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Abstract
A model for the pseudodecagonal approximant PD4 in the Al–Co–Ni system was deduced from single crystal X-ray diffraction data. The space group is Bbmm with a = 101.3, b = 32.1 and c = 4.1 Å. Atomic positions of 133 unique atoms in the unit cell with a reasonable geometry were found by direct methods and the difference Fourier syntheses. The obtained structure model is in good agreement with high-resolution electron microscopy images of PD4. Diffuse scattering observed along the a * direction in the hkl layers with l = 1/2, 3/2 etc. indicates a superstructure with a doubling of the periodicity along the c -axis and a lamellar disorder along the a -axis. If this diffuse scattering is taken into account, c = 8.2 Å.
Acknowledgements
This project is supported by the Swedish Research Council. X. D. Zou is a research fellow of the Royal Swedish Academy of Sciences supported by a grant from the Knut and Alice Wallenberg Foundation. L. Demchenko is supported by a grant from the Royal Swedish Academy of Sciences.