MECHANISM OF THE PHASE TRANSFORMATIONS IN DECAGONAL QUASICRYSTALS. Walter Steurer, Matthias Honal and Torsten Haibach, Laboratory of Crystallography, ETH Zentrum, CH-8092 Zurich, Switzerland

A geometrical model for the mechanism of the transformation of decagonal quasicrystals to crystalline phases explaining the experimental diffraction phenomena has been developed.

Most decagonal phases studied experimentally so far have been found to be stable in only a small high-temperature range. Increasing the temperature transforms the decagonal quasicrystals to simple crystalline phases or leads to their peritectic decomposition. By decreasing the temperature high-order approximants are formed which can be obtained experimentally in the form of oriented nanodomain structures only. It is surprising that the coherence length of these phases, consisting of domains with ~100-400 Å diameter, reaches up to ~10000 Å [1].

Decagonal quasicrystals are built up from columnar clusters with fivefold orientational symmetry and ~20 Å diameter. These clusters are packed quasiperiodically, i.e. they occupy special positions on a quasiperiodic tiling (quasilattice). In the approximant phases the same clusters decorate a translational lattice. Due to the incommensurability of a quasilattice with fivefold symmetry and a translational lattice of any kind, the geometry of a phase transformation is not simple. The particular structural properties of the clusters, however, allow a purely displacive transformation leading to a nanodomain structure.

The characteristics of these transformation mechanism in the three- and the higher-dimensional description, respectively, will be presented.

[1] Kalning, M., Kek, S., Burandt, B., Press, W. and Steurer, W. J. Phys.: Condens. Matter 6, 6177-6187 (1994).