Piezoelectricity

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When these crystals are subjected to mechanical stress, electricity is produced due to displacement of ions. The electricity thus produced is called piezoelectricity. But if an electric field is applied to these crystal there will be atomic displacement causing mechanical strain.

These crystals act as mechanical-electrical transducers.

Uses

As pick-ups in record players.

Pyroelectricity

Some polar crystals when heated produce a small current or pyroelectricity.

Ferroelectricity

In some piezoelectric crystals the dipoles are permanently lined up even in the absence of an electric field. However, on applying electric field the direction of polarization changes.

Example: Barium titanate (BaTiO₃), Sodium potassium tartrate (Rochelle Salt) and Potassium dihydrogen phosphate ( KH₂PO₄).

Antiferroelectric

If there is no net dipole moment it is said to be an antiferroelectric crystal. This is because the dipoles are alternately arranged.

Example: (PbZrO₃) Lead zirconate.