REVERSIBLE TRANSFORMATION BETWEEN AN ICOSAHEDRAL AL-PD-MN PHASE AND AMODULATED STRUCTURE OF CUBIC SYMMETRY

A new Al-Pd-Mn phase, called F2(M), and its reversible transformation into an icosahedral structure at high temperatures were studied by tra nsmission electron microscopy (TEM) and by in-situ X-ray diffraction u sing synchrotron light sources. The phase F2(M) appears to be closely related to the F2 super-ordered icosahedral phase identified by Ishima sa and Mori (1992, Phil. Mag. Lett., 71, 65) and has almost the same c hemical composition. As identified by TEM, its structure is of cubic s ymmetry and non-periodic. An overall icosahedral symmetry results with crystallographic orientational relationships between domains of cubic symmetry. The room-temperature X-ray diffraction pattern presents fir st- and second-order satellite reflections around the main and superst ructure Bragg peaks of the F2 phase. They are located along directions parallel to threefold axes with a wave-vector equal to a quarter of a six-dimensional reciprocal-lattice vector of the icosahedral Al-Pd-Mn lattice. In a first approximation, these satellites reflections can b e interpreted considering the superposition of ten independent cosine phason waves having a polarization along threefold axes in the perpend icular space. Above 740 degrees C, the F2(M) phase transforms into an icosahedral structure. Below 740 degrees C, the reverse transformation was observed with kinetics in agreement with an atom-diffusion-contro lled mechanism. When going through the phase transition from the high- temperature phase, all high-Q(perpendicular to) reflections have their intensity strongly decreased whereas satellites reflections appears a round them in agreement with a scheme of a phason-driven phase transit ion. Annealing of the sample at 650 degrees C after rapid cooling from 750 degrees C led to an F2 phase with a large amount of Q(perpendicul ar to)-dependent diffuse scattering instead of satellites reflections.