Topotactic redox chemistry of NaFeAs in water and air and superconducting behavior with stoichiometry change

We report experimental evidence that shows superconductivity in NaFeAs occurs when it is Nadeficient. The oxidation of NaFeAs progresses differently in water and in air. In water the material oxidizes slowly and slightly retaining the original anti-PbFCl structure. In air NaFeAs oxidizes topotactically quickly and extensively transforming to the ThCr₂Si₂ structure type. Water acts as a mild oxidizing agent on the FeAs layer by extracting electrons and Na⁺ cations from the structure, while oxidation in air is more extensive and leads to change in structure type from NaFeAs to NaFe ₂As₂. The superconducting transition temperature moves dramatically during the oxidation process. Exposed to water for an extended time period NaFeAs shows a substantial increase in T_c up to 25 K with contraction of unit cell volume. NaFe₂As₂, the air oxidized product, shows T_c of 12 K. We report detailed characterization of the redox chemistry and transformation of NaFeAs in water and air using single crystal and powder X-ray diffraction, magnetization studies, transmission electron microscopy,Mössbauer spectroscopy, pOH and elemental analysis.