Uniform Patterns of Fe-Vacancy Ordering in the K_x(Fe,Co)_2–ySe₂ Superconductors

The Fe-vacancy ordering patterns in the superconducting K x Fe 2– y Se 2 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 ThCr 2 Si 2 -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 √5 ( a s is the unit cell parameter of the parent ThCr 2 Si 2 -type structure) supercell and I 4/ m space group. Superstructure II reduces the symmetry to orthorhombic with the a II = a s √2, b II = 2 a s √2 supercell and the Ibam space group. This type of superstructure is observed for the first time in K x Fe 2– y Se 2 . 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 K x Fe 2– y Se 2 (0 < y < 0.5) solid solutions.