Answer to Question #120617 in Inorganic Chemistry for Yuki

Question

Why does a ferroelectric material require the absence of an inversion centre in its crystal structure?

Answer 1

If a material's crystal structure has centre of symmetry ,it means that for every atom, there exist a point in the unit cell through which inversion will bring the atom to the same type of yet another atom.

1. Whereas some materials like ferroelectric materials have absence of centre of symmetry or have acentric points. All of the crystalline materials whose structures possess these point groups (with exceptions ) exhibit the phenomenon of piezoelectricity, which means that stress will generate a charge separation on the faces of the crystal (the direct piezoelectric effect) and will undergo mechanical strain when subjected to an electric field (the converse piezoelectric effect).
2. Moreover these ferroelectric crystals have unique axis of geometry and they are piezoelectric only because they are accentric . Example$-ZnO$
3. Ferroelctrics are subsets of these piezoelectric crystals on which when electric field is applied ,it will cause sufficient polarization to switch to different stable direction.Upon removal of field,they return to original position.

Hence,the ferroelectric crystals show their basic properties of piezoelcctricity and dielectrics only because they lack inversion centre in crystal symmetry.