DIELECTRIC SPECTROSCOPY AND MICROWAVE CONDUCTIVITY OF BISMUTH

STRONTIUM MANGANITES AT HIGH FREQUENCIES

S. N. Mathad1, R. N. Jadhav, R. P. Pawar Vijaya Puri*

1K.L.E Institute of Technology, Hubli, India.

*Thick and thin film device lab, Department of Physics,

Shivaji University, Kolhapur-416004, India.

E mail: vijayapuri1@gmail.com

Abstract: Bismuth strontium manganites (BSM) with variation in strontium content have been

synthesized by simple solid state reaction method. X-ray diffraction data show that the samples are

orthorhombic structure with crystallites size ~40 to 60 nm. SEM studies reveal morphology of micro-rods

and micro-flakes. The dielectric constant (ε’) and the dielectric loss (tan δ) decreases with the strontium

content and the applied frequency which has been attributed to Maxwell–Wagner polarization. The

microwave conductivity of samples decreases from 8.9 S/cm to 2.29 S/cm at 8.2GHz as strontium

increases. The penetration depth has also been reported at microwave frequencies from 8-18GHz.

KeyWords; XRD, dielectric constant, AC conductivity, penetration depth, microwave

conductivity.

Corresponding Author:

Dr. Vijaya Puri

Thick and Thin Film Device Lab. Department of Physics,

Shivaji University, Kolhapur, India 416004.

Email: vijayapuri1@gmail.com

1. Introduction

The progress of human civilization has been strongly influenced by the progress in the field of

material science. The rapid improvement of mobile devices require smaller components, which can be

obtained by designing multifunctional materials which possess properties like ferromagnetism and

ferroelectricity. Both these materials are used for data storage, however ferroelectric materials are used in

random-access memories (FeRAMs).

The simple perovskite oxide ABMnO3, has many different types of ferroic phases including

ferroelectrics, anti-ferroelectrics, ferro-elastics, ferro-magnetics, anti-ferromagnetics, and coupled forms

of these. The materials with the general formula of ABMnO3, where A site is a trivalent ion (La, Bi, Pr)

and the B site a divalent ion (Sr, Ba, Ca) known as manganites, have been studied widely in the recent

years [1–2]. Rare-earth mixed-valence manganites (Ln1−xAxMnO3) with perovskite structure have been

focused on during the last few years because of their colossal magnetoresistance (CMR) and charge

ordering (CO) phenomena [3]. When hole carriers are introduced in the parent LaMnO3, the

antiferromagnetic (AFM) insulating phase is converted into ferromagnetic (FM) metallic phase mainly

due to the double exchange interaction. On the other hand, if Ca2+ is substituted, the intriguing CO state

emerges near x ∼ 0.5, which is usually considered as a spatially ordered distribution of Mn3+ ↔Mn4+ ions

in the lattice in a purely ionic picture accompanied by orbital ordering. Recently Ca or Sr doped BiMnO3

have presented quite different properties from the other manganites, even though the ionic size of Bi3+

(1.24 Å) is almost same as that of La3+ (1.22 Å). When Sr or Ca is substituted, FM state weakens rapidly

and the CO phase is induced [4]. In contrast to lanthanide manganites, the bismuth manganites

Bi1-xAxMnO3 (A=Ca, Sr) have not been studied much, may be because of their lack of spectacular CMR

properties [5-6]. It was the charge order (CO) phenomenon displayed by many of these compounds

which has centered the attention of many research works on these compounds. CO is usually understood,

in a purely ionic picture, as a spatially ordered distribution of Mn+3↔Mn+4 ions in the lattice. To explain

the ferromagnetism in BiMnO3 an orbital ordering of dz2 and dx

2-y2 of Mn ions in the perovskite lattice has

been discussed [7]. High temperature charge ordering [8], magneto-optic effects [9], Raman spectroscopy

[10], dc and I-V characteristics of resistive manganites [11] have also been reported. The intrinsic

properties of ceramic materials are influenced by shape, size, orientation, voids, inhomogeneities, etc. of

the grains, the large percentage of their atoms in grain boundary environments and the interactions

between grains.

The main purpose of the present work is synthesis of Bi(1-x)SrxMnO3 where x=0.20, 0.25, 0.40 and

0.50 (BSM) using well known solid state reaction. These samples were named Sr20, Sr25, Sr40 and Sr50

respectively. Detailed structural and surface morphological, transport properties of samples have been

studied. The X and Ku-band microwave transmittance, absorbance and conductivity of Bi(1-x)SrxMnO3 are

also reported. To the authors knowledge the transport and microwave properties of Bi(1-x)SrxMnO3 have

been reported for the first time in this work.

2. EXPERIMENTAL PROCEDURE

The rod structured Bi(1-x)SrxMnO3 (BSM) where x=0.20, 0.25, 0.40 and 0.50 sample was prepared by a

low cost, conventional solid state synthesis technique. The starting materials were bismuth oxide (Bi2O3),

strontium chloride (SrCl2.6H2O) and manganese chloride (MnCl2.4H2O) were of high purity AR grade

Sd-fine chemicals (99.9% of purity). The mixture was ball milled in ethanol for 1 hr, dried and then

sintered in oxygen atmosphere at 900°C for 7h. The obtained material was then ball milled and calcinated

again under same conditions at 10500C for 12 hrs. The powder was homogenized by grinding it up to 30

min in agate mortar. The disk-shaped pellets, adding proper amount of PVA binder, was prepared by

pressing the powder under a load of 10 ton cm-2 for 10 min. The dimensions of the pellets were ∼1 cm in

diameter with thickness ∼0.125–0.20 cm. The addition of binder not only imparts strength and good flow

properties to the material but also strengthens the compacted part so that it can be handled without

damage prior to firing. The structural and phase analysis of the calcinated powder and sintered pellets

were done by XRD (Philips-3710) with Cr Kα1 radiation in the 2θ range from 200 to 90°. Surface

morphology was studied using SEM model JEOL JSM 6360.

The dielectric measurements were made on polished ceramic discs with silver coating on both sides,

which replicates a capacitor. The relative dielectric constant (ε’) and dielectric loss (tanδ) were measured

as parametric functions of frequency. The dielectric constant of the samples was measured using air dried

silver paint coated on the flat surfaces to form electrodes for dielectric measurements using the

commercially available impedance analyzer (SOLARTRON 1260A) in the frequency range 20Hz to

2.5MHz at room temperature. Transmission of microwaves due to BSM was measured point by point

using transmission and reflection method with rectangular waveguide consisting of the X and Ku band

generator, isolator, attenuator, directional coupler and RF detector. The microwave conductivity and

penetration depth of BSM samples have been calculated at microwave frequencies using the complex

permittivity factors.

3. RESULTS AND DISCUSSION:

3.1 XRD STUDIES:

The X-ray diffraction (XRD) technique was used to confirm the presence of crystalline phases and to

study the influence of atmosphere on the calcinations process. The orthorhombic structure of BSM

ceramics was confirmed by X-ray spectrum shown in Fig. 1. The average crystallite size for the different

compositions was calculated by Debye Sherrer’s formula [12] which is in range of ~40 to 60 nm.

θβ

λ

cos

9.0=D Where D is the diameter of the crystallites of powder, λ is the wavelength of the Cr-

Kα line, β is the FWHM in radians and θ is Bragg’s angle.

The lattice parameter of each sample was calculated from the (hkl) values of the diffraction peaks.

The cell parameters were found to vary with change in concentration of strontium. The cell parameters a,

b and c lie between 5.635-5.538 Å, 5.657-5.452 Å and 3.893- 3.704 Å respectively. These cell parameters

are in good agreement with the results of JCPDS (JCPDS Card No. 49-0774). The observed profile

reflects the variation of lattice parameters, which depends only on the direction of the reciprocal lattice

vectors. We conclude that the observed effect unequivocally indicates a variation of lattice parameters in

BSM. The anomaly of the lattice parameters is due to the development of a static Jahn-Teller distortion of

the Mn+3O6 octahedra [2].

3.2 Scanning Electron Microscopic studies

The morphology of the BSM samples is shown in Fig. 2. Strontium concentration plays a crucial role in

the morphology, the decrease in strontium content vividly changes growth mechanism of these ceramics.

Sr50 has dense rod like morphology with length from 3µm to 45µm, however Sr40 shows same rod like

morphology, with variation of rod length ranging from 4µm to 60 µm. Another interesting aspect in Sr40

is that small beads and irregular particle growth on the rods are observed. Further decrease in strontium

content i.e Sr25 shows randomly oriented different shaped dense micro-rods like structure of length 4µm

to 50µm with larger beads. On the other hand saturation of rods into flakes and bulky (3D) structures at

lesser content of strontium Sr20 is observed. The flakes observed in Sr20 of different sizes may be due to

too much dense growth of rods leading to this transformation resulting in cube like morphology at some

places (inset). Decrease of strontium concentration leads to increase in grain growth of BSM hence

morphology changes from micro-rods to flakes, beads and agglomeration of these to form cubic identities.

These rods like structure may be due to tunnel-structured manganese oxides behavior [13-14].

3.3 Dielectric Studies:

The dielectric properties of BSM ceramics at room temperature in the frequency range 20Hz to

2.5MHz were studied. The variations of dielectric permittivity (ε′) with log of frequency for these systems

are shown in Fig. 3. Dielectric permittivity of BSM decreases with increase in frequency and Sr content.

It is observed that dielectric constant of BSM decreases from 3.73 x 108 to 4.85 x 105 for Sr20 and Sr 50

at 20Hz respectively.

The relative dielectric permittivity decreases steeply at lower frequencies and remains constant at

higher frequencies which indicates the usual dielectric dispersion. This may be attributed to the

polarization due to changes in valence states of cations and space charge polarization. At higher

frequencies the dielectric constant remains independent of frequency due to the inability of electric

dipoles to follow the fast variation of the alternating applied electric field. It is found that the ε’ decreases

on increasing frequency which indicates a normal behavior of the dielectric materials. The higher values

of ε’ at lower frequency are due to the simultaneous presence of all types of polarizations like space

charge, dipolar, ionic, electronic etc. which is found to decrease with the increase in frequency. At high

frequencies electronic polarization only exists in the materials. The polarization in manganates is via a

conduction mechanism due to electron-hopping between Mn3+ and Mn4+ ions. At high frequencies, the

electron-exchange between Mn3+ and Mn4+ ions cannot follow the alternation of the applied ac electric

field hence ε´ and tan δ fall to smaller values. The hopping mechanism appears to be favourable at lower

applied ac electric field frequencies. Thus at lower frequencies, the grain boundaries are more active than

the grains, while at higher frequencies only grains are active in electrical conduction[15-17].

The decrease in permittivity with frequency can be explained on the basis of Koops’

phenomenological theory since dielectric dispersion is the dependence of the permittivity of a dielectric

material on the frequency of an applied electric field. Because there is always a lagging relation between

change in polarization and change in an electric field the permittivity of the dielectric is a complicated,

complex-valued function of frequency of the electric field. According to Maxwell-Wagner polarization,

in dielectric spectroscopy, large frequency dependent assistance to the dielectric response particularly at

low frequencies may come from charge build-up. This is due to either at inner dielectric boundary layers

on a mesoscopic scale or at the external electrode-sample interface on a macroscopic scale. In both cases

this leads to a separation of charges through a depletion layer. The charges are often separated over a

considerable distance and the contribution to dielectric loss can therefore be orders of magnitude larger

than the dielectric response due to molecular fluctuations. The power dissipation in material is directly

proportional to dielectric loss factor. The dielectric loss shows similar behavior as dielectric constant as

seen from Fig.3. This loss factor curve is considered to be caused by ion migration losses, electron

polarization losses and dipole relaxation losses. At higher frequencies the losses are found to be low if

domain wall motion is inhibited and magnetization is forced to change by rotation due to domain wall

resonance [18].

The variation of relative dielectric permittivity can be related to the collective behaviour of both types

of electronic charge carriers, electrons and holes. The presence of Sr3+ and Sr2+ ions render BSM ceramics

to be dipolar. As we know that rotational displacement of dipoles results in orientational polarization

hence rotation of Sr2+ ↔ Sr3+ dipoles may be visualized as the exchange of electrons between the ions so

that the dipoles align themselves in response to the alternating field. The polarization switch of as

function of frequency [19]. The same thing is observed, as frequency decreases the polarization

decreases, reaches a constant value due to the fact that beyond a certain frequency of external field the

electron exchange Sr3+ ↔ Sr2+ cannot follow the alternating field. In the present study, it is observed that

the dielectric permittivity goes on decreasing with the increase of Sr2+ concentration. However these

materials have high dielectric constants, extensively applicable in microwave telecommunications,

microelectronics technologies, discrete and multilayer capacitors applications and microwave integrated

circuits and microwave dielectric resonators.

3.4 AC Conductivity

To know the conduction mechanism in BSM samples the ac conductivity is studied. The variation of

ac conductivity with frequency is as shown in Fig. 4. The ac conductivity (σac) is calculated from

dielectric data using the relation, where ε’’ is dielectric constant, ‘ε 0’ is

permittivity of free space.

The plots are almost linear confirming small polaron type conduction. Frequency dependant ac

conductivity increases with increase in frequency indicating that conduction occurs due to hoping of

charge carriers among the localized states. In ionic solids, the electrical conduction is due to migration of

ions and this ionic transport depends on the angular frequency. This is the normal behavior of a

ferroelectric material. The sample Sr50 shows more conductivity compared to other samples and Sr40

shows least conductivity. As the frequency of the applied field increases, the conductive grains become

more active, thereby promoting electron hopping between two adjacent sites. At lower frequencies the

grain boundaries are more active and hence the electron-hopping between Mn3+ and Mn4+ and Sr2+ and

σ a.c = ω ε 0ε’’

Sr3+ ions is less at lower frequencies. As the frequency of the applied field increases the conductive grains

become more active thereby promoting hopping conduction. At higher frequencies the frequency of the

hopping ions might not be able to follow the applied field frequency and it lags behind. Thus conductivity

decreases at higher frequencies. This behavior can be attributed to the relaxation process associated with

the domain reorientation, domain wall motion, and the dipolar behavior. Thus ac conductivity is

proportional to angular frequency confirming linear nature [18, 20].

3.5 Microwave Measurements:

The electromagnetic transmittance, absorption and conductivity studied at X band (8-12GHz) and

Ku band (13-18GHz) frequencies using waveguide reflectometer technique as shown in Fig.5. The

transmittance of bismuth manganite with strontium content in 8-18 GHz frequency range is plotted in Fig.

6. The composition dependent variations are observed in microwave transmittance. The microwave

transmittance increases with strontium from 20% -60% at 12GHz. From SEM and microwave

transmittance it can be said that the rod like morphology of BSM is useful to increases the

transmittance. From transmittance and reflectance of BSM the microwave

absorption is calculated which is plotted in Fig.7as a function of frequency in the X-band (8–12 GHz) and

Ku-band (13–18 GHz). From Fig.7 it is observed that as Sr concentration increases the microwave

absorption decreases from 45% - 14% at 12GHz. However oscillatory behavior in microwave absorption

from 9-10.8 GHz is obtained and the absorption is in the range 40-80%. The high absorption in a large

band of frequencies indicates potential for microwave applications [21]. The absorption is achieved

through dielectric loss mechanisms that convert electromagnetic energy into heat. Materials based on

purely dielectric phenomenon and electric losses are often considered unusable as a broadband absorber

due to their inability to absorb power for low frequencies. Absorption is the heat loss under the action

between electric dipole in material and the electromagnetic field [22].

In case of BSM as Sr content increases the rod like morphology also increases and might be due to

this absorption decreases because rod like structure increase the transmittance of microwave. From

σ =ωε’’ε0

dielectric measurement it is observed that loss decreases with Sr and due to this energy dissipation

decreases which are also responsible for the decrease in absorption.

3.6 Penetration Depth and Microwave Conductivity:

Microwave energy is transferred to the material by interface of the electromagnetic field at the

molecular level. The dielectric properties determine the effect of the electromagnetic field on the material.

When electromagnetic radiation is incident on the surface of a material, it may partly reflect from that

surface and there will be a field containing energy transmitted into the material. This electromagnetic

field interacts with the atoms and electrons inside the material. Depending on the nature of the material,

the electromagnetic field might journey very far into the material, or may pass on out very rapidly. For a

given material, penetration depth will generally be a function of wavelength. The penetration depth is

used to denote the depth at which the power density has decreased to 37 % of its initial value at the

surface. Materials with higher imaginary part of the complex permittivity show faster microwave energy

absorption. The penetration depth is calculated according to equation, which depends on the dielectric

properties of the material [23]. D= c/ ω {2 ε’(1+( ε”/ ε’)2)1/2-1}1/2

Where c is speed of light, ω is angular frequency ε’ relative dielectric constant and ε” is dielectric loss of

material that describes the behavior of a dielectric material under the influence of the microwave

field. The penetration depths of all the samples are shown in Fig. 8. The microwaves penetration

depth of Sr25 is relatively more compared to other samples and Sr20 shows least penetration depth. The

nature of penetration depth of all the samples show frequency dependent as well as common dispersion

behavior and penetration depth decreases as function of frequency. The value of penetration depth of Sr25

is about 11.6 X 10-5 for 8.2GHz.

The microwave conductivity of ceramics was calculated using the equation [24]

Where ω is angular frequency, εo and ε” is permittivity of free space and dielectric loss of material.

The change in microwave conductivity of BSM with frequency (8-18GHz) and strontium content is

shown in Fig. 9. It is observed that for all the samples of the Bi(1-x)SrxMnO3 ,the conductivity decreases

with strontium content. The microwave conductivity of samples (Sr20 and Sr50) decreases from 8.9 S/cm

to 2.29 S/cm at 8.2GHz with strontium concentration and 4.18 S/cm to 0.287 S/cm at 18GHz.Thus Sr20

has more conductivity compared to other samples.

4. Conclusion:

Bismuth strontium manganites have been successfully synthesized by cost effective solid state

reaction. X-ray diffraction data reveals the formation of orthorhombic structure phase of Bi(1-x)SrxMnO3.

Further different sized micro-cubes and micro-rods like morphology of all the samples are confirmed by

scanning electron microscopy. All of them depict similar types of variation in dielectric constant and loss

factor decreases with an increase in frequency as well as strontium concentration, dielectric constant

varies in wide ranges of 3.73X108 and 4.85X105 for Sr20 and Sr50 at 20Hz which obeys Koop’s

phenomenological theory of dielectric dispersion. The AC conductivity of BSM was calculated from the

dielectric measurements. The ac conductivity measurement with frequency leads us to conclude that the

conduction mechanism in the present due to small polarons. Bi(1-x)SrxMnO3 illustrates absorbing nature

over a large range of frequency in the X and Ku band region of the electromagnetic spectrum .To the

author’s knowledge the dielectric, transport studies, and microwave studies at X and Ku band have been

reported for the first time. The microwave conductivity of samples decreases 8.9 S/cm to 2.29 S/cm at

8.2GHz as strontium doping increases and penetration depth also shows variations with strontium content.

Acknowledgement

One of the authors Vijaya Puri gratefully acknowledges the UGC India for Award of Research

Scientist ‘C’. S. N. Mathad very much thankful to Management, and Principal K.L.E Institute of

Technology, Hubli. R. N. Jadhav would like to express thanks to DST for Award of Women Scientist-A.

All authors acknowledge the UGC-SAP and DST-PURSE.

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Figure Captions

Fig. 1. X-ray diffraction spectra of the Bismuth strontium manganites.

Fig. 2. Surface morphology of BSM.

Fig.3. Variation of relative dielectric permittivity (ε’) and dielectric loss factor (tan δ) of

Bi(1-x)SrxMnO3 with frequency.

Fig.4. Variation of ac conductivity of Bi(1-x)SrxMnO3 with frequency of BSM samples.

Fig. 5. Schematic of the waveguide reflectometer set up for microwave measurement.

Fig.6. Transmittance of Bi(1-x)SrxMnO3 in X and Ku band.

Fig.7. Microwave absorption of Bi(1-x)SrxMnO3 in X and Ku band.

Fig.8.Penetration depth of Bi(1-x)SrxMnO3 in X and Ku band.

Fig.9.Variation of microwave conductivty with Sr in Bi(1-x)SrxMnO3 in X and Ku band.

Fig. 1. X-ray diffraction spectra of the Bismuth strontium manganites.

Fig. 2. Surface morphology of BSM.

Fig.3. Variation of relative dielectric permittivity (ε’) and dielectric loss factor (tan δ) of

Bi(1-x)SrxMnO3 with frequency.

Fig.4. Variation of ac conductivity of Bi(1-x)SrxMnO3 with frequency of BSM samples.

Fig. 5. Schematic of the waveguide reflectometer set up for microwave measurement.

Fig.6. Transmittance of Bi(1-x)SrxMnO3 in X and Ku band

Fig.7. Microwave absorption of Bi(1-x)SrxMnO3 in X and Ku band.

Fig.8.Penetration depth of Bi(1-x)SrxMnO3 in X and Ku band.

Fig.9.Variation of microwave conductivty with Sr in Bi(1-x)SrxMnO3 in X and Ku band.