A combined Nuclear Magnetic Resonance and X-ray Absorption Fine Structure study on the local structure of Ge and Pb in PbO - GeO2 glasses and their relationships with thermal properties and devitrification products

Thermal and structural properties of xPbO/(1 - x)GeO2 glasses with 0.0 less than or equal to x less than or equal to 0.50 were studied by means of differential scanning calorimetry (DSC), X-ray absorption fine structure (XAFS) on Pb L-III and Go K edges, and Pb-207 solid-state NMR. Extended X-ray absorption fine structure (EXAFS) evidences on Go indicated that most of the Go is in tetrahedral coordination, which exhibits an increasing distortion toward higher distances for x greater than or equal to 0.10. Concerning the Pb environment, both EXAFS and NMR have shown that in all glasses most of the Pb ions are at the apex of a trigonal (PbO3) or square (PbO4) pyramid or both as in crystalline PbGeO3. NMR has also shown that Pb enters as a modifier and, by increasing x, the PbO3/PbO4 ratio increases. The thermal behavior of the system can be interpreted by dividing the investigated compositional range into three regions: (i) 0 < x &LE; 0.05; (ii) 0.05 < x < 0.25; (iii) 0.25 &LE; x &LE; 0.50. In both region i and region ii, a nanoscale separation can be inferred because of the presence of nanocrystalline GeO2 dispersed in the glassy matrix. In region iii, a complete miscibility was observed. The Pb environment and the nanoscale separation can clarify the complex devitrification processes found in this system.