A Fe[Nb]-Li center in stoichiometric LiNbO$_3$ crystals: Mechanism of formation

A new iron center in stoichiometric lithium niobate crystals has been studied by the EPR method. The angular dependences of the EPR spectrum of the center have been used to derive the parameters of its spin Hamiltonian. The data amassed on the variation in the concentrations of two iron centers in lithium niobate crystals annealed in a Li$_2$CO$_3$ powder have provided an insight into the mechanism of formation of the new center, as well as corroborated its model proposed by us earlier. According to this model, the center represents a complex of two defects aligned with the polar axis in the crystal: the iron ion at the niobium site and an interstitial lithium ion filling the nearest structural vacancy (Fe$^{3+}$[Nb]–Li$^{+}$[V]). The structure of other Fe$^{3+}$ centers revealed earlier in LiNbO$_3$ crystals, in which the iron ion occupies the niobium site, has been discussed.