Explain ionic polarization.
Ionic polarization is polarization that is caused by relative displacements between negative and positive ions in ionic crystals (for illustration, NaCl).
When crystals or molecules do not has only atoms of the same type, the distribution of charges around an atom in the molecules or crystals leans to negative or positive. When a result, as lattice vibrations or molecular vibrations induces relative displacements of the atoms, the centers of negative and positive charges might be in various locations. These center positions are influenced by the symmetry of the displacements. If the centers don't correspond, polarizations arise in molecules or crystals. That polarization is termed as ionic polarization.
Ionic polarization causes ferroelectric transition including dipolar polarization. The transition that is caused by the order of the directional orientations of permanent dipoles beside a particular direction is termed as order-disorder phase transition. The transition that is caused by ionic polarizations in crystals is termed as displacive phase transition.