Transcript Corner Sharing Octahedra
Types of Ferroelectric Materials
Ferroelectric Materials can be structurally categorized into 4 groups: 1.
Corner Sharing Octahedra: 1.1 Perovskite-Type Compounds (such as BaTiO 3 , PT, PZT, PMN, and PLZT) 1.2 Tungsten-Bronze-Type Compounds (such as PbNb 2 O 6 ) 1.3 Bismuth Oxide Layer Structured Compounds (such as Bi 4 Ti 3 O 12 and PbBi 2 Nb 2 O 9 ) 1.4 Lithium Niobate and Tantalate (such as LiNbO 3 and LiTaO 3 ) 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 O 2 Ions
Inside each Octahedron
Cation B b+ Space between the Octahedra
Corner Sharing Octahedra
In prototypic forms, A a+ , B b+ , and O 2-
ions geometrically coincide
Non-Polar Lattice
Phase Transitions
Changes in Lattice Structure
A a+ and B b+
ions displaced w.r.t. O 2-
Polarized Lattice
ions
Perovskite-Type Compounds
Perovskite
Mineral Name of Calcium Titanate (CaTiO 3 ) B A O General Chemical Formula
ABO 3 A
Cation with Larger Ionic Radii B
Cation with Smaller Ionic Radii O
Oxygen
Perovskite-Type Compounds
Perovskite
Perovskite
Three-Dimensional Network of BO 6 Octahedra Cubic-Close-Packed of A and O ions with B in interstitial positions Most Ferroelectric Perovskites A 2+ B 4+ O 3 or A 1+ B 5+ O 3 Non-Ferroelectric Perovskites A 3+ B 3+ O 3
Perovskite-Type Compounds
Structural Classifications of A 2+ B 4+ O 3 by A 2+ and B 4+ ionic radii compounds
Perovskite-Type Compounds
Barium Titanate (BaTiO 3 ) Ti Ba O Ti
Ba
6 coordinated to Oxygen (Octahedron) 12 coordinated to O (Cubic-Close-Packed) O
4 coordinated to Ba AND 2 coordinated to Ti (Distorted Octahedron)
Perovskite-Type Compounds
Barium Titanate (BaTiO 3 ) Cubic-Close-Packed (CCP)
OR
Face-Centered-Cubic (FCC) (abc-abc-abc arrangement)
Barium Titanate (BaTiO
3
)
Ti Ba O Ti
Ba
6 coordinated to Oxygen (Octahedron) 12 coordinated to O (Cubic-Close-Packed) O
4 coordinated to Ba and 2 coordinated to Ti (Distorted Octahedron)
Crystal Chemistry of BaTiO
3
Phase Equilibria of BaTiO
3
(BaO-TiO
2
)System
Very First Phase Equilibria Effects of BaO/TiO 2 Ratio
• Very little solubility of excesses BaO or TiO 2 • Excess TiO 2 results in Ba 6 Ti 17 O 40 separated phase (melt at 1320 C) liquid phase sintering below 1350 C wide grain sizes (5 –50 m m) • Excess BaO results in Ba 2 TiO 4 separated phase (melt at 1563 C) solid insoluble phase acts as grain growth inhibitor below 1450 C smaller grain sizes (1 –5 m m)
Phase Transitions in BaTiO
3
Cubic (m3m)
Tetragonal (4mm) Orthorhombic (mm2) Rhombohedral (3m) 120 C 0 C -90 C
Paraelectric Phase
Ferroelectric Phase
Phase Transitions in BaTiO
3 Lattice Parameters Variation with Temperature during the Phase Transitions Through X-Ray and Neutron Diffractions, during the Cubic-to-Tetragonal Phase (Structural) Transition, Ba 2+ , Ti 4+ , and O 2 (w.r.t. center O 2 ) displaced along the c-axis +0.06 Å, +0.12 Å, and –0.03 Å, respectively
Phase Transitions in BaTiO
3 Spontaneous Polarization (P s ) versus Temperature I. No Spontaneous Polarization (P s = 0) II. P s III. P s along [001] directions of the original cubic along [110] directions of the original cubic IV. P s along [111] directions of the original cubic (P s ~ 26
m
C/cm 2 at room temperature)
Phase Transitions in BaTiO
3 Relative Permittivity of Single Crystal BaTiO 3 Measured in the a and c Directions versus Temperature
BaTiO
3
Ceramics and Modifications
BaTiO 3 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 BaTiO 3 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 BaTiO
3 Curie Point and Phase Transitions Shifters This 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 BaTiO
3
Ceramics (T
c
Suppressors)
Ba(Ti 1-x Zr x )O 3 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 T c (as compared to Curie point adjusted compositions above)
Effects of Grain Sizes
large grain At Curie Point
multiple domains more domain wall motions
higher K small grain
single domain
less domain wall motions due to grain boundary
lower K large grain At Room Temp
larger domains
internal stress
lower K less small grain
smaller domains
internal stress relieved internal stress
higher K
less larger