Influence de divers types de substitution sur les températures de transition des oxyfluorures ferroélectriques de type chiolite

A great number of oxyfluorides with a chiolite related structure have been prepared. They show all a ferroelastic, ferroelectric-prototype, paraelectric transition. The highest Curie temperature (T_c = 800 K) has been obtained for the tungsten compound Na₅W₃O₉F₅. The value of T_c is found to vary as a function of the substitutions from Na₅W₃O₉F₅. The largest decrease is observed for the composition Na₄Lu (WNb₂)O₉F₅ with T_c = 88K. Pyroelectric and piezoelectric applications are expected. The influence of the composition on other transitions observed in these materials has been determined.