X-RAY DIFFUSE-SCATTERING IN THE DECAGONAL PHASES AL70NI15CO15, AL72.5NI11CO16-CENTER-DOT-5 AND AL62CU20CO15SI3 UP TO 1150 K

The tide compounds have an isotypic averaged structure but differ in s ignificant detail with respect to disorder. Disorder diffuse scatterin g has been studied in situ up to 1150 K with a heating camera and X-ra y 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 di ffuse layers and diffuse phenomena, in particular diffuse streaks, wit hin 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 fo rmer decrease in intensity above 770 K and vanish completely at about 1070 K, whereas the intensity of the diffuse streaks decreases between 1120 and 1150 K. Both processes are time dependent but fully reversib le. This decrease in disorder in the quasicrystalline packing occurs i n the same temperature regime where an extension of the stability fiel d of the decagonal Al70Ni15Co15 phase was reported earlier.