Structure and Properties of Infrared Transmitting Chalcohalide Glasses.

There have been a great deal of demands placed on the materials which show high transmittance in the mid -IR region compared to oxide glasses. Halide and chalcogenide glasses have long been studied as candidates for this purpose. There is yet a third family of glasses made by the mixture of halide and chalcogenide glasses which will be referred to here as "chalcohalide glasses". The purpose of the present work is to investigate the structure and properties of new chalcohalide glasses in the Ge-S-Br and Ge-S-I systems. The role of halogen atoms on the structure of GeS_2 and GeS_3 glasses has been discussed in detail based on the results obtained from IR, Raman and x-ray photoelectron spectroscopy. Effect of halogen addition on such properties as IR-transmittance, density, glass transition temperature and refractive index has also been studied. As the halogen components were added to binary Ge-S glasses, there would be a formation of Ge-Br bonds substituting sulfur atoms in the GeS_4 tetrahedra. Sulfur atoms removed from the GeS _4 tetrahedra would then form S_8 rings and chains. In general, the structure of Ge-S-Br(or I) glasses can best be described as a solid solution of S_8 rings dispersed throughout the network structure formed by GeBr _{rm x}S_ {4-rm x}(or GeI_ {rm x}S_{4- rm x}) groups and thereby forming a molecular phase separation. It could be expected that the network connectivity of Ge-S glasses decreases with the formation of Ge-Br (or I) bonds due to the network terminating role of monovalent halogen atoms. Consequently, structure of these glasses would become more open and eventually lead to the formation of chain-like structures. The observed changes in the properties of chalcohalide glasses in the Ge-S-Br and Ge -S-I systems support the proposed structural model.