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-Na 3 Fe 2 (PO 4 ) 3 and Li 3 Fe 2 (PO 4 ) 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. Na 3 Fe 2 (PO 4 ) 3 crystallizes in the space group R 3̄ c 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, τ M(1) = 1) to 393 K (β-form, τ M(1) = 0.91) and then to 473 K (γ-form, τ M(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 FeO 6 octahedra become less distorted. Ion exchange from Na 3 Fe 2 (PO 4 ) 3 leads to the rhombohedral form of Li 3 Fe 2 (PO 4 ) 3 (space group R 3̄; 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 Li 3 Fe 2 (PO 4 ) 3 which is isotypic with Li 3 In 2 (PO 4 ) 3 . Removal of Na + from M(1) results in a strong increase of the c parameter due to stronger repulsions between adjacent FeO 6 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 6 Li spectrum at 152 ppm. Two crystallographic sites are clearly distinguished for iron: Fe(1)O 6 shares three of its vertexes only with LiO 4 tetrahedra and is far more distorted than Fe(2)O 6 which shares three edges with LiO 4 tetrahedra.