15

CHAPTER 2

PROPERTIES OF DIELECTRIC MATERIALS AND THEIR

APPLICATIONS

2.1 BASICS OF DIELECTRICS

A dielectric is an insulator or a non conductor of electricity such as

rubber, plastic, glass and ceramic with a very high electrical resistance greater

than 106 ohms. These materials can hold electrostatic fields for a long time.

The non-conducting / insulating material in which an applied field causes the

displacement of charges without causing any flow of charges is called

dielectric materials.

An important application of dielectrics is the parallel plate

capacitor. In a vacuum capacitor (Figure 2.1) with plates of a given area, has

an interfacial charge

Qo = o E (2.1)

where E is the electric field between the metallic plates and o is the

permittivity of free space (8.854 x 10-12 Fm-1). If the field varying with time is

applied, the charge Qo follows exactly, there will be no inertia in the vacuum

medium (Jonscher 1977). If the capacitor is filled with a dielectric medium as

shown in Figure 2.2, the induced charge is increased by the polarization P of

the medium,

Q = Qo + P = o (1 +) E = E (2.2)

16

where is the dielectric permittivity that characterizes the materials ability to

store charge and is the susceptibility of the dielectric medium. The

polarization of the medium cannot follow instaneously with the varying field;

therefore there will be a delay between the polarization and the field. It is

convenient to define the time response to a step function field,

Figure 2.1 Parallel plate capacitor with free space between plates

Figure 2.2 Parallel plate capacitor with dielectric medium

17

E (t) = 0 for t < 0

E (t) = E0 for t 0 (2.3)

The current i (t) = dp/dt = o E f (t) (2.4)

where f(t) is the dielectric response function. The vacuum response is

represented as a function at t = 0 and it cannot be resolved experimentally.

The response of polarization to an arbitrarily time varying signal

E (t) is given by the convolution integral

P (t) = o f () E (t - ) d (2.5)

which indicates that the system retains memory of past excitations. In a

harmonically varying field with angular frequency = 2f,

f being the linear frequency in Hz. Fourier transformation of equation (2.5)

gives the frequency dependence polarization.

P() = o () E() (2.6)

where P() and E() are Fourier components of P(t) and E(t) respectively,

and () is the complex Fourier transform of f(t).

f (t) exp (it) dt = () – i ()

= () (2.7)

where i = (-1)1/2.

The real component , gives the component of polarization in

phase with the field. The imaginary component , known as the dielectric

loss. Equation (2.7) shows that () and () are interrelated and they are

in fact Hilbert transforms of one another, known as the Kramers – Kronig

18

relation which are valid in the most general conditions, subject only to the

linearity of response.

In a peak field of peak amplitude Eo,

the energy lost per radian = o () 20E / 2 (2.8)

the power loss = () 20E /2 (2.9)

and this define the alternating current (ac) conductivity

() = o -o () (2.10)

showing the relationship between ac conductivity and dielectric loss with

direct current (dc) conductivity, o being concluded. It follows that any one of

the three functions , and f (t) fully determines the response of the system

and the other two may be derived from it (Jonscher 1977).

The classical expression relating the complex permittivity to the

frequency is first based on Debye (1945) in which the susceptibility function

can be expressed by

* () = (0) / [1 + i (/p)] (2.11)

where p = 1/ is the loss peak frequency and is the relaxation time, which

depends on the temperature and the viscosity of the medium but independent

with time.

However it is difficult to find the pure Debye response in any

experimental data of dielectric materials so other empirical expressions are

modified and such modification was proposed by Cole et al (1941), followed

by another modification ie Davidson and Cole function (1951). Either of these

two expressions is not enough to describe a large number of dielectric

19

behaviour and therefore a generalization has been introduced by Havriliak

et al (1966) that consists of the combination of both Cole – Cole and

Davidson – Cole expressions.

The new concept of power law frequency response of dielectric

relaxations was introduced by Jonscher (1983) and the equations were found

suitable for fitting experimental data for over a wide range of dielectric

materials. According to this universal law, the complex capacitance and

corresponding susceptibility can be written as

*() C*() = B(i)n-1 (2.12)

where C* is the complex capacitance, B is the proportionality constant and n

the exponent defines the frequency dependence that lies between 0 and 1.

2.2 POLARIZATION MECHANISMS IN DIELECTRICS

The most important property of dielectrics is their ability to be

polarized under the action of an external electric field. There are various

possible mechanisms for polarization in a dielectric material. Electronic

polarization, ionic or atomic polarization, orientation polarization and

interfacial polarization (Figure 2.3).

Electronic polarization is found in all materials and arises from a

shift of the center of gravity of the negative electron cloud in relation to the

positive atom nucleus in an electric field. This happens at high frequency

range 1014 to 1016 Hz.

Ionic or atomic polarization mechanism is the displacement of

positive and negative ions in relation to one another. This happens at the

frequency range of 1012 to 1013 Hz.

20

The orientation polarization is associated with the presence of

permanent electric dipoles which exist even in the absence of an electric field.

An unequal charge distribution of molecules or complex ion tends to line up

with the electric dipoles in the direction of the field. This occurs at the

frequency range of 103 to 108 Hz.

The interfacial polarization is due to the trapped mobile charges

which are impeded by interfaces. This occurs at the frequency range of 10-2 to

10+4 Hz. Space charges resulting from these phenomena appear as an increase

in capacitance as far as the external circuit is concerned.

The total polarization P is the sum of the polarization resulting from

all the four mechanisms.

Ptot = Pe + Pa + Pd + Ps (2.13)

where Pe is the electronic polarization, Pa is the ionic / atomic polarization, Pd

is the orientational polarization and Ps is the space charge (or) interfacial

polarization.

Electronic and atomic polarizations respond so rapidly that they are

effectively instaneous below GHz frequencies, contributing a purely real

value to the permittivity. On the other hand, permanent dipoles, ionic

defects of dipolar types and mobile hopping charge carriers respond slowly.

So the permittivity of a medium containing sum of all these can be expressed

as

o( ) ( )

(2.14)

where labels the various mechanisms and are the complex susceptibility

(Jonscher 1977).

21

Figure 2.3 Different mechanisms of polarization

2.3 DIELECTRIC CONSTANTS OF IONIC CRYSTALS

Based on classical mechanics, Clausius and Mossoti formulated a

relation between macroscopic dielectric constant () and atomic

polarisability ( ) (Levy 1972)

o

1 N2 3 V

(2.15)

where V is the volume of atomic sphere. This equation is applicable to all

isotropic materials. If changes with temperature we can attribute this to

22

temperature dependence of and V. Bosman et al (1963) studied the

temperature and pressure dependence of the dielectric constant of a number of

cubic halides and oxides. Differentiation of formula (2.15) yields

P P P

1 1 V 1 V1 2 T 3V T 3 T

+ T P V

1 V 13 V T 3 T

= A + B + C (2.16)

This equation consists of three physical effects described in terms

of A, B and C representing respectively the effect of decrease in the number

of polarisable ions per unit volume accompanying the thermal expansion, the

effect of increase of an atomic polarisability owing to the increase of available

inter-atomic distance with the rise of temperature and the temperature

dependence of the polarisabilities at constant volume. The sum of A and B,

which describes the total effect of volume expansion, can be written as

T P

1 VA B1 2 V T

=

P

T

T

1 V1 V T

1 V1 2 PV P

(2.17)

This equation can be divided by differentiating equation (2.15) with

respect to volume at constant temperature. From equation (2.16) and (2.17),

A, B and C can be determined separately by measuring the dielectric

constant its temperature dependence P

,T

thermal expansion coefficient

23

1V

P

VT

and the compressibility factor T

1 V .V P

If A + B + C is

positive, PT

is positive and vice versa. The theory of ionic crystals and

dielectrics is also discussed by Williams (1952).

2.4 ELECTRICAL PROPERTIES

Perovskites exhibit diverse electronic conductivity behaviour. Some

compounds are insulators with good dielectric properties while other show

metallic conductivity, but a majority is semiconductors.

Electrical properties of perovskite oxides have been explained by a

consideration of one electron energy band diagram proposed by Goodenough.

Electrical and various other properties of perovskite oxides have been

extensively reviewed in the literature (Rao et al 1970, Rao et al 1978, Khattak

et al 1979, Parker 1978).

Goodenough’s approach to explain the properties of transition

metal oxides and related materials is essentially based on the principles of

chemical bonding. Goodenough considers the cation-cation and

cation-anion-cation interaction to be of importance in describing the

behaviour of electrons in oxide materials of a given crystal structure. In ABO3

compounds the one-electron energy diagram essentially pertains to the BO3

array and the central A atom acts as an electron donor and has only minor

effect on the original diagram. Such energy diagrams can be constructed

considering the most probable hybridization of the anions and cationic

orbitals. In perovskite the closest interaction is 180 cation-anion-cation (cac)

interaction, as cation-cation (cc) distance is larger than cac. In cac interaction

B-O-B the s and p orbitals of oxygen gives rise to filled valance bands and

24

that are separated from an empty * conduction band (made up from the

cationic s and p orbitals) by a large energy gap. The tenfold degenerate

cationic d orbitals would be split by the octahedral crystal field into t2g and eg

orbitals, and the extent of splitting will depend on the covalent mixing of

anionic and near – neighbour cationic orbitals.

We should emphasize now that cation – cation overlap (and to a

smaller extent the cation-anion-cation orbital overlaps) has got a great

influence in deciding whether t2g and eg levels remain localized or become

transformed into band orbitals. Thus if the overlap integral cac cac C , all

the levels remain localized ( C is the critical overlap integral). If

cac C cac , only t2g levels remain localized but eg orbitals spread out into

a band and finally if C cac cac , then all the levels become bands. In the

latter case, if the band is occupied, it should show metallic behaviour.

Table 2.1 lists the type of conductivity behaviour found in LaBO3 (B = 3d

element) compounds with their d electronic configuration and the

corresponding situation of the overlap integrals. It is to be noted that

Goodenough’s model satisfactorily explains the observed electrical properties

of perovskite oxides and can be further exploited in predicting the electrical

properties of new isostructural compound. Substitution of a suitable higher

valent 3+ ion at the A site may force part of the transition metal ions into a

lower oxidation state so as to maintain charge neutrality, there by greatly

modifying the conductivity behaviour. Substituting at the B site in LnBO3

oxides by other transitional metal cations has been studied by Rao et al

(1975). In the case of LaNi1-xFexO3 a metallic to semiconducting transition

occurs at x = 0.2.

25

Table 2.1 Properties of LaBO3 perovskites (Rao et al 1975)

Compound Symmetry Electronic Configuration

Spin State (S)

Electrical Conduction Overlap integral

LaTiO3 O *1 *0 12

Metal c<cac<

cac

LaVO3 T t*2 *0 1 S.C cac<c<

cac

LaCrO3 O t*3 *0 32

S.C cac<c=<

cac

LaMnO3 O t*3 e*1 2 S.C cac<

cac <c

LaFeO3 O t*3 e*2 52

S.C cac<

cac <c

LaNiO3 R t* *1 12

Metal cac<c<

cac

2.4.1 Polarons and Hopping Conduction

Consider a delocalized (band) electron with most of its amplitude

near a particular position in the primitive cell of an ionic crystal. The electron

being negatively charged can lower its energy by inducing a lattice

polarization which brings extra positive charge near the position where its

amplitude is large and which pushes away some negative charge into region

where its amplitude is small. This means that there must be a strong coupling

between the electrons and the longitudinal optical phonons. This problem is

handled by introducing a fictitious particle, the polaron, which is an electron

that always moves around together with the associated lattice polarization,

that minimizes its energy. The electronic problem is replaced by polaronic

problem and the remaining polaron - phonon interaction is assumed to be

small. The polaron clearly has lower energy than the electron alone, but it has

a larger effective mass since it must carry its lattice distortion with it as it

moves.

26

Polaron theory has been analyzed in various limits, depending on

the strength of the electron – phonon interaction, the extent of the polarization

and the bandwidth of the electrons. In the limit in which associated lattice

deformation extends over a large number of lattice parameters, the lattice may

be replaced by a continuum. It can be shown that the extent of this

deformation is given by

12

o or h / 4 m* (2.18)

o is the average frequency of the longitudinal phonons, r0 is called the

polaron radius. (Frohlick 1954). If ro is much larger than the lattice spacing,

the polaron is called large. When ro is small compared to the interatomic

spacing, the continuum approximation breaks down. Further more if the

energy Ep given by

op o2

r1E = α h / 2π ωπ a

(2.19)

is large compared to the electronic bandwidth , small polaron theory applies

(Holstein 1959). The quantity Ep defined in equation (2.19) is the approximate

reduction in energy due to small polaron formation.

Holstein found that for a perfect crystal small polaron states overlap

sufficiently to form a polaron band in which ordinary band conduction can

take place. This conduction should predominate at low temperatures. In

equation (2.19) is the parameter determining the strength of the electron-

phonon interaction. The width of this band can be shown to decrease

exponentially with increasing temperature and in the vicinity of half the

Debye temperature, the bandwidth becomes less than the uncertainty in

energy due to the finite life time of polaron states. Above this temperature

electrical transport in the polaron band is negligible and the small polaron can

27

be thought of as localized. This localization comes about because the electron

can form a bound state with the local deformation it induces. The deformation

extends only to nearest neighbours in the small polaron limit.

Once trapped, the only means by which the polaron can contribute

to conduction is by hopping from one lattice position to an equivalent one.

But for an equivalent position to exist either the lattice around an unoccupied

site must distort or the lattice around the polaron must undistort or some

combination of the two must take place. All of these lattice deformations

require energy in the form of longitudinal optical phonons. At low

temperatures there are few such phonons present, and the hopping probability

is small, but at high temperatures there are exponentially more phonons

present and the hopping probability is larger. Hopping conduction can be

considered simply as a diffusion of carriers through the lattice with the

assistance of phonons. Since the diffusion constant is related to the mobility

by an Einstein relation, it is clear that the mobility of localized carrier

conduction by hopping from site to site is of the form

-W/KT0µ =µ e (2.20)

where w is the minimum energy necessary to obtain two equivalent sites. The

energy w can be easily evaluated in terms of the small polaron binding energy

Ep by noting that for small deviations around any equilibrium configuration

the potential must vary quadratically with a suitably defined distortion

parameter. Thus the minimum energy to obtain an equivalent site must be the

situation in which the region around the carrier undistorts half way; this only

requires a total of half the polaron binding energy, since the latter requires a

full ‘undistortion’ of the polaron site and the energy increases quadratically

with distortion. Thus equation (2.20) can be written

-Ep/2KT0µ =µ e (2.21)

28

The pre exponential 0 can be somewhat temperature dependent.

An algebraic factor of T-1 enters from Einstein relation relating mobility

to diffusion

µ = e D / KT (2.22)

where D is the diffusion constant. For small polaron at high temperature

Holstein finds an additional factor of T-½ in D so that 0 varies as -3/2T . Often

the exponential itself dominates the temperature dependence of the

conduction.

Hopping conduction via localized states is a very different process

from conduction through delocalized states. But since both processes require

temperature activation in semiconducting solids they cannot be distinguished

by electrical conduction measurements alone. The electrical conductivity is

expressed as the product of the carrier concentration n and the mobility

= ne (2.23)

For both hopping conduction and semi conduction, we expect

= 0 eEa/KT (2.24)

where Ea is the experimentally determinable activation energy. The unique

feature of hopping conduction however is equation (2.21), activation energy

of the mobility. Essentially all of the contribution to Ea in band conduction

comes from the carrier concentration and Ea can be associated with half the

energy gap in the intrinsic region.

For hopping conduction, there still can be a contribution to Ea from

n and there must also be contribution from . Thus for hopping we may write

29

Ea = Eb + Ep/2 (2.25)

where Eb represents the energy necessary to force the carrier from a defect

site.

In an ordinary semiconductor, the thermoelectric power

measurement can be used to separate mobility from carrier concentration.

Analogous results can be derived for hoping conductivity. The thermoelectric

power becomes

bkS= E / KT + aq

(2.26)

where a could be very small if no vibrational energy is transferred when

energy is transferred by the hopping. When predominant conduction

mechanism is by hopping of small polaron holes, the sign of the

thermoelectric power should be p type.

Hopping conduction is not associated only with small polaron

formation, and the same process can occur whenever the carriers are in

localized state. At low temperatures, in any imperfect semiconductor or

insulator, there are always carriers bound to donors or acceptors and in

localized state. If there are both donors or acceptors present and some donor

levels are above some of the acceptor levels, thermal equilibrium requires that

equal concentration of donors and acceptors are ionized even at T = 0K. Let

us assume a concentration Nd, of shallow donors and Na shallow acceptors

with Nd > Na. Then all Na acceptors and Na of the donors per unit volume will

be ionized at a very low temperature while a concentration (Nd - Na) of the

donors will be localized. The random electric fields from the ionized donors

and acceptors will spread the donor levels somewhat. Further more the

environment of an occupied and of unoccupied donor are very likely quite

different because of the presence or absence of the electron itself. In such a

30

situation phonon assisted hopping of the electrons through the localized donor

levels becomes a possible conduction mechanism. The activation energy w is

a measure of the spread in energy of the donor state, and should be ordinarily

comparable to the ionization energy of the donor. Thus at sufficiently low

temperatures this mechanism of conduction, known as impurity conduction

should predominate.

2.4.2 Band Conduction

The energy levels available to any electron in solid form quasi

continua called bands which are separated by regions of forbidden energies

called gaps. Insulators are those solids in which there are sufficient numbers

of electrons to just fill all the lowest energy bands while leaving all the higher

energy bands empty. If there is a relatively large (>2eV) gap between the

highest filled band known as the valence band and the lowest empty band

known as the conduction band, then the material is an insulator.

2.4.3 Hubbard Model

The Hubbard model is the simplest model of interacting particles in

a lattice, with only two terms in the Hamiltonian a kinetic term allowing for

tunneling (hopping) of the particles between sites of the lattice and a potential

term consisting of an on-site interaction. The Hubbard model is a good

approximation for particles in a periodic potential at sufficiently low

temperatures that all the particles are in the lowest Block band, as long as any

long- range interactions between the particles can be ignored. For electrons in

a solid, this model can be an improvement on the tight binding model, which

includes only the hopping term. For strong interactions, it can give

qualitatively different behaviour from the tight binding model, and correctly

predicts the existence of Mott insulators, which are prevented from becoming

conducting by strong repulsion between the particles.

31

In the tight binding approximation, electrons are viewed as

occupying the standard orbitals of their constituent atoms, and then hopping

between atoms during conduction. Mathematically, this is represented as a

hopping integral or transfer integral between neighbouring atoms, which can

be viewed as the physical principle that creates electron bands in crystalline

materials. However, the band theories do not consider interactions between

electrons. By formulating conduction in terms of the hopping integral, the

Hubbard model is able to include the so called onsite repulsion, which stems

from the coulomb repulsion between electrons. This sets up a competition

between the hopping integral, which is a function of the distance and angles

between neighbouring atoms, and the onsite repulsion. Therefore it can

explain the transition from conductor to insulator in certain transition metal

oxides as they are heated the nearest neighbour spacing increases, which

reduces the hopping integral and the onsite potential is dominant. Similarly

this can explain the transition from conductor to insulator in systems such as

rare earth compounds. As the atomic number of the rare earth metal increases

the lattice parameter increases or the angle between atoms can also change,

thus changing the relative importance of the hopping integral compared to the

onsite repulsion.

2.4.4 Mott Insulators

The band theory of conduction is based on two major assumptions

that the adiabatic approximation and the one-electron approximation are both

valid. The adiabatic approximation assumes that the ion cores are too massive

to react rapidly to the motion of the electrons, but that the electrons

immediately adjust to any ionic motion. This simplification enables the

electronic problem to be decoupled from that of the vibration of the ion cores.

First the electronic energy bands with the ions at their equilibrium position are

determined, and then solutions are found for the normal modes of vibration of

32

the ions (i.e. phonon modes) about these positions. The total energy of solid is

then the sum of the ground state energy, the energy of any excited electron

and the vibrational energy of the oscillating ion with negligible coupling

between the two excitations.

The one electron approximation assumes that each electron, as it

propagates through the solid, interacts only with the time-averaged negative

charge due to all the other electrons. This clearly over estimates the energy,

since it does not allow for the possibility that the other electrons can correlate

their motion to avoid being in the same region of space at the same time as the

electron under consideration. Thus the so called correlations are neglected.

Although the one electron approximation is appropriate for the vast

majority of ordinary metals and semiconductors, this is not the case for most

insulators. If the widely separated atoms are brought further together, the

electrostatic repulsion between two electrons on the same proton is reduced

by screening effects due to the polarization of all the other electrons.

Concomitantly, this increase of density results in an increasing bandwidth .

Thus the bandwidth is a measure of the validity of the one-electron

approximation. Mott (1974) presented physical arguments which suggest that

for every solid a critical value of exists below which the one-electron

approximation becomes invalid. In such a material even the outer electrons do

not spread into bands but remain localized around a particular ion core. If this

occurs, the solid is an insulator, independent of the fractional occupancy of

the outer most bands in the one-electron approximation. Such solids are called

Mott insulators.

2.5 THERMOELECTRIC PROPERTIES

Thermoelectric power is the property of a solid subjected to a

temperature gradient. This temperature gradient results in the establishment of

33

a thermoelectric voltage across the specimen. Thermoelectric power or the

Seebeck coefficient is defined as the thermoelectric field per unit temperature

gradient.

ESgrad T

(2.27)

if the electric current density J is zero.

Considering a one – dimensional temperature gradient,

dTgradT ,dx

the Seebeck voltage is also in the x direction and

x

J 0

ESdT / dx

x

(2.28)

where Jx is the electrical current density along the x axis.

The figure of merit (z) of a thermoelectric material is defined by the

expression

2SZK

(2.29)

where S is the Seebeck coefficient (µV/deg)

is the electrical resistivity (ohm. met) and

K is the thermal conductivity (Watt m-1 K-1)

Thus a good thermoelectric material should have a high Seebeck coefficient,

low electrical resistivity, low thermal conductivity and good physical and

chemical stability at high temperatures (Kiselov et al 2009). The most

34

important group of thermoelectric materials is semiconductor compounds. A

number of laboratories in search for better materials, conducted studies on

perovskite type compounds for thermoelectric applications. (Galasso 1969).

Thermoelectric measurements however have helped to understand better the

mechanism of conductivity in many perovskite type compounds.

2.6 REVIEW OF PROPERTIES OF DIELECTRIC MATERIALS

AND THEIR APPLICATIONS

Azough et al (2006) studied the dielectric properties of

1/3 2/ 3 3Ba Me Nb O (Me = Zn, Co, Ni and Mg) ceramics. Q values of the

ceramics depend on the degree of cation ordering and the additives. Slow

cooling leads to 1:2 ordering of the B sites and enhanced the dielectric Q

values. Additions of BaO-4WO3 or V2O5 yield higher Q values. The

temperature coefficient of resonant frequency of these ceramics is reported.

Dielectric properties of ceramics in lead zirconate titanate - lead magnesium

niobate system are reported to have ferroelectric behaviour (Rattikorn

Yimnirum et al 2004). Dielectric properties of Ca based perovskites

3 51/ 2 1/ 2 3Ca B B ' O B Al,Cr, Mn,Fe B' Nb,Ta 2 5

1/3 2/3 3and Ca B B ' O (B = Mg,

Ca, Co, Ni, Cu, Zn) (B = Nb, Ta) are reported (Hiroshik Kagata et al 1994).

Ca based complex perovskite have lower dielectric constant when compared

with well known barium based perovskite dielectrics. Dielectric properties of

high curie temperature ferroelectrics (l-x)Pb(Fe0.5Nb0.5)O3 – xPb(Zr0.2Ti0.8)O3

are reported (Fang et al 2009). These ceramics are found to exhibit first order

ferroelectric phase transition. Dielectric characterization of novel perovskites

of the type (Ln1/2Na1/2)TiO3 (Ln = Dy, Ho, Er, Tm, Yb, Lu) are studied

(Shan et al 1998). The dielectric loss and the electrical properties are found to

be sensitive to the synthesis route. The perovskite compound Ba3MnNb2O9

synthesised via solid state reaction route showed reasonable dielectric

35

properties than the sample synthesized via aqueous solution process (Yun Liu

et al 2005).

Strontium deficient perovskites with composition

Sr0.86(Ga0.36Ta0.64)O3 are prepared and its dielectric properties are reported

(Takahashi et al 1997). Ga3+ is replaced by Sc3+, In3+, Y3+, Nd3+ and La3+ and

reported that the dielectric constant of these ceramics indicated no distinct

dependence on the Bsite cation species. Higher microwave Q factor are

obtained for these compounds. Various electrical properties, dielectric

constant, hysteresis, pyroelectric and piezoelectric response of the solid

solution x Pb(In1/2Nb1/2)O3 – (1-x) Pb(Sc1/2Ta1/2)O3 have been reported

(Edward Alberta et al 1996). The temperature dependence and the anomaly

present in the oxide system La0.53Na0.41-xLixTiO3 has been reported (Tetsuhiro

Katsmata et al 2002). The temperature coefficient of permittivity and its

dependence on the structural changes in the complex perovskites have been

reported by Colla et al (1993). Microwave dielectric properties of

Pb based complex perovskite ceramics have been reported (Yong Scho et al

2003).

LaMnO3 is reported to be an insulator and La1-xSrxMnO3 is found to

be metallic in character. Effect of Sr doping enhances the low frequency

polarization and decreases the activation energy (Chern et al 2005). In the

perovskite oxides NdNi1-xCuxO3, the electrical properties change drastically

with x. The metal - insulator transition temperature decreases as x increases

upto x = 0.03 and for x > 0.03 the system is found to be metallic for all

temperatures (Perez et al 1996). Cubic perovskite type Sr(Mn0.97Nb0.03)O3

(x = 0.03) exhibited a metal - insulator transition at 390 K due to the spin state

of Mn3+ ion. The system is found to be of n type semiconductor in the

temperature range of 80 – 773K (Taguchi et al 2005).

36

Addition of 2 to 6% of La3+ in BaZrO3 has increased its resistivity.

The mechanism of conduction is indicated to be p type, with La3+ improving

resistivity by lowering the hole concentration. Break down strengths is

measured to be very high and this material is found to be a high temperature

capacitor dielectric material (Koenig et al 1964). Low doped samples of

Sr2[Fe(Re1-xWx)]O6 (x = 0.1 – 0.2) exhibits metal - insulator transition. For

sample with x = 0.3 – 1.0 at low temperature it behaves like an insulator with

a weak localization of charge carriers and has a good agreement with Mott’s

hopping theory. At high temperature charge conduction follows thermally

activated semiconductor type (Poddar et al 2004). Temperature dependence

resistivity of polycrystalline Sr2(FeMo)O6 is similar to metallic type and when

doped with V it behaves like an insulator at low temperature and at high

temperature it obeys T2 variation (Chattopadhyay et al 2004). Electrical

insulator behaviour of perovskite type compound Sr2Cu1-xLixW 6O is

reported by Jansen et al (1993). The complex perovskite system Na1-xSrxTaO3

0.0 x 0.4 the electrical resistivity decreases with increasing strontium

content, but metallic conductivity is not reached even for the highest possible

strontium content Na0.6Sr0.4TaO3 (Isotomin et al 2000).

The electrical conductivity of the sample Ba0.5Sr0.5Co0.8Fe0.2O3 is

reported to be suitable for solid oxide fuel cells (Bo Wei et al 2005).

Vijayakumar et al (2008) reported that the complex perovskite compound

Ba2GdSbO6 is suitable for high temperature superconducting films. The

dielectric properties make Zn0.8Sn0.2TiO4 ceramic material very attractive to

applications such as dielectric resonators, filters and substrates for hybrid

integrated circuits (Ioachim et al 2003). Sung – Gap Lee (2002) reported that

the Mg doped (Ba, Sr, Ca)TiO3 ceramics are suitable for phased array

applications, Kulavik et al (2007) investigated the dielectric and electrical

properties of ceramic materials with perovskite structure and their suitable

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applications are reported. Relaxor ferroelectrics Pb(ZrTi)O3, Pb(MgNb)O3

and Pb(ZnNb)O3 are suitable for capacitors, the compositions

(La0.7Sr0.3) (Zr0.5Co2+0.2Co3+

0.3)O3, (La0.8Sr0.2) (Ti0.5Co2+0.3Co3+

0.2)O3 and

(La0.7Sr0.3)(Ti0.3Fe0.7)O3 are reported to be the candidates for low temperature

NTC thermistor, the composition Ca(Ti0.9Y0.1)O3 and (Sr0.9Dy0.1)(Ce0.8Y0.2)O3

for high temperature NTC thermistor and (Sr0.8Ce0.2)MnO3,

(Sr0.8Ce0.1La0.1)MnO3, (Sr0.8Ce0.1Sm0.1)MnO3 and (Sr0.9Ce0.1)CoO3 complex

perovskite oxides are suitable for electrode materials for solid state cells

Ba2LaNbO6 and Ba2ErNbO6 are reported to be the substrate candidates for

superconducting films (Kurian et al 2001) and (Nair et al 2004). Electrical

conductivity and thermoelectric power measurements have been carried out as

a function of oxygen partial pressure and temperature on some pure

perovskite oxides (Selvador et al 1997).