Crystal structures of new silver ion conductors Ag7Fe3(X2O7)4 (X = P, As)

The crystal structures of new Ag7Fe3(X2O7)(4) (X = P, As) compounds, prepared through ion exchange from their sodium analogs, are reported. They adopt the monoclinic crystal system and exhibit an Ag ordering on cooling evidenced by a lowering of the symmetry from C-centered to primitive Bravais lattices. Crystal structures were determined from single-crystal X-ray diffraction at 100 K and 298 K for each composition. The structure consists of FeO6 octahedra sharing their corners with P2O7 dimers to form a three-dimensional framework [Fe-3(P2O7)(4)](7-) into which the silver ions are located. The differences between the four structures lie on the distribution of the silver ions within this framework, at the origin of a strong anisotropy in conductivity. Temperature displacement factors on Ag sites are generally higher in the arsenate than in the phosphate, in good correlation with conductivity data.