Defect structure and charge transport in solid solutions Ba1-xLaxF2+x

In this paper we have investigated the defect structure and the charge-transport properties of solid solutions of the type Ba1-xLaxF2+x. The defect structure has been studied by means of samples which had been doped slightly with trivalent gadolinium ions. These probes have been employed to investigate the surrounding crystal lattice, which contains in some cases large amounts of trigonal La3+-Fi- dipoles. It appears that the Gd3+ impurities do not participate in an eventual clustering process, because we have not observed EPR signals with significant intensities which can be assigned to clusters. In the solid solutions studied in this paper we have observed two different dipolar defects: (a) the nearest-neighbor (NN) or tetragonal La3+-Fi- dipole and (b) the nextnearest-neighbor (NNN) or trigonal La3+-Fi- dipole, the latter being the more dominant defect. The concentration ratio of the NN and NNN dipoles varies with the concentration of La3+ ions in the sample. With increasing La3+ concentration the above-mentioned ratio changes in favor of the NNN dipoles. In our ionic thermocurrent experiments on the system Ba1-xLaxF2+x we have observed three peaks: a weak one at about 137 K, which is associated with NN dipoles; a stronger one at about 190 K, which is due to NNN complexes; and a very strong one, which shifts to lower temperatures with increasing values of x. This strong peak is due to space charges which are produced by the polarizing field. The conductivity mechanism will be discussed in terms of the two-jump mechanism proposed in an earlier paper. In the range of low concentrations the eventual role of monovalent cations and oxide impurities is discussed. In order to obtain more information about the defect structure of the solid solutions Ba1-xLaxF2+x, we have investigated the development of the linewidth of the different resonances observed for trigonal Gd3+ probes which had been introduced into the samples. The observations have been analyzed, and it has been concluded that the Gd3+ probes are perturbed by distant La3+-Fi- dipoles. The broadening of the EPR lines will be calculated using a statistical model; the electrostatic interactions of the dipoles are found to shift the fine lines of the Gd3+ probes. The theoretical model employed here is found to give reasonable agreement with the experimental results.