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.
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