Serial sectioning techniques were used to investigate the self-diffusion of 18F in single crystals which were undoped, or doped with Na+ or La3+. The results for undoped crystals, at temperatures ranging from 250 to 450C, could be described by:

D (cm2/s) = 1 x 10-6 exp[-0.39(eV)/kT]

At 500 to 640C, a sharply increased activation enthalpy was found. The diffusivities for Na-doped samples at lower temperatures were an order of magnitude higher than those for undoped samples, but the activation energy seemed to be exactly the same. The values for La-doped samples at 406C were lower, by a factor of 2, when compared with the Arrhenius line for undoped crystals. The F diffusivity was high when compared with cation diffusion. Its dependence upon doping was characteristic of Frenkel disorder in the anion sub-lattice, in which the F vacancies were more mobile than the F interstitials. From the temperature dependence of the diffusion coefficient, a value of 0.39eV was deduced for the activation enthalpy for the motion of F vacancies and a value of 2.1eV for the formation enthalpy of a Frenkel pair. Fluorine Self-Diffusion in Pure and Doped CdF2. P.Süptitz, E.Brink, D.Becker: Physica Status Solidi B, 1972, 54[2], 713-8

Table 164

Diffusion of 42K in Molten KF