DIELECTRIC AND RELATED PROPERTIES OF FLUORINE-OCTAHEDRA FERROELECTRICS

Ferroelectricity has been reported in six families of complex fluorides that contain fluorine octahedra and in four other families that contain both oxygen and fluorine atoms in the octahedra. The experimental Curie temperatures found are in good agreement with those predicted on the basis of structure in the case of the BaMnF4, SrAlF5, Sr3Fe2F12, Pb5Cr3F19 and K3Fe5F15 fluorine-octahedra containing families and are also in agreement in the case of the Pb5W3O9F10 and Na5W3O9F5 oxygen and fluorine-octahedra containing families. The structures of Na2MgAlF7, Bi2TiO4F2, and K3MoO3F3 in their ferroelectric phases are not known with sufficient accuracy to allow prediction of T(c) hut each family exhibits the experimental attributes of ferroelectricity. Dielectric, calorimetric, and structural properties are presented for all ten ferroelectric families. The range of Curie temperatures and dielectric permittivities in fluorine-octahedra containing ferroelectrics is comparable to that in oxygen-octahedra ferroelectrics.