Anomalous dielectric behaviour of La(III) substituted lead titanate ceramics

In La(III) substituted PbTiO₃ ceramics the behaviour of the reciprocal dielectric constant in the paraelectric phase can be described by the relation $\text{1}\text{?～}\text{(T ? T}{}_{\mathrm{c}}\text{)}{}^{\mathrm{\gamma }}$ with the exponent 1 ?γ<2 depending on the composition. This γ-value is an intermediate of values for Curie-Weiss behaviour (γ = 1) and of values connected with diffuse phase transitions (γ = 2) and increases with increasing La(III) concentration.An excellent agreement between experimental and calculated permittivity curves in the paraelectric phase can be achieved using an alternative equation consisting of a linear and a quadratic term with coefficients varying systematically with the composition. The last equation can be derived and interpreted by means of a physical model assuming the existence of heterophase fluctuations.