Cross-Polarization from Quadrupolar Nuclei to Silicon Using Low-Radio-Frequency Amplitudes during Magic-Angle Spinning

Abstract

The dynamics of cross-polarization from the central transition of a quadrupolar nucleus (27Al or 23Na) to a spin-1/2 nucleus (29Si) during magic-angle spinning and using low-radio-frequency field strengths are analyzed for the mineral low albite. Under these conditions additional complications in the spin-lock behavior of the quadrupolar nucleus and in the cross-polarization process were found experimentally and are examined in detail. A step-by-step procedure for optimizing cross-polarization from the central transition of a quadrupolar nucleus to a spin-1/2 nucleus is described. Significant enhancement of 29Si NMR sensitivity and several applications are demonstrated.

Monitoring crystallization in lithium silicate glass-ceramics using 7 Li → 29 Si cross-polarization NMR

“…Here modern magnetic resonance techniques offer powerful element-selective, inherently quantitative insights into structure and dynamics at the atomic scale [6]. For example, highresolution 6/7 Li and 29 Si NMR have been widely applied for monitoring the formation of lithium disilicate-based glass ceramics [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. While most of the results obtained indicate that stoichiometric disilicate glass transforms directly into the crystalline compound, there have been also some reports of metastable Li 2 Si 2 O 5 (and lithium metasilicate) precursor phases [9,12].…”

“…Furthermore, off-stoichiometric and/or P 2 O 5 -containing glasses -used in commercial glass-ceramicsresult in the additional formation of crystalline phases of lithium metasilicate, lithium phosphate and quartz, accompanied by compositional changes in the residual glass matrix [7,15,20,21]. An accurate quantification of the crystalline content of lithium silicate glassceramics is severely hampered, however, by the low natural abundance of the 29 Si isotope and its relatively small gyromagnetic ratio (γ = −5.319 × 10 6 T −1 rad s −1 ), resulting in low detection sensitivity. This problem is severely compounded by extremely long 29 Si spin-lattice relaxation times [8,21] -both for the glassy and the crystalline component -necessitating signal accumulation times on the order of one week per sample for building up sufficient signal to noise ratios suitable for quantitative analyses.…”

“…An accurate quantification of the crystalline content of lithium silicate glassceramics is severely hampered, however, by the low natural abundance of the 29 Si isotope and its relatively small gyromagnetic ratio (γ = −5.319 × 10 6 T −1 rad s −1 ), resulting in low detection sensitivity. This problem is severely compounded by extremely long 29 Si spin-lattice relaxation times [8,21] -both for the glassy and the crystalline component -necessitating signal accumulation times on the order of one week per sample for building up sufficient signal to noise ratios suitable for quantitative analyses.…”

“…While the overwhelming majority of such applications involve magnetization transfer from 1 H to 13 C nuclei, we have recently reported analogous 7 Li → 29 Si cross-polarization experiments, resulting in significant signal-to-noise gains and spectral editing opportunities in the spectroscopy of various crystalline binary lithium silicides [23,24]. Part of the signal-to-noise benefit arises from the fact that in 7 Li → 29 Si cross-polarization the recycle delay used for signal accumulation depends on the spin-lattice relaxation time of the magnetization source nuclei (here 7 Li), which in general relax much faster than the 29 Si nuclei. On the other hand a serious drawback of cross-polarization methods is their generally nonquantitative character.…”

“…Signal amplitudes generated by crosspolarization are affected by the influence of various superimposed relaxation processes involving the two spin systems and their surroundings, making it difficult to relate signal intensities to spin populations [25]. This complication is particularly serious for spin systems containing quadrupolar nuclei such as 7 Li, where population transfers among the different Zeeman levels during the MAS rotor period can interfere with spin locking [26][27][28][29].…”