Effect of chromium concentration on the structural, magnetic and electrical properties of praseodymium-calcium manganite

Accepted Manuscript Effect of chromium concentration on the structural, magnetic and electrical properties of praseodymium-calcium manganite A. Bettaibi, R. M’nassri, A. Selmi, H. Rahmouni, N. Chniba-Boudjada, A. Chiekhrouhou, K. Khirouni PII:

S0925-8388(15)30060-8

DOI:

10.1016/j.jallcom.2015.05.161

Reference:

JALCOM 34283

To appear in:

Journal of Alloys and Compounds

Received Date: 17 April 2015 Revised Date:

25 May 2015

Accepted Date: 26 May 2015

Please cite this article as: A. Bettaibi, R. M’nassri, A. Selmi, H. Rahmouni, N. Chniba-Boudjada, A. Chiekhrouhou, K. Khirouni, Effect of chromium concentration on the structural, magnetic and electrical properties of praseodymium-calcium manganite, Journal of Alloys and Compounds (2015), doi: 10.1016/ j.jallcom.2015.05.161. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Graphical abstract

1

ACCEPTED MANUSCRIPT

Effect of chromium concentration on the structural, magnetic and electrical properties of praseodymium-calcium manganite A. Bettaibia, R. M’nassri b,c*, A. Selmid, H. Rahmounia, N. Chniba-Boudjadae,f, A. Chiekhrouhoud, K. Khirounia a

SC

RI PT

Laboratoire de Physique des Matériaux et des Nanomatériaux appliquée à l’Environnement, Faculté des Sciences de Gabès cité Erriadh, Université de Gabès, 6079 Gabès, Tunisia b Higher Institute of Applied Sciences and Technology of Kasserine, Kairouan University, B.P. 471, 1200 Kasserine, Tunisia c Laboratoire de Physico-Chimie des Matériaux, Département de Physique, Faculté des Sciences de Monastir, Université de Monastir, 5019 Monastir, Tunisia d Laboratory of Physics of Materials, Faculty of Sciences of Sfax, Sfax University, B.P.1171, 3000 Sfax, Tunisie e Institut NEEL, B.P.166, 38042 Grenoble Cedex 09, France. f Faculté des Sciences de Gabès cité Erriadh, Université de Gabès, 6079 Gabès, Tunisia

M AN U

Abstract.

AC C

EP

TE D

The influence of Cr doping on magnetic, magnetocaloric and electrical properties in a polycrystalline sample of Pr0.7Ca0.3MnO3 is investigated. Structural studies show that our samples are single phase. The magnetization shows that the Pr0.7Ca0.3Mn1-xCrxO3 ceramics exhibit a paramagnetic–ferromagnetic transition with a large magnetic entropy change. The relative cooling power (RCP) values are comparable to those of other manganite. DC conductance GDC measurements show that all samples are characterized by a semiconductor behavior. It is found that GDC decreases by two decades when increasing chromium concentrations. For the parent compound, dc-conductance is characterized by the appearance of a saturation region at a specific temperature (Tsat=200K). For the doped compound, Tsat go beyond room temperature. Conduction mechanism is found to be dominated by the small polaron hopping (SPH) process at high temperature and by variable range hopping one (VRH) at low temperature. AC conductance study confirms that the conductivity is governed by hopping process and obeys to the Jonscher universal power law. The exponent ‘n’ variation with temperature is in good agreement with Mott theory. Its variation as a function of chromium content indicates that the material turns from metallic to semi-insulating behavior when chromium composition increases. Impedance analysis proves the presence of electrical relaxation phenomenon in the material and confirms that grain boundaries played a main role in the conduction process. Keywords: Manganites; X-ray diffraction; Magnetic properties; Electrical properties * Corresponding author: Rafik M’nassri. Higher Institute of Applied Sciences and Technology of Kasserine, Kairouan University, BP 471, 1200 Kasserine, Tunisia E-mail: [email protected] (R. M’nassri)

ACCEPTED MANUSCRIPT

1. Introduction Materials showing the magnetocaloric effect (MCE) are a focus of research activities nowadays due to their numerous potential advantages over vapor-compression refrigeration:

RI PT

higher chemical stability, easy fabrication, longer usage time, low noise, softer vibration and absence of freon, etc [1,2,3,4,5]. In these materials the application /removal of a magnetic field affects the magnetic entropy changes associated with the coupling of the magnetic spin system of the solid [6]. At present, materials with a high MCE at different (low, intermediate and high) temperatures can be obtained, and the main problem remains to get the materials

SC

with a high magnetocaloric effect at a low external magnetic field, which is beneficial for application as magnetic refrigerants, working in the fields produced by permanent magnets

M AN U

[7,8]. Manganites may be promising candidates to satisfy this requirement [9,10,11]. The physical properties in manganites are thought to arise from the strong competition among a ferromagnetic double-exchange (DE) interaction, an antiferromagnetic super-exchange interaction and the spin–phonon coupling [12,13,14]. These interactions are determined by intrinsic parameters such as, average cationic size [15], cationic disorder [16], oxygen stoichiometry [17]doping level [18] grain boundary engineering [19] and particle size [20,21]

TE D

There is no doubt that Mn ions play an essential role in the DE interaction. So, it is worthwhile to investigate the influences of the substitution at Mn site by other elements. A number of studies have been made on the effects of the replacement of Mn by various

EP

transition elements [22].They suggested that the introduction of a transition metal Cr [23], Co [24,25] and Fe [26,27] generally affected the magnetic and transport properties in manganites system. It is found that the substitution of Mn in ferromagnetic metal always leads to a

AC C

reduction in TC and magnetization, an increase in the resistivity and ultimately insulating state at low temperatures for high substitutions. Doped manganite materials showing multifunctional properties attract the attention of several research groups due to the rich fundamental physics and their presence in technological applications. In particular, PrCaMnO system shows several physical properties such as charge-orbital ordering, transition from metal to insulator behavior, presence of both ferroelectricity and ferromagnetism, etc. Usually, these phenomena are related to the competition between different interactions (charge, spin, orbital,…). These interactions and their sensitivity to some external perturbation such as applied electric and magnetic fields, irradiation by the X-ray, pressure, etc leads to important physical properties. Magnetocaloric effect and colossal electroresistance,

ACCEPTED MANUSCRIPT magnetoresistance and dielectric constant were observed in these materials. The study of complex properties of these materials and understanding the fundamental physics constitute the prime challenge. Manganite materials are characterized by the presence of mobile eg electrons hopping from Mn3+ to Mn4+ ions. Such interaction is responsible for the ferromagnetism and metallic conductivity in these materials. Magnetic and electrical

RI PT

properties of these compounds depend on several factors such as the elaboration method, the ratio, the ionic radii, the nature and the compositions of the dopant element, etc. Also, the substitution of Mn produces interesting modifications in the physical properties of these materials. In doped manganites, many studies have investigated the central role of the Mn4+-

SC

O-Mn3+ interactions. Various researches [28,29,30] have suggested that doping at Mn-site influences the polaronic transport. The effect of substituting manganese ion by different element such as tin, iron, chromium and titanium is well studied [31,32,33,34,35,36]. When

M AN U

modifying the dopant element, its concentration and the substitution sites, different conduction mechanisms appear. Hopping and percolation process are most observed mechanisms.

In order to continue our search for new materials with possibility to be used in magnetic or electrical devices, we report in this paper the effect of chromium doping on the structural,

TE D

magnetic, magnetocaloric and electrical properties of Pr0.7Ca0.3Mn1-xCrxO3. All samples have been synthesized by a high temperature solid-state reaction. Physical properties were investigated and the results are presented here.

EP

2. Experimental details

Ceramic samples of Pr0.7Ca0.3Mn1-xCrxO3 (x=0, 0.05 and 0.1) were synthesized from high

AC C

purity precursors: Pr6O11, CaCO3, Cr2O3 and Mn2O3 (Aldrich 99.9%; USA) in the desired proportions by the conventional solid state reaction method at high temperature. The starting materials were intimately mixed in an agate mortar. The obtained powders were then pressed into pellets of about 1mm thickness and sintered at 800, 1000, 1100 and 1300 °C for 24h for each cycle to ensure a better crystallization with intermediate regrinding and repelling. A heating rate of 10 °C per minute was used. Finally, those pellets were cooled from high temperature to room temperature following the cooling inertia of the furnace (~8 h). The physical properties depend strongly on the synthesis route and also on the cooling method [37,38,39,40]. For each sample, phase purity, homogeneity and cell dimensions were determined by powder X-ray diffraction (XRD) data, recorded at room temperature on a

ACCEPTED MANUSCRIPT PANalytical X’PERT Pro MPD diffractometer, using θ/2θ Bragg–Brentano geometry with diffracted beam monochromatized Cu Kα radiation. The diffraction patterns were collected by steps of 0.017° over the angle range 10–80°. The phase analysis was carried out using FULLPROF software based on the standard Rietveld method. Magnetization measurements were performed in BS2 magnometer developed at Néel institute. This magnetometer uses

RI PT

extraction technique and can produce a field of 10 T. The sample temperature is controlled by circulating helium gas controlled in temperature. Magnetization versus temperature measurements in the range of 5-300 K and versus applied magnetic field intensities up to 5 T was carried out using a vibrating sample magnetometer. Magnetic entropy changes were

SC

deduced from the magnetization measurements versus applied magnetic field up to 5 T at various temperatures. On the both side of the pellet we deposited a thin silver film (200 nm thick) through a circular mask of 6 mm of diameter. Thus we obtained a configuration of plate

M AN U

capacitor to measure both the conductance and the capacitance. The sample is mounted in a cryostat to vary the temperature from 77K to 300K. Measurements are conducted with an Agilent 4294A analywer under vaccum, in dark and with a signal amplitude of 20 mV. 3. Results and discussions

X-ray diffraction (XRD) patterns recorded at room temperature for Pr0.7Ca0.3Mn1-xCrxO3 (x=0,

TE D

x=0.05 and x= 0.1) are shown in Figure.1. The inset in Figure 1 shows a typical Rietveld refinement for x = 0.1 sample. Sharp peaks are clearly seen in all XRD patterns, indicating the studied samples to be highly crystalline. The data were refined by the

EP

Rietveld technique using the Fullprof program. For all composition, samples are composed by a single phase and have the same perovskite structure. They crystallize in the orthorhombic system with Pnma space group. The quality of the agreement is evaluated through the

AC C

adequacy of the fit indicator χ2. The unit cell and fitting results are computed and given in Table 1. The inset in Figure 1 shows that the doping concentration induced shifting of the principal peak position to the high or low angle indicating the structural modifications. These modifications can be understood on the basis of structural parameters, B-site average ionic radius ‘’, B-site size variance ‘σB2’ and tolerance factor‘t’. Mainly, shifting in peak position towards higher / lower angle is due to the decrease/ increase in unit cell volume x=0 to x=0.05/ x=0.05 to x=0.1. We plot in Figure.2-a, the magnetization evolution versus temperature M (T) of our doped manganites under an applied magnetic field of 0.05 T in the field-cooled (FC) regime. The Cr

ACCEPTED MANUSCRIPT doping in Pr0.7Ca0.3MnO3 is found to destroy the charge ordering (CO) observed in the parent compound and a paramagnetic to ferromagnetic transition takes place. The magnetic transition temperature TC has been determined by differentiating the M (T) curves and corresponds to temperature where (dM/dT-T) is minimum (see in the inset of Figure.2-a). These curves reveal a strong variation of magnetization around the Curie temperature TC. It

RI PT

indicates that there is a possible large magnetic entropy change around TC. The TC is about ~150 K for Pr0.7Ca0.3Mn0.95Cr0.05O3 and ~160 K for Pr0.7Ca0.3Mn0.9Cr0.1O3.

The isothermal magnetization (M-H) curves are measured over a wide temperature range under various external fields up to 5T. The inset in Figure.2-b shows the isothermal

SC

magnetization curves for the Pr0.7Ca0.3Mn0.95Cr0.05O3 . It can be seen that the magnetization measurements versus magnetic applied field up to 5 T at low temperatures (T < TC) confirmed

M AN U

the ferromagnetic behavior of our sample. The magnetization increases sharply with magnetic applied field for µ 0H < 1 T and tends to saturation. We have plotted the so-called Arrott curves (M2 versus µ0H/M) for x = 0.1 compound in the inset of Figure. 2-b. The M2 versus µ0H/M curves exhibit a positive slope indicating that the ferromagnetic-paramagnetic transition is of second order, according to the Banerjee criterion [12]. The TC values deduced from the Arrott curves are close to this deduced from M (T) measurements.

TE D

The isothermal magnetic entropy change (∆SM) which is associated to the magnetocaloric effect (MCE), can be calculated from magnetization measurements as a function of the magnetic applied field at several temperatures (indirect measurement technique of the

EP

magnetocaloric effect). ∆SM can be measured through the adiabatic change of temperature by the application of a magnetic field according to the classical thermodynamic theory based on

AC C

Maxwell's relations using the following equation: ∆S M = ∑ i

M i − M i +1 ∆H i Ti − Ti +1

where Mi and Mi+1 are the magnetization values measured in µ 0H, at temperature Ti and Ti+1, respectively. Figure. 2.b shows the magnetic entropy change as a function of temperature at µ 0H= 1T and 5T for x=0, 0.05 and 0.1 samples. Pr0.7Ca0.3MnO3 compound exhibits a positive entropy change due to the dominance of the antiferromagnetic interactions with a maximum value of 0.8 Jkg-1K-1 upon a magnetic field change of 5T. Figure. 2-b shows the magnetic entropy change evolution as a function of temperature of Pr0.7Ca0.3Mn1-xCrxO3 samples. The ∆S exhibits a broad negative peak around TC (normal MCE) for x=0.05 and x=0.1. The is found to be 2.76 Jkg-1K-1 and 2.81 Jkg-1Kmaximum of the magnetic entropy change ∆Smax M

ACCEPTED MANUSCRIPT 1

in a magnetic field change of 5T for x=0.05 and x=0.1 respectively. In addition to the

magnitude of the ∆SM, the relative cooling power (RCP) is another key parameter to characterize the efficiency of the magnetic refrigerant, which is proportional to the area under the curve of ∆SM versus T. This parameter corresponds to the amount of heat that can be transferred between the cold and hot parts of the refrigerator in one ideal thermodynamic

RI PT

cycle. The RCP allows an easy comparison of different magnetic materials for applications in magnetic refrigeration; hence, larger RCP values lead to better magnetocaloric materials max [41,42]. The RCP is evaluated as RCP = ∆S M (T, H) × δTFWHM where δTFWHM is the full-

width at half-maximum of ∆SM(T). For our compound, the RCP values are respectively 10

SC

J/kg, 406J/kg, 270J/kg at 5T for x= 0, 0.05 and 0.1. The compound with x=0.05 has the better RCP. The RCP value for 5% of Cr is about 98% of that obtained in gadolinium metal at the

M AN U

same magnetic field change of 5T [43], known as the most important material for magnetic refrigeration. This investigation suggests that this compound can be used as a potential magnetic refrigeration material with excellent performance and can thus be used as an active magnetic refrigerator materials suggested by Barclay [44].

The large MCE in manganites can originate from the spin–lattice coupling related to the magnetic ordering process[45,46,47]. This strong coupling is evidenced by the lattice changes

TE D

accompanying the magnetic transitions in these materials; the lattice structural change in Mn –O– Mn, bond angles and Mn–O bond distances with temperature, which results in a variation of the volume, can cause an additional change in the magnetic properties of the material [48]. In this study , the chromium substitution causes a decrease of the number of hopping electrons

EP

and available hopping sites between Mn3+ and Mn4+, which greatly weakens the double exchange interaction of Mn3+–O–Mn4+. The replacement of partial Mn3+ with Cr3+, Mn3+–O–

AC C

Cr3+ superexchange ferromagnetic interaction is weaker than the double-exchange interaction of Mn3+–O–Mn4+. In addition, it exists an antiferromagnetic Cr3+–O–Cr3+ interaction[49]. For the above reasons, the chromium substitution suppresses the charge ordering state and the ferromagnetic coupling is weakened, and the magnitude of the maximum magnetic entropy change had been depressed in Pr0.7Ca0.3Mn1-xCrxO3 series. Figure.3-a shows the temperature dependence of the conductance (GDC) of PrCaMn1-xCrxO3 samples with x=0, 0.05 and 0.1. The materials are characterized by a semiconductor behavior. For all chromium concentrations, GDC increases with the rise in temperature. For free compound, a saturation value is reached at high temperature region. GDC(T) curves are characterized by a change in the slope at a specific temperature Ts. Such change occurs at Ts

ACCEPTED MANUSCRIPT =120K, 149K and 160K for x=0, 0.05 and 0.10 respectively. As it is seen, the Ts values are closed to those of the Curie temperature. These values decrease when increasing chromium concentration. In the whole explored temperature range, Figure.3-a shows that GDC decreases by two decades when increasing chromium concentrations from x=0 to x=0.10. In manganite systems,

RI PT

electronic structure of the doping element is very related to the temperature dependence of conductivity. In such systems, the chromium and manganese ions exist in Mn3+, Mn4+ and Cr3+ form [50]. The electronic structures of these ions are respectively tg3/2eg1, tg3/2 and tg3/2. Consequently, only the eg1 electron of Mn3+ ion is electronically active. As the radius of

SC

chromium ion is near the Mn3+ one [51], the latter is directly replaced by the Cr3+ form. Then, introducing chromium in manganite reduces the Mn3+/Mn4+ ratio and the lattice will be

M AN U

disturbed due to the presence of Cr3+ in Mn-O-Mn chains. In addition, it is well known that the strength of the interaction between Mn3+ and Cr3+ ions is smaller than that of the interaction between Mn3+ and Mn4+ [52]. So, in chromium doped manganite, the interaction between Mn3+ and Mn4+ became weaker, due to the localization of the eg1 active electron of Mn3+ ion. These factors explain the decrease of the conductivity when introducing the chromium element. Increasing chromium content localizes more and more the Mn3+ eg1

TE D

electron which explains the observed decrease of conductivity with Cr concentration. In addition to the decrease of dc-conductance with chromium content, we note in Figure.3-a, the appearance of dc-conductance saturation region at a temperature Tsat. For the free

EP

compound, Tsat = 200K but for the doped ones, Tsat go beyond room temperature. We suggest that the density of free carriers was enhanced when the temperature increases. Also, carriers acquire a sufficient thermal energy to exceed the encountered barriers when the temperature

AC C

increases. The rise in temperature leads to the reduction of the efficiency of the capture of charge traps. At T>200K and for free compound, we think that around Tsat=200K, the available density of trapped charge is vanished and a plateau appears in GDC(T) curves. When introducing chromium, the Tsat increases. We estimate that charge carriers are trapped by the defects created by chromium. The release of such carriers needs higher thermal energy. This explains the increase of Tsat when increasing chromium concentration. In order to study the conduction process present in chromium doped PrCaMnO3, experimental data of Figure.3-a were fitted using different hopping models. At high temperature, Figure.3-b shows that experimental data are well fitted by the small polaron hopping (SPH) model. The

ACCEPTED MANUSCRIPT latter is expressed by the equation, GDC.T = G0 exp (Ea/kBT), where T is the absolute temperature, G0 is a pre-exponential factor, Ea is the activation energy of conduction and kB is the Boltzmann constant. At low temperature, the inset of Figure.3-b indicates that experimental data are well fitted by the variable range hopping model (VRH). Such model is described by the relation GDC=C exp (-T0/T)¼, where C is a constant and T0 is the temperature

RI PT

factor. In the low temperature region, transport of charge carriers was governed by the distributed localized trap centers characterized by multiple activation energies. When temperature increases, a sufficient thermal energy is gained. Consequently, the traps emit carriers and the conduction mechanism is dominated by thermally activated SPH process.

SC

Figure.3-b shows that the activation energy is sensitive to chromium concentrations. It increases from Ea = 26 meV for x = 0 to Ea = 140 meV for x = 0.1. Such evolution can be attributed to the decrease of the density of charge carriers with increasing Cr content. In one

M AN U

hand, it is known that the electron-lattice interaction induced by the Jahn-Teller distortion leads to polaron formation [53]. In the second hand, lattice distortion and activation energy are strongly related. So, when doping manganite with the small chromium ion, the carriers will be localized by the distortion. Also, the activation energy and the lattice constant vary in the same manner. In addition, as mentioned earlier, when the Cr content increases, some of

TE D

Mn-O-Mn chains are replaced by Mn-O-Cr one. So, electron hop between Mn ions becomes difficult and induces the rise in the activation energy. Also, conduction process is affected by the difference between the Cr-Mn and Mn-Mn interactions. For PrCaMn1-xCrxO3 compounds with x=0, 0.1 and 0.2, the conductance spectra are shown in

EP

Figure.3-(a-c). For free compound (Figure.4-a), the frequency dependence of the conductance is characterized by the occurrence of a frequency independent region for each temperature. In

AC C

the higher frequency region (f > 100 kHz) and for T < 110K, the conductance starts to increase with increasing frequency. But, for T > 110K, the conductance decreases at a specific frequency which shifts to lower frequency side when increasing temperature. For x=0.05 and x=0.10 (Figure.3-(b,c)), a plateau appears at low frequency, then, the conductance increases with increasing frequency. In the high temperature region, such plateau is observed in the whole explored frequency region. The conductance spectrum is characterized by a change in the slope at a frequency called "hopping frequency". The variation of conductance as a function of frequency is typical of a conductivity which is governed by hopping process. At low frequencies, the temperature dependence of the conductance is a proof of thermally activated conductance process in these materials. For these chromium compositions (x=0.05

ACCEPTED MANUSCRIPT and x=0.1), the semi-insulating behavior dominates. Conductance spectrum of chromium doped compounds obeys to the Jonscher universal power law [54] G(ω)=GDC+B ωn(T) where GDC represents the DC conductance, B is a pre-exponential factor and n is a constant which is temperature dependent and ω is the angular frequency . From the frequency range

RI PT

where the conductance is described by the law G=B ωn(T), we deduced the exponent ’n’ values as a function of temperature for different chromium concentrations. As shown in Figure.5, the exponent ‘n’ decreases with increasing temperature. Such result is in good agreement with

equation n=1-(6kBT/Wm)

SC

Mott theory where the temperature dependence of the exponent ‘n’ is described by the

Wm represents the necessary energy to move completely from one site to another one.

M AN U

From Figure.5, it is also found that the exponent "n" changes with chromium concentration. It increases with increasing chromium content in the whole explored temperature range. Such result indicates that the material turns from metallic to semi-insulating behavior when chromium content increases. This evolution is in good agreement with the dc-conductivity analysis and the activation energy variation as a function of chromium composition.

TE D

At some representative temperatures, the complex impedance plots of Pr0.7Ca0.3Mn1-xCrxO3 (x=0, 0.05 and 0.1) are shown in Figure.6. For all chromium concentrations, a semicircle arc appears in the spectra for each temperature. When increasing temperature, it is noticed that the diameters of semicircles diminish. Such behavior indicates that conduction process is

EP

thermally activated and proves the semiconducting character of these compounds in the measured temperature range. It is well known that the complex impedance analysis permits the correlation between the microstructure of the sample and its electrical properties. Usually,

AC C

the material is modeled by an electrical equivalent circuit. According to the complex impedance plots shown in Figure.6, all samples can be modeled by an electrical circuit composed by a grain resistance Rg connected in series with a parallel combination grain boundaries resistance Rgb and capacitance Cgb. The Rg and Rgb values can be determined from impedance spectra. The Rg value is given by the left intercept of the semicircle with real part of impedance axis. The right intercept gives the total resistance value R = Rgb +Rg. As shown in Figure.6, the Rg values are very weak when compared with the Rgb ones. In manganite materials, the grains are more conductor than grain boundaries. The most factors which contribute to such behavior are the presence of dangling bonds and the non-stoichiometric distribution of oxygen on the grain boundaries. These factors act as a barrier layers and as a

ACCEPTED MANUSCRIPT charge carrier traps. From the experimental data, Rgb values were determined at each temperature for all investigated compounds. Then, the SPH model was applied to study the conduction process present in these samples and to test the contribution of grain boundaries in such process. As shown in Figure.7, the grain boundary resistance decreases with increasing temperature for all chromium concentrations. This observation indicates that the process

RI PT

involved in the conduction is thermally activated. The activation energy values deduced from the SPH model are shown in the inset of Figure.7. We note that these values are closed to those inferred from the experimental data of dc-conductivity. This result confirms that the grain boundaries played a main role in the conduction process in this kind of materials.

SC

Figure.8 shows the variation of the real part of impedance Z’ at different temperatures for Pr0.7Ca0.3Mn1-xCrxO3 (x=0, 0.05 and 0.1). In the low frequency region, the Z’ spectra are

M AN U

characterized by a higher values. It is noticed that Z’ decreases with increasing both temperature and frequency. The temperature and frequency dependence of Z’ can be attributed to the rise of the mobility of charge carriers and to the reduction of the density of trapped charges. Also, the presence of space charge in these compounds is confirmed by the observed merge of Z’ values in the high frequency region.

The variation of imaginary part of impedance as a function of frequency and temperature for

TE D

different chromium concentrations is shown in Figure.9. The Z’’ spectra are characterized by the appearance of a peak at a specific frequency (fr) conventionally called “relaxation frequency”. When the temperature increases, it is observed that such frequency (fr) shifts to the high frequency region and the peak heights was reduced. Such observation indicates the

EP

presence of electrical relaxation phenomenon in the material. We add these observations to the previous results to confirm the semiconducting behavior of these samples and to prove

AC C

that charge carriers implicated in conduction mechanism are thermally activated. When chromium content increases, the magnitude of the imaginary part of impedance increases and the peak frequency shifts to low frequency region. This behavior can be attributed to the presence of space charge in chromium doped compound. This hypothesis is confirmed by the observed merging in the Z”(ω) spectra at high frequency region for all chromium concentrations. Using the equation 2π frω= 1 and the obtained relaxation frequency fr values, the relaxation time τ was calculated. The variation of log(τ) versus 1000/T is shown in Figure.10. The activation energy values deduced from relaxation time are equal to those deduced from conductivity analysis which permits to conclude that some relaxation species, such as defects, may be responsible for electrical conduction in the materials.

ACCEPTED MANUSCRIPT 4. Conclusion We have studied the structural, magnetic, magnetocaloric and electrical properties in Pr0.7Ca0.3MnO3 manganites partly substituted at the Mn site by Cr. The X-ray diffraction analysis revealed that all samples exhibit single perovskite phase with orthorhombic Pnma structure. The Cr substitution drives the system from charge order state to ferromagnetic

RI PT

second order PM–FM phase transition with a large magnetic entropy change over a wide range of temperature, which is comparable to those of other manganites. It is found that the compound with 5% of Cr has the highest relative cooling power. DC conductivity measurement indicates that samples have a semiconductor character. A significant decrease in

SC

GDC was observed when introducing Cr element. For undoped sample, a saturation region appeared at a specific temperature (Tsat=200K) in GDC (T) curves. For the doped compound,

M AN U

Tsat go beyond room temperature. Conduction mechanism is well described by SPH model at high temperature and by VRH one at low temperature. The dominance of hopping process was also confirmed by ac-conductance study. Conductance spectrum of doped samples obeys to the Jonscher law. The temperature dependence of the exponent ‘n’ is in good agreement with Mott theory. The increase of the exponent ‘n’ with increasing Cr concentration indicates that the material turns from metallic to semi-insulating behavior. Complex impedance

TE D

analysis permits to model the compounds by an electrical equivalent circuit. These analyses prove the presence of electrical relaxation phenomenon in the material and confirm the contribution of grain boundaries in the conduction mechanism.

EP

5. Acknowledgments

This study is supported by the Tunisian Ministry of Higher Education and Scientific Research

AC C

and the Neel Institute

ACCEPTED MANUSCRIPT

Figures caption

Figure 1: Room temperature XRD pattern for Pr0.7Ca0.3Mn1-xCrxO3 samples. Figure 2:

RI PT

(a) Temperature dependence of magnetization under 0.05T for Pr0.7Ca0.3Mn1-xCrxO3 compounds. The inset is the dM/dT curves for x=0.05 and 0.1.

SC

(b) Magnetic field dependence of magnetic entropy change for Pr0.7Ca0.3Mn1-xCrxO3. Isothermal magnetization curves at various temperatures for Pr0.7Ca0.3Mn0.95Cr0.05O3 and The M2 versus µ 0H/M isotherms for Pr0.7Ca0.3Mn0.9Cr0.1O3

M AN U

Figure 3 :

(a) The temperature dependence of the conductance (GDC) of Pr0.7Ca0.3Mn1-xCrxO3 samples with x=0, 0.05 and 0.1

(b) Plot of log (GDC T) vs 1000/T. The inset is the plot of log (GDC) vs T-1/4 Figure 4: Conductance spectra of Pr0.7Ca0.3Mn1-xCrxO3 compounds with x=0, 0.1 and 0.2

TE D

Figure 5: Temperature dependence of the exponent "n" as a function of chromium concentration

Figure 6: Complex impedance plots of Pr0.7Ca0.3Mn1-xCrxO3:

a) Pr0.7Ca0.3MnO3 ; b)

EP

Pr0.7Ca0.3Mn0.95Cr0.05O3 ; c) Pr0.7Ca0.3Mn0.9Cr0.1O3 Figure 7: The temperature dependence of grain boundary resistance (Rgb). The inset is the

AC C

variation of log(Rgb/T) vs 1000/T. Figure 8 : Variation of the real part of impedance Z’ at different temperatures for Pr0.7Ca0.3Mn1-xCrxO3 a) Pr0.7Ca0.3MnO3 ; b) Pr0.7Ca0.3Mn0.95Cr0.05O3 ; c) Pr0.7Ca0.3Mn0.9Cr0.1O3 Figure 9: The variation of imaginary part of impedance as a function of frequency and temperature

for

different

chromium

concentrations:

Pr0.7Ca0.3Mn0.95Cr0.05O3 ; c) Pr0.7Ca0.3Mn0.9Cr0.1O3 Figure 10 Variation of relaxation time (τ) versus 1000/T

a)

Pr0.7Ca0.3MnO3 ;

b)

ACCEPTED MANUSCRIPT References

AC C

EP

TE D

M AN U

SC

RI PT

[1] Tishin A M and Spichkin Y I (2003) The Magnetocaloric Effect and its Applications Bristol: Institute of Physics Publishing. [2] V.K. Pecharsky, K.A.Gschneidner, Jr. Phys. Rev. Lett. 78, (1997) 4494. [3] V.K. Pecharsky, K.A.Gschneidner, Jr., Adv. Mater. 13, (2001) 683. [4]R. M’nassri, A. Cheikhrouhou J Supercond Nov Magn 27 (2014) 1059 [5] H. Mbarek, R. M’nasri, W. Cheikhrouhou-Koubaa, and A. Cheikhrouhou. Status Solidi A 211, 975 (2014) [6]R. M’nassri, N. Chniba Boudjada , A. Cheikhrouhou, J. Alloys Comp 640 (2015) 183. [7]R. M’nassri, A. Cheikhrouhou J Supercond Nov Magn 27(2014) 421 [8] R. M’nassri, A. Cheikhrouhou Journal of the Korean Physical Society 64 (2014) 879 [9] A.K.M. Akther Hossain, L.F. Cohen, T. Kodenkandeth, J. Mac Manus-Driscoll, N. Mc Nalford, J. Magn. Magn.Mater. 195 (1999) 31. [10]S. Choura Maatar , R. M’nassri, W. Cheikhrouhou Koubaa, M. Koubaa , A. Cheikhrouhou, J. Solid State Chemistry 225 (2015) 83. [11]R. M’nassri, W. Cheikhrouhou-Koubaa, N. Chniba-Boudjada,A. Cheikhrouhou, J. Appl. Phys. 113 (2013) 073905 [12]Zener C 1951 Phys. Rev. 82 403. [13] Millis A. J, Littlewood P. B, Shraiman B. I 1995 Phys. Rev. Lett. 74 5144. [14]Goodenough J. B, Wold A, Arnott R. J, Menyuk N Phys. Rev 124 (1961 )373. [15] R. M’nassri, W. Cheikhrouhou-Koubaa, M. Koubaa, N. Boudjada, A. Cheikhrouhou, Solid State Comm. 151 (2011) 1579. [16]R. M’nassri, W. Cheikhrouhou-Koubaa, M. Kouba, A. Cheikhrouhou IOP Conf. Series: Materials Science and Engineering 28 (2012) 012050 [17]R. M’nassri, A. Cheikhrouhou . J. Supercond. Nov. Magn. 27 (2014) 1463 [18]R. M’nassri, W. Cheikhrouhou-Koubaa, N. Boudjada, A. Cheikhrouhou . J. Supercond. Nov. Magn. 26 (2013) 1429 [19]R. M’nassri, A. Cheikhrouhou , J Supercond Nov Magn 27 (2014) 421 [20] P. Lampen, A.Puri, M-H. Phan, H.Srikanth J. Alloys Compd 512 (2012) 94 [21]R. M’nassri, N. Chniba Boudjada , A. Cheikhrouhou J. Alloys Comp.626 (2015) 20 [22]A. Selmi, R. M’nassri, W. Cheikhrouhou-Koubaa, N. Chniba Boudjada, A.Cheikhrouhou, J. Alloys Comp. 619 (2015) 627. [23]C. Osthöver, P. Grünberg, R.R. Arons, J. Magn. Magn. Mater. 854 (1998) 177. [24]A. Selmi, R. M’nassri, W. Cheikhrouhou-Koubaa, N. Chniba Boudjada, A.Cheikhrouhou, J. Ceramics International, 41 (2015) 7723. [25]P. Barahona, O. Pena, A.B. Antunes, C. Campos, G. Pecchi, Y. Moreno, C. Moure, V. Gil, J. Magn. Magn. Mater. 320 (2008) e61 [26] S. Mahjoub, Mohamed Baazaoui, R. M’nassri, H. Rahmouni, N. Chniba Boudjada, M.Oumezzine J. Alloys Comp 608 (2014) 191 [27] Y.L. Chang, Q. Huang, C.K. Ong, J. Appl. Phys. 91 (2002) 789. [28] R. W. Li, Z. H. Wang, X. Chen, J. R. Sun, B. G. Shen, C. H. Yan, J. App. Phys 87 (2000) 5597. [29]J. L. Alonso, L. A. Fernandez, F. Guinea, V. Laliena, V. Martin-Mayor, Phys. Rev. B 66 (2002) 104430. [30] S. M. Yusuf, M. Sahana, K. Dorr, U. K. Robler, K. H. Muller, Phys. Rev. B 66 (2002) 064430. [31]H. Rahmouni, R. Jemai, M. Nouiri, N. Kallel, F. Rzigua, A. Selmi, K. Khirouni, S. Alaya, J. Mag. Mag. Mat 316 (2007) 23

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[32]H. Rahmouni, R. Jemai, M. Nouiri, N. Kallel, F. Rzigua, A. Selmi, K. Khirouni, S. Alaya, J. Cryst. Growth 310 (2008) 556 [33]H. Rahmouni, R. Jemai, N. Kallel, A. Selmi, K. Khirouni, J. Alloys Comp. 497 (2010)1. [34]H. Rahmouni, A. Selmi, K. Khirouni, N. Kallel, J. Alloys Comp. 533(2012) 93. [35]H. Rahmouni, A. Dhahri, K. Khirouni. J. Alloys Comp. 591 (2014) 259-262 [36]H. Rahmouni, B. Cherif, M. Baazaoui, K. Khirouni. J. Alloys Comp. 575 (2013) 5–9 [37]W. Boujelben, M. Ellouze, A. Cheikh-Rouhou, J. Pierre, J.C. Joubert J. Solid State Chemistry, 165, (2002) 375 [38]W Boujelben, A Cheikh-Rouhou, J Pierre, J.C Joubert J. Alloys Comp 314, ( 2001) 15 [39]J.H. Miao, S.L. Yuan, G.M. Ren, X. Xiao, G. Q.Yu, Y.Q.Wang, S. Y. Yin, J. Alloys comp 448 (2008) 27-31 [40]T. Barbier, C. Autret-Lambert, C. Honstrette, F. Gervais, M. Lethiecq, Mat .Res. Bulletin 47(2012) 4427 [41] Pecharsky, V.K., Gschneidner, K.A., Tsokol, A.O.: Rep. Prog. Phys. 68, 1479 (2005) [42] Gschneidner, K.A. Jr., Pecharsky, V.K.: Annu. Rev. Mater. Sci. 30, 387 (2000) [43]O. Tegus, E. Brück, K. H. J. Buschow , F. R. de Boer, Nature 415 (2002)150 [44]J.A. Barclay, J. Alloys Comp. 207–208 (1994) 355. [45]R. Thiyagarajan, S. Esakki Muthu, R. Mahendiran, S. Arumugam J. Appl. Phys. 115, 043905 (2014) [46]M. H. Phan, S. C. Yu, N. H. Hur, Y. H. Yeong, J. Appl. Phys. 96 (2004) 1154. [47]P. G. Radaelli, D. E. Cox, M. Marezio, S.W. Cheong, P. E. Schiffer, A. P. Ramirez, Phys. Rev. Lett. 75 (1995) 4488. [48]Z. B. Guo, Y. W. Du, J. S. Zhu, H. Huang, W. P. Ding, D. Feng, Phys. Rev. Lett. 78 (1997) 1142. [49]Z.M. Wang, G. Ni, Q.Y. Xu, H. Sang, Y.W. Du J. Magn. Magn. Mater, 234 (2001)371. [50]J. Zhang, Q. Yan, F. Wang, P. Yuan and P.Z. Hang, J. Phys.: Condens. Matter 12 (2000) 1981. [51]P.V. Vanitha, R.S. Sing, S. Natarajan and C.N. R. Rao, J. Solid State Chem. 137 (1998) 365. [52]A. Maignan, C. Martin, F. Damay, M. Hervieu and B. Raveau, J; Magn. Magn. Mater 188 (1998) 185 [53]A. J. Millis, Nature, London 392, (1998) 147. [54]A.k.Jonscher, Nature 276(1977) 673

ACCEPTED MANUSCRIPT Table Caption Table 1 : Structural data obtained after Rietveld refinement for Pr0.7Ca0.3Mn1-xCrxO3

Pr0.7Ca0.3Mn1-xCrxO3 0

0.05

0.1

a (Å)

5.459(8)

5.430(0)

5.429(3)

b (Å)

7.674(1)

7.667(9)

c (Å)

5.430(3)

5.457(2)

V(Å3)

227.52(3)

227.21(9)

O1-Mn

1.924(5)

1.925(6)

1.922(3)

O2-Mn

2.04(3)

2.04(4)

2.06(4)

O2-Mn

1.94(3)

1.92(4)

1.88(4)

< O-Mn >

1.970(1)

1.964(5)

1.956(7)

Mn-O1-Mn

171.44(20)

169.5(3)

172.06(11)

Mn-O2-Mn

150.6(11)

153.4(17)

154.9(16)

< Mn-O-Mn >

157.55(4)

158.78(8)

160.63(1)

χ2

1.21

1.37

1. 38

Bragg R-factor

4.88

5.03

5.75

RF-factor

7.325

6 .79

8.52

7.669(1) 5.455(2)

227.14(3)

SC

M AN U

TE D

EP AC C

RI PT

x

ACCEPTED MANUSCRIPT

Figure 1

AC C

EP

a)

TE D

M AN U

SC

RI PT

Pr0.7Ca0. 3Mn1-xCrxMnO3

Figure 2 a) and b)

b)

ACCEPTED MANUSCRIPT

M AN U

SC

RI PT

a)

AC C

EP

TE D

b)

Figure 3 a) and b)

ACCEPTED MANUSCRIPT

RI PT

a)

TE D

M AN U

SC

b)

AC C

EP

c)

Figure 4 a) b) and c

AC C

EP

Figure 5

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Figure 6 a) b) and c)

AC C

EP

Figure 7

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Figure 8 a) b) and c)

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Figure 9 a) b) and c)

AC C

Figure 10

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

Highlights

Pr0.7Ca0.3Mn0.95Cr0.05O3 has the highest relative cooling power.

RI PT

Pr0.7Ca0.3Mn1-xCrxO3 manganites phases crystallize in an orthorhombic (Pnma) structure.

DC conductivity measurement indicates that samples have a semiconductor character.

SC

Conduction mechanism is well described by hopping processes.

AC C

EP

TE D

M AN U

Conductance spectrum of doped samples obeys to the Jonscher law.