Electrical impedance measurements on a congruent LiNbO 3 single crystal were performed as a function of both temperature and frequency. The measurements were carried out in the directions along the cand a-axes of the crystal. The temperature and frequency dependence of various dielectric properties have been studied. The result has revealed two remarkable dynamic relaxations: dielectric dipolar relaxation and ionic conductivity relaxation. The dipolar relaxation peaks were found at frequencies around 4 × 10 6 and 2 × 10 6 Hz for the c-axis and a-axis, respectively, and they were only slightly temperature dependent. The ionic conductivity relaxation was found at the lower-frequency end but it was temperature dependent. The temperature dependence of the dc electrical conductivity follows the Arrhenius law. It corresponds to the longrange ionic motion of Li + ions which are thermally activated with activation energy of 0.90 and 0.87 eV along the cand a-axis directions, respectively. The dc conductivities measured along the cand a-axes are very close to each other, and the value increases from 1.7 × 10 −6 to 1.9 × 10 −3 −1 cm −1 as the temperature is raised from 300 to 700 • C. The sample crystal becomes an ionic conductor as the temperature is raised.
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°C. The results show that the crystal starts to decompose at around 160°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°C. The frequency dependence of conductivity follows Jonscher’s dynamical law with the relation σ(ω)=σ(0)+Bωn, 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 0.12eV 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 0.83eV. 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 3.01eV.
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