Probing Metal-Bridging Oxygen and Configurational Disorder in Amorphous Lead Silicates: Insights from ¹⁷O Solid-State Nuclear Magnetic Resonance

The detailed degree of mixing between framework Si and metal cations and the nature of oxygen that only links metal cations (metal-bridging oxygen) in archetypal metal oxide glasses have been among the unsolved problems in physical chemistry. Binary lead silicate (PbO-SiO 2 ) glass is an ideal model system for exploring the extent of cation disorder and metal-bridging oxygen because of its peculiar glass-forming ability at high PbO concentration. Here we report the first high-resolution 17 O solid-state NMR spectra for binary lead silicate glasses with varying composition (near that of orthosilicate, i.e., Pb/Si = 2), where peaks due to metal-bridging oxygen (Pb-O-Pb), as well as Pb-O-Si and Si-O-Si are clearly resolved. The spectra also reveals that the metal-bridging oxygen possesses a greater topological disorder due to structural complexity in Pb coordination environments. Together with a statistical thermodynamic model of oxygen speciation proposed to describe mixing behavior in oxide glasses with any oxygen coordination number, the current 17 O NMR results allow direct quantification of the degree of Pb/Si disorder and estimation of relative energy difference among the oxygen clusters [Pb-O-Pb + Si-O-Si = 2(Pb-O-Si)] of ∼−6 kJ/mol. This corresponds to a degree of mixing ( Q ) between Pb and network Si of ∼0.9, which implies that it deviates from random distribution ( Q = 0) but shows a tendency toward Pb/Si order ( Q = 1) that favors the formation of the Pb-O-Si cluster. The calculated configurational enthalpy of lead silicate glasses based on 17 O NMR data showed a negative deviation and is consistent with those estimated from experimental solution calorimetry, unveiling atomistic origins of bulk macroscopic properties in the amorphous oxide. The results and methods shed light on a unique opportunity to explore the nature of intermixing and topological disorder in diverse complex amorphous oxides with varying cation types and oxygen coordination numbers by estimating the exact proportion of metal-bridging oxygen.