Influence of germanate anomaly on elastic, structural, and optical properties of xNa2O-(99–x)[80GeO2:20PbO]-1Er2O3 lead–germanate glasses

Abstract

Abstract A sodium–lead–germanate glass system, with a composition of xNa2O-((100–y)–x)[80GeO2:20PbO]-yEr2O3 (x = 0–25 mol.%, y = 0, 1 mol.%), was prepared by melt quenching and used to investigate the effect of Na2O and Er2O3 on the germanate anomaly. The structural and optical properties of the glass samples were investigated using X-ray diffraction, Fourier transform infrared, and UV–Vis spectroscopy analyses. Elastic properties of Er2O3-doped glasses (y = 1) were studied by measuring longitudinal and shear velocities through the pulse-echo method at 5 MHz. Based on Fourier transform infrared spectroscopy analysis of Er2O3-doped glasses, the conversion of GeO4 into GeO6 indicates that the glass system possesses the germanate anomaly characteristic but has no density anomaly. Longitudinal, shear, bulk, and Young's moduli (C L, μ, K, and E, respectively) increased to their maximum values at x = 10 mol.% but decreased with increasing amount of Na2O added. This finding reveals the elastic nature of the germanate anomaly. Increase in elastic moduli indicates enhanced network rigidity of the glass system in the germanate anomaly region, where the coordination number increased with the transformation of GeO4 to GeO6. Subsequent decrease in elastic moduli (x > 10 mol.%) denotes weakened network rigidity of the glass system because of enhanced formation of non-bridging oxygen. Furthermore, analysis using bulk compression and ring deformation models reveals the nonlinear trends of K bc /K e ratio and average ring size diameter as a result of the germanate anomaly. The anomaly also influenced optical properties of both Er2O3-doped (y = 1) and Er2O3-free (y = 0) glasses, where the optical energy gap (E opt) decreased with the addition of Na2O up to 10 mol.% and slightly increased with more than 10 mol.% Na2O. By contrast, Urbach energy (E U) and refractive index (n) showed opposite trends to that of E opt. The behavior of E U indicates changes in defect concentration, which affects E opt and n.