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 Na deficient. The oxidation of NaFeAs progresses differently in water and in air. In water the material oxidizes slowly and slightly retaining the original anti-PbFCI 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 NaFe2As2. 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, Mossbauer spectroscopy, pOH and elemental analysis.