ARE TERNARY HALIDES USEFUL MATERIALS FOR NONLINEAR-OPTICAL APPLICATIONS

Ternary halides are potential materials for nonlinear optical applicat ions in the mid infrared because of their transparency. We discuss phy sical, chemical and crystallographic aspects and develop a concept for preparing nonlinear optical halides. Based on the bond-charge model o ptical hyperpolarizabilities are calculated for more than one hundred A-X bonds where X = Cl, Br or I. The calculations are tested as far as possible by a comparison with experimental data. The tests show that the listed hyperpolarizabilities are a sound basis for the calculation of nonlinear optical susceptibilities in halides. A list of cations i s given which form bonds with large hyperpolarizabilities and which sh ow the tendency to form acentric structures. Phase diagrams of the two selected systems TlI-AsI3 and TlI-BiI3 are studied experimentally. La rge single crystals of the new acentric compound Tl7Bi3I16 and of Tl3P bCl5 are prepared. In both crystals twinning prevents an effective SHG effect.