Na(4)FeFe(PO(4))(3), a new synthetic NASICON-type phosphate

This paper reports the crystal structure of tetra-sodium diiron tris(phosphate), Na(4)Fe(2+)Fe(3+)(PO(4))(3), which has been synthesized hydro-thermally at 773 K and 0.1 GPa. The crystal structure has been refined in the space group Rc and is identical to that of γ-NASICON. The heteropolyhedral framework is based on a regular alternation, in three dimensions, of corner-sharing PO(4) tetra-hedra and FeO(6) octa-hedra, constituting so-called 'lantern units' stacked along the c axis. The Na(+) cations are distributed over two crystallographic sites: the six-coordinated Na1 site which lies between two 'lantern units', and the eight-coordinated Na2 site which lies at the same z value as the P site.