The diffuse scattering located close to Bragg reflections has been stu
died as a function of the temperature on a single grain of the AlPdMn
icosahedral phase. Between 600 degrees C and 800 degrees C there is a
reversible and continuous transition. When the temperature increases,
the diffuse scattering diminishes whereas high-Q(perpendicular to) ref
lections have their intensity increased. All results are well interpre
ted within the elasticity theory of quasi-crystals: the changes in dif
fuse scattering shape and intensity when lowering the temperature are
due to a softening of the K-1 phason elastic constant. The time decay
of these long-wavelength phason fluctuations is larger than 5 ns.