A new type superstructure in the Al11TM4(TM=Ir, Rh) 1/0 quasicrystalline approximants
K. Nishimoto1, N. Yamaguchi2, R. Tamura2, M. Terauchi1, A.P. Tsai1
1 Institute of Multidisciplinary Research for
Advanced Materials (IMRAM), Tohoku Univ.,
Katahira 2-1-1, Aoba-ku, Sendai-shi, Miyagi, 980-8577, Japan
2Department
of Materials Science and Technology, Tokyo Univ. of Science, Niijuku 6-3-1, Katsushika-ku, Tokyo, 125-8585, Japan
nishi-mt@tagen.tohoku.ac.jp
The Al11TM4(TM=Ir, Rh) compounds are binary 1/0 quasicrystalline approximants for Al-TM quasicrystals. The structure is a primitive cubic lattice with the space group P23 or Pm3 (the lattice constant a = 0.7674 and 0.76692 nm, respectively) [1]. Figure 1 shows the schematically illustration of the Al11TM4 compounds. The unit cell contains “pseudo-Mackay” icosahedral cluster and there is a TM atom in centre of the TM icosahedron. But that possesses disordered Al site inside of the TM icosahedron. On the other hand, the ternary Al-Cu-TM 1/0 approximants possess the FCC lattice with the lattice constant =1.5nm due to the ordered Cu site inside of the TM icosahedron. However, we observed very weak superlattice reflections in the selected area electron diffraction patterns for binary Al11Ir4 (ref.2). And Mihalkovič reported the superstructure for Al11Ir4 compounds by theoretic calculation. In this work, we investigated the superstructure and the composition dependence of the Al11Ir4 and Al11Rh4 by the powder X-ray diffraction method and transmission electron microscopy (TEM) study.
Figure 1. Schematic illustration of Al11TM4 phase.
Al11TM4 with composition ranging from 70 to 75 % Al were prepared by arc melting under an argon atmosphere. And the samples were annealed at 1373K for about 72 hours in the quarts tube under the argon atmosphere, followed by water quenched. Powder X-ray diffraction experiments were carried out using Cu Kα to examine the phase constitution. Additionally, TEM study were performed by JEOL 2010F or TKP2 operating at 200kV. The samples for TEM were crushed by agate mortor and dispersed on micro-grid mesh.
Figure 2 shows selected area diffraction patterns of Al72.5Rh27.5 and Al72.5Ir27.5 taken along [011] and [111] direction. We observed the superlattice reflections at h/2 k/2 l/2 for Al72.5Ir27.5 and Al72.5Rh27.5. In the case of [111] direction of Al72.5Ir27.5, the superlattice reflections appeared at the point to break the 3-fold symmetry. It shown that the type of superstructure of Al72.5Rh27.5 is different from that of Al72.5Ir27.5. Additionally, XRD superlattice peaks disappeared in the poorer Al compositions for Al-Rh system. The composition dependence on the superstructure suggests that the number of Al inside the TM icosahedron contribute to the superstructure.
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Figure 2. Selected area electron diffraction patterns of (a)Al72.5Rh27.5 and (b)Al72.5Ir27.5. |
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1. Y.Grin et. al.,Z.Kristallogr. 212, (1997), 439.
2. A. Oishi, K. Nishimoto, and R. Tamura, Z. Kristallogr. 224, 115 (2009)