Reversible transformation between an icosahedral Al-Pd-Mn phase and a modulated structure of cubic symmetry

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Abstract
A new Al-Pd-Mn phase, called F2M, and its reversible transformation into an icosahedral structure at high temperatures were studied by transmission electron microscopy (TEM) and by in-situ X-ray diffraction using synchrotron light sources. The phase F2M appears to be closely related to the F2 super-ordered icosahedral phase identified by Ishimasa and Mori (1992, Phil. Mag. Lett., 71, 65) and has almost the same chemical composition. As identified by TEM, its structure is of cubic symmetry 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 first- and second-order satellite reflections around the main and superstructure 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 be interpreted considering the superposition of ten independent cosine phason waves having a polarization along threefold axes in the perpendicular space. Above 740°C, the F2M phase transforms into an icosahedral structure. Below 740°C, the reverse transformation was observed with kinetics in agreement with an atom-diffusion-controlled mechanism. When going through the phase transition from the high-temperature phase, all high-Q, reflections have their intensity strongly decreased whereas satellites reflections appears around them in agreement with a scheme of a phason-driven phase transition. Annealing of the sample at 650°C after rapid cooling from 750°C led to an F2 phase with a large amount of Q-dependent diffuse scattering instead of satellites reflections.