On 29Si NMR relaxation as a structural criterion for studying paramagnetic supermicroporous silica-based materials: Silica-based materials incorporating Mn2+ ions into the silica matrix of SiO2-Al2O3-MnO

Supermicroporous paramagnetic materials SiO2-Al2O3-MnO and SiO2-MnO with different manganese concentrations have been probed by solid-state 29Si NMR and magnetic susceptibility measurements. The 29Si T1 and T2 experiments, performed in static and spinning samples, have resulted in determination of electron relaxation times, providing, in turn, quantitative interpretations of 29Si T1 times in terms of distances Mn-Si. The NMR relaxation data have revealed 29Si T1 time distributions, which are close to Gaussian and observed as different T1 values obtained in MAS NMR experiments for isotropic 29Si resonances and their sidebands. Such 29Si T1 distributions, being a common phenomenon in paramagnetic silica-based materials, can be however masked by the bulk magnetic susceptibility (BMS) effects increasing with concentrations of paramagnetic centers. The presence of T1 distributions, in itself, is not a criterion for incorporation of paramagnetic ions into the silica matrix or its surface. However, a quantitative analysis of the experimentally-observed short 29Si T1 components, based on the well-determined electron relaxation times, can provide such a criterion.