ferroelectric crystals   In a ferroclectric crystal the centres of  positive  and negative ions in the unit cell do not coincide, as  a  result  it  exhibits  electric dipole  moment even in the absence  of  external  electric  field. The plot of polarization* versus  electric  field  for  such  crystal results in hysteresis loop. Ferroelectric domains,  which  are  regions in which dipoles are aligned,  give rise  to  hysteresis  (  as  in  ferromagnetism). There   exists   a   transition temperature   beyond  which  the ferroelectric state disappears. This  happens because  the  positions  of the ions shift, and the centres  of  negative  and positive  ions  coincide  giving null electric  dipole  moment.  Ferroelectric crystals  also  exhibit piezoelectric   property*.  Ferroelectric  crystals  like barium titanate   find   wide  application  in  microphones,  phonograph pickups,  strain  gauges, sonar devices etc. They are also used in manufacturing   small  size  capacitors  because  of  their  high dielectric constant.