Dislocation polarization and space-charge relaxation in solid solutions Ba1-xLaxF2+x.

Abstract

Concentrated solid solutions ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{La}}_{\mathrm{x}}$${\mathrm{F}}_{2+\mathrm{x}}$, x\ensuremath{\le}0.492, are studied by means of thermally stimulated depolarization [ionic thermal current (ITC)] or polarization [thermally stimulated polarization current (TSPC)] techniques. At high temperature two peaks are reported whose maxima shift to lower temperatures as x increases. The energies calculated for these relaxation processes are compared to the enthalpies deduced from conductivity measurements. The first high-temperature peak, D, has been already reported in a more restricted concentration range and has been attributed to either a space-charge accumulation near the electrodes or the polarization of the dislocations present in the crystal.We show here that it is the highest-temperature peak reported here for the first time and labeled as peak E which is best related to the displacement of free interstitial fluorines through the bulk of the crystal; that is, to the building up of a space charge near the electrodes. At low concentrations the behavior of peaks D and E is understood if the Debye-H\uckel interaction energy among unassociated defects is taken into account. For the more concentrated region (x\ensuremath{\gtrsim}0.05), the enhanced ionic motion model proposed to explain the variation of the ionic conductivity slopes as a function of x also applies here; the parameters present in the model are calculated and are found to be in agreement with those previously reported. The energy difference observed between peaks D and E is almost constant for x\ensuremath{\ge}0.05. This energy difference equal to 0.08 eV is attributed to the binding energy of the interstitial fluorines forming the dislocation charge cloud to the dislocation line. TSPC experiments are carried out on the same crystals and they show that peak D is present in all the TSPC spectra, thus confirming the localized displacement of the charges, while the highest-temperature peak E is replaced by a steep increase of the ionic current. Therefore, the origin of peak E observed in ITC must be attributed to the accumulation of charges near the electrodes; that is, the build up of the space-charge layer.