The melilite family are new xoide ion conductors with high mobility at low temperatures. Average structures suggest that the ability of the structure to accomodate high levels of interstitials and allow these defects to move with high mobility is due to the presence of tetrahedral centres which are not completely condensed to form the oxide framework. This structural feature would allow relaxation when the defect binds to this centre, and may also, by analogy with the negative thermal expansion materials, produce dynamical features associated with rigid unit modes that enable defect motion. The collection of variable temperature total scattering data will allow the static structure around the defect and the framework dynamics to be investigated using RMC analysis.Lead-free relaxor ferroelectrics are important materials. The local structure of a new example will be analysed.