Effect of point defects on Curie temperature of lithium niobate

The effect of point defects on the Curie temperature (T_c) of LiNbO₃ (LN) was investigated by combining T_c measurements with an analysis of the defect structures of LN doped with impurities having various valences. The data show that T_c of congruent LN increases with the impurity concentration up to around 3 and 2 mol% for divalent and trivalent impurities, respectively, whereas it decreases continuously with increased concentrations of tetravalent impurities. These T_c variations were examined with respect to the defect structures of impurity-doped LN, which are expressed in the form chemical formulae using Kröger-Vink notation. The defect structures of divalent and trivalent impurity-doped LN are (Formula presented.) and (Formula presented.), respectively (Nb_Li: Nb at Li sites; V_{L i}: vacancies at Li sites, M_{L i}: impurities at Li sites, and Li/Nb = the congruent ratio). The defect structure in the case of tetravalent impurities is (Formula presented.). Analyses of the defect structures indicated that the Nb_Li concentration decreases with divalent or trivalent impurity doping, which increases T_c. In contrast, the Nb_Li concentration increases with tetravalent impurity doping, which decreases T_c. In addition, the divalent or trivalent impurity concentrations at which the Nb_Li concentration becomes zero were found to correspond to the concentrations at which T_c is maximized, suggesting that T_c of LN depends on the Nb_Li concentration.