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Transport properties of silvercalcium doped lanthanum manganite B. Cherif a , H. Rahmouni a,n , M. Smari b , E. Dhahri b , N. Moutia a , K. Khirouni a a Laboratoire de Physique des Matériaux et des Nanomatériaux appliquée à lEnvironnement, Faculté des Sciences de Gabès, Université de Gabes, cité Erriadh, 6079 Gabès, Tunisia b Laboratoire Physique Appliquée, Faculté des Sciences, Université de Sfax, B.P.1171, Sfax 3000 Tunisia article info Article history: Received 20 June 2014 Received in revised form 9 October 2014 Accepted 23 October 2014 Available online 25 October 2014 Keywords: Transport properties Electrical properties Dielectric properties abstract Electrical properties of silvercalcium doped lanthanum manganite (La 0.5 Ca 0.5x Ag x MnO 3 with 0.0 ox o0.4) were investigated using admittance spectroscopy in a wide range of temperature (80700 K). As silver concentration increases from x ¼0.0 to x ¼0.2, the resistivity decreases throughout the whole explored temperature range. For x ¼0.3 the resistivity increases due to the existence of secondary phases. The metallic phase may be the dominant one for x ¼0.4 which explains the decrease of the resistivity for this composition. For x r0.3, a metalinsulator transition was observed at 120 K and does not change with Ag content. With x ¼0.4, the transition is observed at 200 K. This variation is attributed to the MnOMn bond angle effects. From conductivity analysis, it is found that the conduction process is dominated by small polaron hopping at high temperature and by variable range hopping at low tem- perature. The deduced activation energy is found to be sensitive to the Ag composition. The variation of the conductivity exponent with temperature conrms the presence of hopping in the conduction pro- cess. For x ¼0.4, a percolation process may be the dominant one. & Elsevier B.V. All rights reserved. 1. Introduction Perovskite manganites with the general formula RAMnO 3 (R (rare earth): La, Nd, Pr; A (divalent ion): Ca, Sr, Pb, Ba) have been of considerable recent interest due to their magnetic, electric and magnetocaloric properties. They can be used as magnetoresistive transducers, magnetic sensors, computer memory systems, mag- netic refrigerants and infrared detectors [13]. These properties can be improved by choosing dopants [412], substitution sites [13], preparation route [1416] and insertion of nanostructures [17]. The effects of substitution of silver ion for A ion have been reported [18,19,11]. The electrical and dielectric properties have rarely been investigated. Such a work completes structural and magnetic studies and helps understand the interplay among magnetic, electric and lattice interactions. It also yields optimized physical parameter which can be useful in detecting or sensing devices. In this paper, we have synthetized a set of samples of La 0.5 Ca 0.5 x Ag x MnO 3 with different silver contents. The structural analysis shows a segregation of silver at the grain boundaries. We studied the electrical properties of the sample by admittance spectroscopy in a wide range of temperature [80700 K]. Such a temperature range is rarely explored. 2. Experimental techniques The powder of calcium dopant lanthanum manganite was prepared using the conventional solid state reaction method. The details of the preparation and thermal treatment are described in previous work [19]. The powder is sintered in pellets of 10 mm diameter and approximately 2 mm thickness. On both sides of the pellets we deposit a thin aluminum lm (200 nm thick) through a circular mask of 6 mm diameter. The obtained aluminum disks are used to measure the electronic transport across the compound and the capacitance in a plate capacitor conguration. The sample is mounted in a cryostat which allows the variation of temperature from 77 to 700 K. An Agilent 4294A analyzer is used to measure the conductance and the capacitance. We took the measurements in parallel mode for the equivalent circuit at signal amplitude of 20 mV. All measurements are conducted in vacuum and in dark. Chemical and structural properties of the samples were pre- sented in a previous work [19]. We summarize the main results that are relevant for electrical and dielectric properties. The sam- ples are stoichiometric in oxygen. The Mn 4 þ content is slightly smaller than theoretical values for the samples with x r0.2. The difference becomes important for x ¼ 0.3 and 0.4 (ΔMn 4 þ ¼ 11.05 and 24.4%, respectively). Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B http://dx.doi.org/10.1016/j.physb.2014.10.022 0921-4526/& Elsevier B.V. All rights reserved. n Corresponding author. Fax: þ216 75 392 421. E-mail address: [email protected] (H. Rahmouni). Physica B 457 (2015) 240244

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Page 1: Transport properties of silver–calcium doped lanthanum manganite

Physica B 457 (2015) 240–244

Contents lists available at ScienceDirect

Physica B

http://d0921-45

n CorrE-m

journal homepage: www.elsevier.com/locate/physb

Transport properties of silver–calcium doped lanthanum manganite

B. Cherif a, H. Rahmouni a,n, M. Smari b, E. Dhahri b, N. Moutia a, K. Khirouni a

a Laboratoire de Physique des Matériaux et des Nanomatériaux appliquée à l′Environnement, Faculté des Sciences de Gabès, Université de Gabes, cité Erriadh,6079 Gabès, Tunisiab Laboratoire Physique Appliquée, Faculté des Sciences, Université de Sfax, B.P. 1171, Sfax 3000 Tunisia

a r t i c l e i n f o

Article history:Received 20 June 2014Received in revised form9 October 2014Accepted 23 October 2014Available online 25 October 2014

Keywords:Transport propertiesElectrical propertiesDielectric properties

x.doi.org/10.1016/j.physb.2014.10.02226/& Elsevier B.V. All rights reserved.

esponding author. Fax: þ216 75 392 421.ail address: [email protected] (H. Rahm

a b s t r a c t

Electrical properties of silver–calcium doped lanthanum manganite (La0.5Ca0.5�xAgxMnO3 with0.0oxo0.4) were investigated using admittance spectroscopy in a wide range of temperature (80–700 K). As silver concentration increases from x¼0.0 to x¼0.2, the resistivity decreases throughout thewhole explored temperature range. For x¼0.3 the resistivity increases due to the existence of secondaryphases. The metallic phase may be the dominant one for x¼0.4 which explains the decrease of theresistivity for this composition. For xr0.3, a metal–insulator transition was observed at 120 K and doesnot change with Ag content. With x¼0.4, the transition is observed at 200 K. This variation is attributedto the Mn–O–Mn bond angle effects. From conductivity analysis, it is found that the conduction process isdominated by small polaron hopping at high temperature and by variable range hopping at low tem-perature. The deduced activation energy is found to be sensitive to the Ag composition. The variation ofthe conductivity exponent with temperature confirms the presence of hopping in the conduction pro-cess. For x¼0.4, a percolation process may be the dominant one.

& Elsevier B.V. All rights reserved.

1. Introduction

Perovskite manganites with the general formula RAMnO3 (R(rare earth): La, Nd, Pr; A (divalent ion): Ca, Sr, Pb, Ba) have been ofconsiderable recent interest due to their magnetic, electric andmagnetocaloric properties. They can be used as magnetoresistivetransducers, magnetic sensors, computer memory systems, mag-netic refrigerants and infrared detectors [1–3]. These propertiescan be improved by choosing dopants [4–12], substitution sites[13], preparation route [14–16] and insertion of nanostructures[17].

The effects of substitution of silver ion for A ion have beenreported [18,19,11]. The electrical and dielectric properties haverarely been investigated. Such a work completes structural andmagnetic studies and helps understand the interplay amongmagnetic, electric and lattice interactions. It also yields optimizedphysical parameter which can be useful in detecting or sensingdevices.

In this paper, we have synthetized a set of samples ofLa0.5Ca0.5�xAgxMnO3 with different silver contents. The structuralanalysis shows a segregation of silver at the grain boundaries. Westudied the electrical properties of the sample by admittance

ouni).

spectroscopy in a wide range of temperature [80–700 K]. Such atemperature range is rarely explored.

2. Experimental techniques

The powder of calcium dopant lanthanum manganite wasprepared using the conventional solid state reaction method. Thedetails of the preparation and thermal treatment are described inprevious work [19]. The powder is sintered in pellets of 10 mmdiameter and approximately 2 mm thickness. On both sides of thepellets we deposit a thin aluminum film (200 nm thick) through acircular mask of 6 mm diameter. The obtained aluminum disks areused to measure the electronic transport across the compound andthe capacitance in a plate capacitor configuration. The sample ismounted in a cryostat which allows the variation of temperaturefrom 77 to 700 K. An Agilent 4294A analyzer is used to measurethe conductance and the capacitance. We took the measurementsin parallel mode for the equivalent circuit at signal amplitude of20 mV. All measurements are conducted in vacuum and in dark.

Chemical and structural properties of the samples were pre-sented in a previous work [19]. We summarize the main resultsthat are relevant for electrical and dielectric properties. The sam-ples are stoichiometric in oxygen. The Mn4þ content is slightlysmaller than theoretical values for the samples with xr0.2. Thedifference becomes important for x¼0.3 and 0.4 (ΔMn4þ¼11.05and 24.4%, respectively).

Page 2: Transport properties of silver–calcium doped lanthanum manganite

B. Cherif et al. / Physica B 457 (2015) 240–244 241

The X-ray diffraction (XRD) analysis shows that samples withxo0.2 are composed of orthorhombic perovskite structure pha-ses; the samples with xZ0.2 have three phases: magnetic per-ovskite phase is the major phase, metal Ag and Mn3O4 are theminor phases.

3. Results and discussions

3.1. Resistivity and metal–insulator transition

Fig. 1 shows resistivity versus temperature curves ofLa0.5Ca0.5�xAgxMnO3 with x¼0, 0.1, 0.2, 0.3 and 0.4. As shown inFig. 1, for x¼0, 0.1, 0.2 and 0.3, only one metal–insulator transition(TMI) was observed at around 120 K. The value of TMI does notchange with increasing Ag concentration and is nearly identical tothat of the free compound. This result is in good agreement withthe literature [11]. For x¼0.4, the TMI was 200 K. In general, thetransition was correlated to the deviation of Mn3þ–O–Mn4þ bondangle. In previous studies [19], the results reported by Smari et al.show that the values of this angle are very close for x¼0, 0.1,0.2 and 0.3 (161.16°, 159.95°, 160.59° and 162.72°, respectively). Forx¼0.4, the bond angle is greater than the others (165.03°). Such aresult explains the significant variation of the transition tem-perature for x¼0.4. The magnetic properties could be related toboth bond angle and chemical composition. It is found in a pre-vious work [20] that the parent compound exhibits paramagneticto ferromagnetic transition at Tc¼222 K and a ferromagnetic toantiferromagnetic (which is charge ordered) transition atTco¼92 K. It is also shown that the introduction of silver destroysthe charge ordered phase and transforms the compound to a fer-romagnetic phase. On the other hand, according to Tao, Pi andBattabyal et al. [21–23] the solubility of silver in perovskite doesnot exceed x¼0.2. Hence for the x¼0.4 compound, both the dis-appearance of charge ordering and the appearance of silver pre-cipitates could be the origin of the metal–insulator transition at200 K. Similar variations of structural and magnetic properties ofdoped La0.5Ca0.5MnO3 with other elements are obtained. Dhimanet al. [24] introduced Sr in the parent compound and found that along range ferromagnetic ordering occurs at x¼0.4 in the range of180–250 K and for all temperatures below 310 K at higher valuesof x.

From Fig. 1, a decrease of resistivity is observed throughout thewhole explored temperature range for Ag content increasing fromx¼0 to x¼0.2. This behavior was observed by Battabyal and Dey

0 100 200 300 400 500 600 700

0,1

1

10

100 00% 10% 20% 30% 40%

ρ (Ω

cm

)

T (K)

Fig. 1. Temperature dependence of resistivity of La0.5Ca0.5�xAgxMnO3 for0rxr0.4.

[23] in Ag substituted LaMnO3 system. It is well known that Ag is agood conductive metal. Hence, the existence of Ag between thegrains opens a new conduction channel for electron transport.Also, the segregation of silver on the grain surface or grainboundaries increases the atomic structure disorder. This reducesor suppresses the barriers encountered by carriers and leads to areduction of electron scattering and an enhancement of crossingby tunneling [25,26]. These factors cause the decrease of resistivitywith increasing Ag content. For x¼0.3, the resistivity increases.This behavior can be attributed to the existence of secondaryphases. Previous work [19] shows that the sample with x¼0.3 hasthree phases (magnetic perovskite phase, metallic Ag and Mn3O4).For x¼0.4, the resistivity decreases again. For this Ag concentra-tion, the metallic phase may be the dominant one; a percolativeprocess is establish that leads to a reduction of resistivity.

3.2. Conductivity spectra, conduction mechanism and activationenergy

Fig. 2 shows typical conductivity spectra at different tempera-tures for all investigated compositions. We have different beha-viors of the conductivity with variations of frequency and tem-perature. At low frequency (fo103 Hz), the conductivity is fre-quency independent and thermally activated. In the frequencyrange between 105 Hz and 106 Hz, the conductivity increases withfrequency. In this dispersive region, the conductivity can beroughly described by a power law: s(ω)¼αωn with 0ono1. Thevariation of the conductivity exponent ‘n’ versus temperature andAg concentration is discussed below. The conductivity has a peakin the frequency range between 105 Hz and 106 Hz. Beyond thepeak, it decreases with frequency.

From the conductivity spectrum the dc conductivity (sdc) wasextracted from the low frequency plateau for each temperature. Tounderstand the transport properties for La0.5Ca0.5�xAgxMnO3

samples, the experimental s�T curves are fitted to the followingequations [27]:

σ

σ

• = −

• = −

T A E k T

B T T

exp( / ) (at high temperatures)

exp( / ) (at low temperatures)

DC a B

DC 01/4

where A and B are the pre-exponential factors, Ea is the activationenergy, kβ is the Boltzmann constant and T0 is a constant. Fig. 3ashows a linear variation of log(sT) versus 1000/T at high tempera-tures. Such a behavior proves that conductivity is dominated bythermally activated hopping of small polarons. Also Fig. 3b showsa linear variation of log(s) versus T�1/4 at low temperature,indicating that electronic conduction is dominated by the variablerange hopping process. The fitting by VRH model is adequate forthe parent compound. As the introduction of silver increases thedisorder, other conduction mechanisms could operate and theVRH model will be limited to a smaller temperature range. We canconclude that as silver content increases, other conduction me-chanisms than the VRH one are involved. The other mechanismscould be space charge zone around silver ion and percolationmechanism.

The deduced values of activation energy are given in the insetof Fig. 3a. We observe that Ea decreases for Ag concentration in-creasing from x¼0 to x¼0.2. The same behavior was observed byBattabyal and Dey [23] in LaAgMnO3 compound. They found thatactivation energy decreases from 168 meV for x¼0.05 to 135 meVfor x¼0.30. The decrease of activation energy may be due to theincrease of charge carriers with increasing Ag content. Since threephases are present in the compound with x¼0.3, charges carrierswill be trapped by the inhomogeneity and activation energy in-creases. Increase of Ea with Ag content was observed by Genceret al. [11] in LCMO–Ag system. They suggested that the dopant was

Page 3: Transport properties of silver–calcium doped lanthanum manganite

Fig. 2. Plots of log(s) versus log(ω)for La0.5Ca0.5�xAgxMnO3 at different temperatures.

B. Cherif et al. / Physica B 457 (2015) 240–244242

mainly distributed at the grain boundary or surface of the LCMOgrains, creating energy barriers to the electrical transport process.For x¼0.4, a space charge zone (SCZ) can be established by themetallic phase at low temperature. So, carriers required significantenergy to cross the SCZ. At high temperatures, the carriers have a

sufficient kinetic energy to easily cross the SCZ. This induced areduction of the activation energy.

Fig. 4 shows the s(ω, T) spectrum, where the frequency andtemperature ranges are 104–105 Hz and 280–400 K, respectively.From such a spectrum, we deduce the conductivity exponent ‘n’ as

Page 4: Transport properties of silver–calcium doped lanthanum manganite

Fig. 3. (a) Variation of (sT) versus (1000/T). The inset shows the activation energyas a function of Ag concentration. (b) Variation of (s) versus (T�1/4).

2x104 4x104 6x104 8x104 105

2

4

6

8

10T(K) The exponent 's'

x=0 x=0.1 x=0.2 x=0.3 x=0.4280 0,8 0,71 0 0,73 0.5320 0,75 0,6 0 0,57 0.2360 0,7 0,42 0 0,41 0400 0,5 0,15 0 0,3 0

σ (S

cm-1)

Frequency (Hz)

280K 320K 360K 400K

x=0,1

Fig. 4. Conductivity spectrum at high frequency for La0.5Ca0.4Ag0.1MnO3. The insetshows the resulting temperature and Ag concentration dependence of the con-ductivity exponent.

B. Cherif et al. / Physica B 457 (2015) 240–244 243

a function of temperature and Ag concentration. It is clear, fromthe inset of Fig. 4, that the conductivity exponent ‘n’ decreaseswith increasing temperature for all Ag concentration. This beha-vior indicates that the conduction process is thermally activated.

The decrease of such parameters with temperature proves thathopping may be the dominating mechanism in the compound. Thedependence of the conductivity exponent ‘n’ on temperature is ingood agreement with Mott's theory [27]. Such a model is generallypresent in perovskite manganite materials [28–31], in ferrites [32]and in ferroelectric materials [33]. At fixed temperature it is clearthat the conductivity exponent ‘n’ decreases with increasing Agcontent from x¼0 to x¼0.2, indicating that the material evolvesfrom semi-insulating to metallic behavior. This result is in goodagreement with the decrease of resistivity when the Ag con-centration increases from x¼0 to x¼0.2. We found that n¼0 in theconsidered temperature and frequency ranges for the compoundwith x¼0.2. At this composition the resistivity starts decreasingwith temperature and phase transition occurs, and we reach thesolubility limit. These phenomena affect conductivity spectra.From Fig. 2, we note that the plateau is extended to long range forx¼0.2. We can conclude that this compound has the best homo-geneity. For x¼0.3 the variation of the conductivity exponent ‘n’ isdue to the effect of the presence of three phases in the compound.The decrease of the exponent ‘n’ for x¼0.4 indicates that higher Agconcentration opens a new channel for electron transport viapercolation regime.

4. Conclusion

We have investigated the electrical properties of silver–calciumdopant manganite (La0.5Ca0.5�xAgxMnO3) with different Ag con-centrations. Ag content strongly affects the resistivity but does notchange the metal–insulator transition for xr0.3. For x¼0.4, TMI

changes due to the variation of the Mn–O–Mn bond angle. Wefound that resistivity and activation energy decrease with Agconcentration increasing from x¼0 to x¼0.2. This result was at-tributed to the good conductivity of Ag metal and the creation ofnew conduction channel for electron transport due to the ex-istence of Ag between grains. The conductivity analysis proves thedominance of hopping model in the conduction process. A per-colation process may be established at high Ag composition.

References

[1] V.S. Kolat, H. Gencer, M. Gunes, S. Atalay, Mater. Sci. Eng. B 140 (2007)212–217.

[2] Z.C. Xia, S.L. Yuan, W. Feng, L.J. Zhang, G.H. Zang, J. Tang, L. Liu, D.W. Liu, Q.H. Zheng, L. Chen, Z.H. Fang, S. Liu, C.Q. Tang, Solid State Commun. 127 (2003)567–572.

[3] N. Khare, D.P. Singh, H.K. Gupta, P.K. Siwach, O.N. Srivastava, J. Phys. Chem.Solids 65 (2004) 867–870.

[4] A. Nucara, F. Miletto Granozio, W.S. Mohamed, A. Vecchione, R. Fittipaldi, P.P. Perna, M. Radovic, F.M. Vitucci, P. Calvani, Phys. B: Condens. Matter 433(2014) 102–106.

[5] A. Krichene, W. Boujelben, A. Cheikhrouhou, Phys. B: Condens. Matter 433(2014) 122–126.

[6] N. Zaidi, S. Mnefgui, A. Dhahri, J. Dhahri, E.K. Hlil, Phys. B: Condens. Matter 450(2014) 155–161.

[7] Y. Bitla, S.N. Kaul, L. Fernández, Barquín, Phys. B: Condens. Matter 448 (2014)223–225.

[8] Sudharshan Vadnala, T. Durga Rao, Prem Pal, Saket Asthana, Phys. B: Condens.Matter 448 (2014) 277–280.

[9] V. Sridharan, L. Seetha Lakshmi, R. Nithya, D.V. Natarajan, T.S. Radhakrishnan,J. Alloys Compd. 326 (2001) 65–68.

[10] L. Seetha Lakshmi, K. Dorr, K. Nenkov, V. Sridharan, V. Sankara Sastry, K.-H. Muller, J. Magn. Magn. Mater. 290–291 (2005) 924–927.

[11] H. Gencer, M. Pektas, Y. Babur, V.S. Kolat, T. Izgi, S. Atalaya, J. Magn. 17 (3)(2012) 176–184.

[12] L. Seetha Lakshmi, V. Sridharan, D.V. Natarajan, Rajeef Rawat, Sharat Chandra,V. Sankara Sastry, T.S. Radhakrishnan, J. Magn. Magn. Mater. 279 (2004)41–50.

[13] R. Ganguly, I.K. Gopalakrishnan, J.V. Yakhmi, Physica B 275 (2000) 308–315.[14] T. Barbier, C. Autret-Lambert, C. Honstrette, F. Gervais, M. Lethiecq, Mater. Res.

Bull. 47 (2012) 4427–4432.

Page 5: Transport properties of silver–calcium doped lanthanum manganite

B. Cherif et al. / Physica B 457 (2015) 240–244244

[15] J.H. Miao, S.L. Yuan, G.M. Ren, X. Xiao, G.Q. Yu, Y.Q. Wang, S.Y. Yin, J. AlloysCompd. 448 (2008) 27–31.

[16] G. Venkataiah, D.C. Krishna, M. Vithal, S.S. Rao, S.V. Bhat, V. Prasad, S.V. Subramanyam, P. Venugopal Reddy, Physica B 357 (2005) 370–379.

[17] P.G. Li, M. Lei, H.L. Tang, Y.F. Guo, W.H. Tang, J. Alloys Compd. 460 (2008)60–63.

[18] T. Tang, K.M. Gu, Q.Q. Cao, D.H. Wang, S.Y. Zhang, Y.W. Du, J. Magn. Magn.Mater. 222 (2000) 110–114.

[19] M. Smari, I. Walha, E. Dhahri, E.K. Hlil, J. Alloys Compd. 579 (2013) 564–571.[20] M. Smari, I. Walha, E. Dhahri, E.K. Hlil, Chem. Phys. Lett. 607 (2014) 25–28.[21] T. Tao, Appl. Phys. Lett. 77 (2000) 723.[22] L. Pi, Solid State Commun. 126 (2003) 229.[23] Manjusha Battbyal, T.K. Dey, Solid State Commun. 131 (2004) 337–342.[24] I. Dhiman, A. Das, K.R. Priolkar, P.S.R. Murthy, Physica B 406 (2011) 1028.[25] X.B. Yang, Y.H. Liu, N. Yin, C.J. Wang, L.M. Mei., J. Magn. Magn. Mater. 306

(2006) 167.

[26] N. Panwar, D.K. Pandya, S.K. Agarawal, J. Phys. Condens. Mater. 19 (2007)456224.

[27] N.F. Mott, E.A. Davis, Electronic Process in Non Crystalline Materials, Clar-endon Press, Oxford, 1979.

[28] H. Rahmouni, A. Dhahri, K. Khirouni., J. Alloys Compd. 591 (2014) 259–262.[29] H. Rahmouni, B. Cherif, M. Baazaoui, K. Khirouni., J. Alloys Compd. 575 (2013)

5–9.[30] H. Rahmouni, A. Selmi, K. Khirouni, N. Kallel., J. Alloys Compd. 533 (2012)

93–96.[31] H. Rahmouni, R. Jemai, N. Kallel, A. Selmi, K. Khirouni., J. Alloys Compd. 497

(2010) 1–5.[32] S. Ghatak, M. Sinha, A.K. Meikap, S.K. Pradhan, Mat. Res. Bull. 45 (2010)

954–960.[33] Lily, K. Kumari, K. Prasad, R.N.P. Choudhary, J. Alloys Compd. 453 (2008)

325–331 (Rh7).