Polarization Rotation in the Monoclinic Perovskite BiCo_(1-x)Fe_O3

摘要

A piezoelectric ceramic, Pb(Ti_(1-x)Zr_x)O3 (PZT), is widely used for various applications, such as transducers and sensors. PZT shows a maximum piezoelectric effect with a composition of about x = 0.5, the boundary separating the tetragonal PAmm (Ti-rich) and the rhombohedral Rim (Zr-rich) phases in the phase diagram. This boundary is known as a morphotropic phase boundary (MPB).Investigation of the structural evolution at the MPB is indispensable for understanding the origin of the enhanced piezoelectric property in PZT. In 2000, Noheda reported the presence of a monoclinic phase at x = 0.50 and 0.52 below 200 and 250 K, respectively, on the basis of powder synchrotron X-ray diffraction (SXRD) studies. The monoclinic phase had a √2a x √2a x a unit cell, where a was the cubic perovskite lattice parameter and the space group was Cm. This finding explains the mechanism of the piezoelectric enhancement as follows. The lack of a symmetry axis in the monoclinic structure allows the rotation of the ferroelectric polarization vector between the polar axes of the tetragonal and rhombohedral phases. This polarization rotation is induced by applying an electric field, leading to the enhancement of the piezoelectric constant.Although the large piezoelectric response of PZT is successfully explained, the presence of a monoclinic phase in PZT as a distinct phase is controversial. The insufficient difference in the pseudo cubic unit cell parameters of tetragonal, rhombohedral, and monoclinic phases inhibits the structural analysis based on diffraction studies. The cla ratio of tetragonal (P4mm) PbTi_(0.48)Zr_(0.52)O3 at 325 K is 1.01, which is quite close to unity, and the a of rhombohedral (Rim) PbTi_(0.40)Zr_(0.60)O3 at 295 K is 90.45°, that is, almost orthogonal. The monoclinic phase has an intermediate structure between the tetragonal and the rhombohedral phases. These three phases are thus so close to cubic that their diffraction peaks overlap extremely.