Results of NMR studies of 23 Na in NaNO 2 confined within molecular sieves MCM-41 with pore size 37 and 20Å and SBA-15 with pore size 52Å are presented. 23 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 NaNO 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 NaNO 2 exhibits bulk-like properties with the ferroelectric phase transition in the vicinity of the bulk transition temperature. The bulk-like NaNO 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 NaNO 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 NaNO 2 caused by confinement. Temperature evolution of the 23 Na NMR line confirms such a suggestion. The amount of NaNO 2 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.

No abstract

Superionic AgI particles embedded in MCM-41 and SBA-15 molecular sieves with different pore sizes were studied using NMR and dielectric methods. The measurements confirmed that the structure of the confined particles did not change noticeably compared to that in bulk AgI. A regular increase in the temperature of the superionic phase transition with decreasing pore size was observed and treated on the basis of the Landau theory of thermodynamic size effects. The maximum increase found upon warming and cooling was about 10 and 8 K, respectively. The ionic conductivity in β-AgI under confinement was shown to be due to silver mobility on the particle surface.

No abstract

No abstract