Definition: When certain crystals become electrically polarized that is electric charges appear on their surface when stressed then this phenomenon is called the piezoelectric effect, or piezoelectricity and the crystals as the piezoelectric crystals.

Examples: Quartz, rochelle salt, tourmaline are piezoelectric substances.

The inverse effect-that these crystals become strained when polarized (i.e. place in an electric field) has also been observed.

Piezoelectric strains are very small, and the corresponding electric fields are very large.

Example. In Quartz a field of 100 V/cm produces a strain of the order of 10^-7.

Origin of piezoelectric effect: Consider the distribution of the ionic charges of crystal about their lattice sites. Normally, the

distribution is symmetrical, and internal electric field is zero. But when the crystal is stressed, the charges are displaced. If a

piezoelectric crystal is stressed, the charges are displaced. In a piezoelectric crystal this displacement distorts the original charge distribution in such a way that it is no longer symmetrical. A net polarization results in such crystals and we observe the piezoelectric effect.

In other crystals, on the other hand, the distribution of charges maintains its symmetry even after the displacement. Such crystals exhibit no net polarization and hence no piezoelectric effect.

It shows that the piezoelectricity is related to crystal symmetry.

The symmetry element involved is the centre of inversion. A crystal can exhibit piezoelectricity only if its unit cell lack a center of inversion. This is because when there is no centre of inversion , only then the charge distribution is distorted so as to produce polarization. However, if the centre of inversion is present, there is no charge distortion whatever, thus no polarization.

Electrostriction: When mechanical stresses change the dimensions of a specimen but do not produce a dipole moment, then this phenomenon is called electrostriction.