Local and overall ionic conductivity in nanocrystalline CaF2

Ionic conductivity data on nanocrystalline CaF2 is evaluated. The d.c. cond uctivity is distinctly larger than in coarse-grained materials. The impedan ce plot exhibits two regimes: a high- and a low-frequency semicircle, the d iameters of which increase with increasing grain size. The high-frequency s emicircle reflects both bulk transport plus transport along the boundaries while the low-frequency semicircle describes the blocking effect of the gra in boundaries. Absolute values and activation energy of the conductivity su ggest dominating transport along space charge layers. The increase of the l ow frequency semicircle is due to increased current constriction because of the appearance of large pores. (C) 2000 Elsevier Science B.V. All rights r eserved.