A Vogel-Fulcher-Tammann temperature dependence of the conductivity was found for single crystals, when measured in the [001] direction. The results could be described by:

(S/cm)  exp[-0.16(eV)/k(T-150)]

At temperatures below 150K, fast ion transport was expected to be frozen.

S.Sigaryov: Journal of Physics D, 1993, 26[8], 1326-7