Origin of Ferroelectricity in BiFeO3-Based Solid Solutions

We investigate the origin of ferroelectricity in the BiFeO3–LaFeO3 system in rhombohedral R3c and tetragonal P4mm symmetries by ab initio density functional theory calculations and compare their electronic features with paraelectric orthorhombic Pnma symmetry. We show that a coherent accommodation of stereo-active lone pair electrons of Bi is the detrimental factor of ferroelectricity. A Bloch function arising from an indirect Bi_6p–Fe_3d hybridization mediated through O_2p is the primary origin of spontaneous polarization (Ps) in the rhombohedral system. In the orthorhombic system, a similar Bloch function was found, whereas a staggered accommodation of stereo-active lone pair electrons of Bi exclusively results in paraelectricity. A giant Ps reported in the tetragonal system originates from an orbital hybridization of Bi_6p and O_2p, where Fe-3d plays a minor role. The Ps in the rhombohedral system decreases with increasing La content, while that in the tetragonal system displays a discontinuous drop at a certain La content. We discuss the electronic factors affecting the Ps evolutions with La content.