Lead silicate glass structure: New insights from diffraction and modeling of probable lone pair locations

Structures of binary PbO-SiO2 glasses have been studied in detail over the compositional range 35 to 80 mol% PbO using high-resolution neutron diffraction, high-energy X-ray diffraction, static Pb-207 NMR, and structural modeling. The changes in the local environment of Pb(II) are subtle; it has a low coordination to oxygen (similar to 3 to 4) plus a stereochemically active electron lone pair and, thus, behaves as a glass network forming (or intermediate) cation over the entire composition range. This conclusion contradicts previous reports that Pb(II) is a network modifier at low concentrations, and is supported by an analysis of lead and alkaline earth silicate glass molar volumes. The Pb-O peak bond length shortens by 0.04 angstrom with increasing PbO content, indicating stronger, more covalent bonding, and consistent with an increase in the number of short (<= 2.70 angstrom) Pb-O bonds, from 3.3 to 3.6. This is accompanied by increased axial symmetry of the Pb(II) sites, and is interpreted as a gradual transition toward square pyramidal [PbO4] sites such as those found in crystalline PbO polymorphs. An attendant decrease in the periodicity associated with the first sharp diffraction peak (FSDP) toward that of beta-PbO, accompanied by increases in the correlation lengths associated with the plumbite network (FSDP) and silicate anions (neutron prepeak), provides evidence of increased intermediate-range order and has implications for the glass forming limit imposed by crystallization. Pb(II) electron lone pairs occupy the natural voids within the silicate network at low PbO contents, while at high PbO contents they aggregate to create voids that form part of the plumbite network, analogous to the open channels in Pb11Si3O17 and the layered structures of alpha- and beta-PbO. Si-O and Pb-O bond lengths have been correlated with Si-29 and Pb-207 NMR chemical shifts, respectively. This is the first time that such correlations have been demonstrated for glasses and attests to the accuracy with which pulsed neutron total scattering can measure average bond lengths.