Ferroelectric materials and piezoelectric materials can be compared with the help of following points :
a) In piezoelectricity, the crystal is polarized by the application of an external stress, whereas in ferroelectricity the source of polarization is the dipole interaction energy itself.
b) Both the phenomena occur in non-centrosymmetrical crystals which are 20 in number. Piezoelectricity occurs in all the 20 crystals, whereas ferroelectricity only in 10, namely, those which provide a favourable axis of a polarity.
c) All ferroelectrics are therefore piezoelectric, but all piezoelectrics are not ferroelectric. For example: tourmaline is piezoelectric, but not ferroelectric.
d) The piezoelectric coefficient is the ratio of the set-up charge to the stress applied along crystallographic axis. The ferroelectrics have very large piezoelectric coefficients.