Electronic and atomic structures of KFe2Se2 grain boundaries

The electronic and atomic structures of the twist and tilt grain boundaries (GB) of the iron-based superconductor KFe2Se2 are studied based on the simulations of the first principles density functional theory. Our results have clarified that the Σ5[0 0 1] twist grain boundary of KFe2Se2 with layered structure has the lower grain-boundary energy. The local structure and the main features of the basic electronic structure within the [0 0 1] twist grain-boundary region have small differences compared with those in KFe2Se2 crystal. The large fluctuations of the charges and magnetic moments are found in the [0 0 1] tilt grain-boundary regions, especially the former are more prominent. The bi-collinear anti-ferromagnetic order is the most stable magnetic order even with grain boundaries in the bulk. The 5a×5a superstructure of Fe-vacancies in K2Fe4Se5 phase is intrinsically related to the coincident-site lattice of Σ5[0 0 1] twist grain boundary.