Dielectric Properties of the Low-temperature Phase Transitions in Divalent Nitrates

The paper is devoted to the elucidation of realization mechanism of the low-temperature phase transitions in divalent nitrates Pb(NO3)2, Sr(NO3)2 and Ba(NO3)2. The investigations results of dielectric permeance ε and tangent of dielectric losses angle tgδ temperature dependencies are presented for lead nitrate Pb(NO3)2. The investigations confirm supposition that low-temperature phase transitions in divalent nitrates occurs because of “freezing” of reorientational oscillations of NO3− groups around Nitrogen atom, which occur at room temperature. A maximum in Pm3 → P213 phase transition region is observed on the ε(T) curves. This maximum decreases with increasing of frequency and at 10 KHz frequency becomes like a “step” by it’s form. Two maximums are observed on the tgδ(T) curves. First of them occurs at the phase transition temperature, another one is moved to low-temperature region. The conclusion, that the first maximum of tgδ is connected with NO3− groups relaxation and second maximum is connected with a formation of domain structure is made. The energy of activation is calculated by the removing of dielectric permeance maximum and this energy is higher than 2 eV. Such value of activation energy is proper for heavy ions. This fact confirms a supposition, that a relaxation of NO3− groups occur at 1 KHz-10 KHz frequencies in the phase transition region.