The effect of the Wyckoff position of the K atom on the crystal structure and electronic properties of the compound KFe₂Se₂.
By means of first-principles electronic structure calculations, we study the effect of the Wyckoff position of the K atom on the crystal and electronic structures of the compound KFe(2)Se(2). When the K atoms take up the Wyckoff positions 2a, 2b and 4c (the related structures of KFe(2)Se(2) are referred to as Struc-2a, Struc-2b and Struc-4c), the calculated lattice constants c lie in the ranges 13.5-14.5, 15.5-16.7 or 18.6-19.1 Å respectively. Three concentric cylinder-like Fermi surfaces emerge around Γ-Z in the Brillouin zone for Struc-2b in the nonmagnetic state, unlike the cases for Struc-2a and Struc-4c. The Fe-Se-Fe angles are 107.8°, 108.8° and 110.7° respectively in the collinear antiferromagnetic state, and the superexchange interactions J(2) between two next neighbor Fe moments are 13.08 meV S(-2), 20.75 meV S(-2) and 11.86 meV S(-2) for the Struc-2a, Struc-2b and Struc-4c structures respectively. Struc-2b and Struc-4c have good correspondence with the newly discovered superconducting phases with T(c) = 40 and 30 K in KFe(2)Se(2). Our findings suggest a reasonable approach for achieving an understanding of the existence of multiple superconducting phases in alkali metal intercalated FeSe superconductor.