Role of defects and oxygen vacancies on dielectric and magnetic properties of Pb2+ ion doped LaFeO3 polycrystalline ceramics

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Role of defects and oxygen vacancies on dielectric and magnetic properties of Pb2 þ ion doped LaFeO3 polycrystalline ceramics K. Devi Chandrasekhar a,n, S. Mallesh a,b, J. Krishna Murthy c, A.K. Das a, A. Venimadhav c a

Department of Physics & Meteorology, Indian Institute of Technology, Kharagpur 721302, India Department of Physics, Indian Institute of Technology, Madras 600036, India c Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur 721302, India b

art ic l e i nf o

Keywords: Magnetization Impedance analysis Oxygen vacancies Polaronic relaxation and Maxwell–Wagner interfacial polarization

. a b s t r a c t We have presented the dielectric/impedance spectroscopy of La1  xPbxFeO3 (x ¼0.15 and 0.25) polycrystalline samples in a wide temperature and frequency range. They exhibited colossal dielectric permittivity and multiple relaxations. Temperature and field dependent magnetization study showed enhancement of magnetization upon Pb doping which has been ascribed to the defect driven magnetization phenomenon. Overall we have emphasized the formation of various kinds of defects and their influence on dielectric and magnetic properties in the system. & 2014 Elsevier B.V. All rights reserved.

1. Introduction Recently, miniaturization of modern microelectronic technology demands high performance and cost effective materials. Colossal dielectric constant (CDC) materials are going to be key electronic components in modern microelectronic technology. Achieving and understanding of CDC is one ongoing problem in condensed matter physics. In this regard, perovskite transition metal oxides have been the focus of scientific interest over a decade; several materials have exhibited CDC behavior in a wide temperature/frequency range [1]. Materials with multiferroic and CDS property are highly desirable for novel spintronic devices [1–4]. In fact multiferroic behavior is demonstrated in several CDC materials [2,5,6]. However, the origin of CDC and the associated multiferroic coupling in these materials remain elusive [6–9]. Recently, much attention has been paid to LaFeO3 which exhibits complex electrical and magnetic behavior with strong correlation among the spin, charge and orbital degrees of freedom. LaFeO3 is an orthorhombically distorted perovskite crystal structure with strong super exchange interaction between the Fe3 þ – Fe3 þ via O2  ion leading to an antiferromagnetic (AFM) ordering Néel temperature TN 740 K [10]. A small deviation in AFM arrangement of spins can develop a weak magnetic moment. Apart from magnetism, the A-site doped LaFeO3 is a state of the art material for gas sensing, catalytic and exchange bias applications [11,12]. For example oxygen ion conduction in A-site doped

n

Corresponding author. Tel.: +886988873428. E-mail address: [email protected] (K. Devi Chandrasekhar).

LaFeO3 can be used to build an efficient solid state fuel cell [13]. Moreover doping the A-site with a divalent cation leads to the charge disproportion of Fe that results in a mixed valence state of iron i.e. Fe4 þ /Fe3 þ [14]. The occurrence of multiferroic nature in LaFeO3 is demonstrated by Acharya et al. These authors have observed both ferroelectric and ferromagnetic behavior in LaFeO3 without any structural transition or dielectric anomaly; however the appearance of such a ferroelectric nature is unusual [15]. Contradictory to this, Idrees et al. have observed the polaronic conduction mechanism in this system and have assigned the CDC to extrinsic effects [16]. These studies demand meticulous experiments to confirm various polarization mechanisms in CDC materials. The polarization of dipolar crystals is influenced by crystallographic defects/ vacancies and formation of long range/localized motion of various charged defects and vacancies. Such defects can also modify the magnetization property [17]. In this paper, we have focused on the effect of divalent Pb2 þ ion on the structural, dielectric and magnetic properties of La1  xPbxFeO3.We have emphasized possible defects and their role on transport and magnetic properties using impedance spectroscopy and SQUID magnetometer.

2. Experimental details Polycrystalline La1  xPbxFeO3 (x ¼0.15 and 0.25) samples were prepared by solid state reaction method using high purity oxides of La2O3, Fe2O3 and PbO (Aldrich 99.99% purity) as raw materials. Stoichiometric amounts were weighed and thoroughly mixed and calcined in air between 650 1C and 1000 1C with intermed-

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iate grindings. The final sintering was carried on pellet samples (10 mm in diameter and 1 mm thickness) at 1000 1C for 24 h. The structural and surface morphology was studied using X-Ray Diffraction (XRD) with Cu-Kα (λ ¼ 1.5414 Å) radiation (Philips Pan Analytical X-pert) and JEOL JSM5800 Field Emission Scanning Electron Microscopy (FESEM), respectively. Dielectric properties were investigated using Agilent 4294A impedance analyzer with an ac excitation of 500 mV in the frequency range of 40 Hz–10 MHz. Electrodes for the dielectric measurements were made by applying wet silver paste on both side of the pellets (diameter 10 mm, thickness 1 mm) and were heated at 200 1C for 5 h. All the dielectric measurements were performed on the home made Liquid Nitrogen (LN2) based cryostat. The temperature and field dependent magnetization were performed with commercial Quantum design SQUID-VSM magnetometer.

3. Results and Discussions 3.1. XRD analysis The room temperature XRD pattern for La1  xPbxFeO3 (x ¼0.15 and 0.25) samples is shown in Fig. 1(a). The structural analysis has been carried out using Rietveld refinement (solid lines in Fig. 1(a)) method. The analysis confirms the orthorhombic crystal structure with Pbnm space group. A minor amount of secondary phase (La2O3) is observed in La0.75Pb0.25FeO3 (LPFO0.25) sample. This is due to volatile nature of Pb and it is indicated by ‘n’ symbol in Fig. 1(a). The obtained fitting parameters for both the samples are shown in Table 1. Fig. 1(b and c) shows FESEM images of La0.85Pb0.15FeO3 (LPFO0.15) and LPFO0.25 samples, respectively. Inhomogeneous grain distribution is observed in both the samples with grain size of the order of 200–500 nm. It can be noticed from

the micrographs that the size of the grains decreases while grain boundaries (GB) increase with the increase of Pb concentration. It is well known that in oxide materials, the transport and magnetic properties are greatly influenced by the presence of various kinds of defects (oxygen vacancies, hole and electron doping and impurities) and crystallographic disorder that in turn depends on various parameters such as synthesis conditions, doping, etc. [17]. Prior information on formation of various kinds of defects would be useful to understand the transport/magnetic properties of the system. Here, we discuss the possible defect formation in the present compound. In most of the cases the formation of oxygen vacancies is inevitable during the synthesis of ceramics unless the reaction is carried out in oxygen atmosphere. During the high temperature sintering process, the thermodynamic reaction of oxidation and reduction of grain and GB happens at different reaction rates that result in oxygen deficient Table 1 The obtained Rietveld parameters for both the samples, where the atomic position of Fe ion is (x y z) ¼ (0 0.5 0). LPFO0.15

LPFO0.25

a (Å) b (Å) c (Å) v (Å3)

5.558 5.563 7.864 243.161

5.5545 5.5647 7.8675 243.18

La O1 O2

x (Å) 0.990 0.012 0.749

o Fe–O4 (Å) 1.9319 o Fe–O–Fe4 (1) 159.42 2 χ 2.81

y (Å) 0.023 0.490 0.196

z (Å) 0.250 0.250  0.025

x (Å) 0.987 0.028 0.728

y (Å) 0249 0.516 0.183

z (Å) 0.250 0.250  0.021

1.762 157.22 4.23

Fig. 1. (a) XRD spectra of La1  xPbxFeO3 (x¼ 0.15, 0.25) polycrystalline samples; (b and c) shows the FESEM images of La1  xPbxFeO3 (x ¼0.15, 0.25) polycrystalline samples.

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grains surrounded by the oxygen excess GB. The oxygen reaction kinetics is described by using Krö ger–Vink notation as follows: Oo -VXO þ 12 O2 ↑ VXO -VOU þe0 VOU -VOU U þ e0 Further, the single and double ionization of oxygen vacancies generate the conduction electrons into the system as described by the above equations. The presence of electrons may stimulate the formation of mixed valence states of transition metal ions by making oxidation and reduction reactions as follows: FeXFe þ e0 -Fe0Fe Alternately, electrons can get trapped at the oxygen vacancy sites ðVOU =VOU U Þ. Further, there is a possibility that the trapped electrons at oxygen vacancies might hop between the neighboring transition metal ions as Fe3 þ –VOU –Fe2 þ . Apart from the formation of natural oxygen vacancies, the doping of divalent Pb2 þ ion into trivalent La3 þ site results in the formation of either oxygen vacancies or promotes the multiple oxidation states of Fe3 þ (i.e. Fe2 þ or Fe4 þ ); this happens to maintain the charge neutrality of the system. The corresponding reaction mechanism is described as 0

2PbO-2PbLa þ 2OX þ VOU U The presence of these defects will significantly influence the dielectric and magnetic properties of the present system as discussed below.

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3.2. Temperature dependent dielectric study Fig. 2(a and b) shows the temperature dependent real part of dielectric permittivity (ε0 ) and the loss tangent factor (tan δ) at different frequencies for LPFO0.15 sample. The dielectric permittivity exhibits distinct regions, a plateau like behavior for T 4290 K with a large dielectric permittivity (  104) and multiple drops are noticed for temperature below 290 K, 260 K and 120 K. The tan δ displays multiple relaxations associated with the drops in ε0 and these relaxation peaks get shifted towards high temperature with the increase of frequency. From now onwards, we call these relaxations as relaxation I (T o120 K), relaxation II (200 K o To 260 K) and relaxation III (T4290 K). It can be noticed that the peak amplitude for relaxation II increases with the increase of frequency whereas relaxation III exhibits a decreasing trend; this hints at different origins of relaxations. In Fig. 2(c), quantitatively similar kind of dielectric properties is observed for the LPFO0.25 sample. We have analyzed the relaxations II and III of both the samples (Fig. 3(a and b)) using thermal activation relation. The fit to relaxation III in LPFO0.25 may not be convincing due to unreliable relaxation time of the order of 10  22 s. The observed activation energies and relaxation times for both the samples are shown in Table 2. Here the activation value of relaxation II and III is comparable to the relaxation of several oxide systems such as Mn doped LaFeO3 and CaCu3Ti4O12, etc. [18,19]. Though the observed activation energy values for both the relaxations are similar, the relaxation times are distinctly different from each other. The observed dielectric properties hint the electrical heterogeneities of samples that in turn are strongly correlated to the microstructure of compound.

Fig. 2. ε0 and tan δ for (a and b) LPFO0.15 and (c and d) LPFO0.25 samples, respectively.

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3.3. Impedance analysis Fig. 4(a–c) shows the frequency dependent modulus spectra for LPFO0.15 sample at different temperatures. The electrical modulus (Mn) function shares the same information as that of the dielectric function and can be related by M n ¼ 1=εn . The representation of dielectric data in modulus formalism has advantages where it neglects the conduction loss contribution and reveals the hidden

relaxations. When the applied frequency matches with time constant ðτ ¼ RCÞ of different contributions, the imaginary part of electrical modulus spectra exhibits a peak. As shown in Fig. 4, unlike temperature dependent dielectric property, the imaginary part of electrical modulus has revealed four relaxations in the temperature window from 85 K to 380 K and all the relaxations are shifted to high frequency side with the increase of temperature indicating thermally activated mechanisms. The presence of more than three relaxations is often noticed in phase separated oxide systems such as manganites, cobaltates, etc. [20,21]. Here, in LPFO, one can notice, presence of intragrain electric inhomogeneities and their corresponding relaxations (grain1 (G1) – T o120 K and grain2 (G2) – 180 KoT o260 K) along with GB (225 Ko To380 K) and electrode (E) (290 KoT o380 K) contributions. In fact the three apparent relaxations in the dielectric spectra fall in between the four modulus relaxations. We have analyzed the modulus relaxation peaks for different contributions using Arrhenius mechanism (Fig. 4(d)). The obtained activation energy and relaxation time for all the relaxations has been listed in Table 2. To get more insight about the electrical conduction mechanisms at grain and GB, we have performed the frequency dependent impedance measurements at several temperatures. Fig. 5(a and b) shows the Nyquist plots at 225 K and 330 K; two semicircular arcs corresponding to different relaxations can be noticed. Moreover, the observed arcs exhibit depression nature which can be correlated to the presence of distribution of relaxation times. On the basis of modulus analysis, we have considered the presence of four relaxations in the impedance by changing temperature from 85 K to 380 K. At low temperature (below 120 K), a clear semicircular arc could not be obtained due to high resistivity. We have analyzed the high temperature relaxations using the electrical equivalent circuits consisting of three parallel RQ elements connected in series, where R denotes the resistance and Q indicates the constant phase element. Q is usually employed to encounter the distribution of relaxation times where the impedance of constant phase element can be described as Z nQ ¼ 1=Q ðjωÞn ð0 rn r1Þ; for an ideal capacitor n-1 whereas for ideal resistor n approaches zero [22]. Z n ¼ Z '  jZ '' ¼

Fig. 3. Plot between ln τ vs. 1/T for (a) LPFO0.15 and (b) LPFO0.25 samples, respectively.

RG RGB RE þ þ 1 þ Q G ðjωÞnG RG 1 þ Q GB ðjωÞnGB RGB 1 þ Q E ðjωÞnE RE ð1Þ

Depending on the temperature range we have chosen two appropriate parallel RQ elements to fit the data. The fit to equivalent circuit with the experimental data is shown as solid lines in Fig. 5(a and b). The goodness of the fit can be verified by plotting the frequency dependent real and imaginary parts of impedance data as shown in the inset of respective figures. The extracted

Table 2 The relaxation time and activation energies obtained from different dielectric functions. Temperature dependent

Frequency dependent

Resistance analysis

LPFO0.15

LPFO0.15

LPFO0.15

LPFO0.25

Tan δ

M″

Relaxation I Relaxation II

E (eV) τ (s)

0.47 5.4  10  15

Relaxation III

E (eV) τ (s)

0.45 1.6  10  12

0.49 3.36  10  15

ε″

Relaxation G1

E (eV) τ (s)

0.071 5.04  10  11

Relaxation G2

E (eV) τ (s)

0.48 2.0  10  15

0.49

Relaxation GB

E (eV) τ (s)

0.44 1.24  10  11

0.43

Relaxation E

E (eV) τ (s)

0.5 5.6  10  9

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Fig. 4. Frequency dependent modulus plots for LPFO0.15 sample for a temperature range of (a) 85–105 K, (b) 180–260 K and (c) 280–380 K; (d) shows the plot between ln τ vs. 1/T.

R and Q values at different temperatures are given in the Supplementary material. We have investigated the electrical conduction mechanisms of various relaxations by analyzing the fitting parameters obtained from the impedance data. Fig. 5(c) shows the temperature dependent resistance behavior of relaxations G2 and GB, where G2 resistance obeys the linear relation between lnðR=TÞ vs: 1=T. This indicates that the adiabatic small polarons are the possible carriers of transport. On the other hand the thermal activated Arrhenius law ðln R vs: 1=TÞ fits satisfactorily for GB relaxation. The activation energies (EC) obtained from the dc transport for these relaxations are listed in Table 2. Activation energy indicates the type of charge carrier involved in the conduction process. It is well accepted that for n-type polaron conduction EC o0.2 eV, whereas for p-type polaron EC 4 0.2 eV [23]. The activation energy of G2 is consistent with the activation obtained from the dielectric spectrum (corresponding to relaxation II) and it indicates the presence of p-type polaron hopping within the grains. Similar kind of hole conduction due to the oxidation state of Fe3 þ to Fe4 þ has been reported in several oxide systems such as BiFeO3 and Bi0.9La0.1Fe0.98Mg0.02O3, etc. [23,24]. For GB, the activation energy matches with the value obtained from modulus analysis and is of the order of 0.43 eV; this can be assigned to the presence of oxygen ion migration in GB. The activation energy of double ionized oxygen vacancies V OU U is of the order of 0.3–1 eV depending on the range of migration; a smaller activation energy in LPFO0.15 sample indicates that the

migration of V OU U is restricted to short ranges; in fact, similar arguments have been reported for polycrystalline BiFeO3 and GaFeO3 ceramic samples [23,25]. Fig. 5(d) shows the variation of Q for both G2 and GB relaxations, where Q of relaxation G2 shows slow increase with temperature, whereas GB shows rapid increase with the increase of temperature. For a quantitative analysis of low temperature relaxation (T o120 K), we have fitted the frequency dependent dielectric spectra using Cole–Cole function given as [26]

εn ¼ ε1 þ

εs  ε1

1 þ ðiωτÞ1  α

ð2Þ

where, εs and ε1 are the static and high frequency dielectric permittivity, respectively; ω is angular frequency and τ denotes the relaxation time. The distribution of relaxation times can be encountered with the term α, and for ideal Debye relaxation α-0. The satisfactory fit to experimental data to Eq. (2) is shown as a solid line in Fig. 6. We have calculated the activation energy and relaxation time using the Arrhenius mechanism as shown in the inset of Fig. 6 and obtained Ea  0.071 eV and τ  5.04  10  11 s, respectively. The obtained activation energy of G1 is very small compared to that of G2 and can be ascribed to the n-type polaron hopping mechanism [27,23,24]. The electron to hop between Fe3 þ to Fe2 þ via single ionized oxygen vacancy ðVOU Þ requires lower activation energy ( o0.2 eV).

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Fig. 5. Nyquist plots for LPFO0.15 sample at (a) 225 K and (b) 330 K; lower insets of respective figures show the fit to equivalent circuit shown in upper inset of the same figure; (c) plot between ln(R/T) vs. 1/T and ln R vs. 1/T and (d) variation of Q vs. T.

Fig. 6. Frequency dependent ε0 and ε″ at 90 K; solid lines show the fit to Cole–Cole formula; inset shows the plot between ln τ vs. 1/T.

3.4. Magnetic properties Fig. 7(a and b) shows the temperature dependence of zero field cooled (ZFC) and field cooled (FC) magnetization (M–T) curves of

LPFO0.15 and LPFO0.25 samples under 500 Oe. The ZFC and FC magnetizations exhibit a large irreversibility starting from 380 K for both the samples; however the strength of irreversibility is strongly suppressed for higher Pb concentration. With the decrease of temperature, the FC magnetization increases smoothly for LPFO0.15 sample, whilst a saturation kind of magnetization has been noticed below 80 K for LPFO0.25 sample. The ZFC magnetization decreases with the decrease of temperature, thereby, exhibiting a broad hump around 100 K for LPFO0.15 sample, whereas for LPFO0.25 sample the hump is suppressed. In perovskite oxides, the local orbital symmetry of the oxygen octahedral, bond angle and bond length determines the magnetic property of the system. In the present case, as discussed above the presence of various defects due to doping of Pb2 þ ions in LaFeO3 induces various FM/ AFM interactions. According to Goodenough–Kanamori rules, strong negative super exchange between Fe3 þ –O2  –Fe3 þ results AFM interactions, whereas moderate superexchange interaction gives rise to weak AFM behavior between Fe3 þ –O2  –Fe3 þ pair [28]. On the other hand, a direct exchange interaction between Fe3 þ /Fe3 þ and Fe3 þ /Fe2 þ can also be possible through VOU that may result in a positive FM exchange interaction [23]. Apart from the defect induced magnetic interactions, the crystallographic disorder due to A-site cation further modulates magnetic

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Fig. 7. ZFC–FC plot for (a) LPFO0.15 and (b) LPFO0.25 samples, respectively. M–H curves for (c) LPFO0.15 and (d) LPFO0.25 samples, respectively; insets in c and d show the magnified view of M–H curves for low fields.

properties by decreasing the Fe–O–Fe bond angle and this aids the stabilization of weak FM behavior. Overall the appearance of several magnetic interactions in La1  xPbxFeO3 weakens the strong AFM nature of the parent LaFeO3 compound and leads to either weak FM or glassy magnetic properties. In fact the presence of random disorder and fluctuating positive/negative magnetic interactions throughout the lattice are the key ingredients to observe the glassy magnetic properties. Fig. 7(c and d) shows the field dependent isothermal magnetization plot (M–H curves) up to 5 T for both the samples. As shown in the figure, magnetization loops at different temperatures exhibit a FM behavior with hysteresis and coercive field. The hysteresis at different temperatures for both the samples is shown in the lower inset of Fig. 7. At high fields, the magnetization shows almost saturation like behavior. Along with temperature dependent magnetization, the field dependent magnetization also indicates the appearance of FM behavior in Pb2 þ doped LaFeO3 samples. Variation of coercive field (HC) with respect to temperatures for both the samples is shown in Fig. 8(a). The HC value decreases with decrease of temperature which clearly signifies the enhancement of FM correlations, whereas for magnetic glassy systems, the HC increases with the decrease of temperature [29]. Moreover, for higher Pb2 þ concentration, the magnetization is enhanced and HC decreases which signifies the role of various defects on bulk magnetization. To further check the possibility of

magnetic glassy behavior, we have performed dynamical magnetic studies such as magnetization vs. time (M vs: t) for LPFO0.25 sample. In this protocol, the sample was first cooled in zero field up to 10 K and the system was kept for aging with a definite waiting time ðt w ¼ 100 sÞ. Then magnetic field is applied and magnetization is recorded with respect to time. The M vs. t graph exhibits no change with time (Fig. 8(b)) that clarifies the absence of glassy magnetic phase in the present system [30].

4. Conclusions We have synthesized polycrystalline La1  xPbxFeO3 (x¼ 0.15, 0.25) samples by the solid state reaction method. Impedance spectroscopy study has revealed electrical inhomgenity and multiple relaxations in the temperature range 80–400 K with intragranular contributions below T o120 K and in the temperature range of 180 KoT o260 K due to n-type polaron hopping between Fe2 þ to Fe3 þ site via oxygen vacancies and polaronic relaxation, respectively. The diffusion of doubly ionized oxygen vacancies at the GB led to dielectric relaxation between 310 oTo 400 K. Temperature and field dependent magnetization confirm the enhanced ferromagnetic interactions upon Pb2 þ doping. Our results clearly demonstrate the important role of defects on the electrical/ magnetic properties of La1  xPbxFeO3 ceramic oxide.

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References

Fig. 8. (a) HC vs. T (K) for both the samples and (b) M vs: t for LPFO0.25 sample at 10 K.

Acknowledgment This work was supported by the Department of Science and Technology, Ministry of Science and Technology fast track project. The authors also acknowledge the use of SQUID VSM facility in CRF of IIT Kharagpur.

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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.physb.2014.04.070.

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