Studies of high-temperature phase transition, electrical conductivity, and dielectric relaxation in (NH ₄)H ₂PO ₄ single crystal

Optical observation, differential scanning calorimetry, thermogravimetric analysis, and differential thermogravimetric measurements have been carried out on (N H4) H2 P O4 single crystal in the temperature range of 30-250 °C. The results show that the crystal starts to decompose at around 160 °C. Upon heating, ammonia N H3 escapes from the surface of the crystal gradually with the production of phosphoric acid. The measured ac impedance data are analyzed as a function of frequency in the temperature range between 40 and 150 °C. The frequency dependence of conductivity follows Jonscher's dynamical law with the relation σ (ω) =σ (0) +B ωn, where ω is the frequency of the applied ac electric field and B is a constant. The obtained values of the exponent n decrease from 1 to 0.25 as the temperature is raised. The electrical conduction at low temperatures below 70 °C is attributed to the hopping of proton on O-H-O hydrogen bonds among hydrogen vacancies. The activation energy of the migration is 0.12 eV in this extrinsic region. At temperatures between 70 and 97 °C, additional defects are created by breaking the stronger hydrogen bond in ammonium groups. The activation energy of defect formation and migration of proton among defects is 0.83 eV. At temperatures above 97 °C, ammonium ions in the crystal are proposed to have the contribution to the electrical conduction with an activation energy of 3.01 eV.