Fe-vacancy ordering in superconducting K_1-xFe_2-ySe₂: First-principles calculations and Monte Carlo simulations

Fe vacancies in the 33 K superconductor K_1-xFe_2-ySe₂ show ordering schemes that may be correlated with its superconducting properties. First-principles calculations and kinetic Monte Carlo simulations lead to a very simple model for vacancy ordering. Repulsive dipolar interactions between Fe vacancies show three ground states: a rhombus-ordered structure for 12.5% vacancies, a squared lattice for 20% vacancies, and a rhombus-ordered structure for 25% vacancies. Other structural states are derived from these three ground states and may contain additional disordered spatial regions. The repulsive interaction between Fe vacancies arises from enhanced Fe-Se covalent bonds, which differs from the well-known attractive interaction of Fe vacancies in body-centered cubic Fe.