CHAPTER - 1

INTRODUCTION

This chapter prov~des an introduction to the work presented in this thesis. It is divided

into two parts: Part A and Part B. In part A, a review on the work done on ferroelectric

ceramics, particularly strontium barium niohate (SBN) and barium sodium niobate

(BNN) ceramics and the effects of alkali metal and rare earth metal doping on these

materials are given. In this work we have concentrated our attention entirely on

ferroelectric ceramics with tetragonal tungsten bronze structure. In part B a brief

introduction to the effect of swift heavy ion irradiation on physical properties of materials

is reviewed. Various mechanisms relating to swift heavy ion (SHI) - material interaction

are given. It also gives a brief description of the technical aspects of 15 UD Pelletron

Accelerator at Nuclear Science centre, New Delhi which has been used in our work.

PART-A

FERROELECTRIC CERAMICS - A REVIEW

1. 1 Introduction

Ferroelectric oxide ceramics form a very broad range of functional ceramic materials and

form the materials base for a good majority of electronic component applications. These

electronic applications account for more than 60% of the total high technology ceramics

market world wide. It is the purpose of this study to examine the range of physical

properties, which make the ferroelectrics attractive for electronics application and the

technologies, which can be used to modify, control and optimize their properties. One of

the fascinating aspects of the field of fenoelectxic ceramics is its interdisciplinary nature.

Major applications of ferroelectric ceramics can be divided into five distinct areas,

which draw upon different combination of properties a s outlined below:

Dielectric applications make use of the very high dielectric permittivity G,,, low

dispersion and wide frequency range of response for compact capacitors in multilayers in

thick and thin film forms [I]. Non linear hysteric response is of interest also to thin film

non volatile semicor~ductor memory [2] and high permittivity films are of interest for

local capacitance in high count DRAMS [3].

Piezoelectric and electrostrictive response in poled and unpoled ferroelectric and

relaxor ferroelectric co~npositions are of importance in transducers [4] for converting

electrical to mechanical response [5] and vice versa [6]. Sensor applications make use of

the very high piezoelectric constant dijk of the converse effect, which also permit efficient

conversion of electrical to mechanical response [7] for very high precision position

control [S]. The possibility of phase and domain switching with shape memory is used in

polarization controlled actuation [9].

Pyroelectric systems rely upon the strong temperature sensitivity of electric

polarization dP, / dT [lo]. 'Ihe pyroelectric effect in ferroelectics for bolometric

detection of long wavelength infrared (IR) radiation [ll] has a number of industrial

applications [12]and there is now a strong focus on imaging systems which may be used

for night vision [13] and for thermal-medical diagnostics [14].

Positive temperature coefficient (PTC) semiconductors are a specialized area of

application of these materials in which the barrier to charge transport at the ceramic grain

boundary in specially processed barium titanate based ceramics is controlled by the

polarization state of the ferroelectic [I51 giving rise to an extremely strong positive

temperature coefficient of resistivity (PTCR effect), controlled by the Curie point of the

ferroelectric composition [16].

In electro-optic applications the properties of interest are the high quadratic [17]

and linear [IS] electro-optic coefficient (r,,,, g,,kl) which occur in ferroelectrics and the

manner in which these can be controlled in modulators, [I91 switches [20], guided wave

structures and photo-refraction devices [2 I].

Although fer~oelectric ceramics have been a subject of great interest for very

many years, much of recent developments in both its understanding and technology have

been stimulated by the increasing commercial importance of the subject. There has, since

years, been demand for newer and better single crystalline products.

The discovery of ferroelectricity in Seignette Salt in 1921 was an ignition event in

the field of materials science. Since that time the list of materials showing this property

has continued to grow rapidly. Ferroeleectric materials form an important group not only

because of the intrinsic ferroelectric property, but because many of them possess useful

piezoelectric, birefrigent and electro-optical properties, which can be applied in devices.

Ferroelectricity is the spontaneous alignment of electric dipoles by their mutual

interaction. This is a process parallel to the spontaneous alignment of magnetic dipoles

observed in ferromagnetism and derives its name from its similarity and features

analogous to that process. The source of ferroelectricity arises from the fact that the local

fields E' increases in proportion to the polarization. For a material containing electric

dipoles increased polarization increases the local field. Spontaneous polarization is to be

expected at some low temperahue at which the randomizing effect of thermal energy is

overcome, and all of the electric dipoles line up in parallel arrays.

An increasing number of materials are being found to exhibit spontaneous

polarization. Barium titanate is the one that has been most widely investigated. Lead

titanate, which has the same perovskite structure as barium titanate, is also ferroelectric.

Other ferroelectrics include Rochelle salt (potassium-sodum tartarate tetrahydrate),

Potassium dihydrogen phosphate (KH2POa), potassium dihydrogen arsenate (KH2AS04)

and a number of other perovskites such as NaTaO3, KTa03, LiTaO3, LiNbO3 etc.

1.2 Advanced ceramics

The term advanced, engineering or technical ceramics refers to ceramic materials, which

exhibit superior mechanical properties, corrosiodoxidation resistance, or electrical,

optical, and /or magnetic properties. While ceramics have been used for over 3000 years,

the materials discussed in this thesis have generally been developed only during the past

30 to 50 years. These materials have the potential to be used in a large number of

applications where resistances to temperature, stress, and environment are required.

Specific applications include electronic sensors and devices, optical elements, magnetic

devices, tribological components, and structural components. The monolithic material

discussed below is most frequently formed by powder processing techniques followed by

firing and consolidation procedures to achieve a dense body.

Advanced ceramics have been developed using a number of basic principles

relating several different levels of structure including atomic, electronic, grain boundary

and microstructure. The interaction of these structural levels result in materials, which

have properties suitable for specific applications. The successful development of these

materials and the~r successors requires an in-depth knowledge and use of

thermodynamics, k~netics, phase equillibria, and crystal structure. An example is the

variety of electronic ceramics in use today. Initial &electric materials were based on

relatively simple materials such as porcelains, glasses and steatites. With the discovery of

barium titanate (BaTiO3), it was found that much higher dielectric constants could be

achieved and controlled over a greater temperature range. Over the past 20-30 year,

additives such as strontium have been used which change the temperature response of it's

dielectric properties as well as Curie temperature. Since BaTiO, is also piezoelectric, it

has led to developments in transducer technology, which in turn has driven development

of related electrostrictive materials such as lead magnesium niobate (PMN) and lead

magnesium titanium niobate (PMTN).

Continued development of these ceramics is based on understanding the

interactions between processing, microstructure, properties and performance. Current

research areas which may prove engineers with the ability to tailor these materials for

specific applications are the measurement of interfacial phenomena including surface

forces; which are important i n structural and electronic ceramics, and for modeling and

simulation techniques for predicting their behavior.

1 .3 Electronic ceramics

The electronics industry relies heavily on advanced ceramic materials such as A1207,

A1203-Ti02, Be0 and AIN for substrates, Barium titanate (BaTiOi) for capacitors, Lead

Zirconate Titanate (PZT) and Lead magnesium niobate (PMN) for actuators and

transducers, Zr02-based ceramics for oxygen sensors, ZnO-based materials for varistors,

NiO and Fe203 for temperature sensors etc. In addition, the ceramics can be combined

with polymers or metals to form composites or hybrids having unique electrical and

structural properties. Ceramics are important in consumer and military electronics,

communication systems, automotive and other forms of transportation, as well as in

computers.

As noted earlier, the successful development of electronic ceramics is the result of

a detailed knowledge of crystal chemistry, phase equilibria, thermodynamics, kinetics,

atomic structure, and electronic structure. These ceramics and hybrides will be the first of

the emerging "smart" materials, which can sense their environment and respond to it.

The ceramic properties which are important for electronic applications result from

a variety of mechanisms which depend on the bulk material, grain boundary properties,

and surface effects. Important properties include &electric constant, dielectric loss

tangent and power loss, Curie temperature, and piezoelectric constant. The dielectric

constant is a measure of the amount of charge that a material can hold. The dielectric

strength is a measure of the voltage gradient the material can withstand before failure.

The critical temperature (T,) is the temperature at which a material becomes

superconductive, that has no resistance to the passage of an electrical current [22].

1.4 Niobates and related relaxor dielectrics.

In recent years many ceramics based on lead niobate, lead titanate, or lead

tungstate have been investigated for use in multilayer capacitor applications. Some of

these systems, studied originally by Russian workers in the early 1960's are ferroelectric,

have peak dielectric constants as high as 20,000, and sinter at or below 1000" C. This

combination of properties make them very attractive for use in multilayer capacitors with

silver electrodes. Materials with high dielectric constant and Curie temperature near 25°C

(e.g., PbMglnNbmOi or slightly above room temperature PbFe112Nb11203 or

PbZn113Nb21303 are of particular interest for capacitor applications because they can be

used with only minor modifications. Also, additions of lead titanate move the Curie point

to higher temperature and increase the dielectric constant. These materials are generally

referred to as relaxor because the dielectric constant peaks at a parhcular temperature and

the magnitude of the peak usually depend on Frequency.

In addition, the Pb(FelnNbln)Oj - Pb(FeznWln)O, with 30-35 mol % lead iron

tungsten has dielectric constant of about 21,000 at 25°C and sintering temperature as low

as 920°C. Additions of lead managanese niobate to th~s system decrease the dielectric

losses. Excess niobate aids densifi cation. An even higher dielectric constant (34,000) has

been reported for lead iron niobate with 18 mol % lead iron tungstate and 2 % barium

copper tungstate, the firing temperature being 900°C. When initial attempts were on to

use relaxor dielectrics in multilayer capacitors with silver electrodes, the capacitor

tended to degrade on test and the parts were mechanically weak.

Although the nature of degradation has been resolved unequivocally the

performance appear to improve if the electrodes contain some pollution and if various

additives (particularly manganese) are used in the dielectric. Th~s increases the insulation

resistance and improves the flexural strength. As a result relaxor dielectrics can now be

used to make mutilayer ceramic capacitors with both high volumetric efficiency and high

reliability.

1.5 Ferroelectrics as electro-optic materials

Ceramics have long been known for their desirable structural, electrical and

electromechanical (piezoelectric) properties. They have found applications in the field of

electro-optics during only the last two decades. Basically electro-optic ceramics are

polycrystalline, ferroelectric (FE) materials which, in addition to their many other

characteristics, possess both high optical transparency and voltage variable electro-optic

(EIO) behavior. Taking together, these two properties have been the key to the successful

utilization of ceramlcs in such electro optic devices such as shutters, modulators, displays

and image storage devices [2'2].

1. 6 Ferroelectric devices

The ferroelectric devices are based on ferroelectricity properties and are produced from

polycrystalline materials. The possibilities of tailoring ferroelectric ceramics according to

the requirement for various devices are discussed.

Ferroelectric devices have a wide field of application in today's technology. This is

due to their special properties resulting from ferroelectricity and the possibilities opened

up by the ceramics for material engineering. The following phenomena are direct

consequences of ferroelectricity or are closely related to it.

P Extra ordinary high permittivities.

> The presence of spontaneous polarization and of areas where this spontaneous

polarization polnt into different directions (domains). The possibility of aligning

spontaneous polarization by applying an electric field (poling) or even switching the

polarization vector (ferroelectric hysteresis).

b Control of electrical conductivity by the mutual influence of spontaneous polarization

of electronic state at interface.

b Pyroelectric effect

> Optical anisotropy, electro-optic and photo refractive effects

As ceramics they offer the following additional possibilities for materials engineering,

> Large range of chemical compositions in homogeneous and inhomogeneous materials

(some materials can only be produced in the polycrystalline state).

P Formation of phase mixtures or solid solutions.

b Control of microstructural parameters like grain size and porosity

i. Great possibility for varying geometric shapes

k Exploration of large number of technologies for the production of ceramics, spec~al

emphasis should be placed on the increasingly important thin film techniques for

structuring, and techniques for production of composites.

In addition, the ceramic form is suitable for mass production, and therefore allows the

realization of inexpensive devices [23].

1.7 Ferroelectric ceramics with tungsten bronze structure

A large or family of materials exist in the tungsten bronze type structure. The structure

consists of a network of oxygen octahedra linked comer to comer in such a way that

different types of interstitials result. Tungsten bronze structure family has been

intensively investigated, but most of the work is on single ctystals.

A large family of AB03 - type oxygen octahedra ferroelectrics crystallize with

structure close to the tetragor~al tungsten bronzes KW03 and Na,W03 [24]. The basic

oxygen octahedral framework is shown in figure 1. 1. The tetragonal unit cell consists of

10 BOs octahedra linked by their comers in such a manner as to form three different

types of tunnels running right through the structure parallel to the c-axis. The rotations of

0 @ 0 0 A t site A 2 sits Bl site BZsita C site

(AI)~"'(A~)~~"(BI kV'(~2)8Yi(~4)4U(~~om

Figure 1.1: Projection down the c(3) axis of a unit cell in the tungsten bronze structure. Site

locations are marked and the structure related formula is given. Roman superscripts mark the

coordinates of the ions at each site location.

oxygen octahedra, evident in the 'ab' plane of the structure in figure 1.1, reduce the point

symmetry to tetragonal (4lmmm) layers stacked in regular sequence along the 4 fold c

axis. The arrangement distinguishes two equivalent 6 fold coordinated B sites at the

centers of inequivalent octahedra with 5, 4 and 3 sided tunnels for the A site cations

extending along the oaxis giving the structure related formula for the bronzes. The

general formula of this structure is (A~)~(AZ)~(B~)~(B~>~(C~)~O~O. These IT3 structured

ferroelectric ceramlcs exhibit Curie temperature reaching up to 5 6 0 " ~ and consists a

family containing more than 85 compounds in the most recent survey [25]. Again there

is very extensive solid solution between end members and the open nature of the structure

as compared to the perovskite permits a very wide range of cation and anion substitutions

without loss of ferroelectricity [23]. The unit cell is only one octahedron high (rc 0.4nm)

in the c-axis direction (with an a axis = b axis dimension of typically 1.25 MI (=a c).

The long chains of oxygen octahedra along the c-axis resemble those in the

perovskites, while normal to this axis the structure consists of slightly puckered sheets of

oxygen atoms. The A-type cations enter the structure in the interstitial tunnel in a variety

of ways depending on the particular composition. The arrangement provides space for up

to four cations in nine co-ordinated bigonal A2 sites, two cations in somewhat smaller 12

co-ordinated A1 sites and four cations in the relatively small three co-ordinated planar c-

sites a s shown in the figure. There are in addition, two different B-cation sites which are

labeled BI and B2 in the figure:.

Only two simple ferroelectric compounds have been discovered with this basic

structure, namely lead metaniobate (PbNb206) and lead meta tantalate (PbTa206) where

the lead atoms are located only in the AI and A2 sites between the NbOs or Ta06

octahedra. Both of these material sites have small orthorhombic distortion from the

prototype tetragonal unit cell. PbNb206 becomes tetragonal at the Curie temperature Tc =

575' C but PbTa206 remains orthorhornbic throughout. As with LiNb03 and LiTa03 solid

solution the substitution of tantalum ion for niobium ions in PbNb206 forms a continuous

solid solution, lowering the Curie temjxrature from 575' to 260' C between the end

members. It is found that the ferroelectric behavior of these materials could be

considerably modified if lead ions are partially replaced by Mg, Ca, Sr, and Ba ions, a

discovery which soon led to a comprehensive study of a large number of alkaline earth

niobate solid solutions.

Only five out of the six available A sites of the tungsten bronze structure are

occupied by lead ions in Pb(Nb/Ta)zO6, so that the structure is to some extent random

even in these simple compounds. Furthermore, both the tantalates and niobates are

thermodynamically stable only at high temperatures (1250 O C in the niobate, 1150 "C in

the tantalate) and the corresponding ferroelectric tungsten bronze phases are obtained

only by quenching crystals rapidly to room temperature from these high temperatures.

Even the original tungsten, NaxW03, and KxW03 are only off- stoichiomeby [30], that is

for x < 1. These compounds are metallic since the cation deficiency is charge

compensated by free carriers. h fact the name 'bronze' describe the metallic lustre of the

compounds. The ferroelectric tungsten bronzes, which are known to be stable at room

temperature, are all solid solutions of at least two compounds such as x (A,) BO3 + y (A4

BOX (where A! and A1 here label two different A cations and are not necessarily

associated with the A, and A2 cell sites of Figure 1.) where neither component material

itself has a stable tungsten bronze structure at room temperature. All this evidence

suggests that the structure is only stable when there is a certain degree of disorder, and

we discuss below how this disorder affects the ferroelectric properties of the materials.

Various dimensions of the structure depend on the particular composition and the

crystal growth. Generally speaking the paraelectric phase has tetragonal symmetry, but

below the Curie temperature both tetragonal distortions may occur. The orthorhombic

cell has dimensions approximately 1.75 X I .75 x 0.8 (nm) where

In most cases the spontaneous polarization appear along the c-axis, but PbNbz06

is an exception to this rule with the polar axis perpendicular to c.

The interest in tungsten bronze ferroelectrics got renewed in the 1960 s because of

the large optical non-linearity of thest. materials. Alternations centered on solid solutions

of alkali and alkaline earth niobates from which transparent crystals could be grown with

a variety of ferroelectric properties depending on the specific cations introduced into the

structure. The general composition may be considered to be close to one of the following

formulae:

(a) (Al)x (A2)5-~ N ~ I O 0 3 0

if both A, and A2 are alkaline earth ions.

(b) (A,) 4, (A21 2-zx ~ I O 0 3 0 .

if A, is an alkaline earth and A2 is an alkali, and

If both AI and A2 are alkali ions, the range of values of x depends on the width of

the tungsten bronze solid solution region. The actual compositions may be more

complicated than these three simple solid solutions since the niobium stoichiometry can

also vary in these equations. However, when the niobates and tantalates are off-

stoichiometry the cation deficiency or excess do not usually give rise to metallic

conductivity as in the non-stoichiometric tungstates. Even with wide departures from

stoichiometry, insulating and transparent crystals may be grown suggesting that, as in

LiNbO3 and LiTa03, charge compensation takes place by ionic rearrangement.

Probably the best known and most widely studied examples of each of the three

categories above are

K6-x-y Lidh N ~ I o + ~ 0 3 0 (KLN)

In SBN the unit cell contains five formula units (10 NbOh octahedra) with only five

alkaline earth cations to fill six interstitial A, sites. Both these ions are too large to enter

the small c-sites. The structure is thus incompletely filled and a certain degree of

randomness is expected. In BNN the AI and A2 sites are completely filled and the C sites

are expected to be filled with the small Li ions in the c sites. These expectations are more

or less born out of the detailed structure measurements and specific composition of each

of these three compounds. It is clear that in SBN (x 3 1.38) the Ba ions prefer the larger

site as may be expected from its larger atomic radius, while the Sr ions are randomly

distributed over the remaining At and A2 sites. In the case of BNN, as before, the Ba

prefers A2 sites and the Na prefer the Al sites so that for x = 0 one would expect the

structure to be completely ordered. (i.e. translationaly invariant). In the case of KLN the

excess Nb ion in the composition displace Li ions from the C sites to the AI. A2 sites.

Again one would expect the structure to become ordered for x = y = 0 with Li on C sites

and K ions filling A1 and A2 sites. However, studies of the phase equillibria of the K20 -

Li20-Nb205 ternary system show that the composition x = y = 0 is not stable within the

tungsten bronzes structure. The structure is only stabilized in the presence of excess Nb

so that complex ordering of this compound is not possible [24].

The important compounds in this group are

(1, Lead (meta) Niobate PbNbzOh

(2) Lead (meta) tantdate PbTa206

(3) Potassium Bismuth Niobate KzBiNb~Ols

(4) Potassium Lanthanum Niobate K2LaNbsOls

(5) Rubidium Strontium Niobate RbSrzNbsOl~

(6) Potassium Strontium Niobate KzSrzNbsO~s

(7) Sodium Strontium Niobate Na,Sr2Nb501s

(8) Lithium Potassium Strontium Niobate L ~ K S ~ ~ V ~ I O O ~ O

(9) Libum S d u m Strontium Niobate LiNaSr&bloO,o

(10) Potassium Barium Niobate KBa~NbsOls

(1 1) Sodium Barium Niobate NaBa?NbsO1~

(12) Lithium Barium Niobate LiBa2NbsOlj

(13) Potassium lead Niobate KPbNbsOls

(14) Barium Magnesium Niobate Ba9MgNb14015

(15) Strontium magnesium Niobate SrgMgNb14045

(16) Barium strontium Niobate Ba2Sr,NbloO3" etc.

These are mainly the family containing niobate or tantalate groups.

1.8 Strontium barium niobate ceramics (SBN)

The physical properties of strontium barium niobate (SBN), which is a ferroelectric solid

solution with the tungsten bronze structure, have been extensively investigated because of

its exceptionally large pyroelectricity [26], and its potential in the non linear optical

application [27]. It is better known to exhibit a peculiar type of diffuse phase transition

(DPT) of displacive type [24]. SrXBal.,Nb2Os (0.25 < x < 0.75) solid solutions are of

immense importance in many technological applications such as pyroelectric detectors

and surface acoustic wave (SAW) devices [28]. Single crystals of varying chemical

composition of SBN (grown by the well-known "Zchochralski" technique) with its

various interesting dielectric, electro-optic etc. properties have been elaborately reported

in the literature [26-331. On the other hand, only relatively scant information is available

on its ceramic counter part. A literature survey has revealed that from 1980 onwards there

have been an increasing trend m research and development of SBN ceramic [34-381 solid

solution. SBN ceramics could be made with large size and more complex shape, therefore

it has received much attention [39]. Recently, several studies have been undertaken to

fabricate SBN ceramics [40-421; however, it seems that a high density and uniform fine-

grained specimen is not easily achieved. A formation mechanism of SBN has been

proposed [41] in which the intermediate phases Ba5Nb4015, Sr5Nb4015, BaNb20~ (BN)

and SrNbzOs (SN) develop and the latter two phases react to form SBN, because the

formation temperature of BN and SN is lower than that of SBN. It is advantages to

fabricate ceramic SBN by the reaction sintering of BN and SN. EXAFS and X-ray

d ihc t ion studies on the structure of Ba,Srl.,NbzO~ have been done [43]. The results

indicate that SBN is in the tetragonal phase belonging to space group c2.,, -P4bm over the

composition range of 0.40 5 x S 0.55 and in the orthorhombic phase with space group

~ ' 2 " -P- over the 0.55.r x 5 0.75 range. The application of electro-optic materials in

such devices as optical integrated circuits, adaptive optical components, optical

resonators etc. have been intensively studied and has recently attracted much attention

[44]. In particular, the potential application of optical phase wnjugators for adaptive

optics is important for high power laser or microwave systems. Optical phase conjugation

has been demonstrated using low-to average-power lasers and ferroelectric single crystals

of strontium barium niobate [45, 461. Optically transparent and electro-optic strontium

barium niobate ceramics have been fabricated. Dielectric studes on all SBN ceramics

with a single phase of TTB structure showed relaxor type behavior [44]. Dielectric

spectroscopy and TEM investigations have been performed on Sr0.75B%.2S%206 for

various thermal histories. Quenclung was found to decrease the degree of relaxor

characteristics. In addition, dark field imaging revealed a decrease in the size of

nanoelastic domains and an increase in the size of nanopolar domain [47]. Dielectric

relaxation characteristic studies on SBN ceramics as a function of temperature have been

reported 1481. The ageing and poling behavior of dielectric response of SBN relaxor

ferroelectric ceramics has been studied. Three distinct features were observed in the

complex dielectric response of tungsten bronze Srl,Ba,Nb2O6 ( x = 0.40, 0.50, 0.60)

relaxor ferroelectric ceramics. It is suggested that the incommensurate phase plays an

important role in the dielectric relaxation of Srl.,Ba,Nb2O6 ceramics.

Metastability of polar microregtons in relaxor ferroelcctrics was confirmed by the

ageing and poling behavior of SBN ceramics [48]. Sr0.~Bao4&b2O6 is a relaxor

ferroelecbic due to tluctuations in the distribution of Ba and Sr over the five fold and four

fold tunnels in the structure 1491. Controversy exists as to the precise nature of these

materials, but they are known to exhibit several types of interesting behavior including a

change in the refractive index due to a transition from a glassy polarization (or super

paraelectric) phase to a conventional paraelectric phase at a temperature far in excess of

the Curie temperature [50].

1. 9 Effect of alkali metal and rare earth metal doping on SBN ceramics

Potassium sodium strontium barium niobate (KNSBN) is a series of ferroelectric crystals

with unfilled and filled tungsten bronze(?TB) type structure. These crystals are

interesting because of their high Curie temperature, large pyroelectric, piezoelectric, and

electro-optic effects. Lanthanum doped SBN crystals, which are dislocation free, are very

good for pyroelectric detector applications [51]. Rare earth modified ferroelectric crystals

with the formula (Srl.,Ba,)~.,~y~R,Nb~O~, (where R = La, Nd, X = 0.5 and Y = 0.02)

exhibit twice the pyroelectric coefficient and four times the dielectric constant of the

unmodified Sr~-~B~~Nb\lb?Os (x = 0.5) at room temperature. Curie temperature decreases,

dielectric constant increases, while loss factor and detector signal to-noise ratio remains

nearly the same with addtion of rare earth atoms,

Effect of different cations like Fez03, MnO3, Cr05 and La203 on the properties of

Sro.~B~,sNbzO6 ceramics have been analyzed [52]. The most remarkable effects of

doping these cations are the broadening of the E' vs To peak, the lowering of the

temperature for E',, ( = 5" C for Fe and nearly 50' C for Mn, Cr, La). Decrease of dc

conductivity and dielectric losses and a higher coercive field, as deduced from 50 Hz

hysterisis loop, have been reported for undoped and Fe and La doped samples [53].

Single crystal growth of several ferroelectric tungsten bronze compositions such as Ba,.

,SrxTiyNb2,06 (BSTN) are reported [54]. The lower dielectric constant and higher Curie

temperature of BSTN relative to SBN-61 combined with its large electro optic coefficient

makes ths material an alternative for low-voltage guided-wave electro-optic device

applications.

A new group of components with composition (Bas,Srx)Nb401s, having high

permittivity and low loss have been prepared and characterized in the Microwave

frequency region. Microwave dielectric properties such as ET and TF show smooth

variation with x, while the unloaded quality factor (Q) show remarkable improvement in

value [55].

The effect of doping rare earth ions on SBN ceramics modified with ~ a + and K'

have been investigated [56]. The general formula of the solid solution is

R ~ . M S ~ O . ~ S B % . ~ ~ N % , ~ ~ & ~ ~ N ~ ~ O ~ , where R = ce3+, ~a '+ , Nd3', sm3+ or ~d" ' . There are A

position vacancies in all of the compositions containing rare earth ions and in pure SBN.

The introduction of K' and Na~', leading to a filled structure without vacancies, raises Tc,

to 21PC implying stabilization of the structure. The most remarkable effect of doping

with rare earth ions is a drastic reduction of Curie temperature T,. Curie temperature

decreases form 217" C in (NaK) SBN to 55' C in lanthanum doped SEN-NaK and the

phase transition broadens. Further more, T, decreases as the ionic radii of rare-earth ion

increase. This effect has been reported by many workers in tungsten bronze structure.

1.10 Barium sodium niobate ceramics (BNN)

Barium sodium niobate (BNN) is a member of a large class of mixed oxide systems with

a tungsten-bronze type crystal structure [57]. It has attracted attention through the

application of its outstanding piezoelectric, electro-optic and non linear optical properties,

the coefficients of which are phase matchable and three times those of LiNbO3 and LiIO3

[58]. Barium sodium niobate ceramics exist in tetragonal tungsten bronze type structure.

This structure owes its name to its close relationship to the structure of potassium

tungsten bronze K,WO,. It consists of a skeleton of oxygen octahedra sharing comers

and forming various types of tunnels, running along the c-direction, in which cations are

located. An approximate symmetry of all the members of the structural family is

represented by the tetragonal space group Pdmbm (D~.I~,) with parameters a E 12.4 A and

c E 4 A (i.e. the height of one oxygen octahedron). In most substances, the phase

observed at room temperature has a structure which is slightly distorted with respect to

the reference structure. It has a polar tetragonal or orthorhombic symmetry. Most of the

studies have however been restricted to the evaluation and optimization of these useful

properties. Less attention has been given to the characterization and understanding of

their structural phase transitions. In particular, for the alkali-alkaline earth niobates the

crystallographic description of the different phases is still incompletely known. Likewise,

physical measurements across the phase transitions are scarce [59].

Barium Sodium Niobate (Ba4Na2Nbfo0,0) is at present, the best characterized of

these compounds, though many of its features are still not clearly understood. An

intricate pattern of phase transitions has been observed in this compound. A standard

ferroelectric transition (4/mmm+4mm) accompanied by a divergence of the dielectric

susceptibility along c direction occurs at about 5 8 0 ' ~ [60]

In recent years ferroelectric ceramic materials have become important as

substitutes for single crystals in various device applications because of their low cost and

ease of fabrication. Barium Sodium Niobate (BNN) has been found to be a very

important and useful materid, particularly for second harmonic generation. Most of the

work carried out on BNN by different workers has been on single crystals [61, 621. The

effect of rare-earth ions on the lattice parameters and Curie temperature of the

ferroelectric BazNa,RNbloO,o(R = La, Eu, Gd, Dy, or Y) [63].

The effect of dysprosium (Dy) on &electric constant . resistivity, piezoelectric

and crystallographic behaviour of BNN have been reported [64]. The room temperature

dielectric constant and broadening of the dielectric constant versus temperature curves

are observed to increase and the ferroelectric Curie temperature (T,) decreases with the

Dy concentration. It has been observed that some members of this niobate family which

contain rare-earth ions are found to be very important and useful because of the existence

of diffuse phase transition in them. Barium sodium niobate (BaNa2Nbl00,o) is known to

show some peculiar phenomena in the incommensurate phase such as memory effect,

which are closely related to a phase transition from a normal tetragonal phase (space

group 14-) to an Incommensurate one at 573 K [65-661. An important feature of this

transition is that Landau theory predicts a change in point symmetry from 4mm to mm2

in the transition [67].

Features of a ferroelastic domain structure in an incommensurate phase of

Ba,NaNb,O,, (BNN) that appear in the cooling process have been investigated with a

transmission electron microscope. The in situ observation reveal that there exists an

abrupt change in the domain structure around 503 K. The ferroelectric domain structure

above 503 K basically consists of two types of lq ferroelecaic micro domains with a size

of about 20 nm while below it large ferroelectric domains with flat domain boundaries

are found [68]. Since this micro domain structure appers reversibly during heating and

cooling cycles, the microdomain structure is directly related to memory effect.

In continuation to the above mentioned literature survey of SBN and BNN

ferroelectric wrarnlcs, we have prepared and investigated some of the physical

characteristics of these materials. In the following chapters we report the microstructural

analysis of the above system by scanning electron microscopy. The structural studies are

done using powder XRD method. Dielectric constant and dissipation factor are measured

as a function of temperature as well as frequency.

PART - B

EFFECT OF HEAVY ION lRRADIATlON ON PHYSICAL PROPERTIES OF MATERLALS

1. 11 Introduction

This section reviews some of the fundamental physics associated with the swift heavy ion

imadiation/implantation in materials. Ion implantation has certain distinct advantages

over the standard method of ion incorporation into materials or substrates by diffusion at

elevated temperatures. Recently, ion implantation has attracted a great deal of interest due

to the possibility to modify the materials to overcome the doping solubility limits, and to

incorporate virtually any element in to the substrate materials. The interaction between

incident ions and substrates causes effects directly connected to surface damage, such as

mechanical stresses, density and composition modifications, and consequent dielectric

and optical property changes.

1. 12 Radiation effects in solids

When a highly energetic particle such as an electron or ion strikes the atom of a target

material, different mechanisms of energy or momentum transfer takes place. The most

important primary radiation effects are:

Electronic excitation or ionization of individual atoms.

Collective electronic excitations, like plasmons.

Breakage of bonds or cross-linking.

Generation of phonons, leading to heating of the target,

Displacement of atoms in the bulk of the target.

Sputtering of atoms from the surface.

The secondary effects are:

Emission of photons, e.g. X-rays or visible light.

Emission of secondary or Auger electrons, leading to a charging of the target.

The importance of these different contributions are reflected by the cross section for

the respective interaction. The energy of the projectile is of parttcular importance as

different phenomena show different energy dependencies. When radration effects in

materials are considered, it is useful to divide these contributions into those that lead to a

displacement of atoms (knock-on effects) and those that do not (excitations). Generally,

with increasing particle energy excitations decrease in importance, where as knock-on

effects increase.

In insulators electronic excitations are induced by swift heavy ions; inelastic

interactions such as ionization can play a dominant role. Molecules are sensitive to all

kinds of damage, particularly bond breakage. In metals, however, several effects, for

example, ionization, are quenched due to the presence of conduction electrons. Radiation

damage in metals is therefore essentially restricted to knock-on atom displacements. This

makes excitation effects less important, so that metals are comparatively stable under

irradiation, in particular at low projectile energies.

When heavier particles such as ions irradiate the target, the cross sections of most

interactions and the energies transformed to the atoms are generally much higher than in

electron beams. The energy of the ions necessary to displace an atom is therefore lower,

corresponding to their higher mass [69].

1. 10. 1 Excitations

The excitation of phonons, leading to a heating of the specimen, is mainly due to inelastic

scattering of the projstile by electrons. Phonon generation by collisions with nuclei at

energy transfers in the MeV range are of less importance. Heating of the target is

governed by the dissipation of plasmons. The mean free path of the projectile in the

sample depends on its mass and energy. With decreasing mass and increasing energy, the

mean free path of the particles in the specimen increases. Ion irradiation causes much

heating since the range of ions in the specimen can, even for ion energies in the MeV

range, be of the order of the size of the nanoparticles. When ions are stopped in the target,

almost all of their kinetic energy dissipates in the specimen and leads to a considerable

temperature rise which can even cause melting of the materials.

1.13 Interaction of heavy ions with materials Swift heavy ions lose their energy in materials via two mechanisms; direct elastic

collision with target atoms (nuclear stopping power S, = (-dE/dx)n), and inelastic

collisions producing electron excitation (electronic stopping power S, = (& / d ~ ) ~ ) .

1.13.1 Elastic interactions

The expressions of the nuclear stopping power is given by

Where n2 is the atomic density of the target, T the energy transferred from an incident ion

of energy E to a target atom which is called a "primary" atom, T,, the maximum value

of T, and do, the differential cross section. From do, it is possible to estimate the target

damage induced by the elastic collisions by using the fact that T must be greater than a

threshold value T,, of about 10-20 eV, to shift one target atom. Let NdT) be the number

of induced displacements by a primary atom with energy T. Then the mean number of

displacements per atom per incident ion (with energy E) per square centimeters is defined

by

a@] = IN^( T ~o,(E, TI (1.2)

and od.n2(cm-') gives the theoretical creation rate of defects.

1.13.2Inelastic interaction

The other energy loss mechanism in the materials is related to the e lectro~c excitation,

which is given by the electronic stopping power S,. Th~s mechanism is complex. There

is presently no general model that will cover all ions at all energies. The energy loss

depends essentially on the speed of the incident ion. The main process of energy loss of

MeV energy ions are due to electronic exc~tation and ionization. In insulators it has been

observed that electronic energy loss influences the sputtering behaviour above a

particular threshold [70].

Electronic excitations such as intraband or interband (electron-hole pair)

excitations are excitations involving energy transfers in the eV range. These excitations

are of significance in insulators and to a less extent in semiconductors. The excited states

can cause local atomic bondng instabilities and rearrangement, leading to bond breakage;

this phenomenon is commonly known as radiolysis. Metals and also graphite are

immune to this type of damage.

Plasmons are collective excitations with the energy loss being spread over several

bonds. The dissipation causes heating but little damage. The energy transfers depend on

the valence electron density; e.g.; J-Plasmons in graphite have an energy of 27eV and in

diamond it is 33 eV.

Ionization is of importance in insulators and semiconductors where the lifetime of the

excited electrons is long enough to cause irreversible bond breaking. In a metal,

ionization is quenched instantaneously (10-'~s) and local perturbations in electric charge

are removed in that time scale.

When a surface atom is knocked by a highly energetic particle or when collision

cascades intersect the surface of the specimen, atoms whose energies exceed the surface

binding energy get ejected. Surface atoms are less tightly bound than atoms well inside

the surface, therefore only the transfer of the sublimation energy is required to eject an

atom.

The interchange of electrons with the nuclei is slow compared to the spreading of

electronic energy, and the dissipation of energy occurs on a time scale which is small

compared to the lattice vibration period.

The generation of X-rays or Auger electrons behave similarly to ionization damage.

For light elements such as carbon, Auger emission dominates over X-ray emission. The

inelastic scattering cross-section for all processes put together decrease slowly with

increasing beam energy and increases with the atomic number of the specimen material.

No primary damage is caused in metals, but pre-existing atomic defects can get affected.

For example. irradiation-induced recombination can occur.

1.13 3 Knock-on atom displacements

Atomic displacements occur by knock-on collisions of highly energetic electrons or ions

with the nuclei of the atoms in the specimen. The knock-on displacement event occurs

within a very short time. The time scales during the production of atomic defects are

0 10-*'s; energy transfer frorn the particles to the nucleus (primary knock-on); 13 . . o 10- s, inter atomic collisions (cascade);

Q 10-'Is; dissipation of epithermal energy (stable defects and clusters);

210'"s; thermal migration of point defects.

1. 14 Swift heavy ion-based material science research at NSC, New

Delhi

Swift heavy ions with state of the art nuclear insfrumentation have opened up exciting

possibilities for characterisation and depth profiling of materials over a wide mass range,

particularly for lighter elements. Surface effects produced by SHI irradiation of materials

have also attracted attention due to observation of surface rippling, electronic imitation

induced desorption of large molecules etc. There are possibilities of various new phases

in materials and also to produce cylindrical channel of controllable diameter filled with

the modfied tnaterials.

At Nuclear Science Centre, New Delhi. there is a 15 UD Tandem Pelletron

Accelerator facility, which is able to deliver ion beams of almost all the elements across

the periodic table in the range of 10-270 MeV [71]. There are two dedicated beam lines

for materials science research. Materials science beam line has three scattering chambers

in series to conduct on-line. in-situ irradiation experiments. At 15'. materials science

beam line has a general purpose high vacuum chamber with facilities for a temperature-

controlled liquid nitrogen cooled multiple sample holder having provision of 120mm

linear motion and rotation of 360'. It is equipped with electrical and optical feed-throughs

for on-line or in-situ electrical transport measurements, ionoluminescence; thickness

monitoring etc. The first is a general purpose high vacuum (HV) chamber, where a

cryopump is installed. With the help of cryopump, we are able to attain oil free clean low

10"mbar vacuum within a short period. A He-Cd laser and a spectrograph have been

installed to perform in-situ photoliminescence and iono-luminescence measurements. A

dedicated large area position sensitive (LAPS) detector for on-line ERD studies is

mounted in HV chamber. This facility is dedicated to user community for experiments.

This chamber is followed by two more chambers as shown in the Figure 1. 2. A scanned

photograph of the material science beam facility is shown in Figure 1. 3.

CP CR10 SUMV P U S R f S l S l l V I W t A T l f f i ICTUP T V C 1. V. C4I I fRA

FOR fVAPORA'IIOY V l l W PORT 01 OOUUK SLIT F f t D I R R W 6 H N A Y - HVC nlsn VACCUM clunara TP TURBO PUMP 1 0 A t - I O f l t C T O R I t L f S C O P t III O m l o MElfR UIIV ULTRA I I I E I I rrcuun cnmsra as* ncsluurr 6 u r u t r n a U M l M U L R A R l 6 H VACUUU S U M l l l l l O r p lo,, IVw

1 U * l l f L I # 6 WICROSCOP& Of DKTfClOR ELtClRDYlCS

mn] BfL,"

W VALVE

Fig. 1. Materials science beam line at NSC.

The first chamber is an ultra high vacuum (UHV) chamber having a provision for

on-line residual gas analyzer (KGA) [72]. It also has an ultra high vaccum (UHV)

scanning tunneling microscope (STM).

The second is an ultra high vacuum (UHV) chamber where typical vacuum of

m bar is attainable. In-situ U I N scanning Tunneling Microscopy (STM) measurements

in UHV chamber can be perfo~med. In the second chamber, a goniometer has been

installed for Rutherford back scattering (RBS) channeling studies. At 45', there is a 1.5

m diameter general purpose scattering chamber. In this chamber there is a provision of

EDR with an hE- E detector telescope for depth profiling of light elements.

Third is a high vacuum chamber which is dedicated to perform wn-channeling

studies and blocking ERD studies. In this chamber a goniometer is installed and X-ray

reflectivity measurement facility is setup. [71].

1. 14.1 00-line Measurements

An on-line EDR facility is being used for depth profiling of light elements. Hydrogen

loss behavior in polymers under heavy ion irradiation i s investigated for different

electronic excitation energies by elastic recoil detection analysis (ERDA) 11731

RBS - channeling facility has been installed with the aim of doing on-line damage studies

and ion beam- induced recrysblization studies.

Figure - 2.Material science beam chamber of 15 UD Pelletron Acce1erator at NSC, New Delhi.

1.14.2 I n Situ Measurements

In the high vacuum chamber of materials science beam line, in sifu measurements

at 77 K and room temperature are regularly being performed. In siru resistivity

measurements are being done in various materials like metals, s u p conducto~ and mi

conductors.

In silu Vf noise measurement facility exists in the high vacuum chamber of the

materials science beam line. A number of studies indicate a possible correlation between

Vf noise and the lattice defects in the material [74, 751. Therefore llf noise can serve as a

tool to investigate the high energy heavy ion-induced defects inside metals,

semiconductors and other materials.

An insitu UHV STM has been installed recently in the materials science beam

line for the studies of inhvidual damage created by swift heavy ion irradiation (SHI) in

semiconductors.

Swift heavy ion (SHI) irradiation of materials provide a tool to modify various

material properties such as physical, chemical and other properties, mainly through the

inelastic energy transfer mechanism. This is in contrast to the familiar elastic scattering of

low energy ions causing direct lattice damage. SHI can also produce lattice damage

through inelastic scattering producing the trail of excited 1 ionised atoms, as it is clearly

evident from the well-known phenomena of ion track production in insulators.

Experiments have shown that when the electronic energy loss exceeds a threshold value,

SHI irradiation produces an amorphized zone along the ion (also known as columnar

tracks) path in many materials. By selecting appropriate ion mass and energy one can

engineer material properties in a desired manner. The columnar tracks produced by SHI

have provided a way to pin the vortices in high TT, superconductors and in the production

of micro filters. Recent studies have shown that the metal-insulator transition temperature

can be tuned to a higher temperature in colossal magneto resistive (CMR) materials using

SHI irradiation.

The structure of Au-implanted LiNbO3 and SrTiO3 has been studied and reported

[76]. Recently, improvement of the photorefractive response of Fe doped KNbO3 crystals

by MeV proton irradiation has iseen studied [77]. Development of dissipative structure in

time and space has been theoretically predicted in metals under ion irradiation [78]; the

irradiating ions provide a radiation damage energy, which is stored in the material.

Due to inelastic energy transfer during ion irradiation, defects are introduced in a

material directly by displacement of the lattice atoms and indirectly through de-excitation

of the electronic sub-system. For high enerby heavy ions (several MeV), achieved by

accelerating ions in a tandem accelerator, the above regions of energy transfer can be

spatially separated. Processes initiated purely by energy transfer to lattice electrons and

their subsequent de-excitation can be investigated in thin specimens or in regions nearest

to the point of entry of such ions in a solid separated from the ion range. Such processes

are called electronic energy loss or electronic stopping (S,) initiated as the ions loose

energy through excitation of lattice electrons.

A general trend has now been established which connects electronic energy loss

induced atomic movement in certain materials. All materials which show phase

transformations under pressure (martensitic or displacive) get modified by ion irradiation.

Hence it would be proper to say that ion irradiation introduces stress field giving rise to

displacive transformations. Dissipating electronic loss energy through atomic

displacements becomes a distinct possibility which under favourable conditions, could

lead to defect structure formation^ [79].

1.14. 3 Effects of SHI on ceramic materials

Ion irradiation has proved to be efficient in modifying the basic properties of the near

surface of materials, and is 1.ncreasingly becoming a technique for the processing of

insulators, more specifically oxide ceramics. The AB03-like compounds are of prime

interest, particularly the ferroelectric oxides because of their non-linear optical properties.

It has it was recently been demonstrated that very high concentration of Ti could be

incorporated into the lattice of LiNbO, followed by an appropriate thermal annealing

process [SO].

Another important work has been recently reported on Pb implantation in ABO,

oxides (CaTiO3, SrTiO3) 1811, for which it was shown that the amorphous layers resulting

from ion bombardment can rec:ryatallize completely by a solid phase epitaxy process after

annealing at moderate temperatures ( 5 500'~).

Effect of H' and 0' ilnplantation on electrical properties of SrBi2Ta209 (SBT)

ferroelectric thin films have been studied and reported [82]. X-ray diffraction patterns of

SBT films show that no difference appears in the crystalline structure of as H+-implanted

SBT films compacted with as gown films. He and 0' co-implanted SBT films show an

obvious degradation of crystalline structure. Ferroelectric property measurements

indicate that both remenant polarization and coercive electric field of H' implanted SBT

films decrease with increasing ~tmplantation dose. The disappearance of ferroelectricity is

found in H', 0' co-implanted SBT films at room temperature.

Ion implantation, as a conventional microelectronics process, has been

extensively used in the fabrication. of microelectronic and optoelectronic devices, and as a

method to alter the properties of oxide materials without constraints imposed by thermal

equilibrium [83, 841. Ions with different oxidation states have been implanted in

TiOz(rutile). The lattice disorder as well as the lattice site location of the implanted ions

are determined using Rutherford backscattering and channeling (RBS-C) spectrometry.

The electrical conductivity of the implanted samples increased by about 12 orders of

magnitude irrespective of the oxidation state of the implanted species [85]. Recent studies

on photo refractive response time in Fe doped KNb03 crystals is reported to be greatly

improved by MeV proton irradiation 1861.

The influence of swift heavy ion irradiation on transport properties of expitaxial

thin films of Lao.75 Cau.25 MnO3 (LCMO) is studied and reported [87]. The films are

irradiated with 90 MeV 160 beams and 250 MeV 1 0 7 ~ g beams at different fluence values.

In the case of 90 MeV 1 6 0 ions LCMO specimens are irradiated to 10"-10" ions/cm2. A

systematic variation in Curie temperature (Tc) or resistivity peak temperature (Tp) has

been observed. It has been no:ted that for both types of ions the Tp increased for the

specimen irradiated at 10'' ionsicrr? fluence. Further increase of fluence decreased the Tp

value and at higher fluences (10~"ions/cm~) for 90MeV 160 and 1013 ions/cm2 for 250

MeV ' O ' A ~ ions) the specimens show non metal-to-insulator transition even at low

temperatures down to 77K.

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