Lithium niobate($LiNbO_3$)is one of the most interesting ferroelectric materials for application in various devices, owing to its favorable electrooptic and piezoelectric properties. The congruently grown $LiNbO_3$ associated with nonstoichiometry leads to the formation of intrinsic defects in its crystal structure. So far, several defferent defect models have been proposed for nonstoichiometric $LiNbO_3$. In particular, the defect structure of the deviation in the Li/Nb and the metal/oxygen stoichiometries have a strong influence on the optical and electrical properties.
We investigated the crystallographic structure, optical and dielectric properties of $LiNbO_3$ and MgO-doped $LiNbO_3$ single crystal grown by Czochralski method.
The crystallographic structure of $LiNbO_3$ and MgO-doped $LiNbO_3$ was refined by four-circle X-ray diffractor method. It was found that the addition of MgO up to 4 mole% to the congruent $LiNbO_3$ leads to a steady increase of the lattice constants $a_H$ and $c_H$. But when the MgO content exceeds this concentration, an abrupt decrease in the lattice constants is observed. This abruptly change in the lattice constants seems to have an intimate connection with the anomalies in several physical properties : We observed that the SHG phase matching temperature, Curie temperature and optical damage resistance show abrupt change at this concentration. Our structure refinements suggested that this shreshold phenomena in the structure and physical properties are deeply related to the displacement Nb atoms in oxygen octahedra, which is, in turn, related to the defect structure. A change in the mechanism of the MgO incorporation at about 4 mole% in congruent $LiNbO_3$ was also confirmed.
