Iron-Doped Sodium-Vanadium Fluorophosphates: Na₃V_2-O_2-Fe(PO₄)₂F₁₊ (y < 0.3)

The sodium-vanadium fluorophosphate family has been actively investigated recently, but few examples tackle chemical doping or the substitution of vanadium. This work presents a series of iron-doped compounds Na₃V_2-O_2-Fe(PO₄)₂F₁₊ (y ≤ 0.3) prepared by hydrothermal synthesis with low iron content. The amount of iron in the structure is confirmed by X-ray and neutron powder diffraction, electronic paramagnetic resonance, magnetic susceptibility measurements, and solid-state nuclear magnetic resonance (ssNMR). The degree of vanadium substitution, together with the solubility limit for iron in sodium-vanadium fluorophosphates, has been calculated by ssNMR and magnetic susceptibility measurements to be y = 0.3 based on the synthetic route used here. The introduction of small amounts of Fe³⁺ to the structure leads to the reduction of a fraction of V⁴⁺ to V³⁺, and the voltage profiles do not change with the introduction of iron to the structure. In situ synchrotron X-ray diffraction demonstrates that the electrochemical-structural changes during charge and discharge are very similar to those observed in the V³⁺/V⁴⁺ mixed-valent Na₃V₂O_1.6(PO₄)₂F_1.4, which could be related to the existence of both iron dopant and V³⁺ in the phase.