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 [...] 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.