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

The Pb-O coordination environment in binary (PbO)(SiO₂)_100- 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 PbO (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 PbO 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 PbO 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.