Crystalline network influence on the variation of TC with the F-O substitution rate in oxyfluorides derived from octahedral-type ferroelectric oxides

Ferroelectric fluorides have been studied in the two last decades in relation to the performance of ceramics sintered at low temperature for various potential applications. Compositions derived from octahedral-type ferroelectrics by progressive replacement of oxygen by fluorine are particularly interesting. Those considered in the present work belong to perovskite, tetragonal potassium tungsten bronze [TKWB] and LiTaO3 families. The variation of the Curie temperature T-C with the F-O substitution rate is strongly dependent on the crystalline network and on the ferroelectricity-type. The decrease of T-C is maximal for TKWB-type materials due to the predominant role of covalency of the M-X (X = O, F) bonds in relation to the same direction for such bonds and for both the displacement vector of the M-atom from the octahedron-centre and the spontaneous polarization vector. Also the ferroelectricity is purely displacive for this type. (C) 1999 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.