title: Structure-composition relations for the partly disordered Hume-Rothery phase Ir7+7deltaZn97-11delta (0.31< or =delta< or =0.58).
abstract: The crystal structure, its variation within the homogeneity range and some physical properties of the new zinc-rich, partly disordered phase Ir7+7deltaZn97-11delta (0.31< or =delta< or =0.58) are reported. The structures of three phases with distinct composition were determined by means of single crystal X-ray diffraction. Ir7+7deltaZn97-11delta exhibits a significant homogeneity range, adopts a complex gamma-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+7delta)Zn97-11delta which quantitatively accounts for the interrelation between substitutional and positional disorder and provides a measure for the homogeneity range in structural terms.
authors.affiliation: Department of Chemistry and Center of Material Sciences, Philipps University Marburg, 35032 Marburg, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Hornfeck
authors.forename: Srinivasa
authors.initials: S
authors.lastname: Thimmaiah
authors.forename: Stephen
authors.initials: S
authors.lastname: Lee
authors.forename: Bernd
authors.initials: B
authors.lastname: Harbrecht
mesh_terms: D000497:Alloys / Q000737:chemistry*; D003460:Crystallization; D004553:Electric Conductivity; D007495:Iridium / Q000737:chemistry*; D008280:Magnetics; D008958:Models, Molecular; D013696:Temperature; D014961:X-Ray Diffraction; D015032:Zinc / Q000737:chemistry*
chemical_list: D000497:Alloys; D007495:Iridium; D015032:Zinc