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Journal of Materials ScienceFull Set - Includes `Journal of MaterialsScience Letters' ISSN 0022-2461 J Mater SciDOI 10.1007/s10853-012-7112-9

Structural, transport, magnetic, anddielectric properties of La1−x Te x MnO3(x = 0.10 and 0.15)

Irshad Bhat, Shahid Husain, Wasi Khan& S. I. Patil

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Structural, transport, magnetic, and dielectric propertiesof La12xTexMnO3 (x 5 0.10 and 0.15)

Irshad Bhat • Shahid Husain • Wasi Khan •

S. I. Patil

Received: 3 October 2012 / Accepted: 22 December 2012

� Springer Science+Business Media New York 2013

Abstract In this study, we have investigated the struc-

ture, temperature-dependent resistivity, magnetization, and

dielectric properties of La1-xTexMnO3±d (x = 0.10 and

0.15). X-ray diffraction analysis confirms the rhombohe-

dral crystal symmetry with space group R3c. For both the

samples, the temperature dependence of magnetization

plots show paramagnetic-to-ferromagnetic phase transition.

The Curie temperature (Tc) and magnitude of magnetiza-

tion increase with the Te concentration. Field-dependent

magnetization produces the asymmetric hysteresis loop

that has been attributed to the magneto crystalline anisot-

ropy induced by lattice distortion and the rare earth spin

coupling at room temperature. Temperature-dependent

resistivity plots exhibit metal–insulator transition (MIT)

and charge-ordering state. These plots have been fitted

using variable range hopping model, and the density of

states [N(EF)] has been estimated. Magnetoresistance is

measured as a function of temperature in the field of 1T,

5T, and 8T. The dielectric constant shows an anomaly near

MIT. The dielectric constant exhibits a peaking behavior

with the applied frequency and the temperature dependence

of dielectric constant attains colossal values at high

temperatures.

Introduction

Perovskite manganites Ln1-xKxMnO3 (Ln = rare earth

ion; K = divalent or tetravalent ions) have been a subject

of vivid interest in recent years because of their exotic

electronic and magnetic properties [1–5]. Most interesting

properties of the manganese perovskite arise from the

competition between ferromagnetic (FM) double exchange

(DE) and antiferromagnetic (AFM) super exchange, with

the ratios of these competing interactions being determined

by intrinsic parameters such as doping level, average ‘‘A’’

size cation radius hrAi, cation disorder, and oxygen stoi-

chiometry. These materials exhibit high magnetoresistance

for certain composition ranges [5]. Besides the colossal

magnetoresistance (CMR), charge-ordering (CO) phe-

nomena [6] has also attracted a lot of interest. Earlier

studies show that CO state and the concomitant spin and/or

orbital-ordering (OO) are favored when the long range

coulomb interaction and/or a strong lattice interaction due

to Jahn–Teller (J–T) distortion overcomes the kinetic

energy of the electron [7–9]. Wider ‘‘eg’’ bandwidth, i.e.,

larger average A site radius (rA) favors lower TCO because

the mobility of the itinerant electrons through the lattice is

higher, while narrower bandwidth induces an opposite

trend. Such, CO phenomenon has been observed when the

concentration of the charge carriers takes the rational val-

ues of the periodicity of the crystal lattice [10–12]. In the

picture of Millis et al. [13] the Jahn–Teller effect leads to

the opening of a gap at the Fermi level, which together with

the DE gives at least a qualitative description of the

experimental observation. The strong electron–phonon

interaction arising from the Jahn–Teller splitting of the

‘‘Mn’’ d level plays a crucial role. Although optical,

magnetic, thermoelectric power, dc conduction measure-

ment studies on these materials suggest a high possibility

I. Bhat (&) � S. Husain

Department of Physics, Aligarh Muslim University,

Aligarh 202002, India

e-mail: bhat.amu85@gmail.com

W. Khan

Department of Applied Physics, Z.H. College of Engg. &

Technology, Aligarh Muslim University, Aligrah 202002, India

S. I. Patil

Department of Physics, University of Pune,

Ganeshkhind 411007, Pune, India

123

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DOI 10.1007/s10853-012-7112-9

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of conduction due to a hopping process of small polarons

[14–16].

In general, doping of LaMnO3 with divalent or tetra-

valent ions derives the manganese ions into a mixed

valence state. The effect of tetravalent ion doping at La site

of the LaMnO3 like Ce4?, Zr4?, Sn4?, and Te4? has been

reported earlier [17–24]. These reports include x-ray pho-

toemission spectroscopy (XPS) and Hall effect measure-

ments have revealed that Te ions are in the tetravalent state

and the manganese ions exist in a mixed valence state of

Mn2? and Mn3? in La1-xTexMnO3 (x = 0.10 and 0.15)

[19–21]. The introduction of Te drives manganese Mn3?

ions of LaMnO3 into a Mn2? ions, which is equivalent to

introducing electron into the eg band. The mixed phase of

Mn3?/Mn2? is the key component to understand the CMR,

charge-ordering effects, and the transition from the FM

metal to paramagnetic (PM) semiconductor in such

systems.

The transition metal oxides (manganites) of the type

ABO3 where (A = rare earth ion and B = transition metal

ion) also show ferroelectric behavior [25–27]. Large

dielectric constants are expected for ferroelectrics in a

narrow temperature range close to transition temperature or

in systems where we observe dielectric response based on

the mechanism of hopping of charge carriers that diverge

toward low frequencies. Recently, a new class of materials

exhibiting a colossal dielectric constant (CDC) (e0[ 103)

has also gained considerable attention because of its

application in high-e0 electronic materials, such as random

access memories. Fundamental interest was initiated by the

observation of CDC behavior in some high Tc compounds

[28–30]. During the last decade, similar observations of

CDC behavior have been reported in an increasing number

of materials, such as transition-metal oxides [31–35].

Several theoretical and experimental results previously

reported have claimed that internal barrier layer capacitor

(IBLC) [36–39] and surface barrier layer capacitor (SBLC)

[40, 41] are the two special influential factors giving rise to

the CDC behavior. It is believed that such barrier layers

may get developed at grain boundaries or at the planar

defects and at the interface between the metallic electrode

and the bulk sample, giving rise to the formation of

Schottky barriers.

The role of grain boundaries is expected to be drastically

enhanced in compounds with a mixed-phase character, not

only owing to their inherent character, but also because

they can act as accumulative pinning centers for structural

defects. Earlier studies [42, 43] on low-frequency dielectric

properties of doped manganite reveal that these properties

are affected by barrier-layer capacitor microstructure, cre-

ated because of the oxidation of grain boundaries, which

forms an insulating layer. Various theories have excluded

the possibility of the intrinsic origin of high dielectric

constant [44, 45]. These studies conclude that the internal

inhomogeneity plays the major role. It is suspected that

such inhomogeneity arises from crystal twinning or some

internal domain boundaries. In view of the above, we have

focused on the problem to study the structural, transport,

magnetic, and dielectric properties of Te-doped LaMnO3.

It is one of the lesser studied materials as far as the

dielectric properties are concerned.

Experimental

The standard solid-state reaction route is used to prepare

the samples of La1-xTexMnO3 (x = 0.10 and 0.15). Stoi-

chiometric amounts of La2O3, TeO2, and MnO2 are mixed,

ground, and heated at 1050 �C for 20 h. After first heat

treatment, these samples are ground again and sintered at

1100 �C, with two intermediate grindings. The powder

x-ray diffraction pattern is recorded using a x-ray diffrac-

tometer with Cu Ka (k = 1.5406 A) radiation at room

temperature. Magnetization versus temperature measure-

ments and the magnetic hysteresis measurements were

carried out using vibrating sample magnetometer. The

resistivity as a function of temperature is measured with

four-probe technique. The dielectric constant measurement

is performed on the Novo control, Alfa-A high-perfor-

mance frequency analyzer. The samples were palletized in

disk shape at a pressure of 6 ton/cm2, and silver paste was

sputtered on their surfaces to ensure good electrical contact

with the electrode capacitor.

Results and discussion

Structural analysis

The room temperature x-ray diffraction (XRD) patterns

confirmed single-phase nature of the samples. The com-

pound La1-xTexMnO3 (x = 0.10 and 0.15) has a rhombo-

hedral crystal structure with space group R3c. The crystallite

sizes of the samples calculated using Debye–Scherrer’s

formula are found to be 346 and 377 A, for x = 0.10 and

0.15, respectively. The structural parameters were refined by

the standard Rietveld technique [46]. The experimental and

calculated XRD patterns for La0.9Te0.1MnO3 and

La0.75Te0.15MnO3 with conventional Rietveld parameters

are shown in Fig. 1. The lattice parameters and the unit cell

volume of both the samples are tabulated in Table 1.

The unit cell volume increases with Te concentration.

This is consistent with the earlier reports [19–21, 47] since

it is known that the radius of Te4? is about 0.097 nm and

that of the La3? is 0.1216 nm. The part of substitution of

Te4? ion for La3? ions should cause the reduction in the

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ionic size hrAi of the rare earth ion, which will cause the

cell constriction and the distortion in the MnO6 octahedron,

resulting in the decrease of Mn–O–Mn bond angle from

180�. However, we have found that an increase in the Mn–

O–Mn bond angle from 160.7� to 163.4� and small changes

in Mn–O bond lengths (dMn–O * 1.96 for x = 0.10 and

dMn–O * 1.95 for x = 0.15) with the increasing Te con-

centration result in the increase in volume. Another reason

for this increase may be the oxygen stoichiometry (d) in

La1-xTexMnO3±d, and consequently the respective valence

states of ions present at A and B sites.

Figure 2 shows the Fourier transform infrared (FTIR)

spectrum for La1-xTexMnO3 (x = 0.10 and 0.15) at room

temperature. Since rare earth manganites of type (ABO3),

such as LaMnO3, are known to have a distorted crystal

structure with Mn (central atom) octahedral surrounded by

its nearest neighbor six O ions. It is believed that MnO6, in

its ideal form has six vibrating modes, two of which are

reported to be IR active [48–50].

The irrational bands present around 606 and 418 cm-1 for

La0.9Te0.1MnO3 and the bands 607 and 409 cm-1 for

La0.75Te0.15MnO3, are the characteristics of the perovskite

structure [48]. The bands at 606 and 607 cm-1 are attributed

to the Mn–O stretching vibrations mode (ts), and the bands at

418 and 409 cm-1 are corresponding to the Mn–O–Mn

deformation (bending) vibrations mode (tb). We have

observed a shift in the tb with the increase in Te doping. The

tb mode shifts slightly toward the lower wavenumber (cm-1)

region, and a very small shift of 1 cm-1 toward higher

wavenumber is observed in ts. Since both the bands are

sensitive to the octahedral distortion in MnO6, the lowering

of symmetry arising from the Jahn–teller effect results in

Mn–O–Mn bond length to vary which produces the slight

shift in bending vibration mode tb. The relative intensity of

two vibrating modes is influenced not only by the distortion

surrounding the MnO6 octahedron, but also by the local

fluctuation of the electron density at the A (La) site.

Magnetization

Fig. 3 shows the temperature dependence of magnetization

plots for Te-doped LaMnO3. The Curie temperatures (Tc) are

found to be 210.15 and 211.43 K for x = 0.10 and x = 0.15

concentrations, respectively. The increase in Tc with the

increasing Te content may be attributed to the strengthening

of the interaction of the lattice constriction and bond angle

increase, which strengthens the FM coupling between Mn

Fig. 1 X-ray diffraction patterns of La0.9Te0.1MnO3 and La0.8Te0.2MnO3.

The experimental data points are indicated by stars, and the calculated

profile by solid traces. The lowest curve shows the differences between

the experimental and the calculated data. The vertical bars indicate the

expected reflection positions for rhombohedral structure

Table 1 Lattice parameters and unit cell volumes (V) of

La1-xTexMnO3 (x = 0.10 and 0.15) at room temperature

Concentration (x) Lattice parameters V (A´

)3

a (A´

) c (A´

)

x = 0.10 5.511 13.321 350.36

x = 0.15 5.514 13.325 350.84

Fig. 2 FTIR spectra for La1-xTexMnO3 (x = 0.10 and 0.15) samples

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ions. The values of metal–insulator transition (MIT) tem-

perature Tp for both the compositions obtained from the

resistivity measurement are lower than the values of corre-

sponding Tc; this could be due to the increase of carrier

density with the increasing Te concentration and is a com-

mon characteristics of electron-doped compounds [17, 51].

Another reason could be the size effects at rare earth site

(A site) and the variation of Mn–O–Mn bond angle, since the

substitution of Te at rare earth site varies hrAi and causes the

distortion in the MnO6 octahedron; consequently, the

Mn–O–Mn bond angle increases, which may influence the

FM-DE and AFM super-exchange interaction differently,

and hence the increment in Tc. The magnetization has dras-

tically increased with Te concentration, and can be attributed

to the competition between the DE and the core spin inter-

action. This competition leads to the parallel alignment of

core spins as the doping level increases. The exchange

interaction of the Mn–Mn depends on the bond angle and the

bond length. The decrease of Mn–O length and increase of

Mn–O–Mn bond angle will make Mn–Mn exchange inter-

action stronger, leading to a higher Tc and higher magneti-

zation value. The decreases in magnetization at lower

temperatures signifies the existence of AFM-insulating

clusters along with the FM metallic clusters, which results in

the magnetic inhomogeneity and domain wall [52, 53].

The hysteresis curves for magnetization versus magnetic

field at room temperature are shown in Fig. 4. The asym-

metric reversal magnetization has been noticed in our

samples La1-xTexMnO3 (x = 0.10 and 0.15), which may

indicate the presence of FM and AFM clusters, since such

coexistence of FM and AFM domains is often the cause for

the asymmetric magnetization hysteresis and their inter-

action can strongly influence the reversal mechanism of the

magnetization, modifying nucleation and annihilation of

the vortex. Our results show a shift of the hysteresis loop

toward negative fields. The magnetization reversal mech-

anism depends upon the orientation of the cooling field

with respect to the twinned microstructure of the antifer-

romagnet, and on whether the applied field is increased to

(or decreased from) a positive saturation field. The reversal

magnetization occurs via either domain wall motion or

magnetization rotation on opposite sides of the same hys-

teresis loop. We believe that the asymmetric magnetization

in both the samples may be due to the magneto crystalline

anisotropy [54–56] induced by lattice distortion and the

rare earth spin coupling at room temperature. There are two

aspects of lattice distortion in the context of the magnetic

anisotropy of manganites: one is the global strain, which is

a general feature for many manganites and determines the

anisotropy of the uniform rotation of the spin magnetic

moments; and the other one is the alternating local dis-

tortion of the magnetic anisotropy energy known as one of

the common mechanisms responsible for the non-collinear

magnetic order. If La spin coupling has a contribution to

the magnetic anisotropy, then it is most likely due to the

interaction of La, 4f electron spin with 3d-electron Mn

spin, which weakens the magnetization as well as induces

the magnetic anisotropy. The charge-ordering and mag-

netic anisotropy, which is induced by lattice distortion and

the rare earth spin coupling at room temperature, could

behave in an AFM-ordered or FM-insulating state. We

have observed an enhancement in the magnetization with

the increasing Te concentration.

Fig. 3 Magnetization versus temperature plots for a La0.9Te0.1MnO3

and b La0.85Te0.15MnO3 samples

Fig. 4 The magnetization versus field at room temperature for

a La0.9Te0.1MnO3 and b La0.85Te0.15MnO3 samples. The insets show

the enlarged view for the same

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Transport Analysis

The temperature-dependent resistivity q(T) plots for

La1-xTexMnO3 (x = 0.10 and 0.15) are shown in Fig. 5.

On decreasing the temperature, these compounds first

undergo a PM (insulating) to FM (metallic) phase transi-

tion at transition temperatures Tp = 203 and 209 K,

respectively, then resume the semiconducting behavior. A

wide charge-ordered region centered between 70 and

140 K is observed in both the samples which could be due

the coexistence of the AFM and FM spin clusters, and the

competition between them yields charge-ordering charac-

teristics. Such phase coexistence features are explained in

the framework of electronic phase separation scenario,

predicted for the manganites [57]. This behavior is attrib-

uted to the structural inhomogeneous characteristic of the

strongly correlated electronic systems that govern the

electric and magnetic properties. The insulating behavior

results from the single ion J–T distortion as well as the

blocking of hopping by strong Hund’s coupling to the

paramagnetically disordered Mn spins. A shift in the MIT

and charge-ordering transition temperature (TCO) is

observed with the increasing Te doping amount. It is well

documented in the literature [58, 59] that the strong

increase of the resistivity with decreasing temperature can

be described by the Mott’s variable range hopping (VRH)

model [60–62]. In this model, it is assumed that the charge

carriers move along a path described by the optimal pair

hopping rate from one localized state to another. To

investigate the electronic transport mechanism, we have

fitted the resistivity q(T) data with thermally activated

conduction (TAC) model, q(T) = q0(T) exp(Ea/kBT);

Small polaron hopping (SPH) model, q(T) = q0(T) T

exp(Ea/kBT); and three-dimensional Mott’s VRH model,

q(T) = q0(T) exp(To/T)1/c—with c = 2 or 4—predicting

the charge transport by tunneling of electrons or holes [63];

c = 4 has been considered as indicative for isotropic

charge transport. Efros and Shklovski [64, 65] proposed an

alternative explanation for c = 2 in Mott’s three-dimen-

sional model where the coulomb interaction between the

charges is taken into account.

We have found that VRH model is much superior to

other models in describing the carrier transport in our

system. The fitted results are shown in Fig. 6 a, b. We have

found VRH model responsible for the transport properties

below (T [ TCO) as well as above (T \ TCO) charge-

ordered regions. The most convincing results (straight line)

are observed for log q against 1/T1/4 below MIT in both the

samples, which are typical for three-dimensional VRH

Fig. 5 lnq(T) versus T plots for La1-xTexMnO3 (x = 0.10 and 0.15)

samples

Fig. 6 lnq(T) versus 1/T1/4 plots for a La0.9Te0.1MnO3 and

b La0.85Te0.15MnO3 samples. Solid line shows Mott’s VRH model

fitting, and the inset shows the VRH fitting below the transition

temperature

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model. The value of resistivity increases above the arbi-

trary temperature (TA) with the temperature decreasing

further. A small metallic region has been observed between

203 and 196 K for La0.90Te0.10MnO3 and between 209 and

198 K for La0.85Te0.15MnO3 before resuming semicon-

ducting behavior with the temperature decreasing further. It

is a known fact that VRH transport behavior does require

the appearance of random potential; hence, due to the large

anisotropic lattice distortion of MnO6 octahedron in

La1-xTexMnO3 (x = 0.10 and 0.15) caused by charge

ordering, it will trap the electrons below TCO, and hopping

across the grain boundaries contributes toward transport

process at low temperatures. The To (characteristic tem-

perature) occurring in the VRH relation can be related to

the carrier localization length, using the expression

To = 24/(kBN(EF)pL3) [66]. The value of the localization

length should be comparable to Mn–O bond distance for

VRH type of conduction; in the present case, the mean

Mn–O bond lengths for x = 0.10 and 0.15 calculated from

XRD data are dMn–O = 1.96 A and dMn–O = 1.95 A,

respectively. The values of To and density of states N(EF)

are tabulated in Table 2. It is clear from the Table 2 that

the characteristic temperature (To) shows the minimum

value in charge-ordered region for both the samples, which

indicate less-disordered state and hence attains higher

values of density of states near Fermi level N(EF). The Chi-

square values indicate the quality of the fitted data. The

magnetoresistance as a function of temperature is measured

in the presence of the magnetic field 1T, 5T, and 8T as

shown in Fig. 7. Sharp and broad peaks can be attributed to

the coexistence of the AFM and FM clusters, contributing

to the spatially metallic and insulating areas in the studied

temperature range, which is believed to occur in materials

where the electron density is commensurate to the number

of lattice sites. Under the applied field, the resistivity gets

suppressed, especially near the (I–M) transition as well as

in the charge-ordered region. Under 8T magnetic field, the

maximum MR ratios reached are about 70 and 67 % for

x = 0.10 and 0.15 concentrations, respectively, in the

vicinity of charge-ordered region.

Dielectric Properties

Low-frequency dielectric properties of doped manganites

have been the subject matter of earlier studies [67, 68]. We

have studied the effects of frequency and temperature

dependence on dielectric functions for both the samples.

Table 2 Values of

characteristic temperature To,

density of states at the Fermi

energies N(EF), and Chi-square

(v2) determined from fitting of

resistivity temperature data for

La1-xTexMnO3 (x = 0.10 and

0.15)

Concentration (x) Temperature ranges To (K) N (EF) (eV-1 cm3) 9 1020 v2

x = 0.10 Below Tp 560 9 105 2.09 0.9996

T \ Tco 485 9 104 24.2 0.9959

Ta [ T [ Tco 7.86 1.49 9 107 0.9800

T [Ta 437.71 2.68 9 105 0.9996

x = 0.15 Below Tp 660 9 105 1.78 0.9996

T \ Tco 677 9 104 17.4 0.9952

Ta [ T [ Tco 7.7 1.52 9 107 0.9240

T [ Ta 1923.48 6.11 9 104 0.9965

Fig. 7 Temperature dependence of magnetoresistance (MR) with

magnetic fields of 1T, 5T, and 8T for a La0.9Te0.1MnO3 and

b La0.85Te0.15MnO3

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Figs. 8 and 9 show the variation of dielectric constant with

frequency and temperature, for La1-xTexMnO3 (x = 0.10

and 0.15). Dielectric constant e0 can be expressed as

e0 = ea ? ed ? el, where ea is associated with the dis-

placement of ionic charge distribution relative to their

nuclei and contribution at higher frequencies; the lattice

contribution (el) arises from displacement of ions and their

charge distributions; and ed gives the dipolar contribution

associated with the charge carrier hopping. Dielectric

constant as a function of frequency shows a peak behavior;

e0 first decreases for specific low-frequency region up to

2 kHz, above which a dispersion maxima starts to develop

giving a broad peak between 400 kHz and 1.25 MHz and

attains the colossal values of dielectric constant (CDC).

The height of the dispersion peak shows a decrease with

the decreasing temperature, the signature of dipolar relax-

ation. The interface and the grain boundaries control the

behavior of e0 at lower frequencies: the thinner the grain

boundary layers, the higher the value of e0. The peaking

behavior of e0 with frequency can be explained using

Rezlescu model [69]. According to this model, the peaks of

e0 curves can be ascribed to the presence of collective

contribution to the polarization from two different types of

charge carriers—both intrinsic and extrinsic effects con-

tribute to the overall dielectric response. Intrinsic effects

are related to charge condensation that takes place in some

systems with cations of variable valances such as Mn3?–

Mn2?/Mn4? and Ni2?–Ni3? [70, 71]. External factors lead

to the interfacial polarization produced in grain boundaries

and contact sample electrodes [72]. An increase in

dielectric constant has been observed with the increasing

temperature as depicted in Figs. 8b and 9b, suggesting the

thermal effects on charge carriers which orient themselves

after getting liberated, thereby enhancing the space-charge

Fig. 8 a. Frequency dependence of dielectric constant (e0) at

different temperatures for La0.9Te0.1MnO3. b Temperature depen-

dence of the dielectric constant (e0) at different frequencies for

La0.9Te0.1MnO3. Inset shows the enlarged view near transition region

Fig. 9 a Frequency dependence of dielectric constant (e0) at different

temperatures for La0.85Te0.15MnO3. b Temperature dependence of the

dielectric constant (e0) at different frequencies for La0.85Te0.15MnO3

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polarization and hence the overall dielectric constant. We

have observed anomaly in dielectric constant near the MIT

temperature. A decrease in dielectric constant near MIT

has been observed, and above which, it rises rapidly to

higher values, upon decreasing the resistivity further,

suggesting a correlation between the two phenomena. It is

pertinent to mention that the higher values of the dielectric

constant is attributed mainly in space-charge polarization

mechanism, known to predominate in heterogeneous

structures, in which a material is assumed to be composed

of different regions (grain and grain boundaries).

The conductivity of grains is considered relatively better

than that of grain boundaries because the charge carriers

encounter different resistances so that accumulation of

charges at separating boundaries occurs, and hence, the

dielectric constant values increase. Earlier reports interpret

high dielectric responses like CDC as an artifact emerging

from Schottky effect at the electrode contacts [69].

M. Sanchez-Andjar et al. [73] suggest the dielectric

behavior as a consequence of the formation of polar entities

at the temperature of the charge condensation, due to an

asymmetric charge distribution, intermediate between site-

centered and bond-centered types as described by Efremov

et al. [74] for real half-doped manganites.

Loss tangent

Figure 10 shows the temperature dependence of loss tan-

gent at different frequencies. We have observed high

dielectric loss, with relaxation dispersion peaks centered at

different temperatures for both the samples, which is the

direct consequence of the semiconducting nature of these

Fig. 10 Temperature dependence of loss tangent (tand) at different frequencies for a La0.9Te0.1MnO3; and b La0.85Te0.15MnO3 samples

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materials, having high dc resistivity. These loss peaks can

be discussed within the framework of the Maxwell–

Wagner model which refers to interfacial polarization

occurring in electrically inhomogeneous systems [75]. The

presence of oxygen non-stoichiometry in our samples can

be thought of as a possible source of inhomogeneity. It is

known that when an electric current passes through inter-

faces between two different dielectric media, because of

their different conductivities, surface charges pile up at the

interface [76], and the system shows a Debye-like relaxa-

tion process under an external alternating field [75]. Dou-

ble-dissipation peak behavior can be seen at the

frequencies of 1.25 MHz, 625 kHz, 312 kHz, and 156 kHz

for La0.90Te0.10MnO3, and can be interpreted by the hop-

ping of localized carriers and the thermally excited relax-

ation processes. We have noticed only single peak at low-

frequency toward low-temperature region, but with the

increase in frequency of the applied field above 7800 Hz, a

secondary dissipation peak starts to develop toward the

higher-temperature region, thereby minimizing the char-

acteristic dissipation peak formed at low-temperature

region. While, in the case of La0.85Te0.15MnO3, a different

scenario has been observed: the characteristic height of the

peak starts to develop with the decrease in frequency, and a

complete phase transition can be observed at 155 kHz near

MIT in La0.85Te0.15MnO3. The dissipation energy (loss)

decreases with the increasing frequency of the applied

field. The decrease in loss tangent with the increasing

frequency can be attributed to the dc conductivity. The loss

tangent increases with the increasing concentration as

depicted in Fig. 10. It should be noted that the temperature

at which the dielectric constant begins to increase rapidly is

approximately equal to that at which the dielectric loss

dispersion starts.

Fig. 10 continued

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Conclusion

We have systematically investigated the structural, mag-

netization, electronic transport, and dielectric properties for

La1-xTexMnO3 (x = 0.10 and 0.15). The samples are

found to be in single phase as confirmed by Rietveld

analysis. Both the samples exhibit the PM–FM phase

transition accompanied by MIT temperature. However,

with further decrease in temperature, the samples show the

insulating phase characteristics accompanied with the

charge-ordered region at low temperature. The Curie

temperature Tc, the charge-ordering transition temperature

TCO, and the MIT have been found to increase with Te

doping. Asymmetric magnetization and the shifting in

hysteresis loop have been noticed. The VRH model has

been found useful in discussing the transport properties

(resistivity). Dielectric constant exhibits a peak response

with frequency and attains colossal values at higher tem-

peratures for both the samples.

Acknowledgements The authors are greatly thankful to the UGC-

DAE Consortium for Scientific Research, Indore and the Center ofExcellence in Materials Science (Nanomaterials), Department of

Applied Physics, Aligarh Muslim University, Aligarh for providing

the experimental facilities. The authors are also thankful to Dr. R.K.

Kotnala, the National Physical Laboratory, New Delhi for providing

facilities in respect of magnetic measurements.

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