Structural and optical properties of AgIn5S8

Compounds of the chalcogenide family Ag-In-VI (VI = S, Se, Te) are interesting materials due to their stoichiometric stability and potential application in nonlinear optics and solar cells. A polycrystalline ingot of AgIn5S8, an ordered vacancy semiconductor, was prepared by direct fusion of the stoichiometric mixture of the elements in an evacuated quartz ampoule. The presence of a single phase with cubic structure was confirmed by X-ray powder diffraction at room temperature. The lattice parameter, a, was calculated, giving 10.821750 angstrom. Samples in evacuated quartz ampoules were used to perform Differential Thermal Analysis measurements, showing congruent melting at 1110 degrees C. Transmittance and reflectivity measurements were used to calculate the absorption coefficient alpha. From the plot of (alpha hv)(2) versus hv, two direct transitions are observed at 1.25 eV and 1.88 eV. While the higher energy direct transition has been observed by other authors, the direct nature of the lower energy transition was confirmed from the fitting of the plot of the reflectivity versus 1/hv between 0.53 eV(-1) (1.89 eV) and 0.55 eV(-1) (1.82 eV), obtaining a value of 1.29 eV. The real refractive index n and the high-frequency dielectric constant epsilon(infinity) were also obtained from the fit of the reflectivity, resulting to be 2.68 and 7.2, respectively.