Observation of a decagonal quasicrystalline phase and quasicrystalline approximants in in situ heated thin films of the Al-Co-Cr-Fe-O system

Quasicrystalline and crystalline phase transitions observed during in situ heating in the transmission electron microscope of amorphous thin films of the Al-Co-Cr-Fe-O system have been studied. In particular, heating the film at 750 degrees C during 2 h, grains with decagonal quasicrystalline structure were developed. This quasicrystalline phase presents a periodicity of 1.4 nm along the c-axis and its tenfold electron diffraction pattern indicates the existence of phason strains in its structure. X-ray energy dispersive spectroscopy microanalysis showed a high relative concentration of Al compared with the concentration of Cr, O and Fe in the quasicrystalline grains, and electron energy-loss spectroscopy confirmed the presence of oxygen in these grains. All this resulted in a nominal composition of Al58Cr28O17Fe5 for this phase (i.e. this is an oxygen-containing decagonal quasicrystalline phase). Cobalt was only detected in grains whose structure corresponds to crystalline phases such as Al13Co4, the well known approximant phase of the decagonal quasicrystal. (C) 1999 Elsevier Science S.A. All rights reserved.