Impact of Transition Metal Cations on the 29Si NMR Signal in Metal Oxide Glasses: A DFT Case Study of Hafnia Silica Glass

We investigate 29Si nuclear magnetic resonance (NMR) chemical shifts δiso of soda-silica and hafnia-soda-silica glass models by structural modeling and the gauge-invariant projector augmented wave (GIPAW) method within density functional theory (DFT). Models of soda-silica glasses with molar ratios Na2O:SiO2 of 1:2 and 1:3 and hafnia content of 0–10 mol % are generated via a melt-quench procedure and ab initio molecular dynamic simulations. By correlating computed chemical shifts with structural data we establish angular correlation functions for Q2, Q3, and Q4 units in soda-silica glasses. Addition of hafnia to soda-silica glasses results in 6-coordinated Hf surrounded by bridging O under avoidance of direct linkages between Hf-centered octahedra. Hf impacts the structural information that can be gathered from 29Si NMR analysis profoundly: each Hf in second coordination to Si shifts the angular correlation function for Si–O–X (X = Si, Hf) angles by 3–5 ppm. Hence, NMR signals of Q2 and Q3 units may overlap depending on the number of Hf surrounding Si. By substituting H for Na we convert the glass models into models of sol–gel derived HfO2–SiO2. The profound impact of Hf on the chemical shift of 29Si calls for a reinterpretation of 29Si NMR peaks observed for hafnia-silica glasses.