Competing structural instabilities in the ferroelectric Aurivillius compound SrBi2Ta2O9 -: art. no. 214111

The Aurivillius family includes many compounds with ferroelectric phases. Despite sharing the basic perovskite structural block with more common ferroelectric oxides such as BaTiO3 or PbTiO3, their phase-transition behavior is rather more complex. In particular, SrBi2Ta2O9 (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.