Refinement of d-AlNico Quasicrystal Structures
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Refinement of d-AlNiCo Quasicrystal Structures Hiroyuki Takakura *, Akiji Yamamoto** and An Pang Tsai* *National Research Institute for Metals, Tsukuba, Japan. CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama, Japan. **National Institute for Research in Inorganic Materials, Tsukuba, Japan. [email protected]
ABSTRACT The models of decagonal Al Ni Co quasicrystal with the space group of
were refined on the basis of single crystal x-ray diffraction data set using the 5D description. The results of a structure model derived from Al Fe -type approximant crystal and Burkov model are compared. The former gives =0.045 and =0.063 for 449 reflections with 103 parameters and a reasonable chemical composition of Al TM (TM=transition metals). The projected structure is consistent with high resolution images of this material. On the other hand, the latter gives =0.161 and =0.193 for 55 parameters and a composition of Al TM . INTRODUCTION Several stable decagonal quasicrystals have been found in different kind of alloy systems: Al-Pd-Mn, Al-Pd-TM (TM=Fe, Ru, Os), Al-Cu-Co, Al-Ni-Co, and Zn-Mg-RE (RE=Y, Dy, Ho, Lu, Tb, Gd) [1]. They show various periods along the axis (tenfold axis); 4, 8, 12, or 16 ˚ . Although the Al-Ni-Co system have been extensively studied among them, its detailed atomic structure is still under discussion. One can obtain highly perfect phase of the decagonal Al-Ni-Co quasicrystals [2, 3]. The representative of this ideal phase has a composition Al Ni Co which shows neither diffuse scattering nor superlattice reflections [3]. Moreover, this phase has shortest period 4 ˚ along the axis among the decagonal quasicrystals. Therefore, the Al Ni Co compound is the best candidate to investigate its detailed atomic structure. The Al Ni Co phase has only been investigated by electron microscopy techniques so far [4-8]. The HAADF images clearly showed that the transition metal (TM) atom positions[4, 7] and a symmetry breaking of so-called !" ˚ tenfold cluster [10]. Several competing structure models have been proposed to interpret these findings [4-8]. A structure model has been proposed $# on the basis of local structures of &%(') -type approximant crystals [5, 9]. Another structure model based on the often-discussed Burkov model [11] has also been proposed for the same material [7]. The main issue is whether or not the symmetry breaking is intrinsic. Meanwhile, in response to these new experimental evidences, a different conceptual model based on the Gummelt prototile decagon [12] has been presented [6, 8]. The present work compares the results of the structure refinements of decagonal Al Ni Co quasicrystal based on a model derived from the approximant crystal with that based on the Burkov model.
K9.9.1
9 8 7 4 5 2 1 10 6 3
TM
23 19 20 1816 21 22 14 1312 11 17 15
TM/Al ~0.5/0.5
Al/TM ~0.8/0.2
Al
Figure 1: The ODs of the best fit model of Al Ni Co . EXPERIMENT An a
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