Dielectric permittivity, ac conductivity and phase transition in hydroxyl ammonium sulfate

The complex dielectric permittivity (epsilon* = epsilon' - j epsilon '') and ac conductivity sigma (omega, T) as a function of temperature (90-375 K) and frequency (0.4 kHz to approximate to 100 kHz) were measured in this work for polycrystalline samples of hydroxyl ammonium sulfate, (NH3OH)(2)SO4. The measured electrical parameters revealed the existence of a structural phase transition at T approximate to 312 K, which was further confirmed by a differential thermal analysis thermogram, where a clear endothermic peak centered at approximate to 312K is observed. Regarding the charge transport mechanism, it is likely that the behavior of frequency-dependent conductivity follows the universal dynamic response sigma(omega, T) = A(T)omega(s(T)). Moreover, the temperature dependence of the frequency exponent s(0 <= s <= 1) suggests the quantum mechanical tunneling model to be the most likely one that describes the electrical transport mechanism. The data correlate with the crystal structure and the hydrogen-bonding system.