The inorganic fluoride and oxyfluoride ferroelectrics

The ferroelectric fluorides belong to six families of type (NH4)(2)BeF4, BaMnF4, SrAlF5, Na2MgAlF7, K3Fe5F15 or Pb3Cr3F19. The oxyfluorides can be separated in two groups: i) the true oxyfluorides with a relatively high F/O value (K3MoO3F3, Na5W3O9F5, Pb5W3O9F10, Bi2TiO4F2,...); ii) the compositions derived from ferroelectric oxides (perovskite, tetragonal tungsten bronze, pyrochlore, LiTaO3,...) characterized by a continuous F-O substitution. The phase transitions and the physical properties are discussed. The origin of the spontaneous polarization is described structurally. The variations of the Curie temperature with the type of substitution are related to chemical bonding. The potential interest of these materials for applications is briefly given.