Modifier to oxygen bonds influence on the thermal and mechanical properties of neodymium laser phosphate glasses

The selection of the right modifier elements in the design of phosphate glass compositions, which simultaneously need to fulfil strict optical requirements while showing excellent thermo-mechanical performance, can be a complicated task. This work has explored the thermal and elastic properties of mixed-alkali metaphosphate glasses with different bivalent modifier elements and obtained the behavior of the main properties involved in the thermal shock resistance of the glasses as a function of the average ionic field strength of the modifiers’ composition. The glasses have been studied by dilatometry, thermal diffusivity and Brillouin spectroscopy measurements and their properties interpreted in terms of the modifiers’ influence on the bonds between phosphorus atoms and the bridging and non-bridging oxygens of their structure, which have been characterized by Raman spectroscopy. Despite showing better glass-forming ability for high contents of the bivalent modifier oxide, the ZnO-containing glasses explored herein present lower values of their mechanical resistance than what corresponded to their ionic field strength. However, an increase of the ZnO content can be used to obtain bigger improvement of the thermal shock resistance than if a bivalent modifier with lower field strength were used, such as BaO. The last has been explained as a consequence of the bigger measured change of the overall polarizability and lower modifier-oxygen bonds anisotropy.