CLUSTERING OF Y-3+ IN BAF2 USING GD-3+ AS PARAMAGNETIC PROBES

Ionic thermocurrent (ITC) and electron paramagnetic resonance (EPR) experiments have been performed on solid solutions of the type Ba1-xYxF2+x, 0.0001≤x≤0.05, additionally doped with a small fixed molar concentration, equal to 10-4, of Gd3+ acting as paramagnetic probes. It is shown that the more abundant polarizable defect is the next nearest neighbor (nnn) dipole, Y3+-Fi-, whose molar concentration reaches a maximum for x∼0.005. Also, the EPR experiments show the coexistence of cubic and trigonal Gd3+ centers for the whole x range studied here. The variation of the population of these two paramagnetic centers peaks around x=0.0002 and the fraction of their relative abundances deduced from the observed EPR intensity is almost constant. The existence of several weaker lines evidences the presence of Gd3+ ions in additional sites to the cubic and trigonal ones. The comparison of the results obtained from the ITC and EPR techniques indicates the presence of Y3+ in larger clusters than the simple nnn dipole. To explain the variation of the abundance of the trigonal center at low Y3+ concentrations, a mechanism is proposed for the creation of the clusters formed by the trapping of an extra Fi- by either the Y3+ or the Gd3+ nnn dipoles. The existence of clustering among the trivalent ions in this system is supported by the variation of the linewidths of the EPR lines as x increases. These linewidths do not increase linearly with x but rather present a saturation effect at high x values. This indicates that a high doping levels the yttrium ions are not mainly dispersed in the lattice as nnn dipoles, but rather they are forming higher clusters whose interaction with the Gd3+ trigonal centers is small. The ITC results show that most of the clusters are either not polarizable or present a very small dipolar moment. As all the evidence presented here indicates that clustering is important in these crystals, one can conclude that the behavior of the high temperature ITC peak is best understood if it is attributed to an interfacial polarization around the dislocations of the crystal and not to a space charge effect. © 1990.