The origin of the spontaneous polarization of CaBi2Nb2O9: The polarization of the two layers and their coupling mechanisms

The first-principles calculations are employed to investigate the origin of the spontaneous polarization (Ps) of CaBi2Nb2O9 (CBNO), composing of the perovskite-like (CaNb2O7)2- layer (CNO) and the fluorite-like (Bi2O2)2+ layer (BO). The Ps along the a-axis exists in the both of the CNO and BO layers, and the direction of the Ps along the a-axis of the two layers is the same, their contribution to the Ps of the ferroelectric phase is almost same. Moreover, the experimental results (12.00 mu C/cm2) are about half value of our calculated the Ps (28.67 mu C/cm2) and previous calculated (24.40 mu C/cm2). The difference between the experiments and the theoretical calculations may be cause by the reason that only the polarization in the CNO layer can switch under the external electric field. Meanwhile, the stable crystal structure of the BO layer result in the polarization cannot switch. Therefore, the polarization of the BO layer does not contribute the remnant polarization. The inter-layer coupling between the CNO and BO layers of the ferroelectric phase enhances the Ps along the a-axis. In addition, the Ps along the caxis of the two layers is almost same, however, their direction is opposite, as is similar to the anti-ferroelectrics.