Physica B 457 (2015) 240244Contents lists available at ScienceDirectPhysica Bhttp://d0921-45
n CorrE-mjournal homepage: www.elsevier.com/locate/physbTransport 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 Matriaux et des Nanomatriaux applique lEnvironnement, Facult des Sciences de Gabs, Universit de Gabes, cit Erriadh,6079 Gabs, Tunisiab Laboratoire Physique Applique, Facult des Sciences, Universit de Sfax, B.P. 1171, Sfax 3000 Tunisiaa 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 propertiesx.doi.org/10.1016/j.physb.2014.10.02226/& Elsevier B.V. All rights reserved.
esponding author. Fax: 216 75 392 421.ail address: firstname.lastname@example.org (H. Rahma b s t r a c t
Electrical properties of silvercalcium doped lanthanum manganite (La0.5Ca0.5xAgxMnO3 with0.0oxo0.4) were investigated using admittance spectroscopy in a wide range of temperature (80700 K). As silver concentration increases from x0.0 to x0.2, the resistivity decreases throughout thewhole explored temperature range. For x0.3 the resistivity increases due to the existence of secondaryphases. The metallic phase may be the dominant one for x0.4 which explains the decrease of theresistivity for this composition. For xr0.3, a metalinsulator transition was observed at 120 K and doesnot change with Ag content. With x0.4, the transition is observed at 200 K. This variation is attributedto the MnOMn 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 x0.4, 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 . These propertiescan be improved by choosing dopants , substitution sites, preparation route  and insertion of nanostructures.
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 ofLa0.5Ca0.5xAgxMnO3 with different silver contents. The structuralanalysis shows a segregation of silver at the grain boundaries. Westudied the electrical properties of the sample by admittanceouni).spectroscopy in a wide range of temperature [80700 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 . 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 . We summarize the main resultsthat are relevant for electrical and dielectric properties. The sam-ples are stoichiometric in oxygen. The Mn4 content is slightlysmaller than theoretical values for the samples with xr0.2. Thedifference becomes important for x0.3 and 0.4 (Mn411.05and 24.4%, respectively).
B. Cherif et al. / Physica B 457 (2015) 240244 241The 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 metalinsulator transition
Fig. 1 shows resistivity versus temperature curves ofLa0.5Ca0.5xAgxMnO3 with x0, 0.1, 0.2, 0.3 and 0.4. As shown inFig. 1, for x0, 0.1, 0.2 and 0.3, only one metalinsulator 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 . For x0.4, the TMI was 200 K. In general, thetransition was correlated to the deviation of Mn3OMn4 bondangle. In previous studies , the results reported by Smari et al.show that the values of this angle are very close for x0, 0.1,0.2 and 0.3 (161.16, 159.95, 160.59 and 162.72, respectively). Forx0.4, the bond angle is greater than the others (165.03). Such aresult explains the significant variation of the transition tem-perature for x0.4. The magnetic properties could be related toboth bond angle and chemical composition. It is found in a pre-vious work  that the parent compound exhibits paramagneticto ferromagnetic transition at Tc222 K and a ferromagnetic toantiferromagnetic (which is charge ordered) transition atTco92 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.  the solubility of silver in perovskite doesnot exceed x0.2. Hence for the x0.4 compound, both the dis-appearance of charge ordering and the appearance of silver pre-cipitates could be the origin of the metalinsulator transition at200 K. Similar variations of structural and magnetic properties ofdoped La0.5Ca0.5MnO3 with other elements are obtained. Dhimanet al.  introduced Sr in the parent compound and found that along range ferromagnetic ordering occurs at x0.4 in the range of180250 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 fromx0 to x0.2. This behavior was observed by Battabyal and Dey0 100 200 300 400 500 600 700
100 00% 10% 20% 30% 40%
Fig. 1. Temperature dependence of resistivity of La0.5Ca0.5xAgxMnO3 for0rxr0.4. 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 x0.3, the resistivity increases.This behavior can be attributed to the existence of secondaryphases. Previous work  shows that the sample with x0.3 hasthree phases (magnetic perovskite phase, metallic Ag and Mn3O4).For x0.4, 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()n 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 La0.5Ca0.5xAgxMnO3samples, the experimental sT curves are fitted to the followingequations :
T A E k T
B T T
exp( / ) (at high temperatures)
exp( / ) (at low temperatures)
DC a B
where A and B are the pre-exponential factors, Ea is the activationenergy, k 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 T1/4 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 x0 to x0.2. The same behavior was observed byBattabyal and Dey  in LaAgMnO3 compound. They found thatactivation energy decreases from 168 meV for x0.05 to 135 meVfor x0.30. 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 x0.3, charges carrierswill be trapped by the inhomogeneity and activation energy in-creases. Increase of Ea with Ag content was observed by Genceret al.  in LCMOAg system. They suggested that the dopant was
Fig. 2. Plots of log(s) versus log()for La0.5Ca0.5xAgxMnO3 at different temperatures.
B. Cherif et al. / Physica B 457 (2015) 240244242mainly distributed at the grain boundary or surface of the LCMOgrains, creating energy barriers to the electrical transport process.For x0.4, 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 asufficient kinetic energy to easily cross the SCZ. This induced areduction of the activation energy.
Fig. 4 shows the s(, T) spectrum, where the frequency andtemperature ranges are 104105 Hz and 280400 K, respectively.From such a spectrum, we deduce the conductivity exponent n as
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 (T1/4).
2x104 4x104 6x104 8x104 105
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
280K 320K 360K 400K
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) 240244 243a 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 . Such a model is generallypresent in perovskite manganite materials , in ferrites and in ferroelectric materials . At fixed temperature it is clearthat the conductivity exponent n decreases with increasing Agcontent from x0 to x0.2, 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 x0 to x0.2. We found that n0 in theconsidered temperature and frequency ranges for the compoundwith x0.2. 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 forx0.2. We can conclude that this compound has the best homo-geneity. For x0.3 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 x0.4 indicates that higher Agconcentration opens a new channel for electron transport viapercolation regime.4. Conclusion
We have investigated the electrical properties of silvercalciumdopant manganite (La0.5Ca0.5xAgxMnO3) with different Ag con-centrations. Ag content strongly affects the resistivity but does notchange the metalinsulator transition for xr0.3. For x0.4, TMIchanges due to the variation of the MnOMn bond angle. Wefound that resistivity and activation energy decrease with Agconcentration increasing from x0 to x0.2. This result was at-tributed to the good conductivity of Ag metal and the creation o...