Dielectric relaxation, ionic conductivity and activation volumes in cubic lead fluoride doped with alkali-metal cations

Point defect parameters in β-PbF2 revisited

“…In accordance with this explanation, the ratio υ act /h act should solely be governed by macroscopic properties of the bulk solid. Actually, it was shown [4] that the migration enthalpy (h f,m ) and volume (υ f,m ) for the free ( f ) fluorine vacancy motion, as well as the corresponding quantities h m,i , υ m,i for the fluorine interstitial (i) migration, which have been determined from earlier conductivity measurements [5,6] under pressure, exhibit ratios υ f,m /h f,m and υ m,i /h m,i that are not only equal to those υ act /h act found by Figueroa et al [3] but also equal to the value predicted by the cBΩ model. In the meantime, however, new conductivity measurements in β-PbF 2 appeared [7] which lead to values of the ratio υ/h for the defect migration as well as for the anion Frenkel defect formation processes.…”

“…In Fig. 1, we plot the experimental values of υ i versus h i from various sources as follows: First, four points (solid dots) correspond to the association parameters from the conductivity studies of Figueroa et al [3] in β-PbF 2 doped with Li, Na, K and Rb (see their Tables III and IV), whereas two other points obtained by the same authors in two undoped samples P 1 and P 2 . Second, three points (open, full and inverted triangle) correspond to the (free) fluorine vacancy migration parameters, as they have been reported by Oberschmidt and Lazarus [5], Samara [6] and Murin et al [7], respectively.…”

“…This has been the subject of a large number of investigations. Among these studies, Figueroa et al [3] have made low temperature dc-conductivity measurements under various pressures on PbF 2 either pure or doped with various alkali metals. At their lower temperatures of these measurements at which the conductivity is due to free vacancies thermally dissociated from the electric dipoles consisting of the impurity and the fluorine vacancy (created for reasons of charge compensation), the activation volume υ act is given by:…”

“…Moreover, Figueroa et al [3] made dielectric relaxation measurements at various pressures, which give the volume υ m,b for the (re)orientation process of the above dipoles. All these volumes determined for all the alkali ions Li + , Na + , K + and Rb + have been found to be positive.…”

“…All these volumes determined for all the alkali ions Li + , Na + , K + and Rb + have been found to be positive. In addition, Figueroa et al [3] noticed that the ratio of the activation volumes to the activation energies (which are, in fact, activation enthalpies designated hereafter with h act ), is the same for all dopants. An explanation on this important experimental finding was then proposed by Lazaridou et al [4], based on an early model termed cBΩ model, that will be summarized later.…”