A Combined Nuclear Magnetic Resonance and X-ray Absorption Fine Structure Study on the Local Structures of Ge and Pb in PbO−GeO2 Glasses and Their Relationships with Thermal Properties and Devitrification Products

Thermal and structural properties of x PbO/(1 − x )GeO 2 glasses with 0.0 ≤ x ≤ 0.50 were studied by means of differential scanning calorimetry (DSC), X-ray absorption fine structure (XAFS) on Pb L III and Ge K edges, and 207 Pb solid-state NMR. Extended X-ray absorption fine structure (EXAFS) evidences on Ge indicated that most of the Ge is in tetrahedral coordination, which exhibits an increasing distortion toward higher distances for x ≥ 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 (PbO 3 ) or square (PbO 4 ) pyramid or both as in crystalline PbGeO 3 . NMR has also shown that Pb enters as a modifier and, by increasing x, the PbO 3 /PbO 4 ratio increases. The thermal behavior of the system can be interpreted by dividing the investigated compositional range into three regions: (i) 0 < x ≤ 0.05; (ii) 0.05 < x < 0.25; (iii) 0.25 ≤ x ≤ 0.50. In both region i and region ii, a nanoscale separation can be inferred because of the presence of nanocrystalline GeO 2 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.