Composition and Structure Dependence of the Photoelastic Response of Oxide Glass
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The isotropy of a glass can be broken by the application of a mechanical stress giving rise to a phenomenon of birefringence. Some lead-containing glass compositions are known to prevent this phenomenon and they are called zero-stress optic glass. Mueller’s theory of photoelasticity attempts to explain the structural origin of the photoelastic response in glass and crystal. Zwanziger’s empirical model is able to predict the photoelastic response of a glass based on its composition and the crystal structure of its constituents. Lead-, tin-, antimony-, zinc-, and cadmium-containing glasses were investigated in the binary silicate, borate, and phosphate systems. The stress optic coe?cient of these binary glasses was measured experimentally using the S´enarmont method or found in the literature. Solid-state Nuclear Magnetic Resonance spectroscopy and M¨ossbauer spectroscopy were mainly used to investigate the local environment of the cations. The photoelastic response of a glass and its structure were correlated, and the results were compared with the expectations arising from Mueller’s theory and Zwanziger’s empirical model. The theory and the model were both tested and their reliability was discussed. Zero-stress optic glasses are of technological interest, but new environmental regulations forbids the use of lead in materials, including glass. From experimental results and literature, a global strategy to design new zero-stress optic glasses was established. New lead-free zero-stress optic glasses were discovered with properties similar to the lead-containing zero-stress optic glass (high index of refraction, transparency, no coloration). The study of the structural dependence of the photoelastic response of oxide glass contributed to identify new parameters in?uencing the photoelasticity, such as covalency, polarizability and natural deformation of the additive.