Structure determination of structurally complex Ag36Li64 gamma-brass

The crystal structure of the Ag(36)Li(64) gamma-brass was determined by analyzing the powder diffraction pattern taken using a synchrotron radiation beam with wavelength 0.50226 A. It turned out that the compound contained 52 atoms in its unit cell with the space group I43m and that the Li atom enters exclusively into inner tetrahedral (IT) and cubo-octahedral (CO) sites, whereas the Ag atom enters into those on outer tetrahedral (OT) and octahedral (OH) sites in the 26-atom cluster. Small amounts of Li also exist in OT and OH sites, resulting in chemical disorder. We discovered that the volumes of the IT and CO polyhedra shrink, while those of the OT and OH polyhedra expand relative to those of the corresponding polyhedra in the original b.c.c. (body-centered cubic) structure. This feature is universal and is found in other gamma-brasses such as Cu(5)Zn(8) and Al(8)V(5), for which the structure data are available. Among these gamma-brasses, we revealed the unique bond-length distribution for pairs connecting the atom on OH sites and that on CO sites, depending on the degree of d-p orbital hybridization between the transition metal elements such as Ag, Cu and V on OH sites, and the non-transition metal elements such as Li, Zn and Al on CO sites. It is suggested that this may hold a clue to resolving why some gamma-brasses such as the present Ag-Li and Cu-Zn possess a finite solid solution, but others such as Al(8)V(5) and Mn(3)In exist as line compounds.