Effect of co-substitution on structural, optical, dielectric and magnetic behavior of LaFeO3

Accepted Manuscript Effect of Co-substitution on structural, optical, dielectric and magnetic behavior of LaFeO3 Atma Rai, Awalendra K. Thakur PII:

S0925-8388(16)33890-7

DOI:

10.1016/j.jallcom.2016.11.407

Reference:

JALCOM 39904

To appear in:

Journal of Alloys and Compounds

Received Date: 1 September 2016 Revised Date:

24 November 2016

Accepted Date: 30 November 2016

Please cite this article as: A. Rai, A.K. Thakur, Effect of Co-substitution on structural, optical, dielectric and magnetic behavior of LaFeO3, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2016.11.407. 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.

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Effect of Co-substitution on structural, optical, dielectric and magnetic behavior of LaFeO3

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Atma Rai and Awalendra K Thakur Department of Physics, Indian Institute of Technology, Patna, 800013, India,

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Abstract:

Effect of co-substitution by Na at La site and Mn at Fe site of perovskite type LaFeO3 on its structural,

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optical, dielectric and magnetic properties at room temperature have been studied. Single phase nanocrystalline La1-xNaxFe1-yMnyO3 (x=y=0-0.2) have been prepared by modified Pechini route. Substantial effect of substitution on structural property has been observed, which is clearly visible in view of changes in Fe/Mn-O-Fe/Mn bond angle and lattice strain. The substitution concentration dependent structural changes have also been confirmed by Fourier transform infrared (FTIR) spectrum in terms of

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upward shift of stretching mode (Fe/Mn-O) and downward shift of bending mode of vibrations (Fe/MnO-Fe/Mn). Typical nanocrystalline surface morphology has been observed with visible change in grain size on changes in concentration, which is also confirmed by X-ray diffraction (XRD). These changes

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have profound effect on magnetic property tailoring and appear to be correlated with Fe/Mn-O-Fe/Mn bond angle deviation and indirect exchange interactions between Fe and Mn ions via intervening Oxygen

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ion. Dielectric response (εr ~ 103) at room temperature was observed on Na and Mn co-substituted LaFeO3. The dielectric spectrum shows typical non Debye type behavior. Modulus spectrum reveals shift of peak position toward higher frequency side with increasing concentration i.e. concentration dependent relaxation time. Electrical conductivity shows thermally activated Arrhenius type response with a maximum of~ 2.5 ˣ 10-4 S.cm-1 at 450 K for La0.85Na0.15Fe0.85Mn0.15O3. Both grain and grain boundary contribution to conduction mechanism has been observed.

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_______________________________________________________________________________ Keywords:

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Perovskite, X-ray diffraction, Rietveld refinement, Magnetic measurements, Dielectric measurements

1. Introduction

Rare earth based ferrites with chemical formulae RFeO3 (R is rare earth element) attracted flurry of

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research activity due to their unique structural, electrical and magnetic properties. Among rare earth based ferrites, LaFeO3 (LFO) is widely studied and reported for different applications such as oxide fuel cells

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[1-3], sensors [4-5], catalyst [6-8] and magnetic applications [9-12]. Recently few reports in literature also suggested multiferroic characteristics at room temperature [13, 14]. The colossal dielectric (εr ~ 103) response of RFeO3 also attracted substantial attention due to feasible applications in miniaturization of high performance electronic devices like multilayer capacitors, resonators and memories. The electronic and magnetic properties of LFO are tailored either by oxygen partial pressure or by appropriate

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substitution of foreign elements. The steady and stable performance of perovskite of the type RFeO3 depends on the Goldsmith tolerance factor‘t’ [15]. The tolerance factor in the case of LFO was found to be (~ 0.9546) which is less than 1. Therefore a typical perovskite cubic structure undergoes a symmetry

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lowering and deforms into orthorhombic unit cell structure. It causes a deviation of Fe-O-Fe bond angle from 180o, resulting in distortions of FeO6 octahedra. Consequently, property tailoring becomes a handy

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tool by proper substitution at La and Fe sites. Earlier reports on RFeO3 mentioned that the substitution of divalent ions like Ca2+, Sr2+, Ba2+ [16-18] and monovalent ions like Na+, Ag+ [19, 20] at La site leads to the charge imbalance in the system resulting in mixed valence state of Fe (Fe3+/Fe4+). This in turn causes oxygen non stoichiometry in the crystal lattice.

Several reports of divalent and trivalent cation

substitution at La site and transition metal substitution at Fe site exist in literature with an aim to tailor the electronic and magnetic properties for wide range of applications. To enhance the magnitude of mixed cation effect it is believed that monovalent substitution at La site would have better impact. M. B.

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Bellakki et al. [20] suggested that Ag substitution at La lattice site leads to the creation of ions of mixed valency (Fe3+/Fe4+) and also noted that such an approach affects the electronic as well as magnetic properties strongly, which is attributed to the presence of mixed valency of Fe in Ag doped LFO.

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Motivated from the above facts, an attempt has been made to synthesize Na and Mn co substituted LFO via modified Pechini route. The ionic radii of Na+ (1.39Ȧ) is very close to La3+ (1.36 Ȧ) in twelve coordinated geometry and ionic radii of Fe is closer to Mn in six coordinated geometry of Perovskite [21].

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Considering these facts in mind, Na substitution at La site was taken up to get twice the amount of hole concentration as compared to the aliovalent substitution. Tong et al. [22] studied LaMn1-xFexO3 (x= 0-0.4)

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system and demonstrated that there is definite probability of double exchange interaction between Fe3+ ions and Mn3+ ions. Anjum et al. [23] studied the system La0.7Bi0.3Fe1-xMnxO3 and demonstrated possible indirect exchange interactions between Fe and Mn ions. Feasibility of such an interesting indirect exchange interaction between Fe and Mn on substitution is expected to strengthen magnetic properties in

distortion on substitution.

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this class of perovskite in addition to a strong dielectric order present inherently due to displacive

Based on the idea described above, we have attempted to study and analyze the effect of monovalent cation substitution at A (La3+) site and multivalent cation substitution at B (Fe3+) site in LaFeO3. In the

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present study, the effect of co substitution of Na and Mn for structural, dielectric, optical and magnetic property at room temperature has been studied. The result so obtained is discussed below.

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To the best of our knowledge, no systematic study of Na and Mn co- substitution in lanthanum orthoferrite has yet reported. 2. Experimental

A novel chemical method called modified Pechini route has been adopted for the preparation of Na and Mn co substituted lanthanum ferrite, i.e., La1-xNaxFe1-yMnyO3 (x = y = 0.00, 0.05, 0.10, 0.15, 0.20). Stoichiometric amount of acetates of lanthanum, sodium and manganese (Sigma Aldrich) and nitrates of

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iron (Merck) were dissolved in a small amount of distilled water (milli Q grade). Appropriate amount of citric acid and ethylene glycol was added in the homogenious solution. Citric acid having one hydroxyl group and three corboxyl group is a weak acid which easily decomposes in aqous solutions and acts as a

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chelating agent. It traps the cations of precursor material. Ethylene glycol acts as gelating agent.The molar ratio of citrate/nitrate has been taken to be 1/1.5 in the course of experiment which is sufficient to provide exothermicity to obtain nanocrystalline powder. In this technique, cations bonded with citric acid

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followed by its hydrolysis to ethylene glycol to produces an internal easter linkage. Homogenized solutions are kept on a hot plate at temperature 60 oC for 4h with constant magnetic stirring. The

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temperature is further increased upto 120 oC in order to evaporate the residual liquid if any .The solution becomes viscous, foamy and finally auto combustion starts. The autocumbustion induces redox reaction, in which carboxilic group plays the role of reducing agent and nitrate ion acts as an oxidant, while citric acid and ethyline glycol acts as organic fuel. As a consequence, residual powder is left finally, which is grinded and calcined later after calcination temperature is optimised. Thermogravimetric analysis (TGA)

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was done by using thermogravimetric analyser(STA 6000, Perkin Elmer, USA) to estimate optimized phase formation temperature. It indicated that sample phase formation occour at ~ 850 oC. Accordingly as synthesized powder was calcined in a programmable furnace at 850 oC for 2 hour at rate of 5 oC / minute.

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In order to understand phase formation and crystallinity, X-ray differaction (XRD) analysis has been carried out with high power X-ray powder diffractometer (model: Rigaku TTRAX III diffractometer 18

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kW) with Cu Kα radiation (λ=1.5418 Å) over Bragg’s angles (2θ) (20o ≤ 2θ ≤ 70o) at a step size of 0.01o. The microstructural evolution of the sample has been recorded with the help of field emission scanning electron microscopy (FESEM) (Model: Hitachi S-4800, JAPAN) with energy dispersive spectrum (EDS) facility. Fourier transform infrared spectrum (FTIR) was recorded using Perkin Elmer (Model USA) spectrophotometer in the ranges 400-1200 cm-1. UV-Visible-NIR absorption spectra (Perkin Elmer Lambda 35 UV/VIS spectrometer) in wavelength range 350 to 1000 nm have been recorded to estimate band gap of the sample. The Dielectric measurements of all samples at room temperature (300 K) have

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been carried out with the help of impedance analyzer (N4L PSM 1735) in the frequency range 100 Hz to

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4 MHz. Magnetic hysteresis (M-H) loop was recorded at room temperature with field varrying ± 60 kOe.

3. Result and discussion 3.1. Thermogravimetric Analysis (TGA)

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Thermogravimetric analysis (TGA) technique was employed to obtain optimal temperature for calcining the as prepared powder in single phase form. Fig.1 shows the thermogram of La1-xNaxFe1-yMnyO3 (x = y =

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0.00, 0.05, 0.10, 0.15 and 0.20). The TGA thermogram depicts two distinct regions of mass loss as marked in Fig.1. The region I of weight loss begins at 100 oC attributed to dehydration and continues progressively at a steady rate upto~350oC. The step loss marked as region II suggesting progressive mass loss (∆m/m0) with rise in temperature up to ~700 oC is assigned to thermal decomposition of precursor compound ( carbonaceous compounds), this progressive mass loss (step loss) up to ~800oC may be

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related to the removal of reaction products prior to formation of stable phase of the product. Beyond 850 o

C, mass loss becomes stable. A typical TG-DTA thermogram (inset) of LaFeO3 depicts peak at ~100 oC

attributed to dehydration, broad peak at ~600 oC corresponds to removal of carbonaceous species and

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peak at ~715 oC are related to onset of crystallization of LaFeO3. So on the basis of TGA result the desired sample has been achieved after calcination at 850 oC for two hours in a programmable furnace at

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the heating rate of 5 oC/ minute. This is the optimal condition for sample formation.

3.2. Structural Analysis

Fig. 2 shows the X-Ray diffraction (XRD) pattern (Rietveld refinement) of Na and Mn co-substituted lanthanum ferrite under study. All peaks have been identified and indicated the formation of single phase La1-xNaxFe1-yMnyO3 (x = y = 0.00, 0.05, 0.10, 0.15and 0.20).

No evidence of secondary or impurity

phase has been observed within the experimental limit of XRD. From XRD pattern, it is noted that the

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main peak (121) shift towards higher angle side with increasing concentration of substituents (Na/Mn). Therefore changes in structural parameters with variation in concentration of substituent have been estimated using Rietveld refinement. Though the results indicate minor change in unit cell parameters (a,

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b, c), an overall effect on changes in cell volume is clearly visible with increasing Na and Mn concentration. Little distortions in crystal structure, as noted from rietveld refinement result seems consistent with average tolerance estimation given in table [1]. This may be attributed to the difference in

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ionic radii of deferent cations causing crystal lattice contraction due to distortion of Fe/MnO6 octahedra or/and by mixed valence state of Mn in order to neutralize the charge imbalance created by substitution of

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trivalent cation by monovalent cations. The structural data obtained from XRD have been refined using Rietveld refined method [24]. The whole pattern refinement was done by taking LaFeO3 to be an orthorhombic crystal system with space group Pnma (62). For simplicity of comparison of refinement results, simulation was performed with the same initial condition as of LFO with space group Pnma. The observed, calculated patterns along with their difference and allowed Bragg positions are shown in Fig 2. 2

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The calculated profiles are in good agreement with observed pattern. Fairly good values of χ were

obtained and tabulated for every sample. The result so obtained after rietveld refinement is presented in table [1] along with refined pattern (observed, calculated and difference plot) shown in Fig [2]. In

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addition, the crystallite size was estimated by using sherrer formula t= 0.9 λ/ β Cosθ, where β is the width of line at half maximum intensity. The lattice strain was estimated by Williamson hall calculation using

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software package and tabulated in Table [1]. 3.3 Fourier transforms infrared spectroscopy (FTIR) To corroborate the findings on structural information from XRD analysis, Fourier transform infrared (FTIR) spectrums of the samples under study has been recorded in the Mid –IR region (400-1200cm-1). The observed FTIR modes in the region of interest are shown in Fig [3]. It shows clear bands at wave numbers~ 435 cm-1, 558 cm-1. Out of these the bands at 435 cm-1 and 558 cm-1 appears to be strong, attributed to metal oxygen (M-O) contribution. The formation of perovskite structure of the type ABO3

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can be confirmed by presence of metal oxygen bond present in the system. The vibrational bands present at 558 and 435 cm-1 indicate the formation of lanthanum orthoferrite. The band at 558 cm-1 in the parent sample LaFeO3 is attributed to the Fe–O stretching vibration and the band at 435 cm-1 correspond to the

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Fe–O–Fe deformation (bending) vibration [25]. A small shift in the vibrational band towards higher wave number can be observed with the increase in Na and Mn co-substitution, it may be attributed to change in

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mass of different co-substituted foreign elements.

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3.4 Surface property analysis

The surface morphology of La1-xNaxFe1-yMnyO3 (x=y=0-0.2) is shown in Fig.4. It depicts a typical nanocrystalline microstructure with non uniform grain distribution throughout the sample. The typical average grain size has been estimated to be ~ 50 nm for undoped sample and co-substituent sample grain size reduces with concentration as confirmed by XRD result earlier. The particles are agglomerated and a

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typical EDS pattern of La0.85Na0.15Fe0.85Mn0.15O3 shows the presence of all elemental compositions in their respected molar concentrations.

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3.5 UV- Vis Spectroscopy

UV-Visible spectrum of La1-xNaxFe1-yMnyO3 (x = y = 0.00, 0.05, 0.10, 0.15 and 0.20) have been recorded

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to estimate the effect on band gap of pure LaFeO3 on co-substitution with Na at La site and Mn at Fe site. The recorded spectrum is shown in Fig. [5]. The diffused reflectance spectrum (DRS) has been recorded in the wavelength range 250-1000 nm. The Kubelka Monk (KM) function is used for diffused reflectance spectra and it has been converted to absorbance in accordance with F(R) =α/s= (1-R) 2/2R [27], where F(R) is the Kubelka-Munk (KM) function, α is absorption coefficient, s is scattering coefficient and R is diffused reflectance. The energy dependence of semiconductors near absorption edge has been estimated using tauc’s equation [28].

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(Αhν)=B (hν-Eg) n

…… (1)

where α is the absorption coefficient, h is the Planck constant, B is a constant related to the effective mass of the holes and electrons, n is 1/2, 3/2, 2 and 3 respectively for allowed direct transition, direct

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forbidden, allowed indirect transition and indirect forbidden transitions, Eg is the band gap energy of optical transition. The Tauc plot is shown in Fig [5]. From the extrapolation of F(R) hν=0, near absorption edge for n=1/2, i.e., direct allowed transitions, the optical energy gap of the La1-xNaxFe1-yMnyO3 (x = y =

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0.00, 0.05, 0.10, 0.15 and 0.20) have been estimated to be ~ 2.1 ± 0.2 eV, 1.8± 0.2 eV, 1.6± 0.2 eV, 1.6± 0.2 eV, 1.6± 0.2 eV respectively. It is to be noted that the band gap of LaFeO3 seems consistent with

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earlier report [29]. There is apparent lowering in band gap with minimum value close to Nanocrystalline Si, on co-substitution at La and Fe site in LaFeO3 by Na and Mn respectively. Such a result suggests clear improvement in conduction properties on co-substitution due to band gap lowering. This may possibly be due to release of excess charge carriers to counter balance coulombic interaction on substitution sites. The smaller band gap can also cause higher photo catalytic activity in co-substituted samples as compared to

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3.6 Magnetic property

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its parent compound LaFeO3 and conventional photocatalyst TiO2 [30, 31].

The room temperature magnetization curves (M-H) of La1-xNaxFe1-yMnyO3 (x=y= 0.00, 0.05, 0.10,

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0.15and 0.20) are shown in Fig 6. All samples exhibits non linear magnetization curve. LFO is known to be antiferromagnetic with G type spin orientation in which antiparallel Fe3+ spins interact via intervening oxygen ion at room temperature with Neel temperature ~ 740 K [32]. The M-H loop observed for pure LaFeO3 and co-substituted counterparts La1-xNaxFe1-yMnyO3 (x = y = 0.05-0.20) represents typical weak ferromagnetic response with varying degree of magnetic strength on co-substitution by Na at La site and Mn at Fe sites. In case of LaFeO3, La3+ exhibits no localized magnetic moments. The presence of little magnetic contribution (weak ferromagnetic feature) arises due to the presence of Fe attributed to the

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uncompensated moment arising out of canted spin. A similar behavior has been reported earlier in literature for YFeO3 and BiFeO3 [33]. A very small, though noticeable, change in magnetic response has been observed on co-substitution at La and Fe sites by Na and Mn respectively. The magnetization curves

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(M-H) for La1-xNaxFe1-yMnyO3 (x = y = 0.05-0.20) at 300 K clearly shows no saturation up to 60 kOe. The maximum magnetization (Mmax), coercive fields (Hc) for all compositions estimated and presented in table [2], and their variation as a function of concentration has been shown in Fig [6]. It shows that there is

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progressive increase in magnetic moment with increase in concentration that reaches maxima at 15% and then decreases. In contrast, coercive field (Hc) is maximum at 5% and then it drops monotonically

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reaching the level of pure LFO for 20% co-substitution. The enhancement in magnetic moment of Na and Mn co-substituted LFO upto x = y = 0.15 and then lowering for x = y = 0.20 are attributed possibly to two reasons. (a) Mismatch of two magnetic sub lattices arising out of the presence of Fe/Mn on cosubstitution, which distorts the Fe/MnO6 octahedra causing a clear change in Fe/Mn-O-Fe/Mn bond angle, thereby reinforcing magnetic strength. Such a possibility seems logical and evident from XRD

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results on rietveld refinement. A systematic increase (Table 1) in in-plane Fe/Mn-O1-Fe/Mn bond angle and out-of plane bond angle Fe/Mn-O2-Fe/Mn upto 15% concentration enhances the magnetic moment. On the other hand out of plane bond angle decreases drastically for 20% concentration of substituent

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(Na/Mn) thereby magnetic moment decreases. This result is also supported by an identical shift in Fe/MnO-Fe/Mn bending mode in FTIR spectrum. So the concentration dependent tunability of magnetic

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moment arises primarily due to strong interaction of M-O on co- substitution (b) Another reason may be substitution of Na at La site leads to the charge imbalance in the system causing mixed valence state of Mn to maintain charge neutrality in the system. In the presence of mixed valence state of Mn ions (+3/+4) in the lattice possibility of double exchange interaction (Fe3+-O-Mn3+, Mn3+-O-Mn4+) seems feasible giving rise to weak ferromagnetism in the system with increasing concentration of Mn. However the possibility of super exchange interaction (Mn3+-O-Mn3+, and Fe3+-O-Fe3) can not be ruled out in the system. So natural competition between FM/AFM sub lattices causing an incommensurate magnetic order

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on substitution leading to the uncompensated (canting) magnetic interface that gives rise to weak

3.7 Dielectric property impedance and modulus spectrum

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ferromagnetism in the co-substituted system.

In order to get the better understanding of dielectric property of La1-xNaxFe1-yMnyO3 (x = y = 0.00, 0.05,

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0.10, 0.15and 0.20) , the variations of dielectric constant (εr) and dielectric loss (tan δ) as a function of frequency at room temperature (RT) are given in Fig. 7. It is clear that the dielectric constant decreases

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with the increase of frequency, i.e., the dielectric constant is strongly dependent on the frequency. The decreasing trend in dielectric constant with increasing frequency is due to the fact that the species contributing was lagging behind with the applied field. In the present frequency region there are non Debye-like dielectric dispersions with a corresponding loss peak in the loss tangent (tan δ). Three possible mechanisms was found to contribute in the frequency range 100 Hz - 4 MHz (a) electrode polarization,

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due to interface barrier between electrode contact and sample, (b) The Maxwell-Wagner-Siller polarization, due to heterogeneity in the sample and (c) true bulk effect which contribute to the observed dielectric relaxation. In general, electrode polarization due to interface between electrode contact and

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sample occurs at low frequency (below~100 Hz) [34]. The maximum value of dielectric constant at room temperature was found to be > 2ˣ103 for La0.85Na0.15Fe0.85Mn0.15O3 at 300 K. In order to understand the

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relaxation/conduction mechanism from different species like grain, grain boundary etc., complex impedance spectroscopy (CIS) analysis has been employed at room temperature (300 K).The magnitude of impedance z and phase angle (φ) has been measured for a wide frequency range (100 Hz - 4 MHz). The real and imaginary part of impedance (Z’ and Z’’) were calculated from following relations.

Z′= z Cos φ and Z″= z Sin φ…………………… (2)

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Fig. 8 shows the Nyquist plot (Z″ Vs Z′) for La1-xNaxFe1-yMnyO3 (x = y = 0.00, 0.05, 0.10, 0.15and 0.20) at room temperature and a typical La0.85Na0.15Fe0.85Mn0.15O3 fitted plot is shown in the inset. Two depressed semi circular arc are clearly shown in the measured frequency range. The semi circle observed

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at higher frequency is due to the contribution from bulk (grain) while the semicircle observed at low frequency corresponds to conduction across grain boundary. Since only two semicircles are obtained in measured frequency range, the possibility of relaxations due to electrode-sample interface is ruled out. In

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general relaxations arising from electrode-sample interface take place below 100Hz. An equivalent circuit comprising parallel combinations of constant phase element (Qg) and grain resistance (Rg) in series with

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grain boundary resistance (Rgb) and constant phase element (Qgb) have been used to fit Nyquist plot. The constant phase element is related to capacitance by following relations.

C=(R) (1-n)/nQ1/n…………………………… (3) where n=1 for pure capacitor and n=0 for pure resistor. The Niquist plot is fitted by using this model and

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fitting parameters for grain and grain boundary is tabulated in table [3].

Fig. [9] shows the variation of imaginary part of modulus (M″) with frequency for all samples. The imaginary part of modulus was estimated by following formulae, M″=2ߨ݂ܼʹ where f is frequency and Zʹ

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is the real part of impedance. The peak in all concentration depicts well defined relaxation phenomena at room temperature (300 K) and asymmetric nature of peak reveals non Debye type relaxation mechanism

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in La1-xNaxFe1-yMnyO3(x = y = 0.0-0.2). It is clear from M″ Vs Frequency plot that relaxation peak shifts towards higher frequency side with increasing concentration of Na and Mn. This indicated to a corresponding decrease in relaxation time with increasing concentration of Na and Mn. The result suggests concentration controlled improvement in the dynamics of charge transfer and such an effect has, infact, been noted in conductivity response described below.

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3.8 dc conductivity The dc conductivity of all samples at different temperatures was evaluated by σdc = t/RA, where σdc is the dc conductivity, t is the thickness and A is the area of the sample. A thermally activated Arrhenius type dc

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conductivity response with temperature is noted. It shows a negative temperature coefficient of resistance (NTCR) effect, which is typically found in semiconductors. The dc conductivity plotted with temperature for all concentrations follows Arrhenius type of behavior, σdc =σ0exp (Ea/KBT) where σ0 is the pre

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exponential factor, Ea is the activation energy of charge carrier, KB is the Boltzmann constant and T is the temperature. Fig. 10 shows the variation of dc conductivity with Temperature for grain and grain

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boundary contribution and variations in activation energy with concentration for grain and grain boundary are also given in the graph itself. Typically the activation energy ≥ 1 are attributed the oxide ion conductor which occurs at high temperatures while the activation energy Ea < 0.2eV are attributed to small n type polaron hoping and Ea > 0.2eV are attributed to p type polaron hoping [35]. So it indicated that p type polaron hoping mechanism may be a key factor in conduction mechanism and conduction

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yields both grain and grain boundary contribution i.e it is not only extrinsic effect as reported in earlier reports[36] but also an intrinsic effect.

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4. Summary and conclusion

In the present studies, single phase nanocrystalline powder of La1-xNaxFe1-yMnyO3 (x = y = 0.00, 0.05,

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0.10, 0.15, 0.20) have been successfully prepared by modified Pechini route. Phase formation temperature was optimized apriori by thermogravimetric analysis. The lattice parameters along with noted changes in Fe/Mn-O-Fe/Mn bond angle obtained from Rietveld refinement of XRD results suggest strain induced structural distortion well within the limits of structural stability. The decrease in lattice volume with cosubstitution may be attributed to creation of ions of smaller radii (Mn3+/Mn4+) at Fe site. The optical band gap decreases with co-substitution. The noted weak ferromagnetism is correlated with Fe/Mn-O-Fe/Mn bond angle changes as well as indirect exchange interaction possibly be causing between Fe and Mn via

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intervening oxygen ion. A maximum dielectric constant (> 2×103) at room temperature and at 100 Hz frequency was found for La0.85Na0.15Fe0.85Mn0.15O3. So the high value of dielectric response and improved magnetic response in co-substituted sample observed was beneficial for various electronic and magnetic

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devices, like multilayer capacitors, resonators, magnetic storage etc. the salient features of the present work may be listed as.

i) Co-substitution by Na at La site and Mn at Fe site in LaFeO3 has strengthen dielectric, magnetic, optical

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and conduction properties.

enhancement in magnetic response.

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ii) Tailored Fe/Mn-O1-Fe/Mn and Fe/Mn-O2-Fe/Mn bond angle on substitution is attributed to

iii) Reinforcement of grain boundary effect is the sole factor attributing to enhancement in dielectric and conduction properties in La0.85Na0.15Fe0.85Mn0.15O3.

Overall effect of co-substitution has strengthened both the dielectric and magnetic properties of LaFeO3

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with maximum enhancement at 15% concentration.

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[21] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751

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programs version, Laboratoire Leon Brillouin, CEA-CNRS, France (2010). [25] C. Shivakumara, Solid State Commun. 139 (2006) 165–169

L.J. Bellamy, the Infra-red Spectra of Complex Molecules, second ed., London, England, 1958

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P. Kubelka, F.Z. Munk, Tech. Phys. 12 (1931) 593–601

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[29] Z. Yang, Z. Huang, L. Ye, X. Xie, Phy. Rev. B. 60 (1999) 23 [30] K.M. Parida, K.H. Reddy, S. Martha, D. P .Das, N. Biswal. Int. J. Hydrogen Energy. 35(2010)12161

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[33] G. Catalan, J. F. Scott. Adv. Mater. 21 (2009)2463–2485 [34] I. Ahmad, M. J. Akhtar, M. Younas, M. Siddique, M.M.Hasan, J. Appl. Phys. 112(2012)074105

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[35]M.Idrees, M. Nadeem, MM Hassan. J. Appl. Phys. 43(2010) 155401

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[36]M.Idrees, M. Nadeem, M. Atif, M. Siddique, M. Mehmood, MM. Hassan, Acta Mater 59(2011)1338

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Figure captions. Fig. 1 TGA thermogram of La1-xNaxFe1-yMnyO3(x = y = 0.0-0.2)

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Fig.2 (a) Rietveld refinement plot with observed, calculated, difference and allowed Bragg positions of La1-xNaxFe1-yMnyO3(x=y=0.0-0.2). (b) The magnified main peak (121) shift Fig. 3 FTIR absorption spectrum of La1-xNaxFe1-yMnyO3(x = y = 0.05-0.2). The absorption spectrum of LaFeO3 is given in inset. Fig. 4 FESEM micrograph of La1-xNaxFe1-yMnyO3(x = y = 0.0-0.2) and EDS spectrum of

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La0.85Na0.15Fe0.85Mn0.15O3.

Fig. 5 Kubelka Monk fit and Reflectance spectrum of La1-xNaxFe1-yMnyO3(x = y = 0.0-0.2).

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Fig. 6 M-H loop of La1-xNaxFe1-yMnyO3(x = y = 0.0-0.2) at Room temperature and Mmax and Hc variations with concentration. Magnified image of the M-H loop also given in inset. Fig. 7 Relative permittivity and Loss tangent of La1-xNaxFe1-yMnyO3(x = y = 0.0-0.2) Fig. 8 Nyquist plot of La1-xNaxFe1-yMnyO3(x = y = 0.0-0.2) at room temperature. A typical fitting of La0.85Na0.15Fe0.85Mn0.15O3 with equivalent circuit is given in inset. Fig. 9 M″ Vs Frequency of La1-xNaxFe1-yMnyO3 (x = y = 0.0-0.2) at room temperature.

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Fig. 10 variation of dc conductivity with temperature for La1-xNaxFe1-yMnyO3 (x = y = 0.0-0.2) at temperature ranging 400 K-500 K for pure LaFeO3 and 325 K-450 K for Na and Mn co-substituted sample. Derived activation energy for grain and grain boundary variation with concentration also given.

Table 1. Structural parameters extracted from Rietveld refinement and calculated tolerance factor

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Table 2. FTIR Peak assignment and magnetic parameters Table 3. Calculated fitting parameters of the Nyquist plot for La1-xNaxFe1-yMnyO3 and dielectric

parameters at room temperature (300 K)

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Table 1

V(Å3)

Goldschmidt tolerance factor

Lattice strain

c (Å)

Crystallite size

b (Å)

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a (Å)

Goodness of fit

Bond Angle (o)

Unit cell lattice parameters

(t) (nm)

ε (%)

˂τ˃

156.90(1) 1.59

42.8

3.06×10-1

0.9546

159.83(1) 1.23

40.0

3.26×10-1

0.9550

χ2

B*-O(1)-B

B-O(2)-B

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La1-x Nax Fe1-y MnyO3 (x=y)

System under study

Structural parameters extracted from Rietveld refinement and calculated tolerance factor.

5.5587(1) 7.8639(4) 5.5484(1) 242.53

155.33(2)

x=y=0.05

5.5405(3) 7.8326(1) 5.5545(9) 241.04

157.54(2)

x=y=0.10

5.5242(2) 7.8237(5) 5.5508(8) 239.90

160.20(1)

161.29(8) 1.56

30.3

4.26×10-1

0.9557

x=y=0.15

5.5128(5) 7.8256(4) 5.5341(3) 238.74

160.77(3)

163.46(1) 1.52

23.2

4.26×10-1

0.9560

x=y=0.20

5.5064(8) 7.8074(1) 5.5268(1) 237.60

165.40(2)

155.44(1) 1.54

22.8

5.49×10-1

0.9567

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where B* is Fe/Mn in Perovskite ABO3

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Table 3 Calculated fitting parameters of the Nyquist plot for La1-xNaxFe1-yMnyO3 and dielectric parameters at room temperature (300k) Sample

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La1-xNaxFe1-yMnyO3

Impedance parameters

Dielectric parameters

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Grain (Bulk)

_________________

Grain Boundary

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εr

x=y=0.05

1.21×106 2.9 9.23×10-11 1.4 0.93 0.2

1.88×107 3.3

3.08×10-10 0.2 0.97 0.02

~236

x=y=0.10

3.35×105 4.4

7.56×106 3.9

2.56×10-10 0.5 0.93 0.1

~242 0.64

2.6 0.94

0.3

2.4 0.89 0.2

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4.22×10 3.4 8.73×10

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0.1

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1.09×10

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1.21×10-10 1.6 0.96

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x=y=0.00

Rg(Ω) Error Qg(F) Error ng Error Rgb(Ω) Error Qgb(F) Error ngb Error (%) (%) (%) (%) (%) (%) 5 -09 7 -10 9.73×10 2.1 1.03×10 1.1 0.97 0.02 4.60×10 4.5 1.12×10 0.3 0.99 0.1

1.41×10

5

5

2.26×10

2.5 9.92×10

-11

2.2 0.80

-10

3.4

4.2 1.13×10

0.1

0.84 0.2

tanδ

(1kHz) ~183 0.37 0.12

~1256 1.01 ~ 752

1.49

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Table 2

Fe/Mn-O-Fe/Mn

Stretching (ν1)

559 564 574 593 596

437 438 435 428 423

1.30 1.31 2.02 2.29 0.28

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Bending (ν2)

Maximum Magnetic moment (Mmax) (emu/g)@60kOe

Coercivity Hc (Oe)

73 310 210 150 70

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Fe/Mn-O

Magnetic Property

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FTIR band positions

La1-x Nax Fe1-yMnyO3

System under study

FTIR Peak assignment and magnetic parameters

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Highlights Co-substitution has strengthened dielectric and magnetic response in LaFeO3.

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Na and Mn Co-substitution strengthened optical and conduction process in LaFeO3.

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Tailored bond angle on co-substitution resulting enhancement in magnetic response.

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Grain boundary has profound effect in dielectric and conduction processes.

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