DIELECTRIC DISCONTINUITIES AND MULTIPOLAR INTERACTIONS IN FERROELECTRIC PEROVSKITES

Ferroelectric perovskites, in particular KNbO3, PbTiO3 and BaTiO3, are characterized by discontinuities in the dielectric constant at the tetragonal-cubic transition (T*) as well as by the abrupt disappearance of the spontaneous polarization which goes from a minimum finite value P-s* =P-s(T) to zero at T=T*. It is shown that, taking into account quadrupolar and higher order terms into the effective field acting on the elementary dipoles lattice, good fits to the observed P-s(T) in a wide temperature range below T* and to the inverse dielectric constant discontinuity (C/T-c)[epsilon(+)(-1)(T*)-epsilon(-)(-1)(T*)] at T similar or equal to T* are obtained for these three perovskites.