Ferroelectric materials - those which exhibit a spontaneous and switchable electrical polarization - have a wide range of applications. However materials which exhibit ferroelectric behaviour are rare because in order to exhibit a spontaneous electrical polarization, materials must adopt a non-centrosymmetric crystal structure.We have been studying a series of layered perovskite phases which adopt non-centrosymmetric structures at room temperature. It is proposed that the non-centrosymmetric structures and resulting ferroelectric behaviour arise due to a novel ‘hybrid improper’ mechanism. In order to confirm this hypothesis we need to determine the structures adopted by these compounds at high temperature, because the nature of the phase transitions between ferroelectric and non-ferroelectric states is characteristic of the mechanism which leads to ferroelectric behaviour.