Types of Ferroelectric Materials

The first ferroelectric material to be discovered was some sodium-potassium tartrate tetra-hydrate crystals prepared in 1672 by a pharmacist, Elie Seignette, who lived in La Rochelle, France—hence the name Rochelle salts. However, their piezoelectric properties were not discovered until 1880 and their ferroelectric properties not until the 1920s. This material is only a ferroelectric between the temperatures of — 18°C and 23°C where it transforms from orthorhombic to monoclinic and back to orthorhombic. The Curie temperature as well as its polarization is increased when the hydrogen is replaced with deuterium. 

Later ferroelectricity was discovered in KH2PO4 (KDP) below its Curie temperature of 123 K where it transforms from its piezoelectric tetragonal d2d (42m) form to the ferroelectric C2v (2 mm) symmetry. RbH2P04 and KH2ASO4 (KDA) also exhibit ferroelectricity at low temperatures. Another member of this class is triglycene sulfate (TGS). It transforms from its monoclinic form C2 (2) to its centrosymetric form C2h (2/m) at its Curie temperature of 49.7°C. The Curie temperatures of these systems are also increased in their dueterated 

BaTiOs molecule above the 120°C transition temperature (a). The thermal expansion of the lattice provides sufficient space for the Ti ion the remain in the center of the cubic unit cell with the perovskite structure (Pm3m). Below 120°C (b) the thermal expansion can no longer maintain the cubic s)unmetry and a solid state transition to a tetragonal lattice (P4 mm) occurs. The four ions squeeze the Ti to one side or the other creating a permanent electric dipole. 

Another class of ferroelectrics is the perovskites such as the titanates (BaTiOs, PbTiOs, SrTiOs, CaTiOs), the niobates (KNbOs, NaNbOs), the ilmentites, (LiNbOs, LiTaOs)), the ternary lead zirconate titanate (PZT), and the quaternary lead lanthanum zirconate titanate (PLZT). These materials undergo a displacive-type phase transformation at their Curie temperature. 

We will examine some properties of BaTiOa to get a feeling for the numbers involved. We will assume the following measured properties (there are some variations in the literature). 

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