Monoclinic distortion, polarization rotation and piezoelectricity in the ferroelectric Na_0.5Bi_0.5TiO₃

Abstract

The relationship between crystal structure and physical properties in the ferroelectric Na_0.5Bi_0.5TiO₃ (NBT) has been of interest for the last two decades. Originally, the average structure was held to be of rhombohedral (<i>R</i>3<i>c</i>) symmetry with a fixed polarization direction. This has undergone a series of revisions, however, based on high-resolution X-ray diffraction, total neutron scattering, and optical and electron microscopy. The recent experimental findings suggest that the true average symmetry is monoclinic (space group <i>Cc</i>), which allows for a rotatable spontaneous polarization. Neither polarization rotation nor its potentially important real role in enhanced piezoelectricity is well understood. The present work describes an <i>in situ</i> investigation of the average monoclinic distortion in NBT by time-resolved single-crystal X-ray diffraction under external electric fields. The study presents a high-resolution inspection of the characteristic diffraction features of the monoclinic distortion - splitting of specific Bragg reflections - and their changes under a cyclic electric field. The results favour a model in which there is direct coupling between the shear monoclinic strain and the polarization rotation. This suggests that the angle of polarization rotation under a sub-coercive electric field could be 30° or more.