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Transport properties of silver–calcium doped lanthanum manganite
Physica B 457 (2015) 240–244
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Physica B 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, Tunisia Laboratoire Physique Appliquée, Faculté des Sciences, Université de Sfax, B.P. 1171, Sfax 3000 Tunisia
b
art ic l e i nf o
a b s t r a c t
Article history: Received 20 June 2014 Received in revised form 9 October 2014 Accepted 23 October 2014 Available online 25 October 2014
Electrical properties of silver–calcium doped lanthanum manganite (La0.5Ca0.5 xAgxMnO3 with 0.0o x o 0.4) were investigated using admittance spectroscopy in a wide range of temperature (80– 700 K). As silver concentration increases from x ¼0.0 to x ¼ 0.2, the resistivity decreases throughout the whole explored temperature range. For x ¼ 0.3 the resistivity increases due to the existence of secondary phases. The metallic phase may be the dominant one for x ¼ 0.4 which explains the decrease of the resistivity for this composition. For x r0.3, a metal–insulator 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 Mn–O–Mn 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 temperature. The deduced activation energy is found to be sensitive to the Ag composition. The variation of the conductivity exponent with temperature confirms the presence of hopping in the conduction process. For x ¼0.4, a percolation process may be the dominant one. & Elsevier B.V. All rights reserved.
Keywords: Transport properties Electrical properties Dielectric properties
1. Introduction Perovskite manganites with the general formula RAMnO3 (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, magnetic refrigerants and infrared detectors [1–3]. These properties can 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 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 La0.5Ca0.5 xAgxMnO3 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
n
Corresponding author. Fax: þ216 75 392 421. E-mail address: [email protected] (H. Rahmouni).
http://dx.doi.org/10.1016/j.physb.2014.10.022 0921-4526/& Elsevier B.V. All rights reserved.
spectroscopy in a wide range of temperature [80–700 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 film (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 configuration. 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 presented in a previous work [19]. We summarize the main results that are relevant for electrical and dielectric properties. The samples are stoichiometric in oxygen. The Mn4 þ content is slightly smaller than theoretical values for the samples with x r0.2. The difference becomes important for x ¼0.3 and 0.4 (ΔMn4 þ ¼11.05 and 24.4%, respectively).
B. Cherif et al. / Physica B 457 (2015) 240–244
The X-ray diffraction (XRD) analysis shows that samples with x o0.2 are composed of orthorhombic perovskite structure phases; the samples with x Z0.2 have three phases: magnetic perovskite phase is the major phase, metal Ag and Mn3O4 are the minor phases.
3. Results and discussions 3.1. Resistivity and metal–insulator transition Fig. 1 shows resistivity versus temperature curves of La0.5Ca0.5 xAgxMnO3 with x¼ 0, 0.1, 0.2, 0.3 and 0.4. As shown in Fig. 1, for x¼ 0, 0.1, 0.2 and 0.3, only one metal–insulator transition (TMI) was observed at around 120 K. The value of TMI does not change with increasing Ag concentration and is nearly identical to that of the free compound. This result is in good agreement with the literature [11]. For x ¼0.4, the TMI was 200 K. In general, the transition was correlated to the deviation of Mn3 þ –O–Mn4 þ bond angle. In previous studies [19], the results reported by Smari et al. show that the values of this angle are very close for x¼ 0, 0.1, 0.2 and 0.3 (161.16°, 159.95°, 160.59° and 162.72°, respectively). For x ¼0.4, the bond angle is greater than the others (165.03°). Such a result explains the significant variation of the transition temperature for x ¼0.4. The magnetic properties could be related to both bond angle and chemical composition. It is found in a previous work [20] that the parent compound exhibits paramagnetic to ferromagnetic transition at Tc¼ 222 K and a ferromagnetic to antiferromagnetic (which is charge ordered) transition at Tco¼ 92 K. It is also shown that the introduction of silver destroys the charge ordered phase and transforms the compound to a ferromagnetic phase. On the other hand, according to Tao, Pi and Battabyal et al. [21–23] the solubility of silver in perovskite does not exceed x ¼0.2. Hence for the x ¼0.4 compound, both the disappearance of charge ordering and the appearance of silver precipitates could be the origin of the metal–insulator transition at 200 K. Similar variations of structural and magnetic properties of doped La0.5Ca0.5MnO3 with other elements are obtained. Dhiman et al. [24] introduced Sr in the parent compound and found that a long range ferromagnetic ordering occurs at x ¼0.4 in the range of 180–250 K and for all temperatures below 310 K at higher values of x. From Fig. 1, a decrease of resistivity is observed throughout the whole explored temperature range for Ag content increasing from x ¼0 to x¼ 0.2. This behavior was observed by Battabyal and Dey
100 00% 10% 20% 30% 40%
ρ(Ω cm)
10
1
0,1 0
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200
300
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T (K) Fig. 1. Temperature dependence of resistivity of La0.5Ca0.5 xAgxMnO3 for 0r x r0.4.
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[23] in Ag substituted LaMnO3 system. It is well known that Ag is a good conductive metal. Hence, the existence of Ag between the grains opens a new conduction channel for electron transport. Also, the segregation of silver on the grain surface or grain boundaries increases the atomic structure disorder. This reduces or suppresses the barriers encountered by carriers and leads to a reduction of electron scattering and an enhancement of crossing by tunneling [25,26]. These factors cause the decrease of resistivity with increasing Ag content. For x¼ 0.3, the resistivity increases. This behavior can be attributed to the existence of secondary phases. Previous work [19] shows that the sample with x ¼0.3 has three phases (magnetic perovskite phase, metallic Ag and Mn3O4). For x ¼0.4, the resistivity decreases again. For this Ag concentration, the metallic phase may be the dominant one; a percolative process is establish that leads to a reduction of resistivity. 3.2. Conductivity spectra, conduction mechanism and activation energy Fig. 2 shows typical conductivity spectra at different temperatures for all investigated compositions. We have different behaviors of the conductivity with variations of frequency and temperature. At low frequency (fo 103 Hz), the conductivity is frequency independent and thermally activated. In the frequency range between 105 Hz and 106 Hz, the conductivity increases with frequency. In this dispersive region, the conductivity can be roughly described by a power law: s(ω) ¼ αωn with 0 on o1. The variation of the conductivity exponent ‘n’ versus temperature and Ag concentration is discussed below. The conductivity has a peak in the frequency range between 105 Hz and 106 Hz. Beyond the peak, it decreases with frequency. From the conductivity spectrum the dc conductivity (sdc) was extracted from the low frequency plateau for each temperature. To understand the transport properties for La0.5Ca0.5 xAgxMnO3 samples, the experimental s T curves are fitted to the following equations [27]:
• σDC T = A exp(−Ea/kBT ) (at high temperatures) • σDC = B exp(−T0/T )1/4 (at low temperatures) where A and B are the pre-exponential factors, Ea is the activation energy, kβ is the Boltzmann constant and T0 is a constant. Fig. 3a shows a linear variation of log(sT) versus 1000/T at high temperatures. Such a behavior proves that conductivity is dominated by thermally activated hopping of small polarons. Also Fig. 3b shows a linear variation of log(s) versus T 1/4 at low temperature, indicating that electronic conduction is dominated by the variable range hopping process. The fitting by VRH model is adequate for the parent compound. As the introduction of silver increases the disorder, other conduction mechanisms could operate and the VRH model will be limited to a smaller temperature range. We can conclude that as silver content increases, other conduction mechanisms than the VRH one are involved. The other mechanisms could be space charge zone around silver ion and percolation mechanism. The deduced values of activation energy are given in the inset of Fig. 3a. We observe that Ea decreases for Ag concentration increasing from x ¼0 to x ¼0.2. The same behavior was observed by Battabyal and Dey [23] in LaAgMnO3 compound. They found that activation energy decreases from 168 meV for x ¼0.05 to 135 meV for x ¼0.30. The decrease of activation energy may be due to the increase of charge carriers with increasing Ag content. Since three phases are present in the compound with x¼ 0.3, charges carriers will be trapped by the inhomogeneity and activation energy increases. Increase of Ea with Ag content was observed by Gencer et al. [11] in LCMO–Ag system. They suggested that the dopant was
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Fig. 2. Plots of log(s) versus log(ω)for La0.5Ca0.5 xAgxMnO3 at different temperatures.
mainly distributed at the grain boundary or surface of the LCMO grains, creating energy barriers to the electrical transport process. For x ¼0.4, a space charge zone (SCZ) can be established by the metallic phase at low temperature. So, carriers required significant energy to cross the SCZ. At high temperatures, the carriers have a
sufficient kinetic energy to easily cross the SCZ. This induced a reduction of the activation energy. Fig. 4 shows the s(ω, T) spectrum, where the frequency and temperature ranges are 104–105 Hz and 280–400 K, respectively. From such a spectrum, we deduce the conductivity exponent ‘n’ as
B. Cherif et al. / Physica B 457 (2015) 240–244
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The decrease of such parameters with temperature proves that hopping may be the dominating mechanism in the compound. The dependence of the conductivity exponent ‘n’ on temperature is in good agreement with Mott's theory [27]. Such a model is generally present in perovskite manganite materials [28–31], in ferrites [32] and in ferroelectric materials [33]. At fixed temperature it is clear that the conductivity exponent ‘n’ decreases with increasing Ag content from x¼ 0 to x ¼0.2, indicating that the material evolves from semi-insulating to metallic behavior. This result is in good agreement with the decrease of resistivity when the Ag concentration increases from x ¼0 to x ¼0.2. We found that n ¼0 in the considered temperature and frequency ranges for the compound with x¼ 0.2. At this composition the resistivity starts decreasing with temperature and phase transition occurs, and we reach the solubility limit. These phenomena affect conductivity spectra. From Fig. 2, we note that the plateau is extended to long range for x¼ 0.2. We can conclude that this compound has the best homogeneity. For x ¼0.3 the variation of the conductivity exponent ‘n’ is due to the effect of the presence of three phases in the compound. The decrease of the exponent ‘n’ for x¼ 0.4 indicates that higher Ag concentration opens a new channel for electron transport via percolation regime.
4. Conclusion
Fig. 3. (a) Variation of (sT) versus (1000/T). The inset shows the activation energy as a function of Ag concentration. (b) Variation of (s) versus (T 1/4).
10 8 6
T(K) 280 320 360 400
x=0 0,8 0,75 0,7 0,5
The exponent 's' x=0.1 x=0.2 x=0.3 0,71 0 0,73 0,6 0 0,57 0,42 0 0,41 0,15 0 0,3
x=0,1
x=0.4 0.5 0.2 0 0
σ (Scm-1)
4
2 280K 320K 360K 400K
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10
Frequency (Hz) Fig. 4. Conductivity spectrum at high frequency for La0.5Ca0.4Ag0.1MnO3. The inset shows the resulting temperature and Ag concentration dependence of the conductivity exponent.
a function of temperature and Ag concentration. It is clear, from the inset of Fig. 4, that the conductivity exponent ‘n’ decreases with increasing temperature for all Ag concentration. This behavior indicates that the conduction process is thermally activated.
We have investigated the electrical properties of silver–calcium dopant manganite (La0.5Ca0.5 xAgxMnO3) with different Ag concentrations. Ag content strongly affects the resistivity but does not change the metal–insulator transition for xr 0.3. For x ¼0.4, TMI changes due to the variation of the Mn–O–Mn bond angle. We found that resistivity and activation energy decrease with Ag concentration increasing from x¼ 0 to x ¼0.2. This result was attributed to the good conductivity of Ag metal and the creation of new conduction channel for electron transport due to the existence of Ag between grains. The conductivity analysis proves the dominance of hopping model in the conduction process. A percolation 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.
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[15] J.H. Miao, S.L. Yuan, G.M. Ren, X. Xiao, G.Q. Yu, Y.Q. Wang, S.Y. Yin, J. Alloys Compd. 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, Clarendon 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).
Contents lists available at ScienceDirect
Physica B 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, Tunisia Laboratoire Physique Appliquée, Faculté des Sciences, Université de Sfax, B.P. 1171, Sfax 3000 Tunisia
b
art ic l e i nf o
a b s t r a c t
Article history: Received 20 June 2014 Received in revised form 9 October 2014 Accepted 23 October 2014 Available online 25 October 2014
Electrical properties of silver–calcium doped lanthanum manganite (La0.5Ca0.5 xAgxMnO3 with 0.0o x o 0.4) were investigated using admittance spectroscopy in a wide range of temperature (80– 700 K). As silver concentration increases from x ¼0.0 to x ¼ 0.2, the resistivity decreases throughout the whole explored temperature range. For x ¼ 0.3 the resistivity increases due to the existence of secondary phases. The metallic phase may be the dominant one for x ¼ 0.4 which explains the decrease of the resistivity for this composition. For x r0.3, a metal–insulator 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 Mn–O–Mn 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 temperature. The deduced activation energy is found to be sensitive to the Ag composition. The variation of the conductivity exponent with temperature confirms the presence of hopping in the conduction process. For x ¼0.4, a percolation process may be the dominant one. & Elsevier B.V. All rights reserved.
Keywords: Transport properties Electrical properties Dielectric properties
1. Introduction Perovskite manganites with the general formula RAMnO3 (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, magnetic refrigerants and infrared detectors [1–3]. These properties can 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 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 La0.5Ca0.5 xAgxMnO3 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
n
Corresponding author. Fax: þ216 75 392 421. E-mail address: [email protected] (H. Rahmouni).
http://dx.doi.org/10.1016/j.physb.2014.10.022 0921-4526/& Elsevier B.V. All rights reserved.
spectroscopy in a wide range of temperature [80–700 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 film (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 configuration. 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 presented in a previous work [19]. We summarize the main results that are relevant for electrical and dielectric properties. The samples are stoichiometric in oxygen. The Mn4 þ content is slightly smaller than theoretical values for the samples with x r0.2. The difference becomes important for x ¼0.3 and 0.4 (ΔMn4 þ ¼11.05 and 24.4%, respectively).
B. Cherif et al. / Physica B 457 (2015) 240–244
The X-ray diffraction (XRD) analysis shows that samples with x o0.2 are composed of orthorhombic perovskite structure phases; the samples with x Z0.2 have three phases: magnetic perovskite phase is the major phase, metal Ag and Mn3O4 are the minor phases.
3. Results and discussions 3.1. Resistivity and metal–insulator transition Fig. 1 shows resistivity versus temperature curves of La0.5Ca0.5 xAgxMnO3 with x¼ 0, 0.1, 0.2, 0.3 and 0.4. As shown in Fig. 1, for x¼ 0, 0.1, 0.2 and 0.3, only one metal–insulator transition (TMI) was observed at around 120 K. The value of TMI does not change with increasing Ag concentration and is nearly identical to that of the free compound. This result is in good agreement with the literature [11]. For x ¼0.4, the TMI was 200 K. In general, the transition was correlated to the deviation of Mn3 þ –O–Mn4 þ bond angle. In previous studies [19], the results reported by Smari et al. show that the values of this angle are very close for x¼ 0, 0.1, 0.2 and 0.3 (161.16°, 159.95°, 160.59° and 162.72°, respectively). For x ¼0.4, the bond angle is greater than the others (165.03°). Such a result explains the significant variation of the transition temperature for x ¼0.4. The magnetic properties could be related to both bond angle and chemical composition. It is found in a previous work [20] that the parent compound exhibits paramagnetic to ferromagnetic transition at Tc¼ 222 K and a ferromagnetic to antiferromagnetic (which is charge ordered) transition at Tco¼ 92 K. It is also shown that the introduction of silver destroys the charge ordered phase and transforms the compound to a ferromagnetic phase. On the other hand, according to Tao, Pi and Battabyal et al. [21–23] the solubility of silver in perovskite does not exceed x ¼0.2. Hence for the x ¼0.4 compound, both the disappearance of charge ordering and the appearance of silver precipitates could be the origin of the metal–insulator transition at 200 K. Similar variations of structural and magnetic properties of doped La0.5Ca0.5MnO3 with other elements are obtained. Dhiman et al. [24] introduced Sr in the parent compound and found that a long range ferromagnetic ordering occurs at x ¼0.4 in the range of 180–250 K and for all temperatures below 310 K at higher values of x. From Fig. 1, a decrease of resistivity is observed throughout the whole explored temperature range for Ag content increasing from x ¼0 to x¼ 0.2. This behavior was observed by Battabyal and Dey
100 00% 10% 20% 30% 40%
ρ(Ω cm)
10
1
0,1 0
100
200
300
400
500
600
700
T (K) Fig. 1. Temperature dependence of resistivity of La0.5Ca0.5 xAgxMnO3 for 0r x r0.4.
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[23] in Ag substituted LaMnO3 system. It is well known that Ag is a good conductive metal. Hence, the existence of Ag between the grains opens a new conduction channel for electron transport. Also, the segregation of silver on the grain surface or grain boundaries increases the atomic structure disorder. This reduces or suppresses the barriers encountered by carriers and leads to a reduction of electron scattering and an enhancement of crossing by tunneling [25,26]. These factors cause the decrease of resistivity with increasing Ag content. For x¼ 0.3, the resistivity increases. This behavior can be attributed to the existence of secondary phases. Previous work [19] shows that the sample with x ¼0.3 has three phases (magnetic perovskite phase, metallic Ag and Mn3O4). For x ¼0.4, the resistivity decreases again. For this Ag concentration, the metallic phase may be the dominant one; a percolative process is establish that leads to a reduction of resistivity. 3.2. Conductivity spectra, conduction mechanism and activation energy Fig. 2 shows typical conductivity spectra at different temperatures for all investigated compositions. We have different behaviors of the conductivity with variations of frequency and temperature. At low frequency (fo 103 Hz), the conductivity is frequency independent and thermally activated. In the frequency range between 105 Hz and 106 Hz, the conductivity increases with frequency. In this dispersive region, the conductivity can be roughly described by a power law: s(ω) ¼ αωn with 0 on o1. The variation of the conductivity exponent ‘n’ versus temperature and Ag concentration is discussed below. The conductivity has a peak in the frequency range between 105 Hz and 106 Hz. Beyond the peak, it decreases with frequency. From the conductivity spectrum the dc conductivity (sdc) was extracted from the low frequency plateau for each temperature. To understand the transport properties for La0.5Ca0.5 xAgxMnO3 samples, the experimental s T curves are fitted to the following equations [27]:
• σDC T = A exp(−Ea/kBT ) (at high temperatures) • σDC = B exp(−T0/T )1/4 (at low temperatures) where A and B are the pre-exponential factors, Ea is the activation energy, kβ is the Boltzmann constant and T0 is a constant. Fig. 3a shows a linear variation of log(sT) versus 1000/T at high temperatures. Such a behavior proves that conductivity is dominated by thermally activated hopping of small polarons. Also Fig. 3b shows a linear variation of log(s) versus T 1/4 at low temperature, indicating that electronic conduction is dominated by the variable range hopping process. The fitting by VRH model is adequate for the parent compound. As the introduction of silver increases the disorder, other conduction mechanisms could operate and the VRH model will be limited to a smaller temperature range. We can conclude that as silver content increases, other conduction mechanisms than the VRH one are involved. The other mechanisms could be space charge zone around silver ion and percolation mechanism. The deduced values of activation energy are given in the inset of Fig. 3a. We observe that Ea decreases for Ag concentration increasing from x ¼0 to x ¼0.2. The same behavior was observed by Battabyal and Dey [23] in LaAgMnO3 compound. They found that activation energy decreases from 168 meV for x ¼0.05 to 135 meV for x ¼0.30. The decrease of activation energy may be due to the increase of charge carriers with increasing Ag content. Since three phases are present in the compound with x¼ 0.3, charges carriers will be trapped by the inhomogeneity and activation energy increases. Increase of Ea with Ag content was observed by Gencer et al. [11] in LCMO–Ag system. They suggested that the dopant was
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Fig. 2. Plots of log(s) versus log(ω)for La0.5Ca0.5 xAgxMnO3 at different temperatures.
mainly distributed at the grain boundary or surface of the LCMO grains, creating energy barriers to the electrical transport process. For x ¼0.4, a space charge zone (SCZ) can be established by the metallic phase at low temperature. So, carriers required significant energy to cross the SCZ. At high temperatures, the carriers have a
sufficient kinetic energy to easily cross the SCZ. This induced a reduction of the activation energy. Fig. 4 shows the s(ω, T) spectrum, where the frequency and temperature ranges are 104–105 Hz and 280–400 K, respectively. From such a spectrum, we deduce the conductivity exponent ‘n’ as
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The decrease of such parameters with temperature proves that hopping may be the dominating mechanism in the compound. The dependence of the conductivity exponent ‘n’ on temperature is in good agreement with Mott's theory [27]. Such a model is generally present in perovskite manganite materials [28–31], in ferrites [32] and in ferroelectric materials [33]. At fixed temperature it is clear that the conductivity exponent ‘n’ decreases with increasing Ag content from x¼ 0 to x ¼0.2, indicating that the material evolves from semi-insulating to metallic behavior. This result is in good agreement with the decrease of resistivity when the Ag concentration increases from x ¼0 to x ¼0.2. We found that n ¼0 in the considered temperature and frequency ranges for the compound with x¼ 0.2. At this composition the resistivity starts decreasing with temperature and phase transition occurs, and we reach the solubility limit. These phenomena affect conductivity spectra. From Fig. 2, we note that the plateau is extended to long range for x¼ 0.2. We can conclude that this compound has the best homogeneity. For x ¼0.3 the variation of the conductivity exponent ‘n’ is due to the effect of the presence of three phases in the compound. The decrease of the exponent ‘n’ for x¼ 0.4 indicates that higher Ag concentration opens a new channel for electron transport via percolation regime.
4. Conclusion
Fig. 3. (a) Variation of (sT) versus (1000/T). The inset shows the activation energy as a function of Ag concentration. (b) Variation of (s) versus (T 1/4).
10 8 6
T(K) 280 320 360 400
x=0 0,8 0,75 0,7 0,5
The exponent 's' x=0.1 x=0.2 x=0.3 0,71 0 0,73 0,6 0 0,57 0,42 0 0,41 0,15 0 0,3
x=0,1
x=0.4 0.5 0.2 0 0
σ (Scm-1)
4
2 280K 320K 360K 400K
4
2x10
4
4x10
4
6x10
4
8x10
5
10
Frequency (Hz) Fig. 4. Conductivity spectrum at high frequency for La0.5Ca0.4Ag0.1MnO3. The inset shows the resulting temperature and Ag concentration dependence of the conductivity exponent.
a function of temperature and Ag concentration. It is clear, from the inset of Fig. 4, that the conductivity exponent ‘n’ decreases with increasing temperature for all Ag concentration. This behavior indicates that the conduction process is thermally activated.
We have investigated the electrical properties of silver–calcium dopant manganite (La0.5Ca0.5 xAgxMnO3) with different Ag concentrations. Ag content strongly affects the resistivity but does not change the metal–insulator transition for xr 0.3. For x ¼0.4, TMI changes due to the variation of the Mn–O–Mn bond angle. We found that resistivity and activation energy decrease with Ag concentration increasing from x¼ 0 to x ¼0.2. This result was attributed to the good conductivity of Ag metal and the creation of new conduction channel for electron transport due to the existence of Ag between grains. The conductivity analysis proves the dominance of hopping model in the conduction process. A percolation process may be established at high Ag composition.
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