ELECTRO-OPTIC CRYSTALS. L. Bohaty, Institut für Kristallographie, Universität zu Köln, Zülpicher Str. 49b, D-50674 Köln, Germany

A very stimulating question for crystallography and one of the most important goals of material science is to find strategies for searching for crystals with desired properties. A direct conversion of physical models into practicable criteria for "tailoring" new crystals usually fails because of the structural complexity of most crystals. Here, the application of crystallographic aspects can be very helpful. As part of our systematic investigation of nonlinear optical and electro-optic properties we have determined the complete linear electro-optic tensors (all components and signs) of more than 60 inorganic and "semiorganic" crystals in the last few years. This broad basis of experimental data enables us to analyse the linear electro-optic effect especially from the crystal-chemical point of view. In particular, we have focused our attention on the elasto-optic-piezoelectric contribution P_ijmn d_kmn of the "unclamped" electro-optic effect r^[[sigma]]_ijk = r^[[epsilon]]_ijk + P_ijmn d_kmn and its importance for the total value of r^[[sigma]]_ijk ([P_ijmn] = elasto-optic tensor, [d_kmn] = piezoelectric tensor, [r^[[epsilon]]_ijk ] = "clamped" electro-optic tensor (at constant strain [[epsilon]]), [r^[[sigma]]_ijk] = "unclamped" electro-optic tensor (at constant stress [[sigma]])). One method of analysis which has proved very successful is the use of longitudinal and transverse effects, their representation as surfaces and the extraction of the extreme values. In addition, statistical analysis of more than 20.000 inorganic crystal structures with respect to the geometry and symmetry of the constituents (ions, molecules, complex groups, etc.) and to the resulting symmetry of the crystal structure make plain the important role of polar constituents and asymmetric groups for noncentrosymmetric crystals and, consequently, for new electro-optic materials. Valuable information which can be used in a systematic search for new inorganic and "semiorganic" electro-optic materials was obtained.