ORIENTATION DEPENDENCE OF THE IONIC-CONDUCTIVITY IN SINGLE-CRYSTALS OF LANTHANUM AND CERIUM TRIFLUORIDE

The impedance spectra of oriented single crystals of LaF3 and CeF3 dop ed with SrF2 (0-16 atom%) were measured for different directions of ch arge transport in the temperature range 290-770 K. At lower temperatur es (T < 420 K), charge transport parallel to the crystallographic c-ax is is accompanied by an additional time constant in the impedance spec tra and can be represented in the equivalent circuit by an additional resistance and capacitance. This feature vanishes completely at temper atures higher than 420 K. It was not observed for charge transport per pendicular to the c-axis. The results are evaluated based-on an impeda nce model of Franceschetti and Shipe. The fluoride ion conductivity si gma(F-) along the c-axis is about a factor of two higher than in a per pendicular direction. The capacitance for transport in c-direction is due to a charge storage effect on trapping F2/F3 sites with longer lif e times. The anisotropy results from different jump frequencies betwee n the three non-equivalent types of F- sites and their different contr ibution along the c-axis and perpendicular to that direction.