Such crystals are called ferroelectric, a term first used by analogy with ferromagnetism.
All ferroelectric crystals are necessarily both pyroelectric and piezoelectric.
Many lose these polar properties at the transition or Curie temperature T c . A nonpolar phase above T c is the so-called paraelectric phase. Ferroelectric single crystals grown in the absence of an electric field are inevitably electrically twinned, with a domain volume containing a given spontaneous polarization direction approximately equal to that containing the antiparallel direction. Ceramics have similar domain structures superimposed on the more general orientational disorder associated with polycrystalline materials. Exposure to an electric field under appropriate conditions can result in complete or partial realignment of the spontaneous polarization.