Chemical Substitution‐Induced Ferroelectric Polarization Rotation in BiFeO3 (original) (raw)

The direction of the ferroelectric polarization vector is a key factor infl uencing the properties of ferroelectric/piezoelectric [ 1 ] and multiferroic [ 2 ] materials. For instance, ferroelectric materials at morphotropic phase boundaries (MPB), where multiple structural phases with ferroelectric polarizations pointing in different crystallographic directions coexist, often display large piezoelectric coeffi cients. [ 3-5 ] It is the ferroelectric distortions, which accompany the polarization rotation that leads to enhancements in the piezoelectric coeffi cient. In multiferroic BiFeO 3 (BFO), it has been shown [ 2 ] that the coupled antiferromagnetic order can be altered by switching the ferroelectric polarization vector. In fact, the ability of a material to display polarization rotation is recognized as an important precursor to occurrence of an MPB. [ 5 , 6 ] Chemical substitution into perovskite BiFeO 3 , which displays room-temperature multiferroic properties, [ 2 , 7 ] has been a subject of much interest since the substitution results in improved ferroelectric properties and enhancement in piezoelectric and dielectric properties. [ 8-10 ] One important consequence of substitution is that a symmetry-lowering structural phase transition from the rhombohedral phase for pure BFO to another structure takes place, displaying the characteristics of an MPB with enhanced dielectric and piezoelectric properties, as observed in Pb-based ferroelectrics. [ 3 , 4 ] Recently, we demonstrated [ 11 ] that substitution of rare earth elements (RE = Sm, Gd, and Dy) into the A-sites of BFO thin fi lms results in a ferroelectric rhombohedral to paraelectric orthorhombic structural transition, exhibiting a double hysteresis behavior in the polarization-electric fi eld (PE) hysteresis loop, and that the occurrence of this transition can be universally described by the averaged A-site cation radius regardless of the substituted rare-earth element. The piezoelectric coeffi cient d 33 and dielectric constant ε 33 are enhanced at the boundary, and the maximum d 33 reaches 110 pm V − 1 , [ 8 ] which is comparable to the value for epitaxial Pb(Zr 0.48 Ti 0.52)O 3 thin fi lms at the MPB. [ 12 ] Based on the results of fi rst-principles calculations, it was proposed [ 11 ] that an electric fi eld-induced structural transformation from the nonpolar orthorhombic to the polar rhombohedral phase is the origin for the double hysteresis behavior and the concomitant enhanced properties at the boundary.