X-ray diffuse scattering from icosahedral Al-Pd-Mn quasicrystals

The diffuse scattering of the thermodynamically stable icosahedral phase i- AlPdMn has been studied by means of x-ray-diffraction technique. The overal l diffuse intensity shows two main characteristics: (1) the global diffuse scattering intensity map is well taken into account by a Huang effect in th e formalism developed by Jaric and Nelson [Phys. Rev. B 37, 4458 (1988)] fo r icosahedral symmetry which reproduces at once both the "background" diffu se intensity and the profile shape around the Bragg peaks; (2) the intensit y level of the diffuse scattering is crucially dependent on the chemical co mposition of the sample; it varies drastically from almost free diffuse sca ttering samples close to an "ideal" composition, up to an overall increase by a factor of 20 of the diffuse scattering for samples with an off-stoichi ometry less than 0.5% in manganese and palladium content. The relative shap es of the diffuse scattering contours around the Bragg peaks are qualitativ ely fairly well reproduced using the sole phason-phason term of hydrodynami cal modes in the formalism of Jaric and Nelson [Phys. Rev. B 37, 4458 (1988 )], suggesting that phasons in these F-type icosahedral quasicrystals induc e minor relaxations in the atomic positions surrounding the flip locations. [S0163-1829(99)06729-6].