Uniform Patterns of Fe Vacancy Ordering in the Kx(Fe,Co)2-ySe2 Superconductors

TL;DR

This study investigates the Fe-vacancy ordering patterns in KxFe2-ySe2 superconductors using electron diffraction and microscopy, revealing two types of superstructures with distinct symmetries and inhomogeneous vacancy distributions.

Contribution

It identifies and characterizes two distinct Fe-vacancy superstructures in KxFe2-ySe2, including the first observation of an orthorhombic superstructure, advancing understanding of vacancy ordering in these materials.

Findings

Two types of Fe-vacancy superstructures identified

Superstructure I retains tetragonal symmetry with a√5 unit cell

Superstructure II is orthorhombic, observed for the first time

Abstract

The Fe-vacancy ordering patterns in the superconducting KxFe2-ySe2 and non-superconducting Kx(Fe,Co)2-ySe2 samples have been investigated by electron diffraction and high angle annular dark field scanning transmission electron microscopy. The Fe-vacancy ordering occurs in the ab plane of the parent ThCr2Si2-type structure, demonstrating two types of patterns. The superstructure I retains the tetragonal symmetry and can be described with the aI = bI = as{\surd}5 (as is the unit cell parameter of the parent ThCr2Si2-type structure) supercell and I4/m space group. The superstructure II reduces the symmetry to orthorhombic with the aII = as{\surd}2, bII = 2as{\surd}2 supercell and the Ibam space group. This type of superstructure is observed for the first time in KxFe2-ySe2. The Fe-vacancy ordering is inhomogeneous: the disordered areas interleave with the superstructures I and II in the same crystallite. The observed superstructures represent the compositionally-dependent uniform ordering patterns of two species (the Fe atoms and vacancies) on a square lattice. More complex uniform ordered configurations, including compositional stripes, can be predicted for different chemical compositions of the KxFe2-ySe2 (0 < y < 0.5) solid solutions.