Dielectric Constant in Paraelectric Perovskites

Abstract

The relative dielectric constant of a series of perovskites such as CaTi${\mathrm{O}}_3$, SrTi${\mathrm{O}}_3$, BaTi${\mathrm{O}}_3$, PbTi${\mathrm{O}}_3$, and KTa${\mathrm{O}}_3$ 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 $\epsilon ={\epsilon }_L+\frac{C}{(T-T_c)}$ is accurately obeyed where ${\epsilon }_L$ is of the order of 50 and represents the dielectric constant in the limit of infinite temperature, and that $T_c$ and $C$, the Curie temperature and Curie constant, increase monotonically with increasing cation mass. ${\epsilon }_L$ 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.