Classifications of Ferroelectric Materials
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The origin of ferroelectricity can be attributed to different mechanisms in different ferroelectrics. Various classifications of ferroelectric materials have been suggested to facilitate the treatments of their properties and applications as listed below (The different colors especially highlight the same materials in different classes):

      i) Classification based on the crystal chemical compositions of ferroelectric materials:
      i.a) Hydrogen-bonded crystals.
         i.a.1) Rochelle salt.
      Rochelle salt has ferroelectric property in the temperature range of -18 °C to 23 °C. This property results in two transition temperatures.
         i.a.2) Triglycine sulfate.
         i.a.3) Potassium di-hydrogen phosphate and arsenates.
      i.b) Double oxides.
         i.b.1) Barium titanate.
         Barium titanate (BaTiO3) is the best known ferroelectric material and is a representative of the oxygen octahedron group of ferroelectric materials.
         i.b.2) Potassium niobate (KNbO3).
         i.b.3) Lead niobate.

      ii) Classification based on the crystal structures of ferroelectric materials:
      ii.a) Corner sharing oxygen octahedra.
      This class of ferroelectric crystals consists of mixed oxides including corner sharing octahedra of O2- ions. For each octahedron, there is a cation Bb+ (3 ≤ b ≤ 6). And, Aa+ ions (1≤ b ≤ 3) occupy the spaces between the octahedra. The geometric centers of the Aa+, Bb+ and O2- ions typically coincide, resulting in a non-polar lattice. The A and B ions are displaced from their geometric centers with respect to the O2- ions when the crystal is polarized. In this type of crystals, the ferroelectricity is achieved by a complex lattice distortion.
      This displacement occurs when the phase transition takes place due to the change of temperature. The formation of dipoles does not induce spontaneous polarization if a compensation of dipoles leads to net zero dipole moment. This class of ferroelectric crystals includes mainly the perovskite type compounds (see v.b.1 below), tungsten bronze type compounds, bismuth oxide layer structured compounds, lithium niobate and tantalate.
      ii.b) Charge ordered ferroelectrics.
      Charge ordering leads to ferroelectricity in only a few materials [2]. Typical examples are Fe3O4 and LuFe2O4. For instance, the ferroelectricity in LuFe2O4 [2] is induced by electron correlations. In this case, the Fe ions in each layer form a triangular lattice with an ordered arrangement of valence states of Fe2+ and Fe3+. This type of charge ordering normally occurs below 350K.
      ii.c) Magnetic ordering ferroelectrics.
      In this type of materials, a long-range spin-spiral magnetic ordering occurs and thus, ferroelectricity becomes possible with a strong coupling to the magnetic ordering [3].
      ii.d) Compounds containing hydrogen bonded radicals.
      ii.e) Organic polymers.
      In many cases, orienting the crystalline phase of the polymer, e.g. polyvinylidene fluoride (PVDF, (CH2-CF2)n) , in a strong poling field can induce remanant polarization, resulting in piezoelectric and pyroelectric properties.
      ii.f) Ceramic polymer composites.

      iii) Classification based on the symmetry related to thermodynamic behavior of ferroelectric materials:
      iii.a) Noncentrosymmetric nonpolar phase.
         iii.a.1) Rochelle salt.
         iii.a.2) Potassium di-hydrogen phosphate.
         iii.a.3) Isomorphous compounds.
      iii.b) Centrosymmetric nonpolar phase
         iii.b.1) Barium titanate.
         iii.b.2) Cadmium (pyro) niobate (Cd2Nb2O7).
         iii.b.3) Triglycine sulfate.

      iv) Classification based on the structural change during phase transition occurring at Curie point Tc:
      iv.a) Transition of order-disorder type.
         This type of ferroelectrics is typically observed in some crystals with hydrogen bonds. In these crystalline systems, hydrogen (H) atoms jump around some multi potential well above Tc and thus, no net dipole moment is formed. Below Tc, H atoms jump around some ordered subset of potential wells, resulting in a net dipole moment in the unit cell. Typical examples are triglycine sulfate and potassium di-hydrogen phosphate (KH2PO4).
      iv.b) Transition of displacive type.
         This type of ferroelectrics is most commonly formed in ionic materials. The ions attain equilibrium positions above Tc and thus, net dipole moment is not present, resulting in a non-polar state. However, below Tc some of the ions shift and thus the centers of positive and negative charges deviate from the equilibrium positions, resulting in a net dipole moment and a consequent polar state in the system. Examples are BaTiO3 and PbTiO3 materials.

      v) Classification based on the number of directions allowed to spontaneous polarization:
      v.a) Spontaneous polarization along single axis.
         v.a.1) Seignette salt.
         v.a.2) Potassium di-hydrogen phosphate.
         v.a.3) (NH4)2SO4.
         v.a.4) (NH4)2BF4.
         v.a.5) Colemanite.
         v.a.6) Thiourea.
         v.a.7) Glycine sulfate.
         v.a.8) Glycine selenate.
      v.b) Spontaneous polarization along multiple axes.
         v.b.1) Perovskite ferroelectric materials.
         These materials include barium titanate (BaTiO3), lead titanate (PbTiO3), lead zirconate titanate (Pb(Zr,Ti)O3), lead lanthanum zirconate titanate ((Pb,La)(Zr,Ti)O3), lead magnesium niobate (PMN), KNbO3, potassium sodium niobate (KxNa1-xNbO3), and potassium tantalate niobate (K(TaxNb1-x)O3).
         For this type of conventional perovskite ferroelectrics, cations have an empty “d” shell configuration [1]. For instance, the empty d state of the transition metal ion Ti4+ in BaTiO3 is used to establish a strong covalent bond with neighboring oxygen atoms. In this case, a shift of the Ti atom from the centre of the O6 octahedra is induced to form the covalent bonds with part of the oxygen atoms at the expense of the other oxygen atoms.
         v.b.2) Pyrochlore ferroelectric materials (e.g. Cd2Nb2O7).
         v.b.3) Niobate ferroelectric materials.
         v.b.4) Alums, such as CH3NH3Al(SO4)2•12H2O and (NH4)2Cd2(SO4)3.
      v.c) Undefined.
         v.c.1) Ilmenite ferroelectric materials such as LiNbO3 and LiTaO3.

      vi) Classification based on the applications of ferroelectric materials (see Table 2381a).

Table 2381a. Ferroelectric materials and their applications.

Function
Application
Material examples

Ferroelectricity

FEDRAM

PZT [Pb(Zr,Ti)O3], PLZT [(Pb,La)(Zr,Ti)O3]

FESRAM

BPZT [BaTiO3-PbZrO3], SBT [SrBi2Ta2O9]
FEMFET BMF [BaMgF4]

High permittivity

Capacitor for high count DRAM

SBT [SrBi2Ta2O9], PZT [Pb(Zr,Ti)O3], PLT [(Pb,La)TiO3]

Pyroelectricity

IR detector PT [PbTiO3], PLT [(Pb,La)TiO3]

Piezoelectricity

BAW/SAW

ZnO, AlN
Filter PLT [(Pb,La)TiO3], PZT [Pb(Zr,Ti)O3]

Electrostriction

Actuator

PLT [(Pb,La)TiO3], PZT [Pb(Zr,Ti)O3]
MEMS ZnO

Acousto-optics

Integrated optics

ZnO, LN [LiNbO3]
Channel switch PLT [(Pb,La)TiO3]
Modulator PLZT [(Pb,La)(Zr,Ti)O3]
Electro-optics

Integrated optics

LT [LiTaO3], LN [LiNbO3]
Coupler BTO [Bi4Ti3O12]
Channel switch PLT [(Pb,La)TiO3]
Modulator PLZT [(Pb,La)(Zr,Ti)O3]

       vii) Classification based on the unit cell structures (see Table 2381b).

Table 2381b. Unit cell structures of ferroelectric materials.
Unit cell structures
Material examples

Layered structure

Bi4Ti3O12

Perovskite or Oxygen octahedral group

BaTiO3, PbTiO3

Pyrochlore group

CdNb2O7

Tungsten bronze group

PbNb2O6, SrNb2O6

 

[1] N. A. Hill, J. Phys. Chem. B, 104 (29), (2000).
[2] N. Ikeda, H. Ohsumi, K. Ohwada, K. Ishii, T. Inami, K. Kakurai, Y. Murakami, K. Yoshii, S. Mori, Y. Horibe and H.Kitô, Nature, 436, 1136, (2005).
[3] M. Mostovoy, Nature Materials, 7, 269, (2008).

 

 

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