Types of Ferroelectric MaterialsFerroelectric Materials can be structurally categorized into 4 groups:

1. Corner Sharing Octahedra:1.1 Perovskite-Type Compounds

(such as BaTiO3, PT, PZT, PMN, and PLZT)1.2 Tungsten-Bronze-Type Compounds

(such as PbNb2O6)1.3 Bismuth Oxide Layer Structured Compounds

(such as Bi4Ti3O12 and PbBi2Nb2O9)1.4 Lithium Niobate and Tantalate

(such as LiNbO3 and LiTaO3)

2. Compounds Containing Hydrogen Bonded Radicals (such as KDP, TGS, and Rochelle Salt)

3. Organic Polymers (such as PVDF and co-polymers)

4. Ceramic Polymer Composites (such as PZT-PE)

Corner Sharing Octahedra

Mixed Oxide Ferroelectrics with Corner Sharing Octahedra of O2- Ions

Inside each Octahedron Cation Bb+ (3 < b < 6) Space between the Octahedra Aa+ Ions (1 <a < 3)

Corner Sharing Octahedra

In prototypic forms, Aa+, Bb+, and O2- ions geometrically coincide Non-Polar Lattice

Phase Transitions Changes in Lattice Structure Aa+and Bb+ ions displaced w.r.t. O2- ions

Polarized Lattice

Perovskite-Type Compounds

Perovskite Mineral Name of Calcium Titanate (CaTiO3)

General Chemical Formula ABO3

A Cation with Larger Ionic Radii B Cation with Smaller Ionic Radii

O Oxygen

BA

O

Perovskite-Type Compounds

Perovskite Three-Dimensional Network of BO6- OctahedraPerovskite Cubic-Close-Packed of A and O ions with B in interstitial positions

Most Ferroelectric Perovskites

A2+B4+O3 or A1+B5+O3

Non-Ferroelectric Perovskites

A3+B3+O3

Perovskite-Type Compounds

Structural Classifications of A2+B4+O3 compounds

by A2+ and B4+ ionic radii

Perovskite-Type Compounds

Barium Titanate (BaTiO3)

Ti 6 coordinated to Oxygen (Octahedron)Ba 12 coordinated to O (Cubic-Close-Packed)

O 4 coordinated to Ba AND 2 coordinated to Ti (Distorted Octahedron)

TiBa

O

Perovskite-Type Compounds

Barium Titanate (BaTiO3)

Cubic-Close-Packed (CCP) OR

Face-Centered-Cubic (FCC)(abc-abc-abc arrangement)

Barium Titanate (BaTiO3)

Ti 6 coordinated to Oxygen (Octahedron)Ba 12 coordinated to O (Cubic-Close-Packed)

O 4 coordinated to Ba and 2 coordinated to Ti (Distorted Octahedron)

Ti

Ba

O

Crystal Chemistry of BaTiO3

Phase Equilibria of BaTiO3 (BaO-TiO2)System

Very First Phase Equilibria

Effects of BaO/TiO2 Ratio• Very little solubility of excesses BaO or TiO2

• Excess TiO2 results in Ba6Ti17O40 separated phase (melt at 1320 C) liquid phase sintering below 1350 C wide grain sizes (5 –50 m) • Excess BaO results in Ba2TiO4 separated phase (melt at 1563 C) solid insoluble phase acts as grain growth inhibitor below 1450 C

smaller grain sizes (1 –5 m)

Phase Transitions in BaTiO3

Cubic (m3m) Tetragonal (4mm) Orthorhombic (mm2) Rhombohedral (3m) 120 C 0 C -90 C

Paraelectric Phase Ferroelectric Phase

Phase Transitions in BaTiO3

Lattice Parameters Variation with Temperature during the

Phase Transitions

Through X-Ray and Neutron Diffractions, during the Cubic-to-Tetragonal Phase

(Structural) Transition, Ba2+, Ti4+, and O2- (w.r.t. center O2-) displaced along the c-axis +0.06 Å, +0.12 Å, and –0.03 Å, respectively

Phase Transitions in BaTiO3

Spontaneous Polarization (Ps) versus Temperature

I. No Spontaneous Polarization (Ps = 0)II. Ps along [001] directions of the original cubicIII. Ps along [110] directions of the original cubicIV. Ps along [111] directions of the original cubic

(Ps ~ 26 C/cm2 at room temperature)

Phase Transitions in BaTiO3

Relative Permittivity of Single Crystal BaTiO3 Measured in the a and c Directions versus

Temperature

BaTiO3 Ceramics and Modifications

BaTiO3 ceramic was the first piezoelectric transducer developed, BUT now use mainly for high-dielectric constant capacitors because

of TWO main reasons:• Relatively low Tc (~120 C) limits its use as high-power transducers• Low piezoelectric activities as compared to PZT

BaTiO3 for capacitor applications require special modifications to suppress its ferroelectric/piezoelectric properties, and simultaneously

to obtain better dielectric features. This is done through additives and compositional modifications, which can produce the following

effects:• Shift of Curie Point and other transition temperatures

• Restrict domain wall motions• Introduce second phases and compositional heterogeneity

• Control crystallite size• Control oxygen content and the valency of the Ti ion

Effects of A and B Sites Substitutions in BaTiO3

Curie Point and Phase Transitions ShiftersThis would enable the peak permittivity to be used in the temperature range

of interest. For example, Sr2+ in the A site would reduce the Curie Point towards room temperature, while Pb2+ would raise the Curie Point. This

leads to tailoring dielectric properties with A and B sites substitutions.

Modified BaTiO3 Ceramics (Tc Suppressors)

Ba(Ti1-x Zrx )O3 Solid-Solution

Low level addition the dielectric peak rises sharply

Higher level addition results in peak broadening (probably

causes by “macroscopic heterogeneity” in the composition

Controlling the Permittivity

Control of K in fine grained BT Control of “dirty” grain

boundary impedance to suppress the Curie Peak at Tc (as compared

to Curie point adjusted compositions above)

Effects of Grain Sizes

At Curie Pointlarge grain multiple domains

more domain wall motions higher K

small grain single domain less domain wall motions due to grain

boundary lower K

At Room Templarge grain larger domains less

internal stress lower K

small grain smaller domains less internal stress relieved larger

internal stress higher K