AbstractTlInSe2and TlInTe2crystallize in a tetragonal cell (space group14/mcm) (MÜLLER et al.). A primitive cell of these ternary crystals contains two formula units. All the Tl atoms in these compounds are univalent. The In atoms are trivalent being in a tetrahedral environment consisting of Se or Te atoms. InSe4or InTe4fragments of a unit cell form the chains extended alongc‐axis coinciding with [001] directions. Such chains are bonded to each other by Tl atoms.TlGaSe2and TlInS2crystals belong to the monoclinic system (space groupC2/c) (MÜLLER, HAHN; WONG‐NG et al.). The primitive cell of these crystals contains eight formula units. The lattice of TlGaSe2crystals consists of strictly periodic two‐dimensional layers arranged parallel to the (001) plane. Each successive layer is turned through a right angle with respect to the previous one. The fundamental structural unit of a layer is the Ga4Se10polyhedron representing a combination of four elementary GaSe4tetrahedra linked together by bridging Se atoms. The Tl atoms are in trigonal prismatic voids resulting from the combination of the Ga4Se10polyhedra into a layer. The Tl atoms form nearly planar chains along the [110] and [110]dir