We present a study of the thermodynamic and physical properties of Tl5Te3,BiTl9Te6 and SbTl9Te6 compounds by means of density functional theory basedcalculations. The optimized lattice constants of the compounds are in goodagreement with the experimental data. The electronic density of states and bandstructures are calculated to understand the bonding mechanism in the threecompounds. The indirect band gap of BiTl9Te6 and SbTl9Te6 compounds are foundto be equal to 0.256 eV and 0.374 eV, respectively. The spin-orbit coupling hasimportant effects on the electronic structure of the two semiconductingcompounds and should therefore be included for a good numerical description ofthese materials. The elastic constants of the three compounds have beencalculated, and the bulk modulus, shear modulus, and young's modulus have beendetermined. The change from ductile to brittle behavior after Sb or Bi alloyingis related to the change of the electronic properties. Finally, the Debyetemperature, longitudinal, transverse and average sound velocities have beenobtained.
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