COMPOSITION VARIATIONS OF LATTICE-PARAMETERS OF TLIN(SE1-XTEX)2, TLIN(SE1-XSX)2, AND TLIN1-XGAXSE2 MIXED-CRYSTALS

TlInSe2 and TlInTe2 crystallize in a tetragonal cell (space group 14/m cm) (Muller et al.). A primitive cell of these ternary crystals contai ns two formula units. All the Tl atoms in these compounds are univalen t. The In atoms are trivalent being in a tetrahedral environment consi sting of Se or Te atoms. InSe4 or InTe4 fragments of a unit cell form the chains extended along c-axis coinciding with [001] directions. Suc h chains are bonded to each other by Tl atoms. TlGaSe2 and TlInS2 crys tals belong to the monoclinic system (space group C2/c) (Muller, Hahn; Wong-Ng et al.). The primitive cell of these crystals contains eight formula units. The lattice of TlGaSe2 crystals consists of strictly pe riodic two-dimensional layers arranged parallel to the (001) plane. Ea ch successive layer is turned through a right angle with respect to th e previous one. The fundamental structural unit of a layer is the Ga4S e10 polyhedron representing a combination of four elementary GaSe4 tet rahedra linked together by bridging Se atoms. The Tl atoms are in trig onal prismatic voids resulting from the combination of the GaSe10 poly hedron into a layer. The Tl atoms form nearly planar chains along the [110] and [11BAR0] directions.