5.3.1 Permanent Dipole Moment and Polarization
These materials consist of net permanent dipole moment i.e. finite vector sum of dipole moment even in the absence of electric field. This requires the material to be non-centrosymmetric whereas dipole moment would be forced to be zero in a centrosymmteric material due to symmetry considerations.
On top of this, there must be a spontaneous polarization as well which means the centers of positive and negative charges in a crystal would never be the same.
The following is the figure showing the crystal structure of a perovskite structured material such as BaTiO3. Here in cubic structure (A), the dipole is zero while in tetragonal form (B), the dipole moment is finite. This displaced position of the central atom is the energetically stable position i.e. the free energy is minimum.
| Figure 5.2 Cubic and tetragonal perovskite structures with dipole moment in latter represented by an arrow due to movement of central B ion up or down along c-axis |
When this kind of ferroelectric material is switched i.e. subjected to a bipolar electric field, it exhibits a polarization vs electric field plot as shown below. You can see that there is finite polarization in the absence of electric field i.e. two equal and opposite values, +Pr or -Pr also called remnant polarization.
These two values can be connected with the position of a central atom in the octahedra in the figure above. So, when you apply field in the direction, the central atom moves up with respect to oxygen octahedral and when you change the polarity, it comes down in the opposite direction, the extent of displacement being similar.
The plot shows a hysteresis which is important for applications such as memory devices. We will explain the details of this phenomenon in the subsequent sections.
| Figure 5.3 A typical ferroelectric hysteresis loop |
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