Displacive-order-disorder crossover at the ferroelectric-paraelectric phase transitions of BaTiO3 and LiTaO3

A detailed comparison between accurate infrared reflectivity data and dielectric constant ε0(T) measurements has been carried out in ABO3 ferroelectrics. In BaTiO3, the ferroelectric mode saturates above the cubic-tetragonal transition, whereas ε0( T) tends to diverge. This supports an intrinsic crossover from displacive to order-disorder behaviour on approaching Tc because the critical dimensionality in these cubic dipolar systems is dc = 4 > d = 3. In the uniaxial LiTaO3 with d = d c = 3, the crossover is less pronounced.