Structure of the pseudodecagonal Al-Co-Ni approximant PD4

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 angstrom. 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 angstrom