Phase transition studies of sodium deintercalated Na2−xFePO4F (0 ≤ x ≤ 1) by Mössbauer spectroscopy

The phase transition in sodium deintercalated Na2−xFePO4F (0 ≤ x ≤ 1) polycrystalline samples was studied by x-ray diffraction and Mössbauer spectroscopy. Sodium deintercalated samples were obtained by chemical deintercalation of pure Na2FePO4F. From the refined x-ray diffraction patterns, the crystalline structure of Na2FePO4F was determined to be orthorhombic with the space group Pbcn. The structure of fully sodium deintercalated NaFePO4F is identical to that of pure Na2FePO4F, differing only in the cell parameters. The changes in the unit cell parameters and atomic positions in Na2−xFePO4F (x = 0, 1) samples originated from a Na2 ion deficiency in the interlayer spaces. The room-temperature Mössbauer spectra of Na2−xFePO4F (x = 0, 1) were fitted with one set of the Fe2+ /3+ doublets. A decrease in the absorption area of Fe2 + in Na2FePO4F with increasing sodium deintercalation was observed in the Mössbauer spectra of Na2−xFePO4F, whereas the area of the Fe3+ doublet in NaFePO4F increased. The large value of ΔEQ for the Fe2+ doublet phase was due to the asymmetric charge distribution of FeO4F2 arising from different lattice and valence state contributions.