Effect of point defects on Curie temperature of lithium niobate

The effect of point defects on the Curie temperature ( T c ) of LiNbO 3 ( 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{ [ Li Li × ] 1 − 5 x − 2 y[ Nb Li ∙ ∙ ∙ ∙ ] x[ M Li ∙ ] y[ V Li ′ ] 4 x + y } [ Nb Nb × ][ O O × ] 3and{ [ Li Li × ] 1 − 5 x − 3 y[ Nb Li ∙ ∙ ∙ ∙ ] x[ M Li ∙ ∙ ] y[ V Li ′ ] 4 x + 2 y } [ Nb Nb × ][ O O × ] 3 , 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{ [ Li Li × ] 1 − 5 x + y[ Nb Li ∙ ∙ ∙ ∙ ] x[ V Li ′ ] 4 x − y } { [ Nb Nb × ] 1 − y[ M Nb ′ ] y }[ O O × ] 3 . 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.