Effect of isomorphic atom substitution on the lattice parameters and the optical absorption edge of TIGaSe2-TllnSe2 mixed crystals

The variation of lattice parameters of TlGaSe2-TlInSe 2 mixed crystals with composition have been investigated by using of X-ray diffraction technique. For the mixed crystals studied, a structural phase transition (monoclinic to tetragonal) due to atom substitution is observed when the sum of the tetrahedral covalent radii of the Ga(In) and the Se atoms in Ga(In)Se4 tetrahedra reaches a critical value of about 0.254 nm. From transmission and reflection measurements, the compositional dependence of the indirect energy band gap was revealed