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

Diffuse layers which are observed in different decagonal quasicrystals perp endicular to the unique 'periodic' axis are explained by one-dimensional (1 D) columns of icosahedral Al clusters along this axis. From results of X-ra y and purely elastic neutron diffraction experiments we give a qualitative explanation by basically off-layer atomic displacements which are grossly u ncorrelated in different columnar clusters. A reversible temperature behavi our of the 1D superstructure which vanishes at high temperatures can be und erstood in the framework of this interpretation. Diffuse intensity modulati ons within the diffuse layers are discussed in terms of atomic displacement s and chemical ordering within the icosahedral clusters which are the basic structural units of the columns. Computer simulations of different columna r models are carried out, their Fourier transforms are compared with the ob served patterns.