On the Structural Chemistry of γ‐Brasses: Two Different Interpenetrating Networks in Ternary F‐Cell Pd–Zn–Al Phases

Abstract

Novel ternary phases, (Pd(1-x)Zn(x))(18)(Zn(1-y)Al(y))(86-delta) (0<or=x<or=0.162, 0.056</=y<or=0.088, 0<or=delta<or=4), which adopt a superstructure of the gamma-brass type (called gamma'-brass), have been synthesized from the elements at 1120 K. Single-crystal X-ray structural analysis reveals a phase width (F43m, a=18.0700(3)-18.1600(2) A, Pearson symbols cF400-cF416), which is associated with structural disorder based on both vacancies as well as mixed site occupancies. These structures are constructed of four independent 26-atom gamma-clusters per primitive unit cells and centered at the four special positions A (0, 0, 0), B (1/4, 1/4, 1/4), C (1/2, 1/2, 1/2) and D (3/4, 3/4, 3/4). Two of these, centered at B and C, are completely ordered Pd(4)Zn(22) clusters, whereas the other two, centered at A and D, contain all structural disorder in the system. According to our single-crystal X-ray results, Al substitutions are restricted to the A- and D-centered clusters. Moreover, the outer tetrahedron (OT) site of the 26-atom cluster at D is completely vacant at the Al-rich boundary of these phases. Electronic structure calculations, using the tight-binding linear muffin-tin orbital atomic-spheres approximation (TB-LMTO-ASA) method, on models of these new, ternary gamma'-brass phases indicate that the observed chemical compositions and atomic distributions lead to the presence of a pseudogap at the Fermi level in the electronic density of states curves, which is consistent with the Hume-Rothery interpretation of gamma-brasses, in general.

Crystal structure determination of the Γ2 phase in the Fe–Zn–Al system by single-crystal synchrotron X-ray diffraction combined with scanning transmission electron microscopy

“…These Zn sites (Zn11-Zn16) are regarded as split sites with partial occupancies when judged from the very short distance among them (~0.3−0.9 Å) and form a tetrapod centred at the 4b site. This kind of positional disorder accompanied by partial occupancies is commonly observed in many Zn-rich compounds [40][41][42][43]. Although the occupancies for the first 13 sites (Fe1/Fe2/Al1/Zn1-Zn10) were allowed to vary, all of them virtually converged to be unity, indicating the absence of vacancies except for the six split sites (Zn11-Zn16).…”

Substitution Patterns Understood through Chemical Pressure Analysis: Atom/Dumbbell and Ru/Co Ordering in Derivatives of YCo5

“…Most often, this involves atoms of two different elements randomly distributed over the same crystallographic sites. Substitutions of larger atom groups can arise as well, such as disorder in the coordination environment of an atom, leading to the possibility of several different polyhedra fitting into the same space. ,− In this article, we began examining how such substitution can be anticipated from the CP scheme of an idealized, unsubstituted parent structure, drawing specific examples from the Y–Co system. We considered two cases: the partial replacement of Y atoms with Co 2 dumbbells in YCo 5 to create the Th 2 Zn 17 -type compound Y 2 Co 17 , and the elemental site preferences in Ru-substituted variants of Y 2 Co 17 .…”

Non-stoichiometric compositions arising from synergistic electronic and size effects. Synthesis, crystal chemistry and electronic properties of A14Cd1+xPn11compounds (0 ≤ x ≤ 0.3; A = Sr, Eu; Pn = As, Sb)

“…Such atomic arrangement is known as augmented gamma-brass or Pearce cluster. 41,[46][47][48][49][50] It is important to note that the bond distance between the Zn atoms (2.7 Å) in this subunit of the gamma-brass cluster compares well with the homoatomic single-bond (2.66 Å). 38 Importantly, those clusters are well known to have the ability to host many interstitial atoms, particularly light elements such as C, H, O.…”