Trends in structural, electronic properties, Fermi surface topology, and inter-atomic bonding in the series of ternary layered dichalcogenides KNi2S2, KNi2Se2, and KNi2Te2 from first principles calculations

By means of the FLAPW-GGA approach, we have systematically studied the structural and electronic properties of tetragonal dichalcogenides KNi2Ch2 (Ch=S, Se, and Te). Our results show that replacements of chalcogens (S→Se→Te) lead to anisotropic deformations of the crystals structure, which are related to the strong anisotropic character of the inter-atomic bonds, where inside the [Ni2Ch2] blocks, mixed covalent–ionic–metallic bonds occur, whereas between the adjacent [Ni2Ch2] blocks and K atomic sheets, ionic bonds emerge. We found that in the sequence KNi2S2→KNi2Se2→KNi2Te2 (i) the overall band structure (where the near-Fermi valence bands are due mainly to the Ni states) is preserved, but the width of the common valence band and the widths of the separate sub-bands and the gaps decrease; (ii) the total DOSs at the Fermi level also decrease; and (iii) for the Fermi surfaces, the most appreciable changes are demonstrated by the hole-like sheets, when a necklace-like topology is formed for the 2D-like sheets and the volume of the closed pockets decreases. Some trends in structural and electronic parameters for ThCr2Si2-type layered dichalcogenides, KNi2Ch2, KFe2Ch2, KCo2Se2, are discussed.