Zn_1−xPd_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 Zn_1−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, Zn_10.65Pd_2.35 (Zn_0.819Pd_0.181) and Zn₂₁₂Pd₆₄ (Zn_0.768Pd_0.232), 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.

Acknowledgements

The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-ENG-82. This work was supported by the Office of Basic Energy Sciences, Materials Sciences Division of the US Department of Energy. Also, a part of this work was supported by the NSF Grants DMR-99-81766 and DMR-02-41092.