Structural basis of ferroelectricity and ferroelastcity

Abstract

The properties of ferroelectric and ferroelastic crystals are examined in terms of atomic arrangement. Ferroelectric crystals are divided into three classes according to the predominant nature of the atomic displacements required by polarity reversal. The one-dimensional class involves atomic displacements parallel to the polar axis; the two-dimensional class displacements are in a plane containing the polar axis; the three-dimensional class involves similar-magnitude displacements in all directions. Ferroelastic crystals are characterized by pseudo-symmetry relations between crystallographically independent atoms, such that the atomic displacements required to interchange identity are small and result in a change in the orientation of the unit cell. The origin, reorientation and temperature dependence of spontaneous polarization of ferroelectric crystals, and the mechanism of simultaneous polarization reversal and orientational change of state in ferroelectric-ferroelastic crystals are analyzed.