Structure–Composition Relations for the Partly Disordered Hume-Rothery Phase Ir7+7δZn97−11δ (0.31≤δ≤0.58)



The crystal structure, its variation within the homogeneity range and some physical properties of the new zinc-rich, partly disordered phase Ir7+7δZn97−11δ (0.31≤δ≤0.58) are reported. The structures of three phases with distinct composition were determined by means of single crystal X-ray diffraction. Ir7+7δZn97−11δ exhibits a significant homogeneity range, adopts a complex γ-brass related cubic structure (cF403–406), is stable up to 1201(2) K, and transforms sluggishly below 1048(4) K into a phase with 394 atoms in the monoclinic primitive unit cell. It is a diamagnetic, moderate metallic conductor. Six distinguishable clusters consisting of 22–29 atoms comprise the structure. The clusters are situated about the 16 high symmetry points of the cubic F lattice. The structure can be subdivided into two partial structures, one with constant composition IrZn5 and 192 atoms per unit cell and a second being significantly richer in zinc with variable composition and 211–214 atoms per unit cell. The meandering triply periodic minimal surface of two interpenetrating diamond-like nets separates the compositionally variable from its complementary invariant part. The phase width is coupled with substitutional and positional disorder. A comprehensive analysis of composition-dependent site occupancy factors reveals a linear correlation between the various types of disorder which can be conclusively interpreted in terms of an incoherent intergrowth of distinctive partial structures in variable proportions on a length scale comparable to the size of the approximately 2 nm large unit cell. On the basis of the structural findings we derive the structure chemically meaningful formula Ir7+7δZn97−11δ which quantitatively accounts for the interrelation between substitutional and positional disorder and provides a measure for the homogeneity range in structural terms.