Local and distant charge compensation of iron ions in ABO$_{3}$ ferroelectrics

The determination of the lattice sites and charge compensators of non-isovalent impurities are vitally important for both fundamental science and tailoring material properties for various applications. Results of our study of Fe$^{3+}$ centers in LiNbO$_{3}$, LiTaO$_{3}$ and KTaO$_{3}$ (LN, LT and KT) with the help of Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) have shown significant difference in spectroscopic characteristics and, correspondingly, structures of these centers. Two Fe$^{3+}$ dominant centers in cubic KT crystals have axial $<$100$>$ symmetry caused by the presence of local charge compensators (oxygen vacancy for Ta$^{5+}$ substitution and interstitial oxygen for K$^{+}$ substitution). In Li-deficient congruent LT and LN crystals the Fe$^{3+}$ ions substitute only for Li$^{+}$. Since no disorder in the nearest surrounding was found, the distant charge compensation by intrinsic defects in cation sublattice is supposed. In stoichiometric samples of LT and LN the iron ions randomly substitute for both Li$^{+}$ and Ta$^{5+}$ (Nb$^{5+})$ having average self-compensation of excess charges.