Metastable 11 K superconductor Na(1-y)Fe(2-x)As2.

Area of Science:

• Solid-state chemistry
• Materials science
• Superconductivity research

Background:

• Iron arsenides are a class of materials with diverse electronic properties, including superconductivity.
• Conventional synthesis methods for iron arsenides often involve high temperatures, limiting access to certain phases.
• Topochemical reactions offer alternative routes to synthesize novel materials by modifying existing structures.

Purpose of the Study:

• To synthesize and characterize a novel superconducting phase of sodium iron arsenide.
• To investigate the structural and chemical properties of the synthesized material.
• To explore the stability and synthesis limitations of the new superconducting phase.

Main Methods:

• Topochemical deintercalation of Na(+) ions from NaFeAs using iodine in tetrahydrofuran (THF) at room temperature.
• X-ray powder diffraction (XRD) for structural analysis and phase identification.
• High-resolution transmission electron microscopy (HRTEM) for microstructural investigation.
• Solid-state Nuclear Magnetic Resonance ((23)Na NMR) spectroscopy for sodium environment analysis.
• (57)Fe-Mössbauer spectroscopy to probe the local electronic and magnetic environment of iron atoms.

Main Results:

• A metastable superconducting phase, Na(1-y)Fe(2-x)As(2), with T(c) ≈ 11 K was successfully synthesized.
• XRD confirmed the ThCr(2)Si(2)-type structure, but revealed very small coherently scattering domains and compositional fluctuations (Na(0.9(2))Fe(1.7(1))As(2)).
• HRTEM and NMR indicated structural distortions and sodium loss, suggesting disorder within the material.
• (57)Fe-Mössbauer spectroscopy identified a single type of iron atom in tetrahedral coordination.

Conclusions:

• The synthesized superconducting phase is a single crystallographic phase characterized by nanoscale domains and chemical composition variations.
• The material's metastability, decomposing at 120 °C, prevents its synthesis via high-temperature solid-state routes.
• The superconducting properties are attributed to the ordered fractions, likely NaFe(2)As(2), within the sample.