Effect of increasing concentration of Na₂O on structural, elastic and optical properties of (90  −   x )GeO₂– x Na₂O–10PbO glass system in the germanate anomaly region

Ternary germanate glasses (90  −   x )GeO₂– x Na₂O–10PbO ( x   =  10–30 mol%) have been prepared by the melt-quenching method. Density, ρ increased with Na₂O content up to maxima at 20 mol% while molar volume, V _a showed an opposite trend to the density, with a minima at 20 mol% of Na₂O content indicating the presence of the germanate anomaly. Ultrasonic velocity measurements showed both longitudinal, v _l and shear, v _s velocities increased up to 20 mol% before decreasing with further addition of Na₂O. Independent longitudinal, L and shear, G moduli along with Young's modulus, Y , mean sound velocity, v _m, Debye temperature, θ _D, and hardness, H recorded maximum values at 20 mol% of Na₂O content which were suggested to be related to the germanate anomaly. Structural modification occurring due to conversion of six-membered GeO₄ rings to three-membered rings of GeO₄ changed bond density and compactness of the glass systems and caused the increase in rigidity and stiffness of the glasses. Beyond 20 mol% of Na₂O, the decrease in the elastic moduli was due to depolymerization of the glass network. Meanwhile, optical energy gap, E _opt exhibited a minima at 20 mol% whereas Urbach energy, E _U and refractive index, n showed a maxima at the same concentration, thereby indicating variation in polarizability due to changes in concentration of bridging and non-bridging oxygen. (paper)