Preliminary investigations have shown that Sr1.8Na0.2MgSi2O7-d, belonging to the melilite family of oxide, is a potential new oxide ion conductor. The aim of this experiment is to determine the local structure (three non-equivalent oxygen sites are present in the melilite structure) and defects position and distribution (oxygen vacancies) introduced within the structure for charge compensation as a consequence of the Na doping on the parent Sr2MgSi2O7 compound.