Abstract
From the chemical bond viewpoint, second-order non-linear optical (NLO) tensor coefficients of KNbO 3 and LiNbO 3 crystals have been calculated. By using the bond-valence theory of complex crystals and the modified bond-charge model, we were able to determine contributions of each type of constituent chemical bond to the total second-order NLO susceptibility. The tensor values thus calculated are in good agreement with experimental data. From the comparison of NLO tensor coefficients of these two crystals, we found that the major NLO contributors are KO 12 groups and LiO 6 octahedra, not the distorted NbO 6 octahedra. The difference between their NLO properties arises from their different structural characters, and the high coordination number of constituent elements in KNbO 3 makes its valence electrons become more delocalised compared with those of LiNbO 3.
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