Super-ionic transitions in tysonite-structured fluoride compounds were studied by means of combined Brillouin and Raman light-scattering spectroscopy. It was found that there was excellent agreement between the transition temperatures which were determined by using the Brillouin and Raman techniques. The transition temperatures were markedly dependent upon the dopant concentration, and were considerably lower in LaF3-BaF2 than in LaF3. The main result was a large reduction in the temperature of transition, to the super-ionic state, which occurred upon increasing the dopant content. In order to explain this, a cooperative transition was proposed which involved a reduction in the defect formation energy and an increase in the defect concentration and/or mobility above the transition.

C.A.I.Anghel, J.D.Comins: Materials Science Forum, 1997, 239-241, 413-6