Diffuse Scattering and Phason Elasticity in the AlPdMn Icosahedral Phase

The diffuse scattering located close to Bragg reflections has been measured on a single grain of the AlPdMn icosahedral phase using elastic neutron scattering. This diffuse scattering is mainly due to phason disorder. The intensity distribution and anisotropic shape can be reproduced in the framework of the elastic theory of quasicrystals using only the two phason elastic constants whose ratio is found to be equal to $-0.5\mathrm{}$.