1

CHAPTER 1

OVERVIEW OF RELAXOR FERROELECTRIC

MATERIALS

1.1 INTRODUCTION

ICE (Information Communication and Entertainment) era provides

constantly developing environment which is steered by technological

innovations. To face the challenges and for the development of civilization it

demands new materials. Piezoelectric materials are the promising tool for

many modern technologies, in particular data storage, photonics, spintronics,

energy conversion, actuators and transducers. An active research exists for the

development of new actuator materials and optimization of properties for

numerous applications. Relaxor ferroelectric materials are the key

components in signal processing devices, transducers and ultrasonic motors.

1.2 PIEZOELECTRICITY

Piezoelectricity is a property possessed by a selected group of materials. It was discovered in 1880 by Jacques and Pierre Curie during their systematic study about the generation of electric charges by the effect of pressure on crystals, such as quartz, zinc blende and tourmaline (Haertling 1999). The name “piezo” is derived from the Greek word piezen, meaning “to press”; hence, piezoelectricity stands for the generation of electricity as a result of mechanical pressure. This can also be defined as electric polarization produced by mechanical strain in crystals belonging to certain classes, the polarization being proportional to the strain and changing sign with it (Cady

2

1946).The piezoelectric phenomena occurs as both direct and converse manner. Direct effect is the polarization of material by the mechanical force and the converse effect is the macroscopic strain by the applied voltage (Jaffe et al 1971). Both these phenomena can be explained by the physical equations (1.1) and (1.2),

= + (1.1)

= + (1.2)

where D is the dielectric displacement, T the stress, E the electric field, S the strain, d the piezoelectric coefficient, s the material compliance and the permittivity. The superscripts indicate a quantity to be held constant.

The origin of piezoelectric effect is related to an asymmetry in the unit cell. According to the lattice structure described by the Bravais unit cell, thousands of crystals in nature can be grouped together into 230 microscopic symmetry types or space groups based on the symmetry elements (Newnham, 1975). Non-centrosymmetric compounds are of particular interest because of their symmetry-dependent properties such as piezoelectricity, ferroelectricity, and second-order nonlinear optical (NLO) behavior. All non-centrosymmetric point groups, except point group 432, exhibit piezoelectric effect. However, only 10 polar crystals in which the direction of the electric dipole is reversible by means of an electric field can be called as ferroelectrics (Xu 1991; Shiv Halasyamani & Poeppelmeier 1998). The interrelation between the non-centrosymmetry point groups and the functional application is shown in Figure 1.1.

1.3 PEROVSKITE STRUCTURE

Among all the structures, perovskites and tungsten bronze are found to have best piezoelectric and ferroelectric characteristics. The perovskite structured relaxor materials are being widely studied. The main

3

advantage of this structure is that, many different cations can be substituted on both A and B sites without drastically changing the overall structure and also complete solid solution can be achieved between many cations over a range of composition (Cross 1987). Anisotropy in piezoelectric properties is large in perovskites compared to all other structures. The perovskite materials can readily undergo phase transitions (Schumacher et al 1995) and their structure is that of mineral perovskite (CaTiO3), which is orthorhombic. The ideal perovskite is centrosymmetric with general formula ABO3 where ‘A’ site cation valence varying from +1 to +3 and ‘B’ site is occupied by the cations of valence +3, +4 or +5. The schematic representation of ABO3 type perovskite is shown in Figure 1.2 where the B-site ion is in the body center position, A-site cations are in the cubic corner position and the oxygen atoms are at the face centered position and form an octahedron around the B-site (Lee et al 2002).

Figure 1.1 Venn diagram representing to the property and point group

in non-centrosymmetric crystals

4

To get a stabilized structure there is a size constraints like charge

neutrality constraints. The size constraint is described by the tolerance factor

‘t’ (Galasso 1990). For the perovskite structure,

=( )

(1.3)

where rA and rB are the radii of the A-site cation (in 12 coordination) and B-

site cation (in 6 coordination) respectively and rO is the oxygen ion radius. For

the ideal perovskite system ‘t’ should be in the range of 0.95 to 1.04 for cubic

symmetry and larger for the distorted perovskite system (Shannon 1976).

Figure 1. 2 Ideal perovskite structure

5

1.4

FER

RO

EL

EC

TR

ICS

Ferr

oele

ctric

ity w

as d

isco

vere

d by

Val

asek

in

1921

in

Roc

helle

sing

le c

ryst

als (

NaK

C4H

4O6.4

H2O

) (Ja

ffe

et a

l 197

1). F

rom

then

on

inte

rest

in

fabr

icat

ion

of n

ew f

erro

elec

tric

mat

eria

l is

inc

reas

ing

rapi

dly.

A c

ompo

und

exhi

bitin

g pe

rman

ent

dipo

le

mom

ent

is

calle

d as

fe

rroe

lect

rics.

Ferr

oele

ctric

ity c

an b

e de

fined

as

the

mat

eria

l tha

t con

tain

s on

e or

mor

e po

lar

axes

alo

ng w

hich

spo

ntan

eous

pol

ariz

atio

n ca

n be

dev

elop

ed b

elow

the

Cur

ie

tem

pera

ture

(T c

). Si

mila

r to

pyro

elec

tric

mat

eria

l fer

roel

ectri

c m

ater

ials

hav

e

spon

tane

ous

pola

rizat

ion

and

the

dire

ctio

n ca

n be

rev

erse

d by

the

app

lied

exte

rnal

ele

ctric

fie

ld.

The

arra

ngem

ent

of c

atio

ns a

nd a

nion

s w

ithin

the

ferr

oele

ctric

giv

es r

ise

to d

ipol

e m

omen

ts w

ith i

n ea

ch u

nit

cell,

and

the

resu

lting

pol

ariz

atio

n ca

n be

m

easu

red

via

mat

eria

l su

rfac

e cu

rren

t. A

dist

inct

ive

feat

ure

of

ferr

oele

ctric

m

ater

ial

is

hyst

eres

is

beha

vior

in

pola

rizat

ion

vs. e

lect

ric f

ield

. Spo

ntan

eous

pol

ariz

atio

n ex

ists

eve

n af

ter

the

rem

oval

of

elec

tric

field

and

is

calle

d as

rem

nant

pol

ariz

atio

n, P

r.

At

T ctra

nsfo

rmat

ion

from

fer

roel

ectri

c to

par

aele

ctric

is

happ

enin

g.

Para

elec

tric

phas

e m

ater

ials

w

ill b

ehav

e as

a

norm

al

diel

ectri

c w

ith

no h

yste

resi

s

(Kao

200

4).

Det

aile

d di

scus

sion

abo

ut t

he h

yste

resi

s w

ill b

e fo

und

in l

ater

sect

ion.

1.4.

1Ph

ase

Tra

nsiti

ons i

n Fe

rroe

lect

ric

Mat

eria

ls

Ther

e ar

e tw

o ty

pes

of fe

rroe

lect

ric p

hase

tran

sitio

n, o

rder

-dis

orde

r

and

disp

laci

ve

(Lin

es

&

Gla

ss

1977

). In

th

e or

der-

diso

rder

ty

pe

of

ferr

oele

ctric

s, th

ere

is a

dip

ole

mom

ent i

n ea

ch u

nit c

ell.

At h

igh

tem

pera

ture

thes

e di

pole

s are

in ra

ndom

dire

ctio

ns a

nd lo

wer

ing

the

tem

pera

ture

ther

e w

ill

be p

hase

tra

nsiti

on w

here

the

dip

oles

will

ord

erly

arr

ange

d an

d w

ithin

a

dom

ain

all t

he d

ipol

es a

re p

oint

ing

tow

ards

the

sam

e di

rect

ion.

Thi

s ty

pe o

f

trans

ition

ca

n be

ob

serv

ed

in

hydr

ogen

bo

nded

fe

rroe

lect

ric

mat

eria

ls

(Kat

rusi

ak e

t al

200

2; W

ang

et a

l 20

09).

The

disp

laci

ve t

rans

ition

can

be

6

understood in terms of polarization catastrophe, in which, if an ion is

displaced from equilibrium position, the force from the local electric fields

due to the ions in the crystal increases faster than the elastic-restoring forces.

This leads to an asymmetrical shift in the equilibrium ion positions and hence

to a permanent dipole moment. Ionic crystals such as barium titanate (BT) are

displacive ferroelectrics (Ishihara 2010).

Ferroelectric materials will undergo second order or first order

transition. The second order phase transition is characterized by gradual

reduction of spontaneous polarization (Ps), with increase in temperature and

becoming zero at Tc and above. Transition in tri glycine sulphate is second

order type (Jona & Shirane 1962). Whereas in the first order transition there

will be a discontinuous reduction of Ps to zero at Tc. Barium titanate

undergoes first order transition (Merz 1954). The first and second order

transitions are explained schematically in Figure 1.3.

Figure 1.3 Polarization as a function of temperature in (a) first and (b)

second order phase transition

1.4.2 Polarization

Spontaneous polarization is due to the ordering of dipoles under the

influence of internal process in a dielectric material without the effect of

7

external factors. Polarization occurs due to several atomic mechanisms. The

total polarization (Ptotal) can be written as,

Ptotal = Pe + Pi + Po + Psc (1.4)

where Pe, Pi, Po and Psc corresponds to electronic, ionic, orientational and

space charge polarization respectively (Bottger 1962; Barsoum 1997).

Electronic polarization (Pe): The electric field causes deformation or

translation of the originally symmetrical distribution of the electron clouds of

atoms or molecules. This is essentially the displacement of the outer electron

clouds with respect to the inner positive atomic cores.

Atomic or ionic polarization (Pi): In ionic lattice, the positive ions are

displaced in the direction of an applied field while the negative ions are

displaced in the opposite direction, giving a resultant (apparent) dipole

moment to the whole body.

Orientational polarization (Po): Polarization arising from the orientation of

molecules which have permanent dipole moments and these dipole moments

are due to the asymmetric charge distribution. It is also known as dipole

polarization.

Space charge polarization (Psc): At higher fields, carrier injection becomes

important. For materials consisting of a high concentration of charge

carriers, polarization due to the migration of charge carriers to form

space charges at interfaces or grain boundaries becomes important. This type

of polarization is called space charge polarization.

The polarization is not constant rather it will vary with respect to

the measuring frequency (Bottger 1962). The variation of polarization for the

frequency is given in Figure 1.4.

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Figure 1.4 Frequency dependence of polarization (Ralls et al 1976)

1.4.3 Ferroelectric Hysteresis

The polarization of piezoelectric materials varies in a closed curve,

called the hysteresis loop. This behavior is distinguished the ferroelectrics

from other normal dielectric materials. The observation of hysteresis loop is

still frequently used for the identification of ferroelectrics. The rectangularity

of the hysteresis loop is the main requirement for memory cells. The linear

relationship between the electric field and the polarization is given by,

(1.5)

where and are the vacuum dielectric permittivity (8.854 × 10-12 F/m)

and susceptibility of the material respectively. Typical ferroelectric hysteresis

loop is shown in Figure 1.5.

9

Figure 1.5 Ferroelectric hysteresis loop

As the electric field (E) strength increases, number of domains with

different polarizations direction will switch towards the field direction,

producing a rapid increase in polarization (AB). When all the domains are

aligned in the field direction saturation is reached (BC). At this saturation

state, appropriately oriented crystals will be composed of a single domain.

The extrapolation of linear segment on the polarization axis represents the

saturation polarization, Ps (CBE). As the field strength decreases, the

polarization will decrease but does not go back to zero (BD). When the field

is reduced to zero, some of the domains will remain aligned and the material

will exhibit remnant polarization (Pr). Field required to remove the Pr or to

reduce the polarization back to zero is called coercive field (Ec). Further

increase of field in the negative direction will cause dipole alignment in this

direction and the cycle can be completed by reversing the field direction (Jona

& Shirane 1962; Xu 1991).

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1.4.4 Ferroelectric Domains

Ferroelectric materials are composed of domains which is a small

region with uniform polarization. Usually in ferroelectric material there are

many domains and the direction is different for neighboring domains. In a

single domain all the dipoles are aligned in the same direction and this

direction can be reversed by the external electric field. The net polarization

along one particular direction will depend on the ratio of oppositely aligned

domains along that direction. If they are in equal volume then the net

polarization is zero. The domains are separated by the domain walls. The

change in dipole moment can be brought by change in the temperature and by

external electric field. This results in domain wall movement, the nucleation

and growth of new domains (Lines & Glass 1977; Landauer 1957). The

ferroelectric domains were first demonstrated in the study of spontaneous

birefringence (Matthias & von Hippel 1948). Ferroelectric domain structure

can be observed by second harmonic generation (Miller 1964), etching, SEM

(Jona & Shirane 1962), liquid crystal method (Furuhata & Toriyama 1973)

and TEM (Wallace 1970). The usefulness of each technique varies from one

material to another with the shape, size and transparency of the material. The

structure of the domains depends on the structure of the crystal. In a single

crystal there is variety of domain patterns and the number of types of domain

walls depends on the number of orientations of the dipole moment when the

spontaneous polarization occurs. Domain structure is strongly dependent on

the symmetry of the ferroelectric phase.

1.4.5 Poling

Single crystals or poly crystalline ferroelectric materials are having

multiple domains. A single domain can be obtained by domain wall motion

which is possible by the application of an appropriate electric field. A very

strong field that could reverse the polarization in the domain is called as

11

poling or polarization switching (Gray 1947). Simply, the process of

applying electric field to a ferroelectric material in order to orient the dipoles

in same direction is called poling. As already discussed when the electric field

increases, the polarization is also increasing, as the alignment of dipole in the

same direction is boosted. When all the dipoles are aligned in the same

direction maximum polarization value is attained and the material is said to be

saturated then the electric field is reduced to zero. Though the applied electric

field is decreased to zero the dipoles are still aligned in the direction of

applied field with some relaxation due to the remnant polarization. The dipole

behavior during poling is schematically explained in Figure 1.6. The random

orientation of dipoles is represented by the Figure 1.6 (a). At the maximum

field all the domains are aligned in the field direction and the material is said

to be saturated (Figure 1.6 (b)). Even after the removal of external field the

dipoles are aligned in the field direction but with some relaxed orientation

depending on the material property (Figure 1.6 (c)).

Figure 1. 6 Orientation of dipoles in the ferroelectric materials (a)

absence of electric field (b) under electric field and (c) after

removal of electric field

1.5 RELAXOR FERROELECTRICS

Relaxation means a system’s monotonous approach to the

equilibrium state after some excitation. In the case of dielectric relaxation one

12

should consider the response of polarization to an external (usually small)

electric field (Alexei et al 2012). Relaxor ferroelectric or relaxors are a class

of disorder ferroelectrics possessing peculiar structure and properties. At high

temperature they exist in a non-polar paraelectric phase which is similar to the

paraelectric phase of ferroelectric in all the aspects. Upon cooling they

transform into ergodic relaxor state in which polar regions of nanometer size

with randomly distributed directions of dipole moments appear. The

temperature corresponds to the transformation is called Burn’s temperature

(TB). At temperature close to TB polar nano regions (PNR) are mobile and

their behavior is ergodic. On cooling, their dynamics slows down enormously

and at low temperature, Tf (freezing temperature) the PNR become frozen.

Freezing of the dipoles dynamics is associated with a large and wide peak in

the temperature dependence of the dielectric constant ( ) with characteristic

dispersion observed at all frequencies. This peak is of the same order of

magnitude as the peaks at Tc in normal ferroelectric perovskites, but in

contrast to normal ferroelectric it is highly diffusive and its temperature Tm(>

Tf) shifts with frequency due to dielectric dispersion. Because of the

diffuseness in the dielectric anomaly relaxors are often called “ferroelectrics

with diffuse phase transition” though no structural transition really occurs

(Bokov & Ye 2006).

Compositional disorder is the common feature of relaxors i.e. the

disorder in the arrangement of different ions on the crystallographic sites.

The relaxor behavior due to the disorder of non-isovalent ions was first

observed in the perovskite compounds e.g. Pb(Mg1/2Nb2/3)O3 (PMN- lead

magnesium niobate) (Smolenskii et al 1961) and Pb(Sc1/2Ta1/2)O3 (PST- lead

scandium tantalate) (Chu et al 1993). In which Mg2+, Sc3+, Ta5+ and Nb5+ ions

are fully or partially disordered in B-sub lattice of ABO3 perovskite.

13

Relaxor ferroelectric (RFE) includes large group of solid solutions

mostly oxides, with a perovskite or tungsten bronze structure (Kleemann

2006). Relaxors are differentiated from the normal ferroelectrics by the

following properties (Samara 2003),

The P–E hysteresis loop is the signature of a FE in the low

temperature FE phase. The large remnant polarization, Pr, is a

manifestation of the cooperative nature of the FE

phenomenon. A relaxor, on the other hand, exhibits a so-

called slim loop. For sufficiently high electric fields the nano

domains of the relaxor can be oriented with the field leading

to large polarization; however, on removing the field most of

these domains re-acquire their random orientations resulting in

a slim hysteresis loop. Figure 1.7 represents the hysteresis

behavior of RFE and FE materials.

The saturation and remnant polarizations of a FE decreases

with increasing temperature and vanishes at the Curie

temperature (Tc).The vanishing of polarization at Tc is

continuous for a second-order phase transition and

discontinuous for a first-order transition which implies the

absence of polar domains above Tc. By contrast, the field-

induced polarization of a relaxor decreases smoothly through

the dynamic transition temperature Tm and retains finite values

to rather high temperatures and it is shown in Figure 1.8.

The static dielectric susceptibility or dielectric constant of a

FE exhibits a sharp, narrow peak at Tc. The FE response is

frequency independent in the audio frequency range. By

contrast a relaxor exhibits a very broad dielectric peak and

14

strong frequency dispersion in the peak temperature (Tm) and

in the magnitude of below Tm. The broad peak is also referred

to as a ‘diffuse phase transition’ and is associated with

compositional fluctuations leading to many micro FE regions

with different compositions and Tc’s. The sharp transition in

barium titanate (BT) and diffuse transition in PMN single

crystals are shown in Figure 1.9 (Kaatze 2010).

Figure 1.7 Hysteresis behavior in (a) ferroelectric and (b) relaxor

materials (Samara 2003)

The temperature dependence of FE obeys a Curie–Weiss law,

above Tc. By contrast the relaxor exhibits strong deviation

from this law for temperatures above Tm and obeys the

modified Curie-Weiss law. Only at very high temperatures the

linear 1/ versus T response is obtained.

15

Figure 1.8 Polarization vs. temperature in (a) ferroelectric and (b)

relaxor materials (Samara 2003)

The FE transition can be thermodynamically first or second

order and involves a macroscopic symmetry change at Tc.

Transparent FE exhibits a strong optical anisotropy across Tc.

By contrast, there is no structural phase transition across Tm in

a relaxor. This was evidenced by X-ray and neutron

diffraction studies (de Mathan et al 1991). The peak is simply

a manifestation of the slowing down of the dipolar motion

below Tm. For transparent relaxors, there is no optical

anisotropy across Tm. In addition, the relaxor ferroelectric

materials are having ferroelectric to antiferroelectric transition

at Td, called depolarization temperature.

The above discussion makes it very clear that the properties and

physics of relaxors are very different from those of normal FEs.

16

Figure 1.9 (a) Sharp transition in BT and (b) diffuse transition in PMN

single crystals (Kaatze 2010).

By considering the above points it is concluded to reflect the

occurrence of relaxor behavior in perovskites, there appears to be two

essential ingredients (Lu 2004; Gupta & Viehland 1996),

Existence of lattice disorder

Existence of PNR at temperature much higher than Tm

Substitution of ions in A and B site of the ABO3 perovskite with

different polarizabilities, valence state and size will sufficiently produce

dipolar defects. It can leads to high degree of disorder to break the

translational symmetry thereby prevent the formation of long range order.

This supports the formation of PNR (Dai et al 1996). The difference in ionic

radius, electronegativity and valence state in the A and B site can induce

enough charge fluctuation, vacancies and local ordering to introduce the

relaxor property (Chu et al 1993). The existence of nano polar regions has

also been evidenced by TEM (Randall & Bhalla 1989), diffuse X-ray

scattering and neutron diffraction studies (de Mathan et al 1991).

17

Very high response coefficient and an enhanced width of the high

response regime around the ordering temperature Tm (Curie range) make

relaxor as popular systems for application in piezoelectric/ electrostrictive

actuators & sensors (e.g. scanning probe microscopy, ink jet printer, adaptive

optics, micromotors, vibration sensors/attenuators, Hubble telescope

correction etc.) electro or electro optic and photo refractive elements

(segmental displays, modulators, image storage, holographic data storage,

etc.) (Kleemann 2006)

1.5.1 Relaxor Theories

Ferroelectrics provide a convenient system to study the general

properties of phase transitions. Normal ferroelectrics undergo a sharp phase

transition at the Curie temperature. Above TC, each atom within the unit cell

is located on a high symmetry position with no net dipole moment. Atoms are

free to vibrate about these equilibrium positions, exciting the various acoustic

and optical phonon modes.

The origin for diffuse phase transition in RFE is not yet clear. To

explain the relaxor properties various physical models such as

superparaelectric model, order-disorder model, microdomain and

macrodomain switching model, dipolar glass model and quenched random

field model have been proposed by many researchers.

In PMN, Smolenskii et al (1959) believed that the relaxor property

is due to the compositional fluctuation which raises local field variation. But

this model failed to explain the relaxor nature where there is no compositional

fluctuation. Burns & Dacol (1983) proposed the dielectric behavior of RFE

is mainly dependant on concentration of dipole moment of the PNR. They

observed the deviation from the linear response of refractive index with

temperature which is attributed to the nucleation of dipolar nano regions at

18

TB. Also they found the TB is very close to the temperature where the classical

Curie-Weiss law starts to deviate.

Dipolar glass-like behavior is another possible model. Viehland et

al (1990) showed evidence for glassy behavior by analyzing the dielectric

response using the Vogel-Fulcher relationship which also implies activated

dynamics. The glassy nature was believed to be due to the randomly oriented

dipolar fields, and the evidence was seen in the very slow dynamics. Bell

(1993), explained the dielectric behavior is based on ideal superparaelectrics

by considering an ensemble of independent, identical, nano-sized

superparaelectric clusters, with a distribution on the size of the clusters.

Taking the temperature dependent of cluster sizes and interactions into

considerations, the calculations was carried out by employing Landau-

Ginsberg-Devonshire formalism to determine the dielectric function of

clusters.

Lu & Calvarin (1995) assumed an exponential distribution of the

size of polar regions. The model predicts that the dielectric absorption always

increases with increasing frequency, which contradicts the experimental

results in the low-temperature range. Cross proposed the superparaelectric

model for relaxors (Cross 1987). Similar to the superparamagnetic state, it

involves polar micro regions that are dynamically disordered above Tm, the

mean Curie temperature of the different regions. Cations continuously flip

between equivalent directions, activated by thermal energy. The heterogeneity

caused by the mixed B-site creates locally favorable directions, so the local

symmetry is lower than the global. However, the energy barrier separating the

different directions are small, so macroscopic polar domains never form as in

normal ferroelectrics.

Power law model was proposed by Cheng et al (1996). To explain

the behavior of polar clusters, they have proposed a relation between

19

frequency and dielectric constant much lower and higher than Tm by two

exponential functions. The analysis at high temperature gives the information

about production rate and concentration of polar cluster whereas the low

temperature range gives the idea about the freezing temperature. Kleeman

(2006) reported that the substitutional charge disorder giving rise to quenched

electric domain-fields is probably the origin of the peculiar behavior of

relaxor ferroelectrics and proposed that the PMN broad phase transition is due

to the random field interactions which also cause freezing into nanometric

domains.

Samara & Venturini (2006) reported the influence of hydrostatic

pressure on the dielectric properties of compositionally disordered ABO3

perovskites. It has been the discovery of a pressure induced FE to RFE

crossover making the RFE state, the ground state, of these materials at

reduced volume. They have observed that the pressure favors the RFE state

and biasing fields favor the FE state. The combined results provide new

insights into the physics and can be explained in terms of changes in the

correlation length for dipolar interaction among the PNR that exist in these

disordered materials.

1.6 LEAD BASED AND LEAD FREE RELAXOR

FERROELECTRIC MATERIALS

Relaxor behavior has been extensively studied in lead based

complex perovskite systems. All commercially used perovskite piezoelectric

materials are lead based compounds. Smolenskii et al (1959) discovered

relaxor properties in the complex perovskite with general formula A(B B )O3.

The compounds are named as lead complex if A-site is occupied by Pb ions.

The synthesis of B-site modified Pb complexes was first attempted by

Galasso & Pyle (1963) and Galasso & Pinto (1965). Since then lead zirconate

20

titanate (PZT), lead lanthanum zirconate titanate (PLZT), and lead

magnesium niobate (PMN) have been developed and utilized for a variety of

applications. PZT and PMN dominated the world of piezoelectrics because of

its strong piezoelectric effect especially at the composition near the

morphotropic phase boundary (MPB). Beside PZT, some newer generation of

lead based piezoelectrics that exhibit piezoelectric properties were also

created by combining PbTiO3 (PT) with some lead based ferroelectrics such

as Pb(Zn1/3Nb2/3)O3 (PZN) and Pb(Mg1/3Nb2/3)O3 (PMN) to form solid

solutions. The piezoelectric coefficient, d33, of the <001>-cut PZN-PT single

crystal with the near-MPB composition (10% PT) is as high as 2500 pC/N

(Kuwata et al 1982) and for PMN-PT crystals with MPB compositions

(35% PT) is 1500 pC/N (Xu et al 2003). Moreover, these materials also

show very high dielectric constants at room temperature owing to the

broadening of the permittivity peak around the Curie temperature.

Though they exhibit interesting properties, the damaging effects of

lead on neuro and kidney toxicity have long been recognized. Lead exposure

has been linked to the Alzheimer’s diseases and the processing waste from the

consumer products poses a great threat to the developing nervous system in

young children. Exposure to lead is known to cause decreased intelligence,

reading disabilities and motor skills. The toxicity of the lead oxide and its

high vapor pressure during processing has resulted in an increasing demand

for the alternative materials with reduced toxicity (Juberg et al 1997).

The research is motivated towards the lead free relaxor ferroelectric

materials around the globe. Among all the lead free piezoelectric material,

more attention has been paid to bismuth layered and the perovskite structure.

The structure consists of perovskite layers and (Bi2O2)2+ layers. The

perovskite layers are sandwiched by (Bi2O2)2+ layers. It usually have high

Curie temperatures (600 ~ 900 ), much higher than those of the lead-based

21

materials (200 ~ 400 ), making them as a good candidate for applications

at high temperatures. But the anisotropic nature of their structures, the

switching of the spontaneous polarization within the materials during

poling is limited within a two dimensional plane which results poor

piezoelectric properties (d33< 20 pC/N) (Miyayama & Yi 2000; Takenaka & Sakata 1980).

The perovskite materials possess good piezoelectric properties

compared to bismuth layered structure. The bismuth based perovskites are

said to be the best lead free sources. The “lone pair” of electrons in Bi-

based oxides is believed to form due to the hybridization of 6s and 6patomic orbitals with 6s2 electrons filling one of the resulting orbitals in

Bi oxides. The lone pair is then considered to be chemically inactive, not

taking part in the formation of bonds but strictly active. The hybridization

causes the lone pair to lose its spherical symmetry and is projected out

on one side of the cation, resulting in an asymmetry of the metal

coordination and distorted crystal structures. Thus the stereo chemically

active lone pair electrons results into the displacements of the Bi atoms

from the centrosymmetric to the noncentrosymmetric structure and leads

to polarization, consequently ferroelectricity. Bi based compounds are having

more ion off centering than Pb due to its 6s2 lone pair electrons in Bi3+

(Ravindran et al 2006).

Interestingly, some of the lead free material offer comparable

piezoelectric properties to that of PZT. Many reports are available on bismuth

based lead free systems BaTiO3(BT) (Bechmann 1956), K1/2Bi1/2TiO3 (KBT)

(Popper et al 1957), Na1/2Bi1/2TiO3 (Smolenskii et al 1961), Na1/2Bi1/2TiO3-

BaTiO3 (NBT-BT) (Takenaka et al 1991), K1/2Bi1/2TiO3-BaTiO3 (KBT-BT)

(Elkechai et al 1996), K1/2Bi1/2TiO3-Na1/2Bi1/2TiO3(KBT-NBT) (Makiuchi et

al 2005). Alkaline niobates and its solid solutions are also interesting lead free

materials. KNbO3 (KN) (Matthias & Remeika 1951), NaNbO3 (NN) (Wood

1951) and KNaNbO3 (KNN) (Du et al 2006) are studied by many groups.

22

Among these lead free systems NBT-BT is considered as a suitable

and promising candidate. NBT-BT single crystal were grown by many

techniques like top seeded solution growth (TSSG), Bridgman, flux and metal

strip heated zone method (MSHZM). NBT-BT ceramics were synthesized by

solid state reaction method for the easy preparation. Various additives (La,

Nb, Eu, Co, Ce, K and Mn) are also being used to further improve the

performance of NBT-BT system in ceramic as well as in single crystal form.

The dielectric and piezoelectric properties of NBT-BT single crystals grown

by different methods and NBT-BT ceramics are given in Table 1.1 and Table

1.2 respectively. Figure 1.10 shows the variation of dielectric and piezoelectric properties in lead based and lead free compounds.

Table 1.1 Piezoelectric properties of NBT-BT single crystals grown by

different methods

Growth Method

Material DopantPiezoelectric Properties

References d33

(pC/N)k33 tan

Bridgman0.94NBT-0.06BT

160 2500 0.09Guisheng Xuet al 2005

TSSG 0.94NBT-0.06BT

140 650Wenwei Ge et al 2008

TSSG 0.94NBT-0.06BT

253 1230 0.02Qinhui Zhang et al 2010

TSSG 0.94NBT-0.06BT Mn

80 750 0.30 0.040 Hong Liu et al 2008145 850 0.55 0.025

TSSG 0.94NBT-0.06BT Mn

400 1040 0.24 0.019 Qinhui Zhang et al 2011 483 1090 0.52 0.019

MSHZM 0.94NBT-0.06BT

4900Bubesh Babu et al 2008

Flux0.94NBT-0.06BT Ce

4500 0.068 Bubesh Babu et al 2007 5000 0.030

23

Table 1.2 Piezoelectric properties of NBT-BT ceramics

Material Dopant

Piezoelectric Properties

References d33

(pC/N)k33 tan

NBT-BT 125 580 Takenaka et al 1991

0.94NBT-0.06BT

122 601 0.40 Bao-Jin Chu et al 2002

0.94NBT-0.06BT

155 826 36.7 0.025Chenggang Xu et al 2008

0.94NBT-0.06BT

127 1660 0.048

Wei Zhao et al 2007 CeO2 137 1776 0.028

Nb2O5 90 1063 0.045

Co2O3 149 1150 0.044

0.94NBT-0.06BT

117 776 0.43 0.025

Hui-dong Li et al 2004 La2O3 125 1576 0.38 0.046

Nb2O5 118 1614 0.38 0.046

Co2O5 139 1200 0.46 0.023

Figure 1.10 Comparison (a) dielectric (b) piezoelectric properties of lead

based and lead free compounds (Shujun Zhang et al 2007)

24

1.7 CRYSTAL GROWTH TECHNOLOGY

The elegance and beauty of the crystals are always the source of

delight. Crystals are the unacknowledged pillars of modern technology. A

crystal is defined as a solid with a high degree of long range three

dimensional internal order of the component atoms, molecules or ions. The

process of forming a crystalline structure is called as crystallization. The

growth of crystal is a fascinating experimental exercise and it has been the

subject of considerable interest leading to the large number of scientific

investigations in the field of materials science and engineering, electronics

industry, photonic industry, fiber optic communications and piezoelectric

transducers. This has accelerated the progress in the science of crystal growth

and related topics. The process of crystal growth is a controlled change of one

phase to another phase. This transition may occur from solid, liquid, or vapor.

Depending on the material characteristics like melting point, solubility and

physico-chemical properties appropriate growth method has to be decided.

Three basic steps involved in the process of crystal growth from the

disordered phase are (Govindhan Dhanaraj et al 2010),

Supersaturation Nucleation Crystal growth

The success of crystal growth formation depends on the choice of

solvent since the constituents of the material to be crystallized are dissolved in

a solvent and crystallization occurs as the solution becomes critically

supersaturated. The nucleation is an important phenomenon where the

initiation of phase transformation occurs and the propagation of phase

transition is called crystal growth.

Crystal growth process involves phase transition of the type solid-

solid, liquid-solid and gas–solid. In Table 1.3 the different techniques of the

crystal growth are discussed with respect to the phase transition. Crystals of a

25

particular material can be grown by one or more various techniques and the

choice of a particular technique for growing crystal depends on the material

property and the type of application. Crystallization techniques can be

classified into three main categories depending on the phase transition

involved (Laudise 1970; Rosenberger 1979; Brice 1986).

Table 1.3 Different phase transition in crystal growth

S.No Phase Transition Nature Parameter Yield1 Solid-Solid

(Solid growth) Solid Temperature Solid

Devitrification Strain annealing Polymorphic phase change Precipitation from solid solution

2 Liquid-Solid a) Melt growth Molten

material Decreasing temperature

CrystalBridgman-Stockbarger KyropoulosCzochralski Zone refiningVerneuil

b) Flux growth Solids+ Flux

Decreasing temperature

Crystal(s)

c)Solution growth Solid + Solvents

Low temperature

Crystals(s)Evaporation Slow cooling Boiling solutions

26

Table 1.3 (Continued)

d)Hydrothermal growth

Solid+ Solvent

High temperature andHigh Pressure

Crystal(s)Hydrothermal sintering Hydrothermal reactionsNormal temperature gradient Reversed temperature gradient

e) Gel growth Solution + gel medium

Solution

Low temperature

CrystalReactionComplex decomplex Chemical reduction Solubility reduction Counter-flowdiffusionCrystal(s)+ products

3 Gas-Solid (Vapor growth)

Vapor(s) Solid Sublimation- condensationSputtering Epitaxial processes

In the present work, the flux growth, commonly known as high

temperature solution growth, has been chosen to grow NBT-BT and Nd added

NBT-BT single crystals.

1.7.1 High Temperature Solution Growth Technique

The crystal growth can be divided into two categories (Elwell &

Scheel 1975).

27

Growth from single component- only one chemical

component is present in the growth system

Growth from multi component – another component is added

to the growth system

Primary reason for the addition of second compound is to reduce

the melting point of the growth system. The reduction in the crystallization

temperature is necessary for the material which is having incongruent melting

behavior and having decomposition at high temperature or at high vapor

pressure. The second component is called as ‘flux’- term commonly used to

refer oxides in the process of soldering or brazing which is used to reduce the

melting point. Various fluxes used to grow large size single crystals are listed

in the review of Roy & White (1968). The flux can be an ionic salt or oxides

or the combination of two. In this method materials to be crystallized are

dissolved in a suitable solvent and the crystallization occurs once the

supersaturation is achieved. Nucleation in flux growth is heterogeneous since

it tends to occur either at undissolved solute particles or on the surface of the

container. This requires lesser degree of supersaturation than that required for

homogenous nucleation. The nucleation rate increases rapidly on cooling

below melting point.

The prime advantage of flux growth is that the crystal growth

occurs at a relatively low temperature than the required for the pure melt. It

offers smooth temperature gradient and ease of synthesizing new materials.

Crystals grown at relatively lower temperature are often free from defect and

have better quality when compared to the crystals grown from melt. The

major disadvantage is very slow stable growth rate, the chemical

contamination by substitution and inclusions which may occur as the crystal

are grown in the presence of flux. The size of the crystals which can be grown

is also limited compared with the melt growth.

28

1.7.1.1 Choice of flux

Both physical and chemical properties of the flux are important.

The desirable properties of the ideal solvents are,

High solubility for the crystal constituents

Appropriate change in solubility with temperature, viscosity

Low melting point

Lowest volatility at the highest operating temperature

Low toxicity

Low reactivity with the container

Easy separation of flux without any detrimental effect of

grown crystal

Depending on the crystal property and requirement of size the flux

has to be decided. Solvents having common anion or cation with the crystal

usually make up a good solvent. In the present work, self flux of bismuth

oxide (Bi2O3) has been chosen for the growth of NBT-BT and Nd added

NBT-BT single crystals.

1.7.1.2 Apparatus required for flux growth

The primary requirement for the flux growth is the high

temperature resistive or inductive heating system. As the growth depends on

the cooling environment, high precision temperature controller with good

stability even at high temperature and with slow cooling rate is necessary.

Crystal growth from high temperature solution growth carried out in

containers in the shape of either crucible or ampoules. For most of the oxides,

29

platinum has been used as a container material because of its high melting

point and high corrosion resistance. For the present investigations silicon

carbide (SiC) furnace which can go upto 1400 was used for the growth.

Eurotherm 2604 temperature controller with an accuracy of 0.01 was

employed to control the temperature and platinum crucible was used as a

container.

1.8 CERAMIC TECHNOLOGY

The word ceramic is derived from the Greek word “keramikos”

meaning pottery. Ceramic is an inorganic solid prepared by the action of heat

and subsequent cooling. Ceramics are polycrystalline. In other words, a

ceramic can be thought of as an agglomeration of small crystals (or grains)

fitted together in a random manner in terms of the crystalline orientation

within each individual grain as show in Figure 1.11. Ceramics generally

consist of randomly oriented, single-crystalline grains as basic micro

structural building blocks, which are separated by distinct grain boundaries.

At high temperatures in the late stage of sintering, larger crystalline grains

grow at the expense of the smaller ones via an atom-by-atom epitaxial growth

mechanism. This grain growth process strongly influences the properties of

ceramics. In polycrystalline materials such as ceramics, each ceramic grain

will have properties not unlike that of crystals. Because of grain boundaries

and the crystallographic axes of the grains are randomly oriented, the

macroscopic properties of the ceramic will in general differ significantly from

those of a single crystal.

30

Figure 1.11 Ideal (a) single crystal (b) polycrystalline material

The development of ceramic processing and thin film technology

lead to the emergence of new applications of ferroelectrics. Ferroelectricity in

the form of an electrically switchable spontaneous polarization has been first

observed in single crystal materials such as Rochelle salt and around 20 years

later in polycrystalline ceramics such as barium titanate (BaTiO3). Since then

ferroelectric ceramics have been applied in all areas of engineering in the

form of sensors, transducers, and actuators. Particular examples are high-

dielectric-constant capacitors, piezoelectric sonar and ultrasonic transducers,

medical diagnostic transducers, gas igniters, ultrasonic motors, thin-film

capacitors, or ferroelectric thin-film memories (Haertling, 1999). Ferroelectric

ceramics play an increasing role for actuator and sensor applications in smart

structures. Ferroelectric ceramics are characterized by specific domain

structures, which give rise to intrinsic and extrinsic contributions to the

effective material response.

There are many methods to synthesis ceramics. Some of them are

(David Segal 1997)

Solid state reaction method

Sol- gel process

31

Co-precipitation method

Molten salts

Hydrothermal technique

Liquid phase and gas phase reactions

Polymer pyrolysis

Pechini and citrate gel method

Aerosols and

Emulsions

The conventional synthesis for ceramics is solid state reaction

between oxides and / or carbonates of precursors. Repeated cycles of milling

and calcinations are carried out to achieve the solid state reaction between

precursors. Relatively high temperatures are required for solid state reactions

typically around 1200 because of limited diffusion during calcinations. The

properties of ceramics are greatly affected by the characteristics of the

powder, such as particle size, morphology, purity and chemical

composition. Various chemical methods, e.g. co-precipitation, sol–gel,

hydrothermal and colloid emulsion techniques are used to efficiently

control the morphology and chemical composition of the prepared powder.

The citrate gel process offers a number of advantages for the preparation

of fine powders of many complex oxides as quoted in literature. The

main drawback of this process is the possible formation of carbonate during

decomposition of the polymeric gel. Non-conventional methods are used to

get better homogeneous and reactive precursor powder compared to solid-

state method. Usually Ti-alkoxides and Ti-chlorides are used as the Ti-

metal source in chemical routes. Ti-alkoxides/nitrates are relatively costlier

than oxides and carbonates of Ti (Lu & Wen 1999; Shrivastava et al 2005;

Cross 1994).

32

Compared to all other methods, solid state route is less time

consuming and a cheaper method. Hence the solid state reaction was selected

for the synthesis of NBT-BT ceramics in the present investigations. The

reaction rate depends on

Area of contact between the reacting solids i.e. surface area

and density

The rate of nucleation

Rate of diffusion of ions

The main advantage of this method is that it allows direct reaction

of solids to form a final product. In addition to that, solid state reactions are

simple to perform as the precursors are readily available at low cost. Also the

reactions are “clean”, i.e. do not involve other chemical elements. The main

disadvantage is the need of high temperature, possibility of non-homogeneity

and contamination from the containers. In the present work NBT-BT ceramics

are synthesized by conventional solid state reaction method and their

characterizations has been studied.

1.9 SCOPE OF THE THESIS

The thesis deals with the above discussed properties in NBT-BT

relaxor ferroelectric single crystals and ceramics and is broadly classified into

seven chapters. The first chapter discusses about the overview of

piezoelectric, ferroelectric and relaxor ferroelectric properties. Also the

growth of single crystals from flux method and synthesis of ceramics are

discussed in detail.

In Chapter 2, the growth of NBT-BT single crystals at

morphotropic phase boundary (MPB) composition by flux method is

33

discussed. The grown crystals were subjected to structural analysis and

electrical characterizations.

Chapter 3 presents the influence of Nd on the NBT-BT single

crystals. The growth condition for the Nd-NBT-BT is optimized for the flux

technique. The inhomogenity issues in Nd added NBT-BT crystals are

addressed by structural and optical characterizations. The grown crystals are

studied for structural, optical and dielectric characterizations. The IR emission

is observed for the Nd added NBT-BT single crystals.

Synthesis of NBT-BT ceramics in its MPB is presented in

Chapter 4. The perovskite nature of the ceramics is confirmed by the XRD

studies. The dielectric, P-E loop and piezoelectric studies are performed for

the NBT-BT ceramics.

In Chapter 5, the effect of Swift Heavy Ion irradiation on the NBT-

BT ceramics are discussed in detail. Three different ions O7+ (100 MeV), Ni7+

(100 MeV) and Au9+ (120 MeV) are used to irradiate the NBT-BT ceramics.

The irradiated ceramics are studied for their structural, surface modification

using XRD and SEM. The dielectric, piezoelectric and ferroelectric studies

are carried out before and after irradiation.

Chapter 6 deals with the consequence of magnetic additives like

Mn, Ni, Co, Cr and Fe in NBT-BT ceramics. The synthesized ceramics are in

perovskite nature and the variation of morphology is studied by SEM. The

relaxor nature is analyzed using dielectric pattern. Vibrating sample

magnetometer (VSM) is used to study the magnetic property of the samples.

The Chapter 7 presents the summary of present investigations and

suggestion for future work.