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

Abstract

Results of NMR studies of $^{23}\mathrm{Na}$ in $\mathrm{Na}\mathrm{N}{\mathrm{O}}_{2}$ confined within molecular sieves MCM-41 with pore size 37 and 20\AA{} and SBA-15 with pore size 52\AA{} are presented. $^{23}\mathrm{Na}$ 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 $\mathrm{Na}\mathrm{N}{\mathrm{O}}_{2}$ below the bulk sodium nitrite melting point consists of two parts with relaxation times which differ by two orders in magnitude. A portion of $\mathrm{Na}\mathrm{N}{\mathrm{O}}_{2}$ exhibits bulk-like properties with the ferroelectric phase transition in the vicinity of the bulk transition temperature. The bulk-like $\mathrm{Na}\mathrm{N}{\mathrm{O}}_{2}$ 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 $\mathrm{Na}\mathrm{N}{\mathrm{O}}_{2}$ 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 $\mathrm{Na}\mathrm{N}{\mathrm{O}}_{2}$ caused by confinement. Temperature evolution of the $^{23}\mathrm{Na}$ NMR line confirms such a suggestion. The amount of $\mathrm{Na}\mathrm{N}{\mathrm{O}}_{2}$ which possesses high molecular mobility depends on pore size and is maximal for the MCM-41 porous matrix with 20\AA{} 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.