Ionic thermocurrent studies in lithium ammonium sulfate

Abstract

Four distinct peaks denoted as $A$, $B$, $C$, and $D$ are found to occur at temperatures -25, 10, 186, and 235°C in the ionic-thermocurrent (ITC) spectrum measured from -196 to 250°C along the ferroelectric $a$ axis in LiN${\mathrm{H}}_4$S${\mathrm{O}}_4$ single crystal. All the four peaks, with slight changes in temperature for current maxima for peaks $A$ and $D$, appear in the ITC spectra of samples doped with specific amounts of ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Zn}}^{2+}$ as impurities. Detailed investigations of the spectrum as a function of impurity concentration, poling temperature, poling time, and polarizing field have been carried out. The experimental results show that the reorientation of the impurity-vacancy complexes identified as (${\mathrm{Cu}}^{2+}{V}_p^{-}$) and (${\mathrm{Zn}}^{2+}{V}_p^{-}$) are responsible for peak $A$ in doped samples. $B$ does not exhibit characteristic features of an ITC peak. Peak $C$ is found to be due to the protons released during the phase transition occurring in the crystal at this temperature (186°C) and hence is an example of "protonic thermocurrent" which is reported here for the first time. Peak $D$ is accounted for as due to the dissociation of the impurity-vacancy ($I-V$) complexes. The numerical values of the following parameters have been calculated from the experimental results: (a) the activation energies in the various phases of the crystal, (b) the reorientation energy, (c) the association energy, and (d) the frequency factor for the relaxation time of the $I-V$ complexes. There is complete agreement between the present results and the data obtained from earlier electrical conductivity studies.