Optical spectra of ammonium sulfate in the paraelectric phase

The optical spectra of ammonium sulfate in the paraelectric phase are investigated in the visible and near-IR ranges. The temperature dependences of the parameter σ of the Urbach rule and the degree of depolarization of the luminescence spectrum exhibit a minimum in the temperature range 120–140°C. This minimum can be associated with the rearrangement of the electronic subsystem and the corresponding isostructural transformations of the lattice. In the near-IR range, the absorption spectrum contains two bands with maxima at energies of 0.50 and 0.75 eV. The shape of the bands and the temperature dependence of the conductivity allow us to interpret these bands as the result of dissociation of polarons of a large radius with binding energies of 0.12 and 0.17 eV.