Abstract
This paper presents the results of a detailed structural study using synchrotron X-ray diffraction at various temperatures up to 850 °C on K
2
x
Fe
4+
y
Se
5
superconductors. The results confirm that extra Fe atoms begin to fill the empty 4d site and stabilize the structural-phase I4/m symmetry so that the lattice size remains the same even above the order–disorder temperature. The results demonstrate that the addition of extra Fe suppresses the Fe vacancy long-range order and the accompanied magnetic order so that superconductivity emerges. The significance of this research is that it provides unambiguously the structural origin for superconductivity in the K
2−x
Fe
4+
y
Se
5
superconducting system.
The exact superconducting phase of K
2−
x
Fe
4+
y
Se
5
has so far not been conclusively decided since its discovery due to its intrinsic multiphase in early material. In an attempt to resolve this mystery, we have carried out systematic structural studies on a set of well-controlled samples with exact chemical stoichiometry K
2−
x
Fe
4+
x
Se
5
(
x
= 0–0.3) that are heat-treated at different temperatures. Using high-resolution synchrotron radiation X-ray diffraction, our investigations have determined the superconducting transition by focusing on the detailed temperature evolution of the crystalline phases. Our results show that superconductivity appears only in those samples that have 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 nonsuperconducting parent compound K
2
Fe
4
Se
5
and the superconducting K
2−
x
Fe
4+
y
Se
5
samples. The X-ray diffraction patterned can be well indexed to the phase with I4/m symmetry in all temperatures investigated. However, we need two phases with similar I4/m symmetry but different parameters to best fit the data at a temperature below the Fe vacancy order temperature. The results strongly suggest that superconductivity in K
2−
x
Fe
4+
y
Se
5
critically depends on the occupation of Fe atoms on the originally empty 4d site.