Bonding mechanism and relaxation energy of SrBi2B2O9 (B=Ta,Nb): First-principles study

Using first-principles calculations, we have studied the electronic structures, the relaxation energy and the bonding mechanism of SrBi2B2O9 (B=Ta,Nb). Unlike the case of prototype perovskites, the covalent bonding between cations and oxygen of SrBi2B2O9 (B=Ta,Nb) does not directly stabilize their ferroelectric phase through reducing their total energy. Whereas, the ferroelectric stability of SBN and SBT is determined by a large relaxation energy derived from a larger distortion along with their phase transition. The origin of the spontaneous polarization of the materials is also briefly discussed in our present work.