Temperature-driven phase transitions in SrBi$_2$Ta$_2$O$_9$ from   first-principles calculations
  The phase transition sequence of SrBi$_2$Ta$_2$O$_9$ is investigated using a shell model with parameters fitted to first-principles calculations. We show that the complex interplay between polar and nonpolar instabilities leads to the presence of two phase transitions, corroborating the existence of an intermediate orthorhombic paraelectric phase. This phase is characterized by the rotation of the TaO$_6$ octahedra around the a-axis. We show that this phase can be also detected from the dielectric response of the material. The present approach constitutes a powerful tool for a theoretical prediction of intermediate phases, not yet observed experimentally, in other Aurivillius compounds. 
R. Machado, M. Sepliarsky and M.G. Stachiotti