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.

Kinetics of physical aging of a silicate glass following temperature up- and down-jumps.

In this article, we investigate the structural relaxation of lithium silicate glass during isothermal physical aging by monitoring the temporal evolution of its refractive index and enthalpy following relatively large (10-40 °C) up- and down-jumps in temperature. The Kohlrausch-Williams-Watts function aptly describes the up- and down-jump data when analyzed separately. For temperature down-jumps, the glass exhibits a typical stretched exponential kinetic behavior with the non-exponentiality parameter ß < 1, whereas up-jumps show a compressed exponential behavior (ß > 1). We analyzed these datasets using the non-exponential and non-linear Tool-Narayanaswamy-Moynihan (TNM) model, aiming to provide a comprehensive description of the primary or α-relaxation of the glass. This model described both up- and down-jump datasets using a single value of ß ≤ 1. However, the standard TNM model exhibited a progressively reduced capacity to describe the data for larger temperature jumps, which is likely a manifestation of the temperature dependence of the non-exponentiality or non-linearity of the relaxation process. We hypothesize that the compressed exponential relaxation kinetics observed for temperature up-jumps stems from a nucleation-growth-percolation-based evolution on the dynamically mobile regions within the structure, leading to a self-acceleration of the dynamics. On the other hand, temperature down-jumps result in self-retardation, as the slow-relaxing denser regions percolate in the structure to give rise to a stretched exponential behavior.

Elemental and cooperative diffusion in a liquid, supercooled liquid and glass resolved.

The diffusion mechanisms controlling viscous flow, structural relaxation, liquid-liquid phase separation, crystal nucleation, and crystal growth in multicomponent glass-forming liquids are of great interest and relevance in physics, chemistry, materials, and glass science. However, the diffusing entities that control each of these important dynamic processes are still unknown. The main objective of this work is to shed some light on this mystery, advancing the knowledge on this phenomenon. For that matter, we measured the crystal growth rates, the viscosity, and lead diffusivities in PbSiO3 liquid and glass in a wide temperature range. We compared our measured values with published data covering 16 orders of magnitude. We suggest that above a certain temperature range Td (1.2Tg-1.3Tg), crystal growth and viscous flow are controlled by the diffusion of silicon and lead. Below this temperature, crystal growth and viscous flow are more sluggish than the diffusion of silicon and lead. Therefore, Td marks the temperature where decoupling between the (measured) cationic diffusivity and the effective diffusivities calculated from viscosity and crystal growth rates occurs. We reasonably propose that the nature or size of the diffusional entities controlling viscous flow and crystal growth below Td is quite different; the slowest is the one controlling viscous flow, but both processes require cooperative movements of some larger structural units rather than jumps of only one or a few isolated atoms.