Zn1− x Pd x ( x =0.14–0.24): a missing link between intergrowth compounds and quasicrystal approximants

Abstract

Our investigations in the Zn-rich portion of the Zn–Pd system are related to the idea that a prototypic 1-dimensional (1D) quasicrystal, as defined by the Fibonacci sequence, can be regarded both as a quasicrystal and as an incommensurately modulated crystal. Consequently, there are two different ways to achieve the atomic structure. Six different new structures, closely related to the γ-brass structure, in the Zn1−x Pd x system have been synthesized and contain Pd-centred Zn icosahedra. A misfit character of all structures with two distinct main subsets and satellite reflections is clear from the diffraction patterns. Among these six structures, two specific examples, Zn10.65Pd2.35 (Zn0.819Pd0.181) and Zn212Pd64 (Zn0.768Pd0.232), have been refined by single crystal X-ray diffraction using traditional refinement methods but also using the (3 + 1)D formalism (superspace group Xmmm(00γ)0s0). Analogies with the Fibonacci sequence are possible if we identify dimers of Pd-centred Zn icosahedra (DI) as short segments, S, and trimers of Pd-centred Zn icosahedra (TI) as long segments, L. By defining these S and L segments, Zn11Pd2 and Zn212Pd64 correspond to the two first simplest approximants of a hypothetical 1D quasicrystal.