Optical studies of (NH 4) 2SO 4 single crystal in the paraelectric phase

Abstract

Transmittance and absorbance spectra of (NH 4) 2SO 4 single crystals along [010] direction were measured at different temperatures (296, 308, 318, 328 and 348 K) in the paraelectric phase. The absorption coefficient was computed and the analysis of the data revealed the existence of two optical transitions in (NH 4) 2SO 4 single crystals. The direct and indirect band gaps were shifted towards the longer wavelength with increasing temperature. The data on the allowed indirect transition was analyzed and interpreted in terms of two valence bands originated by spin orbit interaction and crystal field splitting. The momenta E p were calculated as the difference between E g1, the first valence to conduction band, and E g2 for the second valence band at different temperatures. The results of extinction coefficient ( k), the refractive index ( n), and dielectric constants ( ε) were also discussed and calculated as a function of wave length ( λ). The heat treatment of the crystals proved that the variations of these optical parameters can be consequence of the internal microstructure changes caused by annealing.