Composition variations of lattice parameters of TlIn(Se 1 – x ) 2 , TlIn(Se 1 – x S x ) 2 , and TlIn 1 – x Ga x Se 2 mixed crystals

TlInSe 2 and TlInTe 2 crystallize in a tetragonal cell (space group 14/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. InSe 4 or InTe 4 fragments of a unit cell form the chains extended along c ‐axis coinciding with [001] directions. Such chains are bonded to each other by Tl atoms. TlGaSe 2 and TlInS 2 crystals belong to the monoclinic system (space group C2/c ) (MÜLLER, HAHN; WONG‐NG et al.). The primitive cell of these crystals contains eight formula units. The lattice of TlGaSe 2 crystals 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 Ga 4 Se 10 polyhedron representing a combination of four elementary GaSe 4 tetrahedra linked together by bridging Se atoms. The Tl atoms are in trigonal prismatic voids resulting from the combination of the Ga 4 Se 10 polyhedra into a layer. The Tl atoms form nearly planar chains along the [110] and [110] directions.