Structural and theoretical investigations on the “coloring” scheme of γ-brass type phase Ag5Cd8

Abstract

The γ-brass type phase in the binary Ag–Cd system has been synthesized by high-temperature method, and its composition and crystal structure were analyzed by energy dispersive X-ray spectroscopy and X-ray diffraction techniques. The compound Ag5Cd8 crystallizes in the I4¯3m (cI52) space group, having cell parameter of ∼9.9 ​Å, and the unit cell contains 52 atoms distributed over 4 crystallographically independent positions. Due to the weak X-ray scattering contrast between Ag and Cd, the first principle DFT calculations using the Vienna Ab Initio Simulation Package (VASP) code have been employed to address the “coloring problem” in the structure of Ag5Cd8. Like the ideal cubic γ-brass type phases, the structure can be conveniently described by 26-atom γ-cluster made of four concentric shells: inner tetrahedron (IT), outer tetrahedron (OT), octahedron (OH), and cuboctahedron (CO). Theoretical calculations guide that in Ag5Cd8, Cd prefers IT and CO sites and Ag in OT and OH sites of the γ-cluster. The electronic structure calculation for the model Ag5Cd8 shows the appearance of a pseudogap near the Fermi level, which validates the phase stability by the Hume-Rothery mechanism as a result of the Fermi surface-Brillouin zone (FS-BZ) interaction. Bonding analysis (COHP) using the LOBSTER package shows the Ag–Cd heteroatomic interaction being the highest contributor towards the overall stability of the γ-brass type Ag5Cd8.