Lithium niobate (LiNbO₃: LN) is a piezoelectric material, and it has been used as nonlinear optical device. A crystal is grown from the congruent melt to obtain homogeneous composition. During the crystal growth, antisite defect (Nb_Li) is generated because of excess Nb in LN. The Nb_Li causes photorefractive damage, which hinders optical application of LN. Regarding this problem, Zhong et al. (1980) found that MgO doping increased the damage resistance. However, it has not been fully understood how doping of impurities such as MgO affects to the defect formation which lead to optical damage. In the present study, site structures and Nb_Li behavior have been investigated by measuring lattice constants and curie temperatures (T_c) of LNs with changing impurity concentration.  Lattice constants of congruent LN(c-LN) doped with respective impurities (MgO, ZnO, Fe₂O₃, TiO₂, ZrO₂) were measured by XRD. Site structure changes caused by impurity doping were analyzed with variation of lattice constants with impurities. The analyses show that Mg, Zn and Fe were incorporated into Li-site, which decreased Nb_Li, in contrast Zr and Ti were incorporated into Nb-site, which increased Nb_Li.  T_cs of impurity-doped LNs were measured by DSC with changing a composition of LN along with two different series of A and B. In A series, impurities were doped to stoichiometric LN keeping Nb₂O₅ concentration to be 50 mol%. In B series, they were doped to c-LN keeping Li/Nb to be constant. In A series, T_c didn’t change, in which Li/Nb changed and Nb_Li was constant. In B series, on the other hand, T_c changed, in which Li/Nb was constant and Nb_Li changed. T_c has been believed to depend on Li/Nb in the non-doped LN (O'Bryan et al. 1985). However, from these results, we can conclude that T_c of doped LN depends on Nb_Li concentration instead of Li/Nb. Measurement of lattice constants and T_c reveal that the Nb_Li to be decreased by doping of Mg, Zn and Fe whereas increased by Zr and Ti.

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