Evidence of phase transition in decagonal Al–Co–Ni at low temperatures

Abstract

Powder samples of Al70.7Co13.3Ni16.0 were investigated by X-ray diffraction in the temperature range 15–300 K. At T=150 K at least three additional reflections were observed. The onset temperature and hysteresis-like behaviour was monitored more closely. The reflections cannot be indexed using integers of reflections of the known decagonal lattice. Moreover, near this temperature lattice parameters of the decagonal phase showed deviations from a smooth curve. We suggest the occurrence of an intermediate phase, which, however, seems to be a property of the powdered sample, i.e. caused by milling single crystals. We will solely describe the experimental facts as they are and make a suggestion for the transition mechanism. It is assumed that the new reflections are due to a periodic superorder of the decagonal lattice as described for a Ni-rich alloy in the literature. From the distance of those satellite reflections to main (Bragg) reflections in reciprocal space we estimate the modulation length of the quasicrystalline substructure to be 10a, i.e. ∼38 Å.