A non centrosymmetric crystal, which acts under mechanical force, causes deformation and makes relative displacement of charged particles. Therefore, positive and negative binding charges appear on the surface of crystals. This called piezoelectric crystals.
The cause of the piezoelectricity is related to the Crystal Filter structure. The common characteristic is the crystal point group has no symmetric center.
In the 32 point group of the crystal, the 21 point groups do not have the symmetry center,, crystals of the 20 point groups have the piezoelectricity.
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