Understanding the Structure and Stability of a Partially Disordered “Push–Pull Alloy” Ni 2 Zn 11– x Ga x (0.4 ≤ x ≤ 1.25)

This study identifies two new cubic γ-brass-type phases in the Ni-Zn-Ga system: (a) Ni2Zn11-xGax (0 x ≤ 0.3) exist in a narrow homogeneity range, and the compounds crystallize in a cubic crystal system with space group of I4̅3m (a ≈ 9 Å), (b) Ni2Zn11-xGax (0.4 ≤ x ≤ 1.25) has a broader homogeneity range and the compounds in this region adopt the primitive cubic lattice with the space group P4̅3m (a ≈ 9 Å). The Ga insertion into Ni2Zn11 is site specific, and at the limiting composition, Ni2Zn9Ga1.25, the structure is completely ordered and comprises the two distinct clusters─a 26-atom γ-brass-type cluster (Ni4Zn18Ga4) and a 23-atom defect body-centered cubic (bcc)-type cluster (Ni4Zn18Ga1). The stability of these compounds can be rationalized by favorable enthalpies of formation obtained from the theoretical calculations, which correlate strongly with the coexistence of specific clusters within the primitive cubic unit cell. Electronic structure calculations indicate that these phases are stabilized by the Hume-Rothery mechanism, characterized by the presence of a pseudogap at the Fermi level. The chemical bonding analysis demonstrates that the interactions between Ni-Zn and Ni-Ga are crucial to the structural stability.