  Optical reflectivity as a simple diagnostic method for testing structural
quality of icosahedral quasicrystals 2 The optical reflectivity of Al-based and
Ti-based quasicrystalline and approximant samples were investigated versus the
quality of their structural morphology using optical reflectometry, X-ray
diffraction and transmission electron microscopy. The different structural
morphologies were obtained using three different preparation processes :
sintering, pulsed laser deposition and reactive cathodic magnetron sputtering.
The work demonstrates that the canonical behaviour of icosahedral state in
specular reflectivity is extremely sensitive to different and very fine aspects
of the microstructure : sizes of grains smaller than 50 nm, slight local
diffuse disorder and shifts away from the icosahedral crystallographic
structure (approximants). The work explains why the optical properties of the
same kind of quasicrystals found in literature sometimes reveal a different
behaviour from one author to another. The study then confirms the work of some
authors and definitely shows that the canonical behaviour of icosahedral state
in specular reflectivity over the 30000-50000 cm-1 domain is characterized by a
decreasing function made of steps. It also shows that this behaviour can be
interpreted thanks to the cluster hierarchy of the model of Janot.
