8612(5), 8.8664(3) , 8.8745(4), and 8.8829(7) A, respectively, for Cu5.59Zn7.41, Cu5.31Zn7.69, Cu5.00Zn8.00, Cu4.56Zn8.44, and Cu4.21Zn8.79. The results indicate specific site substitutions on both sides of the ideal composition "Cu5Zn8". In all cases, the 26-atom cluster building up the gamma-brass structure shows a constant inner [Cu4Zn4] tetrahedral star with compositional variation occurring at the outer octahedron and cuboctahedron. First principles and semiempirical electronic structure calculations using both a COHP and Mulliken population analysis were performed to understand the observed compositional range and to address the "coloring problem" for the site preferences of Cu and Zn atoms for this series of compounds.