Ionic Radical Theory of Crystal Electro-Optical and Nonlinear Optical Effects. I. Computation of BaTiO sub 3 Crystal Electro-Optical and Harmonic Generation Coefficients by Using a Distortion Model of the Oxygen Octahedron

摘要

An MO sub 6 ionic radical model is proposed for the mechanism of electro-optical (EO) and Second Harmonic Generation (SHG) effects of BaTiO sub 3 type crystals. Starting with the ionic bond orbital of the TiO sub 6 radical under the interaction between the TiO sub 6 ionic pseudo-molecular orbital and the potential energy of the crystal lattice field, the various electro-optical and SHG coefficients of BaTiO sub 3 are computed individually, using the ABDP theory. The computed results indicate that under the circumstances no adjustable parameters are introduced, there is good agreement between the experimental values and the computed values of the electro-optical and SHG coefficients from TiO sub 6 ionic radical crystal lattice field theory. The contribution of the pseudo-molecular orbitals to these fields is rather small. From this are drawn three conclusions: (1) the electro-optical and SHG effects of BaTiO sub 6 are mainly produced by the distortion of the oxygen octahedron. The bigger the distortion, the bigger are the effects; (2) when BaTiO sub 3 is in the ferro-electric phase, a strong odd term lattice field is produced due to the spontaneous polarization of the crystal. This is the main cause of the large electro-optical and SHG effects of BaTiO sub 3 . Due to mutual cancellation, the covalent bond component of the TiO sub 6 ionic radical contributes little to the effects; (3) there is a simple, linear relationship between the electro-optical and SHG coefficients of BaTiO sub 3 and its spontaneous polarization P/sub s/. (ERA citation 07:011358)