Conductivity data on nanocrystalline CaF2 were evaluated. The direct-current conductivity was found to be clearly larger than that of coarse-grained materials. The impedance plot exhibited 2 regimes. These were high-frequency and a low-frequency semi-circles whose diameters increased with increasing grain size. The high-frequency semi-circle reflected both bulk transport, and transport along the boundaries. The low-frequency semi-circle described the blocking effect of the grain boundaries. The absolute values and activation energies of the conductivity implied the dominance of transport along space charge layers. The increase in the low-frequency semi-circle was attributed to an increased current constriction, due to the appearance of large pores.

Local and Overall Ionic Conductivity in Nanocrystalline CaF2. W.Puin, S.Rodewald, R.Ramlau, P.Heitjans, J.Maier: Solid State Ionics, 2000, 131[1-2], 159-64