Convergent-Beam Electron Diffraction and Electron Microscope Studies of Decagonal Quasicrystals

Decagonal quasicrystals have been investigated by convergent-beam electron diffraction (CBED) and transmission electron microscopy. The CBED method has revealed that there exist two quasicrystals which belong to the space groups of noncentrosymmetric P^^^-m2 and centrosymmetric P10_5/mmc. The dark-field microscopy has revealed the existence of inversion domains with an antiphase shift of c/2 at the domain boundaries in the alloys with the space group P^^^-m2. High-resolution electron microscopy has revealed the existence of specific pentagonal atom clusters in all the alloys except Al-Ni-Rh and Al-Ni-Ir alloys. The screw axes in the quasicrystals with the space group of P10_5/mmc exist at the centers of the clusters in Al-Ni-Rh and Al-Ni-Ir alloys, but exist between the neighbouring two pentagonal atom clusters in the other alloys. A phase transformation from P^^^-m2 to P10_5/mmc occurs through the mixing of atom clusters with the positive and negative polarities