Crystal structures and sodium/silver distributions within the ionic conductors Na5Ag2Fe3(As2O7)4 and Na2Ag5Fe3(P2O7)4

The crystal structures of new Na5Ag2Fe3(As2O7)(4) and Na2Ag5Fe3(P2O7)(4) compounds, prepared through ion exchange from Na7Fe3(X2O7)(4) (X = P, As) are reported. They crystallize in the monoclinic C2/c space group and exhibit a Na/Ag ordering on cooling. Crystal structures were determined from single crystal X-ray diffraction at 100 and 298 K for each composition. The structure consists of FeO6 octahedra sharing their corners with X2O7 dimers (X = P, As) to form a three-dimensional framework [Fe-3(X2O7)(4)](7-) into which the sodium/silver ions are located. The differences between the four structures lie on the distribution of the sodium/silver ions within this framework giving a developed insight of the ionic diffusion paths.