Deviations from Vegard's Law in the Curie Temperature of Mixed Ferroelectric Solid Solutions

Composition dependence of the Curie temperature in mixed ferroelectric- ferroelectric solid solutions with isomorphous crystalline lattices, like the perovskite solid solutions LiTaO3/LiNbO3 or PbZrO3/PbTiO3, has been analysed by means of a classical effective field approach using a two sublattice system. The observed non-linear relationship between the critical temperature and the composition, deviating from a Vegard's law behavior, is shown to be explained in terms of coupling strength between the individual dipoles of the two basic sublattices. In the case of LiTa1-xNbxO3, the Curie temperature is lower than that given by Vegard's law, but for PbZr1-xTixO3 is higher. This means that in the second case the coupling strength between the different individual dipoles is stronger, while in the first case the main interaction is due to the identical dipoles in each sublattice.