On the one-dimensional 8Ǻ periodic superstructure in decagonal phases

Diffuse layers which are observed in different decagonal quasicrystals perpendicular to the unique ‘periodic’ axis are explained by one-dimensional (1D) columns of icosahedral A1 clusters along this axis. From results of X-ray and purely elastic neutron diffraction experiments we give a qualitative explanation by basically off-layer atomic displacements which are grossly uncorrelated in different columnar clusters. A reversible temperature behaviour of the 1D superstructure which vanishes at high temperatures can be understood in the framework of this interpretation. Diffuse intensity modulations within the diffuse layers are discussed in terms of atomic displacements and chemical ordering within the icosahedral clusters which are the basic structural units of the columns. Computer simulations of different columnar models are carried out, their Fourier transforms are compared with the observed patterns.