Structural mechanism of iodine incorporation in As-Se-I glasses and its effects on physical properties

The structural evolution of ternary AsxSe100 − x − yIy glasses (x = 20, 30 and y = 10, 20) upon progressive substitution of Se with I is studied using a combination of high-resolution, two-dimensional 77Se nuclear magnetic resonance (NMR) and Raman spectroscopy. The results suggest that the As-Se bonds in the As-Se-Se linkages in these glasses are severed and terminated by As-I bonds with a concomitant increase in the relative fraction of Se-Se-Se linkages and, in the case of As30Se70 − yIy glasses, also in the relative fraction of As-Se-As linkages. This structural modification decreases the average connectivity of the network and its packing efficiency, which are manifested in an increase in molar volume and fragility and a decrease in glass transition temperature Tg and isothermal viscosity. Additionally, Tg and fragility are found to be only a function of average coordination number regardless of chemical composition. When taken together, these results indicate a relatively homogeneous distribution of the I atoms in the glass structure in “mixed-anion” AsSe(3 − x) / 2Ix pyramidal units.