  We have proposed the polarization rotation theory to understand the
underlying physics of the large piezoelectric effect in ferroelectric crystals
by referring to the coherent rotation model in ferromagnetism. When both the
electric field energy and crystalline anisotropy energy are taken into account,
the polarization in BaTiO3 crystal can be rotated toward the field direction.
The numerical results have indicated the highly anisotropic polarization
rotations and the field-induced-phases with different symmetries in the
crystal. This theory is helpful for understanding the polarization reversal and
the electromechanical effect in ferroelectric materials.
