Role of the Extra-Fe in K2-xFe4+ySe5 superconductors

TL;DR

This study systematically investigates the structural phases of K2-xFe4+ySe5 superconductors, revealing that superconductivity depends on high-temperature treatment and Fe atom occupation, with phase composition being crucial.

Contribution

The paper provides detailed structural insights into K2-xFe4+ySe5, clarifying the relationship between annealing conditions, phase composition, and superconductivity, which was previously unresolved.

Findings

Superconductivity appears only after high-temperature annealing and quenching.

The volume fraction of superconductivity depends on annealing temperature.

Superconductivity is linked to Fe atom occupation on specific lattice sites.

Abstract

The exact superconducting phase of K2-xFe4+ySe5 has yet conclusively decided since its discovery due to its intrinsic multiphase in early material. In an attempt to resolve the mystery, we have carried out systematic structural studies on a set of well controlled samples with exact chemical stoichiometry K2-xFe4+xSe5 (x=0~0.3) that are heat-treated at different temperatures. Our investigations, besides the determination of superconducting transition, focus on the detailed temperature evolution of the crystalline phases using high resolution synchrotron radiation X-ray diffraction. Our results show that superconductivity appears only in those samples been treated at high enough temperature and then quenched to room temperature. The volume fraction of superconducting transition strongly depends on the annealing temperature used. The most striking result is the observation of a clear contrast in crystalline phase between the non-superconducting parent compound K2Fe4Se5 and the superconducting K2-xFe4+ySe5 samples. The x-ray diffraction patterned can be well indexed to the phase with I4/m symmetry in all temperature investigated. However, we need two phases with similar I4/m symmetry but different parameters to best fit the data at temperature below the Fe-vacancy order temperature. The results strongly suggest that superconductivity in K2-xFe4+ySe5 critically depends on the occupation of Fe atoms on the originally empty 4d site.