Abstract
The decagonal Al70Mn17Pd13 quasicrystal was studied by high-voltage high-resolution transmission electron microscopy (HRTEM) at 800 KV with a point-to-point resolution of 1.4 Å. Images normal to the tenfold plane revealed a non-periodic arrangement of identical decagonal clusters with a diameter of 20 Å located at the vertices of a tiling. From images normal to the twofold axes it is inferred that the structure in the direction of the tenfold axis is a periodic stacking of layers with a period of 12.5 Å. Starting from the structure originally determined for decagonal Al-Mn by Steurer in 1991 a structure model including Al, Mn and Pd atoms was proposed. Comparison of the simulated images with the real HRTEM images allowed the determination of the most probable positions for the replacement of Pd atoms. High-resolution images normal to the tenfold axis revealed a triangular pattern near the centres of all decagonal clusters, indicating that the structure at the decagonal clusters is reconstructed in the thinnest part of the quasicrystal fragments. A probable model for the reconstruction was obtained, by considering slight shifts of atoms in the puckered layers. The reconstruction could be observed mainly owing to the high resolution of the electron microscope used. It is suggested that this reconstruction is related to a general instability of the quasicrystal in the thinnest specimen region. Because of this instability the thinnest edge of a quasicrystal fragment is generally amorphous.