Results of NMR studies of Na23 in NaNO2 confined within molecular sieves MCM-41 with pore size 37 and 20 and SBA-15 with pore size 52 are presented. Na23 spin-lattice relaxation and line shape were measured in a large temperature range up to 535 K covering the bulk ferroelectric phase transition point. It is shown that confined NaNO2 below the bulk sodium nitrite melting point consists of two parts with relaxation times which differ by two orders in magnitude. A portion of NaNO2 exhibits bulk-like properties with the ferroelectric phase transition in the vicinity of the bulk transition temperature. The bulk-like NaNO2 prevails below and near the ferroelectric phase transition and its amount decreases strongly when temperature approaches the bulk melting point. Fast nuclear relaxation in another portion of confined NaNO2 revealed very high molecular mobility. This portion increases with increasing temperature and dominates above 510 K. It was suggested that fast relaxation corresponds to the melted or premelted state of confined NaNO2 caused by confinement. Temperature evolution of the Na23 NMR line confirms such a suggestion. The amount of NaNO2 which possesses high molecular mobility depends on pore size and is maximal for the MCM-41 porous matrix with 20 pore size. The correlation time of electric field gradient fluctuations was found for this part to be similar to those in viscous liquids with the activation energy of about 0.42 eV.
|期刊||Physical Review B - Condensed Matter and Materials Physics|
|出版狀態||Published - 2005 九月 1|
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