Coexistence of melted and ferroelectric states in sodium nitrite within mesoporous sieves

Results of NMR studies of ²³Na in NaNO₂ confined within molecular sieves MCM-41 with pore size 37 and 20Å and SBA-15 with pore size 52Å are presented. ²³Na 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 ²³Na 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 20Å 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.