Evidence of phase transition and measurement of thermal expansion in decagonal Al–Co–Ni at low temperatures

Abstract

Powder samples of decagonal AL 70.7Co 13.3Ni 16.0 were investigated by X-ray diffraction (XRD) in the temperature range 15–300 K. We evaluated the temperature dependence of lattice parameters a and c, along and perpendicular to the quasiperiodic plane, and observed indications of a phase transition. Temperature dependences were modelled best by second order polynomials leading to linear thermal coefficients whose anisotropy ratio γ( T)/ α( T) was found to decrease with increasing temperature from 1.60 (at 15 K) to 1.05 (at 300 K). At T=150 K the occurrence of an intermediate phase is indicated by at least three additional reflections, which cannot be indexed using integers of reflections of the decagonal lattice. Furthermore, near this temperature the lattice parameters of the decagonal phase showed deviations from a smooth polynomial fit. We will solely describe the experimental facts as they are and make the suggestion that this transition is due to a periodic superorder of the decagonal lattice.