Retarded ionic motion in flourites

Metals halides with the fluorite structure attain conductivity values typical of ionic melts far below their melting points, and also go through a second-order transition. Conductivity data for the fluorites are reviewed, and it is shown that the anion vacancies have a large and unique mobility value at the transition temperatures Tc. At Tc only small concentrations of defects are present, which strongly contradicts the concept of anion-sublattice melting. The conductivity mechanism above Tc is discussed in relation with the enhanced ionic motion model developed for concentrated anion-excess solid solutions based upon flourites. As a consequences of unusually large defect mobilities at Tc it is noted that retard ionic motion occurs above Tc