A Powder Neutron Diffraction Investigation of the Two Rhombohedral NASICON Analogues:  γ-Na3Fe2(PO4)3 and Li3Fe2(PO4)3

The crystal structures of polycrystalline rhombohedral γ-Na3Fe2(PO4)3 and Li3Fe2(PO4)3 obtained through Na ↔ Li ion exchange have been determined for the first time, from neutron diffraction data recorded at 473 and 300 K, respectively. Na3Fe2(PO4)3 crystallizes in the space group R3̄c at 473 K (γ-form, a = 8.7270(2) Å, and c = 21.8078(5) Å). A progressive transfer of Na+ ions from the M(1) (6-coordinate) to the M(2) (8-coordinate) sites occurs upon raising the temperature from ambient (α-form, τM(1) = 1) to 393 K (β-form, τM(1) = 0.91) and then to 473 K (γ-form, τM(1) = 0.85). This is associated with a significant increase of Fe−Fe distances through the M(1) site and with relaxation of the framework such that the FeO6 octahedra become less distorted. Ion exchange from Na3Fe2(PO4)3 leads to the rhombohedral form of Li3Fe2(PO4)3 (space group R3̄; a = 8.3162(4) Å, and c = 22.459(1) Å). The M(1) and M(2) sites of the NASICON structure are empty in Li3Fe2(PO4)3 which is isotypic with Li3In2(PO4)3. Removal of Na+ from M(1) results in a strong increase of the c parameter due to stronger repulsions between adjacent FeO6 octahedral faces along [001]. Lithium is located on one single-crystallographic site, M(3), with four Li−O distances ranging between 1.91 and 2.09 Å which span a regular tetrahedral geometry and gives rise to one signal in the 6Li spectrum at 152 ppm. Two crystallographic sites are clearly distinguished for iron: Fe(1)O6 shares three of its vertexes only with LiO4 tetrahedra and is far more distorted than Fe(2)O6 which shares three edges with LiO4 tetrahedra.