Structural characterization of heavy metal SiO2–Bi2O3 glasses and glass–ceramics

• xSiO2 ∙(100 − x)Bi2O3 heavy metal glasses and glass–ceramics
• Structural changes evidenced by 29Si MAS NMR, XRD and SEM
• Increasing SiO2 content leads to structural relaxation of vitreous network.
• High local structure stability for glasses with 30 ≤ x ≤ 60 mol%.

29Si MAS NMR spectroscopic data obtained for heavy metal SiO2–Bi2O3 glasses and partially crystallized samples of similar composition were used to investigate the changes induced in their local structure as the ratio between Bi3 + and Si4 + cations decreases from 18:1 to 6:7. Progressive increase of SiO2/Bi2O3 ratio contributes to the structural relaxation of vitreous network and enhances the glass stability. In glass samples with high silica content the increasing number of Si–O–Si bridges proves the three-dimensional polymerization of silicate network. The shape of NMR signals for glass–ceramic samples depends on the crystalline phases formed for different SiO2/Bi2O3 ratios. The crystalline phases developed by heat treatment of vitreous matrices were identified by X-ray diffraction as Bi12SiO20, Bi5.6Si0.5O9.4 and Bi2SiO5. Scanning electron microscopy analysis was used to examine the morphology of the crystals developed in the glassy silica rich matrices.