ORDER-DISORDER TRANSFORMATION IN AN AL-NI-CO DECAGONAL QUASI-CRYSTAL

The temperature dependence of the degree of order has been studied by X-ray diffractometry for the superlattice-type decagonal quasicrystal recently found in the Al-Ni-Co alloy system. On raising the temperature, all the superlattice peaks start to lose intensity at about 750-degrees-C, above which two different behaviours have been observed, the superlattice peaks of one group keep about 40% of the original intensity at 850-degrees-C, while those of the other vanish almost completely at the same temperature. The appearance of the two different behaviours of the superlattice peaks is attributable to the existence of two independent order parameters in this system. The superlattice peaks regain the lost intensity by subsequent low-temperature annealing, indicating that the intensity changes are reversible. A possibility of realization of a novel type of ordering is suggested for the present superlattice decagonal quasicrystal.