INTRODUCTION -...
Transcript of INTRODUCTION -...
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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