Effect of increasing Na₂O structural, elastic and optical properties of (90- x)GeO₂-xNa₂O-10PbO glass system in the germanate anomaly region / Che Noraida Mohd Zainudin

Ternary germanate glasses with the chemical formula (90- x)GeO₂-xNa₂O-10PbO (x : 10, 15, 20, 25, 30 mol %) have been prepared by melt-quenching method. XRD patterns showed the amorphous nature of the glass samples for x ≤ 20 mol % and emergence of minor crystalline peaks in a mainly amorphous matrix for x > 20 mol %. Density, ρ increased with Na₂O content up to 20 mol % before decreasing beyond 20 m0l % while molar volume, Vₐ, showed an opposite trend to the density, with a minima at 20 mol % of Na₂O content indicating the presence of the germanate anomaly below 20 mol %. Ultrasonic velocity measurements showed both longitudinal, vι, and shear, vs velocities increase 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, vm, Debye temperature, Θᴅ, and hardness, H also recorded maxima at 20 mol % of Na₂O content. The initial increase in L and G up to 20 mol % of Na₂O content was suggested to be related to structural modification occurring due to the conversion of six-membered GeO₄ rings to three-membered ring of GeO₄ which changed the bond density and compactness of the glass systems. The increased in elastic moduli also indicates that the structural change was accompanied by an increase in rigidity and stiffness of the glass. Beyond 20 mol % of Na₂O, the decreased L and G could be due to depolymerization of the glass network. Further analysis using bulk compression and ring deformation models also showed the ratio of ideal bulk modulus to the experimental bulk modulus, Kbc/Kₑ increased from 10 to 20 mol % addition of Na₂O accompanied by reduction in average atomic ring size, Ꙇ in the germanate anomaly region. The minimum Kbc/Kₑ values was observed at x : 25 mol % which coincides which minima for Ꙇ. The optical properties the glass system Showed optical energy gap, Eορϯ decreased with addition of Na₂O up to 20 mol % and increased beyond 20 mol %, while Urbach energy, Eu and refractive index, ռ showed opposite trends to Eορϯ. The changes in the optical behaviors are suggested to be related to the changes in N80 concentration in the glass samples.