Heterovalent isomorphous replacement is one of the main methods to increase the ionic conductivity of solid electrolytes. Understanding the nature of these impurity defects and their relaxation can help in establishing the connection between the electrophysical properties and crystallochemical peculiarities of doped superionic conductors. EPR spectra of Gd³⁺ in PbF₂ show a well resolved fine structure as well as spin forbidden transitions with ΔM_s>1. The angular dependence in both cases indicates that Gd³⁺ occupies a purely cubic symmetric site. This stands in contrast to earlier results and is indicative of the importance of the history of a sample, especially its thermal and pressure treatment, as shown by X-ray analysis and conductivity data. A method of investigating dipole defects and polarizing phenomena is proposed, whereby RE-F_i dipole defects are fixed and their relaxation is studied by means of EPR-spectroscopy.