SHORT AND MEDIUM-RANGE ORDER IN ZINC HALIDE BASED GLASSES

Low frequency Raman spectra have been studied for zinc halide based gl ass-forming systems and particular attention was devoted to the 50ZnBr (2).40KI.10BaBr(2) and 99ZnBr(2).1AS(2)O(5) compositions. These system s have two low frequency depolarized bands. at similar to 20 cm(-1) an d similar to 65 cm(-1), that may in part be due to acoustic modes, whi ch have been analyzed according to Martin and Brenig's (MB) model. In the framework of this model, structural correlation lengths of similar to 0.7 nm and similar to 3 nm were calculated from the frequencies of the two bands, respectively. These results are compared with data for other halide, oxide and chalcogenide glass-forming compositions. To d etermine the properties of the two low frequency bands, their depolari zation ratios and temperature dependence have also been investigated. We see that the results can be explained by the existence of both acou stic and optical modes in the low frequency region of the Raman spectr a of zinc halide based glasses. (C) 1998 Elsevier Science B.V. All rig hts reserved.