晶体电光和非线性光学效应的离子基团理论(Ⅰ)——利用氧八面体畸变模型计算BaTiO3晶体电光及倍频系数

本文对钙钛矿型晶体的电光及倍频效应的机理提出了一个(MO₆)离子基团模型。并从(TiO₆)离子基团的准分子轨道及晶格场位能作用下的(TiO₆)离子基团的离子键轨道出发,利用ABDP理论,分别计算了BaTiO₃的各个电光及倍频系数。计算结果表明,在没有引入任何可调整参量的情况下,用(TiO₆)离子基团的晶格场理论所得到的电光及倍频系数的计算值和实验值符合得相当好,而准分子轨道对这些效应的贡献很小。由

AN IONIC GROUPING THEORY OF THE ELECTRO-OPTICAL AND NON-LINEAR OPTICAL EFFECTS OF CRYSTALS (Ⅰ)——A THEORETICAL CALCULATION OF ELECTRO-OPTICAL AND SECOND OPTICAL HARMONIC COEFFICIENTS OF BARIUM TITANATE CRYSTALS BASED ON A DEFORMED OXYGEN-OCTAHEDRA

In order to elucidate the relationship between the structure and the electro-optical and nonlinear optical effects in crystals of the perovskite type, we have here proposed a deformed oxygen-octahedra model for these crystals. On the basis of quasi-molecular orbitals and ionic bond orbitals for the ionic grouping TiO₆ formulated under the imposed crystal field, we have calculated the various electro-optical and second optical harmonic coefficients for the BaTiO₃ crystal without introducing any adjustable parameters. It is gratifying to note that the calculated values of the electro-optical and second optical harmonic coefficients agree satisfactorily with the experimental values. As is to be expected, the quasi-molecular orbitals contribute very little to these optical effects. We have been led to the following conclusions:1) The electro-optical and second optical harmonic effects of the BaTiO₃ crystal arise as a direct consequence of the deformation of the TiO₆ oxygen-octahedra. The more these octahedra are deformed, the larger are these optical effects.2) When BaTiO₃ is present as the ferroelectric phase, a strong odd ordered-term crystal field arises as a result of the spontaneous polarization within the crystal. This explains why BaTiO₃ possesses such large electro-optical and second harmonic optical effects. The covalent bonds in the ionic group TiO₆ tend to cancel each other in their contribution to these effects.3) The electro-optical and second harmonic optical coefficients are directly proportional to the degree of spontaneous polarization.