EVIDENCE FOR THE FORMATION OF SUBSTOICHIOMETRIC SPECIES DURING INTERNAL OXIDATION OF AG-MG ALLOYS

Lattice parameters of Ag-Mg (0.5 and 1 at.% ) were measured. A comparison between these results and data previously obtained by gravimetry shows that (a) a high dilatation is due mainly to excess oxygen by comparison with the amount necessary to form stoichiometric MgO oxide, and (b) the earliest stage of Mg oxidation occurs in a non-expanded layer beneath the expanded one, with the formation of substoichiometric species (O/Mg < 1). The formation of substoichiometric species is explained by taking account of strain fields close to O and Mg atoms in the silver lattice. Indeed, the strong deformation introduced by oxygen in interstitial position delays the addition of oxygen on the MgO* and Mg2O* initial species and favors the formation of substoichiometric species.