Silicon-29 echo train coherence lifetimes and geminal ²J-couplings in network modified silicate glasses

Area of Science:

• Materials Science
• Solid-State NMR Spectroscopy
• Glass Chemistry

Background:

• Nuclear magnetic resonance (NMR) spectroscopy is crucial for understanding glass structure.
• Silicon-29 (²⁹Si) NMR is sensitive to the local environment in silicate glasses.
• Echo-train coherence lifetimes provide insights into dynamic processes and couplings within the material.

Purpose of the Study:

• To investigate the factors controlling ²⁹Si echo-train coherence lifetimes in network-modified silicate glasses.
• To correlate these lifetimes with the nuclear magnetic properties of modifier cations.
• To assess the influence of static and magic-angle spinning (MAS) conditions on ²⁹Si coherence.

Main Methods:

• ²⁹Si solid-state NMR experiments were performed under static and MAS conditions.
• Echo-train coherence lifetimes were measured.
• The impact of MAS speed and pulse sequences on spectral linewidths was analyzed.
• ²⁹Si J coupling distributions were determined.

Main Results:

• ²⁹Si coherence lifetimes varied by up to three orders of magnitude, strongly dependent on modifier cations.
• Alkali silicates exhibited shorter lifetimes due to heteronuclear dipolar couplings.
• Alkaline earth silicates showed longer lifetimes, limited by paramagnetic impurities.
• MAS and pulse trains effectively reduced heteronuclear broadening.
• Geminal ²⁹Si J couplings were measured, with values around 4-5 Hz.

Conclusions:

• Modifier cation properties are key determinants of ²⁹Si coherence lifetimes in silicate glasses.
• ²⁹Si J coupling measurements are most challenging in Li-silicates and least in alkaline earth silicates.
• Structural disorder, linked to cation charge-to-radius ratio, influences J coupling variance.