Paradoxical hydrolytic degradation in GeO2-P2O5 glasses at high GeO2 content: A structural transition study

The GeO2-P2O5-based glass systems hold promise for advanced optical applications due to their high refractive index, excellent optical transparency, and tunable mechanical durability. Despite these advantages, phosphate-rich compositions face significant challenges from inherent hydrolytic instability caused by P-O bond cleavage under moisture. While moderate additions of GeO2 (<= 50 mol %) have been shown to enhance structural stability and preserve optical performance by forming robust Ge-O-P and Ge-O-Ge linkages, an unexpected decline in hydrolytic durability is observed at higher GeO2 concentrations (> 55 mol %). This study systematically investigates this paradox across a compositional range of 45-75 mol % GeO2 using Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS). Our findings reveal that increased GeO2 content induces a structural transition from six-coordinated to four-coordinated Ge, which diminishes Ge-O-P bonds and P-O-P long polymer chains. This shift results in poor hydrolytic stability due to enhanced water molecule interactions and terminal P-OH bond formation. By elucidating these mechanisms, this study provides critical insights into optimizing GeO2-P2O5 glass scanning electron microscope for applications in extreme environments, aligning empirical observations with mechanistic understanding.