The conductivity was studied at low temperatures by using modified electrochemical impedance spectroscopy as well as dielectric measurements. Experiments on samples with a given crystal imperfection density were carried out at 50 to 110K, using frequencies ranging from 10Hz to 0.1MHz. The results confirmed the existence of an almost temperature-independent low-frequency ionic conductivity below a transition temperature of 75K. This was caused by a change in the ion conduction mechanism. At temperatures above 75K, ionic transport occurred through grains and sub-grains crossing grain and sub-grain boundaries. At temperatures below 75K, ion transport took place only via crystal imperfections such as grain and sub-grain boundaries. The results indicated a frequency dispersion of the ionic conductivity, in the mid-frequency range, which obeyed a power-law relationship with an exponent of about unity. This confirmed the so-called new universality law.

E.C.Bucharsky, R.T.Potzschke, G.Staikov, E.Budevski, W.J.Lorenz, W.Wiesbeck: Solid State Ionics, 1999, 124[1-2], 101-8