lead magnesium niobate (pmn) system

Lead Magnesium Niobate (PMN) System


Important Perovskite End Members for Relaxors

Important Relaxors Based on MPB Compositions

Lead Magnesium Niobate (PMN) SystemRelaxor-Based Compositions for MLC


Application Example

Pyroelectrics

Capacitors/dielectrics

Electrostriction/actuators

Medical ultrasound/high

efficiency transducers

Piezoelectrics

Electrooptics

Pb(Sc1/2 Ta1/2)O3

(Ba0.60 Sr0.40)TiO3

Pb(Mg1/3Nb2/3)O3

Pb(Mg1/3Nb2/3)O3

Pb(Zn1/3Nb2/3)O3

Pb[(Mg1/3Nb2/3)1-xTix]O3

Pb[(Zn1/3Nb2/3)1-xTix]O3

Pb[(Sc½Nb½)1-xTix]O3

Pb(Zr1-xTix)O3

Pb[(Zn1/3Nb2/3)1-xTix]O3

Pb[(Sc½Nb½)1-xTix]O3

(Pb1-xLa2x/3)(Zr1-yTiy)O3

Areas of Applications for Relaxors Ferroelectrics and Solid Solutions


Relaxor FerroelectricsRelaxor Ferroelectrics

Pb(B1B2)O3

(B1 ~ lower valency cation : Mg2+, Zn2+, Ni2+, Fe3+)(B2 ~ higher valency cation : Nb5+, Ta5+, W6+)

PMN Pb(Mg1/3Nb2/3)O3

Important Relaxor Ferroelectric with Tc ~ -10 C

Broad diffused and dispersive phase transition on cooling below Tc

Very high room temperature dielectric constantStrong frequency-dependent dielectric properties

Nano-scaled compositional inhomogeniety

Chemically order-disorder behavior observed by TEM study

B-site 1:2 order formula with 1:1 order arrangement in the structure (Most have rhombohedral symmetry due to slight lattice distortion)


Dielectric properties of Pb(Mg1/3Nb2/3)O3

showing diffused phase transition and relaxor characteristics(Tmax ( at 1 kHz) ~ -10 C with r max ~ 20,000)


Property Normal Ferroelectrics Relaxor Ferroelectrics

Permittivity temperature dependence

Permittivity frequency dependence

Permittivity behavior in Paraelectric range

Remnant polarization (Pr)

Scattering of light

X-Ray diffraction

Sharp 1st or 2nd order transition about Tc

Weak frequency dependence

Follow Curie-Weiss Relation above Tc

Strong remnant polarization

Strong anisotropy (birefringent)

Line splitting (cubic to tetragonal)

Broad-diffused phase transition about Tmax

Strong frequency dependence

Follow Curie-Weiss Square Relation above Tmax

Weak remnant polarization

Very weak anisotropy to light (pseudo-cubic)

No line splitting (pseudo-cubic structure)

Comparison of normal and relaxor ferroelectrics


First-Order Phase Transition Second-Order Phase Transition

Spontaneous polarization (Ps)A discontinuity in the first-order phase transition

A continuous change in the second-order phase transition

Relaxor ferroelectric Ps decays continuously with temperature, but does not follow the parabolic temperature dependence

as in the second-order phase transition


Dielectric Behavior Normal Ferroelectrics Relaxor Ferroelectrics

Normal ferroelectrics the onset of spontaneous polarization occurs simultaneously with the maximum in the paraelectric to ferroelectric phase

transition. No Ps above the transition temperature with a valid Curie-Weiss Law

Relaxor ferroelectrics Three regimes : Regime I Above dielectric maximum temperature, Regime II Between Td (depolarization temperature) and Tmax

(dielectric transition temperature), and Regime III Below Td


Regime I : Electrostrictive region with existence of chemically ordered region with no macro-scale ferroelectric domian little or no hysteresis

Regime II : Freezing-out of macro-domain region in which with decreasing temperature the polar regions grow and cluster hysteresis is observed and

becomes more pronounced with decreasing temperature

Regime III : Macro-domain region becomes more stable which results to a large spontaneous polarization and piezoelectric effects with large remnant strain


Ordered and Disordered Perovskite Structures


Ordered and Disordered Perovskite Structures

Fully disorder of the cations in the B-sites occupation

“Normal” ferroelectric materials (such as PZT)

Nano-scale order of the cations in the B-sites occupation

“Relaxor” ferroelectric materials (such as PMN)


Nano-scale ordered region in disordered matrix5 nm

Pb(Mg1/3Nb2/3)O3

Nano-scale ordered region with Mg:Nb = 1:1 (like in NaCl structure)

Non-stoichiometric short range chemical heterogeneity

Different ferroelectric transition temperature regions

Diffused/broad dielectric behavior


Dark field TEM images showing nano-scale ordered region in disordered matrix

PbSc1/2Ta1/2O3

Harmer and BhallaPbMg1/3Nb2/3O3

Randall et al.


Features for Ordered and Disordered Ferroelectrics

Polarization Dielectric

Hysteresis Birefringence


Features for Ordered and Disordered Ferroelectrics

Structural Transition

Ferroelectric properties decay with increasing T


Relaxor FerroelectricsRelaxor Ferroelectrics

PMN Pb(Mg1/3Nb2/3)O3

Strong frequency-dependent dielectric properties

(Tmax shifts to higher temperature with increasing frequency)

(Dielectric losses are at the highest just below Tmax)

Dynamical thermal re-orientation of polar regions with frequency(As frequency increases, the polar regions cannot keep up r and loss )

Dielectric relaxation similar to glass (follows a Vogel-Fulcher model)

However, no certain explanation for relaxor ferroelectrics

Freezing of micro-region and chemical fluctuation

Ordered-disordered region Spin-glass model


One of the difficulties in processing PMN ceramics

Pyrochlore (General formula RNb2O6 where R is a mixture of divalent ions)

Pb1.83Nb1.71Mg0.29O6.39 formed at 700-850 C

Paraelectric with room temperature r of 130

Strong reduction in r if present as inter-granular region in high r PMN region

(Not very significant if only discrete particles disperse in PMN matrix)

Pure Phase PMN with “Columbite Precursor Method” (MgO + Nb2O5 MgNb2O6 MgNb2O6 + PbO PMN)

Example of Pyrochlore Phase

Lead Magnesium Niobate-Lead Titanate (PMN-PT) System

Most widely studied relaxor materials PMN-PT Solid Solutions

High-strain (0.1%) electrostrictive actuatorsHigh dielectric constant (r > 25,000) capacitors


0.65 PMN - 0.35 PT MPB Compositions with normal ferroelectric properties

High dielectric constant capacitors 0.90 PMN - 0.1 PT Relaxor(with Tmax near room temperature with large dielectric constant)

(large “electrostrictive” strain)


Dielectric Behavior of 0.9PMN-0.1PT Relaxor Ferroelectrics


Strain-Field Relation of 0.9PMN-0.1PT Relaxor Ferroelectrics


Electrostriction in Ferroelectric Materials

Basis of electromechanical coupling in all insulatorsx = ME2 and x = QP2

(As compared to x ~ E for piezoelectric effects)

Large in ferroelectrics just above Tc due to electrical unstabability of ferroelectrics (PMN, PZN, and PLZT)

(because of their diffused transition and possible field-activated coalescence of micropolar region to macrodomain of the parent ferroelectric )

“Electrostrictive Mode”

“Field-Biased Piezoelectric Mode”

DC Bias Field Induced Ferroelectric Polarization Normal Piezoelectricd33 = 2Q11P333

d31 = 2Q12P333


Advantages of Electrostriction

Minimal or negligible strain-field dependence hysteresis(in selected temperature range)

More stale realizable deformation than observed in piezo-ceramicsNo poling is required

Longitudinal strain0.1% in PMN

0.3% in PLZT (La/Zr/Ti = 9/65/35)

Disadvantages of Electrostriction

Limited usable temperature range (due to a strong temperature dependence) Small deformation at low electric field

(as a result of a quadratic nature of electrostriction)

PMN-PT and PZN-PT Single Crystals

1-x PMN – x PT Single Crystals x = 35 for MPB compositions

Large piezoelectric strain > 1% High electromechanical coupling

factor (k33 > 90%) Relaxor-based piezoelectric

crystals for next generation transducers

1-x PZN – x PT Single Crystals x = 9 for MPB compositions

Large piezoelectric strain ~ 1.7% High electromechanical coupling

factor (k33 = 92%) Relaxor-based piezoelectric crystals for high performance

atuators


Comparison of field-induced strain for various ceramics and single crystals


Engineered Domain States Initially the domains are aligned as close as possible to the field

direction Increased polarization in rhombohedral structure

As the field is increased to certain values, the domains collapse to the <001> direction, as a result of rhombohedral-to-tetragonal phase

transition Large increase in polarization, hence piezoelectric properties

lead magnesium niobate (pmn) system

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