Formation of γ-brass type pseudo-binary Ni2Zn11-4δXδ (0≤δ≤∼0.13) (X = In and Ga) by an exchange mechanism

The solid solution phases with general formula Ni2Zn11-4 delta X delta (0 <= delta <= similar to 0.13) (X = In, Ga) have been synthesized by high-temperature solid-state synthesis. Their structures have been characterized by X-ray diffraction and elemental analysis. The phases crystallize in the cubic space group 143m and adopt cubic gamma-brass-type Ni2Zn11 structure, which is made of 26 atom gamma-cluster. The unit cell of the Ni2Zn11-4 delta X delta contains 50-52 atoms depending upon the composition of the compounds. The pseudo-binary phases in Ni-Zn-X (X = In, Ga) are formed by the insertion of In or Ga in the structure of Ni2Zn11 by an exchange mechanism. Instead of direct substitution of Zn, In/Ga occupy a the empty centre of gamma-cluster, probably, due to the large size difference in the atomic radii of Zn and In metals. The insertion of trivalent In or Ga is accompanied by the sharp depletion of bivalent Zn atoms from the gamma-cluster to keep the valence electron concentration constant throughout the entire range of solid solution.