Dielectric Constant in Paraelectric Perovskites

The relative dielectric constant of a series of perovskites such as CaTiO₃, SrTiO₃, BaTiO₃, PbTiO₃, and KTaO₃ has been measured at microwave frequencies over a wide range above the Curie temperature. It was found that a Curie-Weiss law of the form =₋+C (T-{T₂) } is accurately obeyed where ₋ is of the order of 50 and represents the dielectric constant in the limit of infinite temperature, and that T₂ and C, the Curie temperature and Curie constant, increase monotonically with increasing cation mass. ₋ consists of contributions from the electronic polarizability, temperature-independent optically active lattice vibrations, and a dominant term stemming from the finite frequency of the temperature-dependent soft mode in the limit of infinite temperature. The limiting frequency of the "soft mode" at high temperatures is estimated.