Nature of the ²³Na spin relaxation increase near the ferroelectric phase transition in bulk and confined sodium nitrite

NMR studies were carried out for ferroelectric NaNO₂ particles embedded into molecular sieves MCM-41 and SBA-15 with various pore sizes. ²³Na spin-lattice relaxation was measured within a large temperature range covering the ferroelectric phase transition in bulk sodium nitrite till sodium nitrite melting. Longitudinal nuclear magnetization restoration in the rigid confined sodium nitrite was found to be described well by the model of thermally activated random reorientations of the NO₂ groups. The activation enthalpy of NO₂ reorientations was evaluated above and below the ferroelectric phase transition in confined sodium nitrite. Its values were shown to be similar to that in bulk sodium nitrite.