Local and overall ionic conductivity in nanocrystalline CaF2

Ionic conductivity data on nanocrystalline CaF2 is evaluated. The d.c. conductivity is distinctly larger than in coarse-grained materials. The impedance plot exhibits two regimes: a high- and a low-frequency semicircle, the diameters of which increase with increasing grain size. The high-frequency semicircle reflects both bulk transport plus transport along the boundaries while the low-frequency semicircle describes the blocking effect of the grain boundaries. Absolute values and activation energy of the conductivity suggest dominating transport along space charge layers. The increase of the low frequency semicircle is due to increased current constriction because of the appearance of large pores.