The conductivity of pure, Y-doped or Na-doped samples was measured between room temperature and 500C. The results showed that the mobile defects were F vacancies, with an activation energy for motion of 0.44eV. An anomalous conductivity curve was observed in crystals which contained Na as an impurity. This could be explained in terms of association effects.

Y.T.Tan, D.Kramp: Journal of Chemical Physics, 1970, 53[9], 3691-8