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

The Fe-vacancy ordering patterns in the superconducting KxFe2-ySe2 and nonsuperconducting K-x(Fe,Co)(2-y)Se-2 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. Superstructure I retains the tetragonal symmetry and can be described with the a(I) = b(I) = a(s)root S (a(s) is the unit cell-parameter of the parent ThCr2Si2-type structure) supercell and I4/m space group. Superstructure II reduces the symmetry to orthorhombic with the a(II) = a(s)root 2, b(II) = 2a(s)root 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.