Reorientation of Eu²⁺-cation vacancy dipolar complex in KCl/KBr binary crystals

In this work we describe the polarization and depolarization process of Eu²⁺-V_C dipoles in Eu²⁺ doped solid solutions of KCl and KBr considering in detail the different jumping paths of the cation vacancy within individual arrangements of minority anions on the first two anion shells surrounding the dipolar complex. Experimental depolarization curves of binary crystals show in contrast to the single depolarization peak in unitary crystals, up to at least two additional depolarization peaks. These two additional peaks are associated with jumps perturbed by one, and two minority anions flanking the jumping path. In crystals for which Cl^- is the minority anion, the formation of the two additional bands is more prominent than for those where Br^- is the minority anion, suggesting that the divalent cation dopant has more affinity towards the Cl^- minority anion than towards the Br^- minority anion. Fittings of our model to the experimental data in which we also contemplate different affinities with the divalent dopant cation for Cl^- and Br^- minority ions, confirm this. In contrast to an earlier work, in which it had been suggested that only the first anion shell contributes to the perturbation of the dipole in order to reconcile the experimentally observed peak intensities and the supposed affinity between the dipole and the Cl^- minority anion, the results of our work suggest that both shells contribute in a similar way. This discrepancy is discussed in terms of the different approaches to the description of depolarization process.