The extended x-ray absorption fine structure (EXAFS) spectra of Al-3 at.% Mg and Al-7 at.% Mg have been measured on the magnesium $K$ edge (1303 eV) using the soft-x-ray beam delivered by the ACO (Anneau de Collision d'Orsay) storage ring, as well as the EXAFS of elemental aluminium (1556 eV) and magnesium. With the use of quasiexperimental phase shifts determined from aluminum and magnesium data, the first-shell radius around a magnesium impurity in an Al-Mg alloy has been determined within ±0.015 Å, yielding a direct measurement of the elastic core effect which turns out to be large ∼0.08 Å. It is discussed in the framework of the elastic theory of continuous media and of the lattice static method, both of which fail to give the correct magnitude of the local relaxation.

• Received 14 January 1981

DOI:https://doi.org/10.1103/PhysRevB.24.5547