First principles investigation of the strain-mode coupling in srbi2nb2o9

The Aurivillius compounds SrBi2Nb2O9 (SBN) and SrBi2Ta2O9 (SBT) present at room temperature isomorphous ferroelectric structures that are stabilized by a complex interplay of three order parameters via a trilinear coupling. In this work we examine the influence of the in-plane misfit strain in the strength of the relevant instabilities and the couplings between them. A mixed strain-stress enthalpy compatible with the symmetry of SBN is developed with all the parameters obtained from first principles. We find that the dominant role of the order parameters varies as a function of the strain and, as a consequence, that the value of the polarization can be tuned by choosing the lattice parameter of the substrate. It is also shown that the sequence of phase transitions and the main features of the phase diagram change qualitatively in terms of the strain.