I believe, compare to piezo and pyro, Ferro got better dielectric properties for high dielectric microwave apps such as in DRA. In terms of grain size, it is depending on atomic radii of the materials, of course will affect perovskite structure for example by introducing dopant into based materials (titanate-based). Polarization occurred due to displacement of an atom from one site to another site, which creates an electric field which induced dipole moment. However, I don't understand, how the material's structure affects the resonance frequency (the mechanism of frequency resonates on grain). Is it grains vibrate with each other and then come out with one nice curve.. ? (I'm newbie here. I'm delightful to hear any ideas and please correct me if I'm wrong.. :)