Characterization of the Pb Coordination Environment and Its Connectivity in Lead Silicate Glasses: Results from 2D ²⁰⁷Pb NMR Spectroscopy

The Pb–O coordination environment in binary (PbO)x(SiO2)100–x glasses with 30 ≤ x ≤ 70 is probed by using two-dimensional 207Pb nuclear magnetic resonance (NMR) isotropic–anisotropic correlation spectroscopy. The isotropic 207Pb NMR spectra show little composition-dependent evolution of the Pb–O nearest-neighbor coordination environment. The systematic variation of the chemical shift tensor parameters offers a unique insight into their local site symmetry and suggests the presence of pyramidal PbO3 and PbO4 sites with sterically active electron lone pairs and with Pb–O bond lengths ranging between 0.23 and 0.25 nm. The PbO3/PbO4 ratio shows a small but monotonic increase from ∼70:30 to 80:20 as the PbO content increases from 30 to 70 mol %. When taken together, the isotropic and anisotropic 207Pb NMR spectra suggest that the majority of the PbOn (3 ≤ n ≤ 4) pyramids in these glasses are connected to the SiO4 tetrahedra via Pb–O–Si linkages. A significant fraction of Pb–O–Pb linkages, where the oxygen is linked only to Pb atoms, appears only in glasses with PbO ≥ 60 mol %. These oxygen atoms appear to be corner-shared between the PbOn pyramids in the structure, and no evidence for edge-sharing between these pyramids is observed in this composition range. We hypothesize that a substantial fraction of the constituent PbOn pyramids start to participate in edge-sharing only at higher PbO contents (>70 mol %), which diminishes the glass-forming ability of the network. This work illustrates the potential of isotropic–anisotropic correlation NMR spectroscopy in structural studies involving nuclides with large chemical shift ranges and anisotropy.