Anomalous-X-Ray Scattering Associated with Short-Range Order in an Al70Ni15Co15 Decagonal Quasicrystal

Hiroshi Abea,b,*, Nobumichi Tamura b, D. Le Bollocfh b, Simon C. Moss b, Yoshie Matsuo c, Yasushi Ishii d, Jianming Bai e

a Department of Materials Science and Engineering, National Defense Academy, Yokosuka 239-8686, Japan
b Department of Physics, University of Houston, Houston, Texas 77204-5506, USA
c Department of Physics, Nara Women's University, Nara 630-8263, Japan
d Department of Physics, Chuo University, Tokyo 112-8551, Japan
e Oak Ridge Associated Universities, Oak Ridge, Tennessee 37831, USA

Mater. Sci.  Eng. A 294-296, 299 (2000).

The diffuse scattering around Bragg reflections was observed in a single decagonal quasicrystal of Al70Ni15Co15 using anomalous-x-ray scattering. The intensities of diffuse scattering vary with the three incident X-ray wavelengths, near the Co K-edge and the Ni K-edge and between them. The contrast of diffuse scattering shows the existence of atomic short-range order (SRO) between Ni and Co. The distributions of diffuse scattering are anisotropically modulated and the diffuse shapes vary from one reciprocal lattice point to another. In quasicrystals, anisotropic diffuse shapes derive from phason strain. It is concluded that both SRO and phason strains exist in Al70Ni15Co15. In addition, as the first neighbor of Ni, Ni is preferred to Co statistically, since SRO intensities appear about zone-centers, i.e. there is local clustering of these species.

Keywords: Short-range order, Phason, Anomalous-X-ray scattering, Decagonal quasicrystal


FIG.1 

Reciprocal space perpendicular to the periodic direction of the decagonal Al70Ni15Co15 phase. The area of the spot is proportional to the intensity of the reflection. The diffuse scattering has been measured around the labeled reflections. 
FIG.2 

ISRO(fNi-fCo) + [ISRO]*[Iphonon+phason] around (a)  10330, (b) 30440 and (c) 12540, where ISRO(fNi-fCo) presents diffuse scattering from short-range order between Ni and Co. Iphonon+phason reveals diffuse intensity from phonons and phasons. Absolute intensity is given in electron units. Calculated distributions around (d)  10330, (e)  30440 and (f) 12540 are based on phason elastic theory.

 		 FIG.3 
 

Pair-correlation function models of the first nearest neighbors. Model (a), (b) and (c) satisfy symmetry. Open circles reveal positive values of a parameters (an = 0.1) and closed circles present smaller positive values (an = 0.05).
Fig. 4 

Calculated diffuse intensity from short-range order between Ni and Co using model in Fig. 3(a), where an=0.1 (1<n<10 ). Diffuse shapes of model (a), (b) and (c) are almost the same, although the actual diffuse intensity depends on proposed models.

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