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 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. 
Sergey M. Kazakov, Artem M. Abakumov, Santiago Gonz\'alez, Juan Manuel   Perez-Mato, Alexander V.Ovchinnikov, Maria V. Roslo...
cond-mat.supr-con