Order/disorder versus or with displacive dynamics in ferroelectric systems

For many years ferroelectrics have been classified as either being of order/disorder or displacive type and thus are strictly distinguishable by their dynamical behaviour. Even though early experiments using different techniques have come to seemingly contradictory results concerning the phase transition mechanism, this issue has not been addressed in detail. in this contribution we show that the classification scheme is mostly invalid in ferroelectrics and that a coexistence of both order/disorder and displacive effects is much more common to these materials than a sharp distinction. The special focus will be on hydrogen-bonded ferroelectrics; however, extensions of these ideas to non-hydrogen-bonded compounds will be presented as well. In addition, experimental methods and results are reviewed and are shown to be excellent tools for testing the dynamics at the phase transition temperature.