Electrical relaxation in rare earth doped cubic lead fluoride

Audio frequency complex impedance measurements are performed over the temperature range 5.5-380 K on lead fluoride doped with ten rare earths and lanthanum at various concentrations. It is found that the complexity of the relaxation spectrum increases as the size of the trivalent ion decreases. Specifically, for the largest rare earths, only one relaxation is found. That relaxation is different from the cases in the alkaline earth fluorides where only one relaxation is observed in that the reorientation enthalpy depends strongly upon ion size. Double-doped samples are studied to determine whether the relaxation is due to a simple site or a cluster. For the smallest rare earths, however, at least nine relaxations are found. The concentration studies indicate multiple relaxations for certain sites. Both simple sites and clusters are observed for small rare earths.