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

Abstract

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 [Fe₃(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.