1 Structural, thermal and optical properties of Ag(In 1-x Ga x ) 5 Te 8 Larissa T. Durán a) , Josefa Estévez Medina a) , Jaime A. Castro a) , José R. Fermín b) , Ricardo J. Morales a) and Carlos A. Durante Rincón, a, *) a Laboratorio de Ciencia de Materiales, Departamento de Física, Facultad Experimental de Ciencias, Universidad del Zulia, Maracaibo, 4005, Venezuela. b Laboratorio de Materia Condensada, Departamento de Física, Facultad Experimental de Ciencias, Universidad del Zulia, Maracaibo, 4005, Venezuela. *Author to whom correspondence should be addressed: durin@cantv.net , cdurante@luz.edu.ve , durincarlos@gmail.com . Tel. 58-412-5476306. Fax: 58-261-7832690. OPT Bella Vista, Apartado 10265, Maracaibo, Zulia, Venezuela. Abstract Ingots of the Ag(In 1-x Ga x ) 5 Te 8 (0≤x≤1) system were prepared by direct fusion of the stoichiometric mixture of the elements in evacuated quartz ampoules. The analysis of the X- ray powder diffraction data showed the presence of a single phase with tetragonal structure at room temperature for all the studied compositions. The lattice parameters a and c were calculated. Melting temperatures, from 696 ºC for AgIn 5 Te 8 to 775 ºC for AgGa 5 Te 8 , were obtained from Differential Thermal Analysis measurements performed on samples in evacuated quartz ampoules. Reflectance measurements were used to calculate the band gap energies and the real refraction indices. A direct band gap, slightly varying from 1.34 to 1.39 eV (0≤x≤0.4), was found in In-rich compounds, while an indirect band gap was found in all the studied compositions varying from 1.11 to 1.14 eV (0≤x≤1). Keywords: semiconductors, powder diffraction, differential thermal analysis (DTA), optical properties, reflectance.