Phase separation in alkali silicate glasses detected through inverse Laplace transform of 29Si nuclear magnetic resonance echo train decay

Abstract

The ratio of silicon-29 nuclear magnetic resonance (NMR) coherence lifetimes for Q⁴ and Q³ sites under magic-angle spinning and a train of π pulses in a series of binary alkali silicate glasses is used to detect phase separation, even at small scales where the glass appears optically homogenous. This approach exploits the dependence of echo train coherence lifetimes on the residual heteronuclear dipolar coupling between Si-29 and the NMR active nuclei of neighboring network modifier cations. The shifted-echo phase incremented echo train acquisition NMR method is used to eliminate modulation of the echo train amplitudes due to J couplings across Si–O–Si linkages. A 2D Fourier and inverse Laplace transform of this dataset provides a correlation of the isotropic ²⁹Si chemical shift to echo train coherence lifetimes, giving a sensitive probe of phase separation as well as chemical composition and local structure of the different phases. The ²⁹Si Q⁴:Q³ mean coherence lifetime ratios are 28.8, 23.8, and 5.8 in the phase-separated glasses, 0.05Li₂O·0.95SiO₂, 0.1Li₂O·0.9SiO₂ and 0.05Na₂O·0.95SiO₂, respectively, while the ratio is reduced to 2.1, 1.6, and 1.6 in glasses not exhibiting signs of phase separation, 0.05K₂O·0.95SiO₂, 0.05Cs₂O·0.95SiO₂ and 0.10Cs₂O·0.90SiO₂, respectively. Phase separation inhibition, through addition of alumina, is also verified in 0.07Li₂O·0.02Al₂O₃·0.91SiO₂.