Questions on ferroelectrics: why tetragonal ferroelectric ceramics are difficult to pole while rhombohedral ceramics easy to?

It may be well known to researchers on ferroelectrics that tetragonal ferroelectric ceramics, such as BaTiO₃, Ti-rich lead titanate zirconate (PZT), are difficult to pole even using a high DC field with long holding time. While the rhombohedral PZT or PZT near the morphotropic phase boundary (MPB) are easy to pole, especially the latter. Why?

 This is a crystal symmetry dependent question. In terms of the deformation modes, a ferroelectric crystal is similar to a ferroelastic crystal or a shape memory alloy which has two independent slip systems for the tetragonal case and three for the rhombohedral. According to the Taylor rule of plasticity, a crystal must have at least five slip systems for a polycrystalline to be ductile. Thus in tetragonal or rhombohedral polycrystalline ferroelectric ceramics, non-180 degree domain reorientation generates large internal stresses because of the spontaneous strain change, i.e., such domain reorientation could be constrained by neighboring grains and may not occur. One may ask: since domain reorientations in both tetragonal and rhombohderal ceramics could be constrained, why tetragonal ceramics differs so much from the rhombohedral?  

The answers of these questions can be found in our latest work (two successive papers) on Acta Mater as attached or  via the following links:

doi:10.1016/j.actamat.2007.08.002  

doi:10.1016/j.actamat.2007.08.003  

Any comments are welcome.