Superconductivity and phase instability of NH3-free Na-intercalated FeSe1-zSz

TL;DR

This study reports the synthesis of a new Na-intercalated FeSe superconductor with a ThCr2Si2-type structure, demonstrating phase stability and superconductivity without NH3 co-intercalation, and explores how interlayer spacing affects these properties.

Contribution

It presents the first example of small alkali metal intercalates with ThCr2Si2 structure without NH3, and investigates the relationship between interlayer spacing and superconductivity.

Findings

Na0.65Fe1.93Se2 has Tc ~ 37 K with ThCr2Si2 structure

NH3-poor and NH3-rich phases show Tc at 45 K and 42 K

Interlayer spacing modulation affects structural stability and superconductivity

Abstract

The discovery of ThCr2Si2-type AxFe2-ySe2 (A = K, Rb, Cs and Tl) with Tc ~ 30K make much progress in iron-based superconducting field, but their multiple-phase separations are disadvantageous for understanding the origin. On the other hand, for small alkali metals, studies on (Li,Na)FeCu(S,Se)2 and NaFe2-{\delta}S2 shows that these compounds possess CaAl2Si2-type structure, implying that ThCr2Si2-type structure is unstable for small alkali-metal intercalated FeSe under high-temperature. Here we report a new intercalate Na0.65(1)Fe1.93(1)Se2 with Tc ~ 37 K, synthesized by low-temperature ammonothermal method. The notable finding is that the Na0.65(1)Fe1.93(1)Se2 shows a ThCr2Si2-type structure, which is the first instance of small-sized alkali metal intercalates without NH3 co-intercalation. Besides, the NH3-poor Na0.80(4)(NH3)0.60Fe1.86(1)Se2 and NH3-rich phase with Tcs at 45 K and 42 K are identified by tuning the concentration of Na-NH3 solutions. The modulation of interlayer spacing reveals the versatile evolution of structural stability and superconductivity in these intercalates.