  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 subbands 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.
