Local Structural Variations in A1 72 M 20 Co 8 Decagonal Quasicrystals
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Local Structural Variations in Al72Ni20Co8 Decagonal Quasicrystals Yanfa Yan National Renewable Energy Laboratory, Golden, CO 80401 S.J. Pennycook Oak Ridge National Laboratory, Oak Ridge, TN 37830 ABSTRACT We investigate local structural variations in Al72Ni20Co8 quasicrystals using first principles density-functional total-energy calculations. We find that local chemical fluctuation can cause large structural variations on the central rings of the 2-nm clusters. Such structure variations may result in ambiguities at the central rings in high-resolution electron microscopy images of Al72Ni20Co8 decagonal quasicrystals. INTRODUCTION It is now well accepted that decagonal quasicrystals can be described by quasiperiodic packing of atomic clusters [1]. For example, all Al-based decagonal quasicrystals (e.g. Al-Ni-Fe, Al-Ni-Co, Al-Cu-Co, …) comprise distinct 2-nm clusters [2-5]. The ideal decagonal quasicrystals can be effectively described by the Gummelt coverage model, i.e., a tiling formed by asymmetric decagons [6-14]. The coverage picture gives some degree of physical insight into how the decagonal quasicrystalline state may actually grow through the sharing of atom positions in neighboring clusters. To understand fully why and how quasicrystals form, it is important to understand the atomic structure of the 2-nm clusters. So far, many structural models have been proposed for decagonal quasicrystals based on X-ray and neutron diffraction, high-resolution electron microscopy and theoretical calculations [15-18]. However, controversies still remain among these models. For example, the structure models proposed by various researchers for Al72Ni20Co8 decagonal quasicrystals show some similarities, but the details still remain controversial, particularly regarding the structure of the innermost ring. A possible reason is that the structure of the innermost ring is sample dependent. For example, a subtle difference in the local chemical composition, temperature, and sample preparation may cause different structure in the innermost ring. Our Z-contrast images showed that even in a single image, the 2-nm clusters exhibit differences at the innermost rings, as shown in Figure 1. In the present paper, we investigate chemical fluctuation-induced local structural variations in Al72Ni20Co8 quasicrystals using first-principles density-functional totalenergy calculations. We find that a subtle local chemical fluctuation (for example, vacancies) can cause large structural variations on the central rings of the 2-nm clusters. Such structure variations may result in ambiguities at the central rings in high-resolution electron microscopy images of Al72Ni20Co8 decagonal quasicrystals.
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Figure 1. Z-contrast image of a Al72Ni20Co8 decagonal quasicrystal, showing structure variations at the central ring marked by white circles. METHODOLOGY Our calculations are based on the density-functional theory, using the Vienna abinitio Simulation Program (VASP) [19]. We used the local density approximation for exchange correlatio
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