Studies of high-temperature phase transition, electrical conductivity, and dielectric relaxation in (NH4)H2PO4 single crystal -: art. no. 044104

Optical observation, differential scanning calorimetry, thermogravimetric analysis, and differential thermogravimetric measurements have been carried out on (NH4)H2PO4 single crystal in the temperature range of 30-250 degrees C. The results show that the crystal starts to decompose at around 160 degrees C. Upon heating, ammonia NH3 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 degrees C. The frequency dependence of conductivity follows Jonscher's dynamical law with the relation sigma(omega)=sigma(0)+B omega(n), where omega 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 degrees 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 degrees 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 degrees C, ammonium ions in the crystal are proposed to have the contribution to the electrical conduction with an activation energy of 3.01 eV. (c) 2005 American Institute of Physics.