X-ray diffuse scattering in the decagonal phases, al₇₀ni₁₅co₁₅ al_72·5ni₁₁and al₆₂cu₂₀co15si₃up to 1150k

The title compounds have an isotypic averaged structure but differ in significant detail with respect to disorder. Disorder diffuse scattering has been studied in situ up to 1150 K with a heating camera and X-ray films and an image-plate system at the synchrotron source Deutsches Elektronensynchrotron/HASYLAB. With the oscillation technique (primary beam perpendicular to the ‘periodic’ direction) the area between the zeroth and the fourth Bragg layer was explored. There are modulated diffuse layers and diffuse phenomena, in particular diffuse streaks, within the Bragg layers. The diffuse layers are related to a superperiod along the ‘periodic’ direction and the streaks arise from disorder in the quasicrystalline packing. A different temperature behaviour of the diffuse layers compared with the diffuse streaks was observed. The former decrease in intensity above 770 K and vanish completely at about 1070 K, whereas the intensity of the diffuse streaks decreases between 1120 and 1150K. Both processes are time dependent but fully reversible. This decrease in disorder in the quasicrystalline packing occurs in the same temperature regime where an extension of the stability field of the decagonal Al₇₀Ni₁₅Co₁₅phase was reported earlier.