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 γ-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 I4̄3m (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) Å,
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
γ-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.