Based on the report of Kim, Cassir et al., nano-ordered CeO₂:Sm³⁺ particles were prepared. Citric acid and ethylene glycol were mixed with predetermined amounts of Ce(NO₃)₃ and Sm (NO₃)₃ aqueous solution, and heated for 12 hours with stirring at 343 K. The organic precursor was obtained by heating at 573 K for 12 hours. Furthermore, the precursor was heated at 383 K for 1 hour under an air atmosphere and calcied at 673 K for 4 hours to obtain Sm³⁺ doped CeO₂ with Sm ratios of 10 and 20 mol% (SDC10 and SDC20). As the liquid (melt) phase, Li₂CO₃ and Na₂CO₃ were dehydrated in CO₂ at 473 K for 48 hours, and the prepared eutectic (Li_0.51Na_0.49)₂CO₃ (LN) was used. Oxide and LN were mixed so as to obtain a predetermined liquid phase volume fraction to obtain CeO₂- LN or SDCs -LN. The mixed sample was pressed at 60 MPa for 30 minutes and fired at eutectic temperature or higher to prepare pellets for AC impedance measurement in the temperature range of 623-773K in 25 Hz-1 MHz.

A temperature dependence of ionic conductivity in ceria/LN coexisting system is shown in Figure 1. At a liquid phase volume fraction of 25 vol% or more, a bending point due to eutectic melting of the molten salt was observed in the vicinity of 720 K. In the region below the inflection point, it was confirmed that the conductivity of the liquid phase volume fraction of 45 vol% was smaller than the value of 15 - 35 vol%, and as the liquid phase amount increased. This is because, as the liquid phase amount increases, the bulk molten salt crystallizes and suppresses ion conduction. At 15 vol%, there was no remarkable change in conductivity even before and after the inflection point, and there was no enthalpy change at 25 vol% or less. Results suggesting that the molten salt existing at the solid interface is in the molten state even below the transition point.