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 KFe2Se2. When the K atoms take up the Wyckoff positions 2a, 2b and 4c (the related structures of KFe2Se2 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 angstrom respectively. Three concentric cylinder-like Fermi surfaces emerge around Gamma-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 degrees, 108.8 degrees and 110.7 degrees 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 KFe2Se2. Our findings suggest a reasonable approach for achieving an understanding of the existence of multiple superconducting phases in alkali metal intercalated FeSe superconductor.