The Role of Trilinear Couplings in the Phase Transitions of Aurivillius Compounds

Some Aurivillius compounds exhibit features beyond the single soft-mode picture valid for other ferroelectrics. In SrBi2Ta2O9 (SBT) two primary distortions are needed to reach the ferroelectric phase, causing the presence of an intermediate phase. However, no intermediate phase has been detected in SrBi2Nb2O9 (SBN), and an avalanche transition seems to happen, with the two order parameters condensing simultaneously. According to ab-initio calculations, the usually assumed triggering mechanism through biquadratic coupling of the two order parameters can be discarded, and we have looked for other possible causes of these avalanche transitions. Using a simple microscopic Hamiltonian (an extended [image omitted] model), mean-field techniques (independent-site approximation) and Monte Carlo simulations, we show that a strong trilinear coupling between the two order parameters and an additional secondary mode could explain the existence of a single first-order transition with the simultaneous condensation of two order parameters.