Competing structural instabilities in the ferroelectric Aurivillius compoundSrBi2Ta2O9

The Aurivillius family includes many compounds with ferroelectric phases. Despite sharing the basic perovskite structural block with more common ferroelectric oxides such as $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ or $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$, their phase-transition behavior is rather more complex. In particular, $\mathrm{Sr}{\mathrm{Bi}}_2{\mathrm{Ta}}_2{\mathrm{O}}_9$ (SBT), one of the best-known members of the family, undergoes a series of transitions which cannot be analyzed within the conventional framework based on a single unstable polar mode. We present symmetry arguments and ab initio calculations which provide a new picture for the behavior of SBT, based on the interplay of several degrees of freedom, surprisingly not all of them associated to unstable or nearly-unstable modes. In fact, a hard, nonpolar secondary mode is seen as playing a fundamental role in the stabilization of the ferroelectric phase. Simple steric and empirical models illustrate these conclusions and help to understand family trends.