Superconducting Phases in Potassium-Intercalated Iron Selenides

TL;DR

This study identifies two pure superconducting phases in potassium-intercalated iron selenides, with distinct critical temperatures and structures, formed via liquid ammonia, advancing understanding of their intrinsic properties.

Contribution

It reports the discovery of two stable pure superconducting phases with specific potassium doping levels and structural parameters, formed through a liquid ammonia route, clarifying their intrinsic properties.

Findings

Two pure SC phases with Tc of 44 K and 30 K identified

Superconducting phases are stable only at specific doping levels

Superconductivity is unaffected by ammonia presence

Abstract

The ubiquitous coexistence of majority insulating 245 phases and minority superconducting (SC) phases in AxFe2-ySe2 (A = K, Cs, Rb, Tl/Rb, Tl/K) formed by high-temperature routes makes pure SC phases highly desirable for studying the intrinsic properties of this SC family. Here we report that there are at least two pure SC phases, KxFe2Se2(NH3)y (x ~ 0.3 and 0.6), determined mainly by potassium concentration in the K-intercalated iron selenides formed via the liquid ammonia route. K0.3Fe2Se2(NH3)0.47 corresponds to the 44 K phase with lattice constant c = 15.56(1) angstroms and K0.6Fe2Se2(NH3)0.37 to the 30 K phase with c = 14.84(1) angstroms. With higher potassium doping, the 44 K phase can be converted into the 30 K phase. NH3 has little, if any, effect on superconductivity. Thus, the conclusions should apply to both K0.3Fe2Se2 and K0.6Fe2Se2 SC phases. K0.3Fe2Se2(NH3)0.47 and K0.6Fe2Se2(NH3)0.37 stand out among known superconductors as their structures are stable only at particular potassium doping levels, and hence the variation of Tc with doping is not dome-like.