Piezoelectric materials          

Figure 11.2 shows the tetragonal pervoskite structure below the Curie temperature which is a characteristic of piezoelectric material. Only 11 groups out of possible 32 crystal groups based on simple lattice geometry and symmetry operation on crystal faces show piezoelectric properties [Cady, 1946]. They are all non-centro-symmetric and a subgroup of such crystals known as ‘ferroelectrics’ always show spontaneous polarization.

Figure 11.2: Tetragonal pervoskite structure of barium titanate below Curie temperature