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Investigation of structure and physical properties of cobalt doped nano-crystalline neodymium orthoferrite
Accepted Manuscript Investigation of structure and physical properties of cobalt doped nano-crystalline neodymium orthoferrite Anand Somvanshi, Shahid Husain, Wasi Khan PII:
S0925-8388(18)34216-6
DOI:
https://doi.org/10.1016/j.jallcom.2018.11.095
Reference:
JALCOM 48321
To appear in:
Journal of Alloys and Compounds
Received Date: 2 October 2018 Revised Date:
7 November 2018
Accepted Date: 8 November 2018
Please cite this article as: A. Somvanshi, S. Husain, W. Khan, Investigation of structure and physical properties of cobalt doped nano-crystalline neodymium orthoferrite, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.11.095. 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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Investigation of structure and physical properties of cobalt doped nano-crystalline neodymium orthoferrite
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Anand Somvanshi, Shahid Husain* and Wasi Khan Department of Physics, Aligarh Muslim University, Aligarh 202002 INDIA *
Corresponding Author
Abstract
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Email: [email protected]
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Cobalt-doped neodymium orthoferrite (NdFe1-xCoxO3) samples have been synthesized through sol-gel combustion method. X-ray diffraction (XRD) analysis reveals that these samples conform in orthorhombic perovskite structure. The crystallite sizes range between 12-14 nm. Bond angle (Fe-O-Fe) and bond length (Fe-O) exhibit variation with cobalt doping as determined by the Rietveld refinement. Scanning electron microscopy (SEM) micrographs with energy dispersive x-ray (EDX) analysis show the uniformly structured surface morphology and confirm the elemental compositions of the
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synthesized samples. Transmission electron microscopy (TEM) images with selected area electron diffraction (SAED) patterns show the crystalline nature of the samples with visibly distinguishable planes. The energy band gap decreases while Urbach energy increases with cobalt doping. The presence of characteristic bands in FTIR spectra confirms the formation of the samples. Néel
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temperature increases with the increase in Co concentration as detected using differential thermal analysis (DTA). The dielectric constant exhibits as increase with the increase in doping concentration.
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The Cole-Cole plots confirm non-Debye type of behavior for all the samples and the dielectric constant deviates from universal dielectric response (UDR) model at lower frequencies. Keywords: Orthoferrites, High resolution transmission electron microroscopy, Urbach energy, Differential thermal analysis, Universal dielectric response model. 1. Introduction Perovskite materials are centre of attraction for research due to their relatively simple crystal structure and many important structural, electrical and magnetic properties. Oxide-based perovskites with the general formula ABO3 have relatively simple crystal structures. They have corner-linked BO6 cation
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centered octahedra with cations A in the three-dimensional structure of the system[1].The existence of crystallographic and magnetic sublattices in the same system correspond to some interesting phenomena due to itinerant electrons, magnetic ordering and because of strong interplay in between localized moments[2].The measurement of distortion or tilting of octahedra in the perovskite structure
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can be understood by the calculation of tolerance factor, τ = (rA +ro)/√2(rB +rO), here rA, rB, and rO are the ionic radii of A site ion, B site ion and oxygen ions respectively. The value of tolerance factor depends on the ionic radii and the matching of equilibrium A-O and B-O bond lengths. Ideal cubic perovskite structures have τ=1. Due to size mismatch between ionic radii of rare earth and Fe ions in
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distorted perovskite structures, τ<1. This distortion leads to the stress of A-O and B-O bonds, which implies the rotation of BO6 octahedra about the crystallographic axis. Ideally, the bond angle of B-O-B is 180o for cubic perovskites, as this value becomes less the degree of distortion increases [3].In
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particular, the rare earth iron oxide perovskite (RFeO3, where R= rare earth) systems have distorted orthorhombic structures [4,5] that have tilt and rotated FeO6 octahedra near around the rare earth ion. The degree of distortion is determined through Fe-O-Fe bond angle that affects the structural, optical and dielectric properties of the system. RFeO3systems are acknowledged by virtue of their interesting properties, such as colossal magneto-optic[6], metal-insulator transition, charge/orbital ordering[7–
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9]with remarkable applications such as spin switching, dielectric resonator, magnetic storage media, gas separators and solid oxide fuel cells (SOFCs)[10–13]. Neodymium orthoferrite (NdFeO3) is a perovskite material having orthorhombic crystal structure with Pbnm space group. It exhibits insulating character at room temperature[14]. The major interactions
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in this system are Fe-Fe, Nd-Nd,and Fe-Nd. Moreover, there are reports[15–17]on different types of transition metal ion doping at the iron siteto improve the various properties of such systems.On the
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other hand, NdCoO3 shows the metallic nature at room temperature. Further, to the best of our knowledge, we do not find reports on cobalt doped NdFeO3 in nano form. This motivates us totake up the synthesis of nanocrystalline NdFe1-xCoxO3(0≤x≤0.4) and investigate their structural, morphological, optical, thermal and dielectric properties[18].
2. Materials and Methods The nanocrystalline NdFe1-xCoxO3(x=0.0, 0.1, 0.2, 0.3, and 0.4) samples were prepared by the sol-gel combustion method. Stoichiometric amounts of Nd(NO3)2.6H2O, Fe(NO3)3.6H2O, Co(NO3)2.6H2O and citric acid were dissolved in ion-free water at room temperature, and then mixed together at a molar
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ratio of n(Nd3+):n(Fe3+): n(Co2+): n(citric)=1: (1-x):x : 4. The appropriate amount of ethylene glycol was added to the mixed solution with continuous stirring at 80 ºC to obtain the gel, then this gel dried at 200 0C for 4 hours and ground at room temperature. The fine powders were calcinated for 2 hours at 700 ºC in a furnace and naturally cooled to room temperature. The samples thus obtained in powder
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form are ground again. The x-ray diffraction (XRD) patterns are recorded using Shimadzu LabX XRD6100 advance diffractometer (CuKα radiation) at room temperature in the 2θ angle range of20 to 80 degrees. All the samples were characterized by scanning electron microscope (SEM) with energy dispersive x-ray (EDX) analysis on JEOL JSM-6510 LV microscope. Transmission electron
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microscopy (TEM) is carried out using JEOL JSM-2100 microscope. UV-Vis absorption spectra have been recorded using UV-Vis NIR spectrophotometer (Lambda 950, Perkin Elmer) in the wavelength
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range of 200-800 nm.Bruker Tensor 37 spectrometer is used to record the FTIR spectra in the transmission mode in the wavenumber range of 400 cm-1 to 4000 cm-1 with a step size of 2 cm-1.Differential thermal analysis has been performed using STA-8000 (Perkin Elmer) in the temperature range from 30 to 1000 oC at a heating rate of 20 oC/min in the nitrogen atmosphere. Dielectric measurements were performed using Agilent 6300A precession LCR meter in the frequency
3. Results and discussion 3.1 Structural Analysis
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range of 75 kHz to 5 MHz.
The x-ray diffraction patterns of NdFe1-xCoxO3(0 ≤ x ≤ 0.4) show single phase orthorhombic crystal
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symmetry with space group Pbnm for all the compositions as displayed in figure 1. The lattice parameters and unit cell volume are determined from the Rietveld refinement analysis using FullProf software. The lattice parameters and unit cell volume are found todecrease with the increase in Co
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concentration as expected because the Co+3(61 pm)ion’s ionic radius is smaller than that of Fe3+ ion (64.5pm)[19]. Lattice parameters for all the samples are tabulated in table 1 and their variation with Co concentration is shown in figure 2. The doping of Co3+ ion results in distortion of the symmetry of FeO6 octahedron and leads to distortion in the orthorhombic structure of the lattice[20]. It is also evident from figure 1 that the most intense peak (121) shifts towards the higher angles with increase in the Co concentration that is the manifestation of the decrease of lattice parameters. In the orthorhombic crystal symmetry, the Nd atoms are fixed at the 4(c) Wyckoff positions: (x, y, 1/4) with their occupation factor and thermal coefficient values, that were also refined. The Fe and Co atoms are in the 4(b) position:
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(1/2, y, z) and oxygen atoms employ two equivalent places, the axial O1 atoms are at 4(c): (x, y, z) and equatorial O2 atoms are at 8(d): (x, y, z). The refined pattern of NdFeO3along with observed x-ray diffraction pattern are shown in figure3. All the structural parameters of pristine and doped samples of NdFe1-xCoxO3 are shown in table 1.
octahedra and
these
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In these types of perovskites, the Nd+3 ions are located in the large cavity formed by these samples have orthorhombic distortion.
According to
the Glazer’s
phraseology[1],this orthorhombic distortion obtained due to three causes: (i) tilting of the anion
octahedra.
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octahedra (O (1) and O(2)) (ii) displacement of the rare earth atoms, and (iii) distortion of the FeO6 Average crystallite size (D) is determined using Debye-Scherrer equation[21]
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D=Kλ/βcosθ
Fig. 1 X-ray diffraction patterns of NdFe1-xCoxO3 (0 ≤ x ≤ 0.4) and selected portion showsa shift of the most intense peak (121) with Co concentration.
(1)
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where, K is a dimensionless shape factor with a value near to unity. The typical value of shape factor is about 0.9, λ is the wavelength of x-rays Cu Kα radiation (λ=1.54 Å), β is the broadening of the most intense peak at half maximum and θ is the diffraction angle for the most intense peak. The average
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crystallite sizes lie in the range of ~ 12-14 nm as tabulated in table 1.
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Fig. 2 Variation in lattice parameters with Co concentration of NdFe1-xCoxO3(0.0 ≤ x ≤ 0.4).
Fig. 3 Rietveld refinement plot of NdFeO3along with the observed x-ray diffraction pattern.
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TABLE 1 Structural parameters ofNdFe1-xCoxO3 as determined from the Rietveld refinement analysis. x=0.1
x=0.2
x=0.3
x=0.4
-0.01738
-0.01113
-0.01161
-0.00978
-0.00978
0.03272
0.03095
0.04653
0.04885
0.04885
Biso (Å )
0.50000
0.50000
0.61525
0.02940
0.18038
Biso (Å2)
0.40767
0.50000
0.53229
0.83577
0.72559
x
0.07189
0.44455
0.29476
0.29476
0.29476
y
0.48428
0.10337
0.10680
0.10680
0.10680
0.25000
0.22890
0.17900
0.17900
0.17900
Biso (Å )
0.50000
0.50000
0.50000
0.50000
0.50000
x
-0.29857
-0.26085
-0.25486
-0.29857
-0.29476
y
0.30168
0.31319
0.30037
0.30168
0.10680
0.03898
0.04119
0.02287
0.03898
0.17900
0.45000
0.45000
0.45000
0.45000
0.50000
1.56
1.18
1.17
1.17
1.23
a (Å)
5.570 (2)
5.546 (3)
5.521 (3)
5.496 (2)
5.474 (2)
b (Å)
5.448 (2)
5.439 (2)
5.430 (3)
5.417 (2)
5.406 (2)
c (Å)
7.746 (3)
7.731 (4)
7.706 (4)
7.678 (3)
7.648 (3)
235.012
233.203
231.018
228.588
226.323
(χ )
1.56
1.18
1.17
1.17
1.23
Crystallite Size
Ds (nm)
12.36
13.71
13.87
14.21
14.65
Fe-O1-Fe
(degree)
138.60
138.85
138.96
139.00
139.05
Fe-O2-Fe
(degree)
151.94
151.81
151.84
151.54
151.05
(Å)
1.9805
1.8785
1.8696
1.8665
1.8608
(Å)
2.0098
1.9834
1.9565
1.9945
1.9891
x y 2
Fe, Co (0.5, 0, 0) O (1) (x, y, z)
z 2
O (2) (x, y, z)
z 2
Biso (Å )
Unit cell volume Goodness of fitting
Fe-O1
3
V(Å ) 2
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Fe-O2
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Lattice parameters
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χ
2
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Nd (x, y, 0.25)
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x=0.0
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Structural Parameters
3.2 Morphological Analysis
3.2.1 Scanning electron microscopy (SEM) The SEM images with energy dispersive x-rays (EDX) analysis of pristine and 30% Co doped NdFeO3 samples are shown in figure 4.These images for both the samples, taken at the 3,000 and 30,000x magnifications reveal the grains of varying sizes with well defined boundaries. The well resolved lines originating from the EDX spectra of constituent elements are recorded in the energy range from 0 to 20 keV.EDX spectra show the presence of only constituent elements (Nd, Fe, Co and O) of NdFe1-xCoxO3.
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Fig. 4(a) SEM images and EDX spectrum of NdFeO3 nanoparticles.
Fig. 4(b) SEM images and EDX spectrum of NdFe0.7Co0.3O3 nanoparticles.
The percentage composition calculated for the entire surface area is listed in the inset of each EDX spectrum. The weight percentage of elements as observed from the EDX is in the close agreement with
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the elemental composition of the used reactants. This confirms the homogeneity of the samples[22,23]. 3.2.2 Transmission electron microscopy (TEM):
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TEM images of pristine and 30% Co doped NdFeO3 at 50,000x magnification are shown in figures 5(a) and 5(b). We have also recorded the high resolution TEM (HRTEM) images with 50,0000x magnification for pristine and 30% doped samples as shown in figures 6(a) and 7(a) respectively. Parallel lines in these images represent interplanar spacing that are found to be 2.83 and 3.89 Å for pristine and 30% doped samples respectively. These interplanar spacings corresponds to (121) and (101) planes as depicted in the XRD patterns (figure 1).
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Fig. 5(a) TEM image of NdFeO3 nanoparticles with particle size distribution.
Fig. 5(b) TEM image of NdFe0.7Co0.3O3 nanoparticles with particle size distribution.
Selected area electron diffraction (SAED) patterns of NdFeO3 and NdFe0.7Co0.3O3 have been shown in figures 6(b) and 7(b) respectively. The well-resolved spotty rings in SAED pattern agree with the allowed Bragg diffraction planes of the orthorhombic structure. The circles are indexed to the Bragg reflections planes (121), (202), (204) and (101). This implies that our TEM findings are consistent with the XRD data.
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Fig. 6 (a) HRTEM image under 5,00000x magnification,and (b) SAED pattern of NdFeO3 nanoparticles.
Fig. 7 (a) HRTEM image under 5,00000x magnification, and (b) SAED pattern of NdFe0.7Co0.3O3 nanoparticles.
TEM images display the well resolved nanoparticles for both of these samples. Average particle size is measured by considering the distinguishable particles in the images. The corresponding histogram shows the frequency of number of particles in the given particle size range (Fig. 5).The Gaussian fit of
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the aforementioned histogram of the TEM images was used to estimate the average particle size of the samples. That is found to be 32.75±0.37and 34.80±0.58 nm for the pristine and 30% Co doped samples respectively. It is observed from the TEM images that the average particle size is larger in Co doped sample as compared to host sample and is in good agreement with the crystallite size calculated by
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Debye Scherrer equation.
Fig. 8 Diffuse reflectance spectra of NdFe1-xCoxO3 (0 ≤ x ≤ 0.4).
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3.3 UV-Visible Spectroscopy
The UV-visible reflectance spectra in the wavelength range of 200 to 800 nm are recorded for
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NdFe1-xCoxO3 (0 ≤ x ≤ 0.4) at room temperature as shown in figure 8. As evident from these spectra that the reflectance of the NdFeO3 increases with the increase in Co concentration. It may be due to the excessive increase in distortion or dangling on incorporation of cobalt ions in the host lattice. Reflection of Co doped samples decreases towards the higher wavelength by following the argument that the synthesized materials seem to be transparent in the visible region of the electromagnetic spectrum. The optical band gap of all the samples has been estimated by Tauc’s relation, ()ℎ = (ℎ − )
(2)
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where F(R) is the Kubelka Munk function proportional to absorbance coefficient (α) and calculated using the relation, ( ) =
()
(3)
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R is the reflectance, hv is the energy of UV-visible radiation in eV, A is a constant and n=1/2 for direct band gap. (F(R)hν)2 versus hν graphs are plotted for all the samples as shown in figure 9. The direct band gap values of pristine and Co doped samples as determined from these plots are depicted in table 2. The values of bandgap show red shift with the increase in Co concentration [24,25].That may be due to the nature of the excitation. The orthorhombic distortion and defects could also be one of the reasons
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for this redshift[26]. S. Singh et al.[27] and A. Mir et al.[28] have reported the energy band gap of NdFeO3 as 3.8 eV and 4.3 eV respectively. These values are much higher as compared to our energy
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band gap values (3.35 eV) and that further decreases to 3.04eV for 40% Co doping. This suggests that
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cobalt doping increases the conductivity and the system is approaching towards metallic state.
Fig. 9(()ℎ) versus energy plots of NdFe1-xCoxO3 (x=0.0, 0.1, 0.2, 0.3 and 0.4).
Urbach tail or Urbach energy is another important parameter that can be determined using UV/Vis. spectra. The higher value of Urbach energy indicates low crystallinity and the disorder in the materials. The disorder in the material develops localized states that extend in the bandgap as schematically shown in figure 10. It can be seen that these defect band states create energy tail measured between the valence band and conduction band edges. In the low photon energy range, the dependence of absorption
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Table 2 Energy band gap and Urbach energy of NdFe1-xCoxO3 (0
Composition
Energy band
≤ x ≤ 0.4).
Urbach Energy(eV)
gap (eV) 3.35
0.910
NdFe0.9Co0.1O3
3.26
1.009
NdFe0.8Co0.2O3
3.20
NdFe0.7Co0.3O3
3.09
NdFe0.6Co0.4O3
3.04
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NdFeO3
1.027 1.403
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1.456
coefficient (α) and photon energy (hν) is called Urbach empirical relation[29] and given by
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=
(4)
Here α0 is a constant and is the Urbach energy. Equation (4) can also be expressed in the following way [30]
ln() = ln( ) !
(5)
Since Kubelka-Munk function F(R) is directly proportional to absorbance coefficient (),
lnF( ) = ln( ) !
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Fig. 10 Schematic diagram of Urbach tail formation.
(6)
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A graph between lnF() and photon energy (ℎ) was plotted and the slope of the fitted straight line is used to estimate Urbach energy (figure 11). The calculated values of Urbach energy for all the samples are tabulated in the table 2. Urbach energy of the NdFeO3 is found to increase with Co concentration that shows the typical inverse dependence with energy band gap as shown in figure 12. The increase in band
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tail width with the doping of Co may be attributed to the number of defect levels produced below the
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conduction band edge of the samples [31].
Fig. 11Plots of ln(F(R)) versus hν to determine the Urbach energy (Eu) for NdFe1-xCoxO3 (0 ≤ x ≤ 0.4).
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Fig. 12 The variation of bandgap and Urbach energy forpristine and Co dopedNdFeO3samples.
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3.4 Fourier transform infrared (FTIR) spectroscopy
In order to determine the presence of various functional groups associated with NdFe1-xCoxO3 (0 ≤ x ≤ 0.4), we have recorded the FTIR spectra of our samples as displayed in figure 13. FTIR spectroscopy is useful to determine the presence of certain functional groups and bonds that absorb
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energy at particular frequencies, thus, it predicts the structure of the material. The broad absorption peaks near 3448 and 3018 cm-1 indicate the presence of O-H bonding. The strong absorption bands near
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1627 and 1402 cm-1 can be attributed to the presence of C=O stretching mode. The band near 492-414 cm-1 is observed due to Fe/Co-O (metal-oxide) stretching vibrations [32].The appearance of absorption band around 680 to 486 cm−1 indicates the presence of neodymium iron oxide phase. The strong absorption peak arising at 563 cm−1 is assigned to the bonding between Nd-O-Fe [33,34]. FTIR spectra of the samples doped with higher concentrations of Co3+ions are shifted towards the higher wavenumbers. As evident from the inset of figure 13, the intense band at 556 cm-1 for NdFeO3 shifts progressively to 561, 572, 578 and 581 cm-1 with the increase in Co concentration. This shift may be explained on the basis of the fact that the increase in concentration of Co+3 ions with smaller ionic
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radius causes tthe shrinkage in unit cell. This shrinkage of the unit cell increases the force constants of
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the vibrating bonds, and hence absorption bands are shifted towards the higher wave-numbers [27, 32].
Fig. 13 FTIR spectra of NdFe1-xCoxO3(0 ≤ x ≤ 0.4) samples.
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3.5 Thermal Analysis
Differential thermal analysis (DTA) of NdFe1-xCoxO3 (x= 0, 0.1, 0.2, 0.3 and 0.4) samples are shown in figure 14. The experiment was performed in a controlled nitrogen atmosphere for the temperature range of 30 to 1000 oC. Figure 14 represents the specific heat and heat flow as a function of sample
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temperature. The heat flow plot clearly indicates an endothermic peak around 246 oC for NdFeO3 and at 277, 297, 300, 303 oC for the Co doped samples. The exothermic peaks are found at the temperature around 785, 808, 479, 392, 374 oC for the pristine and 10, 20, 30, and 40% Co doped samples
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respectively. The endothermic peaks indicate the antiferromagnetic to paramagnetic phase transition that is also known as Néel temperature[35]. The reported Néel temperature of NdFeO3 is 690 K.[28]In our case, the Néel temperature for the pristine and Co substituted samples is found to be 519, 550, 570, 573 and 576 K. This increase in Néel temperature with Co doping may be correlated with the crystallite size calculated by Scherrer formula, as crystallite size increases with the increase in Co doping. Specific heat capacity at constant pressure (Cp) of any system can be calculated by the formula, C$ = ∆Q/(mβ) Here ∆Q is the heat flow in the system, m is the mass of the sample and β is the heating rate.
(7)
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Specific heat capacity at constant pressure and its variation with temperature is shown in figure 14. The values of Cp at the Néel temperature for pristine and Co doped samples are found to be 6.67, 45.85,
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37.79, 3.24 and 13.38 J/g K respectively.
Fig.14 Plots of specific heat and heat flow as a function of sample temperature for NdFe1-xCoxO3 (0 ≤ x ≤ 0.4).
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3.6 Dielectric properties The variation of dielectric constant (ε′) with frequency for NdFe1-xCoxO3 (0 ≤ x ≤ 0.4) has been studied in the frequency range of 75 kHz to 5 MHz. The complex dielectric constant(ε*)can bewritten as the
dissipated energy in the following way [36,37]: * ∗ = * , − -* " The real part of dielectric constant was evaluated by the equation, ε′=Cd/ε0A
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combination of real part ε′that corresponds to the stored energy and imaginary part ε′′ represents the
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* " = * , /01δ
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The imaginary part of the dielectric constant (* " ) can be calculated by the relation,
(10)
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Here, C is the capacitance, d is the thickness, A represents the area of the pallet.The absolute
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permittivity of free space (ε0) to 8.85×10-12 F/m and tanδ is the loss tangent.
Fig. 15(a) Dielectric constant versus frequency plots for NdFe1-xCoxO3(0 ≤ x ≤ 0.4).
The frequency dependence of dielectric constant can be explained with the help of Maxwell-Wagner two-layer Model or the heterogeneous model of the polycrystalline structure given by Koops (1951)[38,39]. As it is clearly evident from figure 15(a) that the trend of dielectric constant decreases with the increase in frequency of applied field rapidly at lower frequencies that may be due to the charge carriers trapped at an interfacial region that is caused due tothe inhomogeneous structure of
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dielectric but the value of dielectric constant becomes almost constant at higher frequencies. This is due to the fact that in the high-frequency region the hopping mechanism is responsible for the dielectric constant and the frequency of the hopping in between ions fails to keep pace with the frequency of applied a.c. field, therefore the value of the dielectric constant decreases at the higher frequencies
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[24,37].
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Fig. 15(b) Dielectric lossversus frequency plots for NdFe1-xCoxO3(0 ≤ x ≤ 0.4)
At the same time the value of dielectric constant shows its dependence on Co concentration too. We have observed an increase in the value of dielectric constant with the increase in Co concentration.
place:
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When we doped Co ions in the NdFeO3 systemthen to conserve charge balance following reaction takes
(11)
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Fe34 + Co4 → Fe4 + Co34
In this reaction, electron hops in between Fe34 → Fe4 and hole in between Co4 → Co34 are responsible for this charge exchange. Generally, Fe2+ ions are considered dominatingto produce polarization as compared to Fe34 ions. This enhancement of Fe2+ ions is one of the reason for the enhancement of dielectric constant[15]. Figure 15(b) represents the dependence of dielectric loss (tanδ) on frequency. Dielectric loss is higher for all the samples at lower frequencies and decreases continuously with the increase in frequency. Dielectric losses at the lower frequencies are due to the space charge polarization, this phenomenon can be explained by Shockley-Read mechanism[40]. According to this mechanism, the
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space charge polarization materializes at lower frequencies due to the capture of surface electrons by the impurities and defects present in the system. While the low dielectric losses at higher frequencies
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are attributed to the emergence of defects in dipoles with the change in valence state of cations[41].
Fig. 16 Cole-Cole plots of NdFe1-xCoxO3 (0 ≤ x ≤ 0.4) at room temperature.
Figure 16 display the Cole-Cole plots between ε′ and ε′′. These plots are used to understand the Debye relaxation behavior for materials. The ideal Debye relaxation behavior suggests the semicircle variation between the parameters ε′ and ε′′. In the present study, there is no semicircular variation is observed in
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the Cole-Cole plots. This implies that our synthesized samples follow the non-Debye behavior in the
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applied frequency range [42].
Fig. 17.Plots of log (ε’×f) versus log (f) with linear fit for NdFe1-xCoxO3 (0≤x≤0.4)
In order to study the response of dielectric constant,we have used universal dielectric response
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(UDR) model [36]. According to this model, the localized charge carriers hopping between spatially fluctuating lattice potentials may give rise to the dipolar effects in addition to conductivity. Thereby,
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the plot between log(ε’×f) and log(f) should be linear. We have plotted log(ε’×f) versus log(f) as shown in figure 17. All the samples follow the UDR model in the high frequency range but show deviation at lower frequencies.The nonlinear behavior at lower frequencies for pristine and doped samples confirms the absence of any other contribution (grain boundary, electrode, or Maxwell-Wagner effect) to the dielectric response [43]. 4. Conclusions NdFe1-xCoxO3(0 ≤ x ≤ 0.4) samples are synthesized by sol-gel method. The Rietveld refinement of observed XRD patterns confirms the formation of single phase orthorhombic crystal structure with
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Pbnm space group. The lattice parameters decreases and unit cell volume contracts with the increase of Co doping on account of smaller ionic radius of Co3+ ion. TEM images confirm the formation of nanoparticles with the average particle size of 34 nm. The direct band gap as determined with Tauc’s relation using from UV-visible reflectance data for theNdFeO3sample is much less than the earlier
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reported values and it further decreases with the increase in Co concentration. The presence of characteristic bands in FTIR spectra confirms the formation of the desired phase. Néel temperature of our samples shifts towards higher values on Co doping.Cole-Cole plots indicate towards non-Debye
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type of behavior and deviation from UDR model is observed at lower frequencies for all the samples.
Acknowledgments
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One of the authors Anand Somvanshi is grateful to the UGC-DAE Consortium for Scientific Research, Mumbai for providing financial support under the project CRS-M-271. Authors are thankful to the University Sophisticated Instrument Facility (USIF), AMU for SEM and TEM facilities. Dr. Mohammad Jane Alam, Spectroscopy lab, Department of Physics, AMU is thankfully acknowledged for optical characterizations.
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The lattice parameters decrease with the increase in Co doping.
•
TEM images confirm the formation of nanoparticles with average size of 34 nm.
•
The energy band gap of NdFeO3 decreases with the increase in Co concentration.
•
Neel temperature shifts towards higher values on Co doping.
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Cole-Cole plots reveal non-Debye type of behaviour for pristine and doped samples.
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•
S0925-8388(18)34216-6
DOI:
https://doi.org/10.1016/j.jallcom.2018.11.095
Reference:
JALCOM 48321
To appear in:
Journal of Alloys and Compounds
Received Date: 2 October 2018 Revised Date:
7 November 2018
Accepted Date: 8 November 2018
Please cite this article as: A. Somvanshi, S. Husain, W. Khan, Investigation of structure and physical properties of cobalt doped nano-crystalline neodymium orthoferrite, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.11.095. 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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Investigation of structure and physical properties of cobalt doped nano-crystalline neodymium orthoferrite
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Anand Somvanshi, Shahid Husain* and Wasi Khan Department of Physics, Aligarh Muslim University, Aligarh 202002 INDIA *
Corresponding Author
Abstract
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Email: [email protected]
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Cobalt-doped neodymium orthoferrite (NdFe1-xCoxO3) samples have been synthesized through sol-gel combustion method. X-ray diffraction (XRD) analysis reveals that these samples conform in orthorhombic perovskite structure. The crystallite sizes range between 12-14 nm. Bond angle (Fe-O-Fe) and bond length (Fe-O) exhibit variation with cobalt doping as determined by the Rietveld refinement. Scanning electron microscopy (SEM) micrographs with energy dispersive x-ray (EDX) analysis show the uniformly structured surface morphology and confirm the elemental compositions of the
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synthesized samples. Transmission electron microscopy (TEM) images with selected area electron diffraction (SAED) patterns show the crystalline nature of the samples with visibly distinguishable planes. The energy band gap decreases while Urbach energy increases with cobalt doping. The presence of characteristic bands in FTIR spectra confirms the formation of the samples. Néel
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temperature increases with the increase in Co concentration as detected using differential thermal analysis (DTA). The dielectric constant exhibits as increase with the increase in doping concentration.
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The Cole-Cole plots confirm non-Debye type of behavior for all the samples and the dielectric constant deviates from universal dielectric response (UDR) model at lower frequencies. Keywords: Orthoferrites, High resolution transmission electron microroscopy, Urbach energy, Differential thermal analysis, Universal dielectric response model. 1. Introduction Perovskite materials are centre of attraction for research due to their relatively simple crystal structure and many important structural, electrical and magnetic properties. Oxide-based perovskites with the general formula ABO3 have relatively simple crystal structures. They have corner-linked BO6 cation
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centered octahedra with cations A in the three-dimensional structure of the system[1].The existence of crystallographic and magnetic sublattices in the same system correspond to some interesting phenomena due to itinerant electrons, magnetic ordering and because of strong interplay in between localized moments[2].The measurement of distortion or tilting of octahedra in the perovskite structure
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can be understood by the calculation of tolerance factor, τ = (rA +ro)/√2(rB +rO), here rA, rB, and rO are the ionic radii of A site ion, B site ion and oxygen ions respectively. The value of tolerance factor depends on the ionic radii and the matching of equilibrium A-O and B-O bond lengths. Ideal cubic perovskite structures have τ=1. Due to size mismatch between ionic radii of rare earth and Fe ions in
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distorted perovskite structures, τ<1. This distortion leads to the stress of A-O and B-O bonds, which implies the rotation of BO6 octahedra about the crystallographic axis. Ideally, the bond angle of B-O-B is 180o for cubic perovskites, as this value becomes less the degree of distortion increases [3].In
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particular, the rare earth iron oxide perovskite (RFeO3, where R= rare earth) systems have distorted orthorhombic structures [4,5] that have tilt and rotated FeO6 octahedra near around the rare earth ion. The degree of distortion is determined through Fe-O-Fe bond angle that affects the structural, optical and dielectric properties of the system. RFeO3systems are acknowledged by virtue of their interesting properties, such as colossal magneto-optic[6], metal-insulator transition, charge/orbital ordering[7–
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9]with remarkable applications such as spin switching, dielectric resonator, magnetic storage media, gas separators and solid oxide fuel cells (SOFCs)[10–13]. Neodymium orthoferrite (NdFeO3) is a perovskite material having orthorhombic crystal structure with Pbnm space group. It exhibits insulating character at room temperature[14]. The major interactions
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in this system are Fe-Fe, Nd-Nd,and Fe-Nd. Moreover, there are reports[15–17]on different types of transition metal ion doping at the iron siteto improve the various properties of such systems.On the
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other hand, NdCoO3 shows the metallic nature at room temperature. Further, to the best of our knowledge, we do not find reports on cobalt doped NdFeO3 in nano form. This motivates us totake up the synthesis of nanocrystalline NdFe1-xCoxO3(0≤x≤0.4) and investigate their structural, morphological, optical, thermal and dielectric properties[18].
2. Materials and Methods The nanocrystalline NdFe1-xCoxO3(x=0.0, 0.1, 0.2, 0.3, and 0.4) samples were prepared by the sol-gel combustion method. Stoichiometric amounts of Nd(NO3)2.6H2O, Fe(NO3)3.6H2O, Co(NO3)2.6H2O and citric acid were dissolved in ion-free water at room temperature, and then mixed together at a molar
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ratio of n(Nd3+):n(Fe3+): n(Co2+): n(citric)=1: (1-x):x : 4. The appropriate amount of ethylene glycol was added to the mixed solution with continuous stirring at 80 ºC to obtain the gel, then this gel dried at 200 0C for 4 hours and ground at room temperature. The fine powders were calcinated for 2 hours at 700 ºC in a furnace and naturally cooled to room temperature. The samples thus obtained in powder
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form are ground again. The x-ray diffraction (XRD) patterns are recorded using Shimadzu LabX XRD6100 advance diffractometer (CuKα radiation) at room temperature in the 2θ angle range of20 to 80 degrees. All the samples were characterized by scanning electron microscope (SEM) with energy dispersive x-ray (EDX) analysis on JEOL JSM-6510 LV microscope. Transmission electron
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microscopy (TEM) is carried out using JEOL JSM-2100 microscope. UV-Vis absorption spectra have been recorded using UV-Vis NIR spectrophotometer (Lambda 950, Perkin Elmer) in the wavelength
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range of 200-800 nm.Bruker Tensor 37 spectrometer is used to record the FTIR spectra in the transmission mode in the wavenumber range of 400 cm-1 to 4000 cm-1 with a step size of 2 cm-1.Differential thermal analysis has been performed using STA-8000 (Perkin Elmer) in the temperature range from 30 to 1000 oC at a heating rate of 20 oC/min in the nitrogen atmosphere. Dielectric measurements were performed using Agilent 6300A precession LCR meter in the frequency
3. Results and discussion 3.1 Structural Analysis
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range of 75 kHz to 5 MHz.
The x-ray diffraction patterns of NdFe1-xCoxO3(0 ≤ x ≤ 0.4) show single phase orthorhombic crystal
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symmetry with space group Pbnm for all the compositions as displayed in figure 1. The lattice parameters and unit cell volume are determined from the Rietveld refinement analysis using FullProf software. The lattice parameters and unit cell volume are found todecrease with the increase in Co
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concentration as expected because the Co+3(61 pm)ion’s ionic radius is smaller than that of Fe3+ ion (64.5pm)[19]. Lattice parameters for all the samples are tabulated in table 1 and their variation with Co concentration is shown in figure 2. The doping of Co3+ ion results in distortion of the symmetry of FeO6 octahedron and leads to distortion in the orthorhombic structure of the lattice[20]. It is also evident from figure 1 that the most intense peak (121) shifts towards the higher angles with increase in the Co concentration that is the manifestation of the decrease of lattice parameters. In the orthorhombic crystal symmetry, the Nd atoms are fixed at the 4(c) Wyckoff positions: (x, y, 1/4) with their occupation factor and thermal coefficient values, that were also refined. The Fe and Co atoms are in the 4(b) position:
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(1/2, y, z) and oxygen atoms employ two equivalent places, the axial O1 atoms are at 4(c): (x, y, z) and equatorial O2 atoms are at 8(d): (x, y, z). The refined pattern of NdFeO3along with observed x-ray diffraction pattern are shown in figure3. All the structural parameters of pristine and doped samples of NdFe1-xCoxO3 are shown in table 1.
octahedra and
these
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In these types of perovskites, the Nd+3 ions are located in the large cavity formed by these samples have orthorhombic distortion.
According to
the Glazer’s
phraseology[1],this orthorhombic distortion obtained due to three causes: (i) tilting of the anion
octahedra.
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octahedra (O (1) and O(2)) (ii) displacement of the rare earth atoms, and (iii) distortion of the FeO6 Average crystallite size (D) is determined using Debye-Scherrer equation[21]
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D=Kλ/βcosθ
Fig. 1 X-ray diffraction patterns of NdFe1-xCoxO3 (0 ≤ x ≤ 0.4) and selected portion showsa shift of the most intense peak (121) with Co concentration.
(1)
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where, K is a dimensionless shape factor with a value near to unity. The typical value of shape factor is about 0.9, λ is the wavelength of x-rays Cu Kα radiation (λ=1.54 Å), β is the broadening of the most intense peak at half maximum and θ is the diffraction angle for the most intense peak. The average
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crystallite sizes lie in the range of ~ 12-14 nm as tabulated in table 1.
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Fig. 2 Variation in lattice parameters with Co concentration of NdFe1-xCoxO3(0.0 ≤ x ≤ 0.4).
Fig. 3 Rietveld refinement plot of NdFeO3along with the observed x-ray diffraction pattern.
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TABLE 1 Structural parameters ofNdFe1-xCoxO3 as determined from the Rietveld refinement analysis. x=0.1
x=0.2
x=0.3
x=0.4
-0.01738
-0.01113
-0.01161
-0.00978
-0.00978
0.03272
0.03095
0.04653
0.04885
0.04885
Biso (Å )
0.50000
0.50000
0.61525
0.02940
0.18038
Biso (Å2)
0.40767
0.50000
0.53229
0.83577
0.72559
x
0.07189
0.44455
0.29476
0.29476
0.29476
y
0.48428
0.10337
0.10680
0.10680
0.10680
0.25000
0.22890
0.17900
0.17900
0.17900
Biso (Å )
0.50000
0.50000
0.50000
0.50000
0.50000
x
-0.29857
-0.26085
-0.25486
-0.29857
-0.29476
y
0.30168
0.31319
0.30037
0.30168
0.10680
0.03898
0.04119
0.02287
0.03898
0.17900
0.45000
0.45000
0.45000
0.45000
0.50000
1.56
1.18
1.17
1.17
1.23
a (Å)
5.570 (2)
5.546 (3)
5.521 (3)
5.496 (2)
5.474 (2)
b (Å)
5.448 (2)
5.439 (2)
5.430 (3)
5.417 (2)
5.406 (2)
c (Å)
7.746 (3)
7.731 (4)
7.706 (4)
7.678 (3)
7.648 (3)
235.012
233.203
231.018
228.588
226.323
(χ )
1.56
1.18
1.17
1.17
1.23
Crystallite Size
Ds (nm)
12.36
13.71
13.87
14.21
14.65
Fe-O1-Fe
(degree)
138.60
138.85
138.96
139.00
139.05
Fe-O2-Fe
(degree)
151.94
151.81
151.84
151.54
151.05
(Å)
1.9805
1.8785
1.8696
1.8665
1.8608
(Å)
2.0098
1.9834
1.9565
1.9945
1.9891
x y 2
Fe, Co (0.5, 0, 0) O (1) (x, y, z)
z 2
O (2) (x, y, z)
z 2
Biso (Å )
Unit cell volume Goodness of fitting
Fe-O1
3
V(Å ) 2
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Fe-O2
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Lattice parameters
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Nd (x, y, 0.25)
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x=0.0
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Structural Parameters
3.2 Morphological Analysis
3.2.1 Scanning electron microscopy (SEM) The SEM images with energy dispersive x-rays (EDX) analysis of pristine and 30% Co doped NdFeO3 samples are shown in figure 4.These images for both the samples, taken at the 3,000 and 30,000x magnifications reveal the grains of varying sizes with well defined boundaries. The well resolved lines originating from the EDX spectra of constituent elements are recorded in the energy range from 0 to 20 keV.EDX spectra show the presence of only constituent elements (Nd, Fe, Co and O) of NdFe1-xCoxO3.
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Fig. 4(a) SEM images and EDX spectrum of NdFeO3 nanoparticles.
Fig. 4(b) SEM images and EDX spectrum of NdFe0.7Co0.3O3 nanoparticles.
The percentage composition calculated for the entire surface area is listed in the inset of each EDX spectrum. The weight percentage of elements as observed from the EDX is in the close agreement with
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the elemental composition of the used reactants. This confirms the homogeneity of the samples[22,23]. 3.2.2 Transmission electron microscopy (TEM):
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TEM images of pristine and 30% Co doped NdFeO3 at 50,000x magnification are shown in figures 5(a) and 5(b). We have also recorded the high resolution TEM (HRTEM) images with 50,0000x magnification for pristine and 30% doped samples as shown in figures 6(a) and 7(a) respectively. Parallel lines in these images represent interplanar spacing that are found to be 2.83 and 3.89 Å for pristine and 30% doped samples respectively. These interplanar spacings corresponds to (121) and (101) planes as depicted in the XRD patterns (figure 1).
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Fig. 5(a) TEM image of NdFeO3 nanoparticles with particle size distribution.
Fig. 5(b) TEM image of NdFe0.7Co0.3O3 nanoparticles with particle size distribution.
Selected area electron diffraction (SAED) patterns of NdFeO3 and NdFe0.7Co0.3O3 have been shown in figures 6(b) and 7(b) respectively. The well-resolved spotty rings in SAED pattern agree with the allowed Bragg diffraction planes of the orthorhombic structure. The circles are indexed to the Bragg reflections planes (121), (202), (204) and (101). This implies that our TEM findings are consistent with the XRD data.
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Fig. 6 (a) HRTEM image under 5,00000x magnification,and (b) SAED pattern of NdFeO3 nanoparticles.
Fig. 7 (a) HRTEM image under 5,00000x magnification, and (b) SAED pattern of NdFe0.7Co0.3O3 nanoparticles.
TEM images display the well resolved nanoparticles for both of these samples. Average particle size is measured by considering the distinguishable particles in the images. The corresponding histogram shows the frequency of number of particles in the given particle size range (Fig. 5).The Gaussian fit of
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the aforementioned histogram of the TEM images was used to estimate the average particle size of the samples. That is found to be 32.75±0.37and 34.80±0.58 nm for the pristine and 30% Co doped samples respectively. It is observed from the TEM images that the average particle size is larger in Co doped sample as compared to host sample and is in good agreement with the crystallite size calculated by
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Debye Scherrer equation.
Fig. 8 Diffuse reflectance spectra of NdFe1-xCoxO3 (0 ≤ x ≤ 0.4).
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3.3 UV-Visible Spectroscopy
The UV-visible reflectance spectra in the wavelength range of 200 to 800 nm are recorded for
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NdFe1-xCoxO3 (0 ≤ x ≤ 0.4) at room temperature as shown in figure 8. As evident from these spectra that the reflectance of the NdFeO3 increases with the increase in Co concentration. It may be due to the excessive increase in distortion or dangling on incorporation of cobalt ions in the host lattice. Reflection of Co doped samples decreases towards the higher wavelength by following the argument that the synthesized materials seem to be transparent in the visible region of the electromagnetic spectrum. The optical band gap of all the samples has been estimated by Tauc’s relation, ()ℎ = (ℎ − )
(2)
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where F(R) is the Kubelka Munk function proportional to absorbance coefficient (α) and calculated using the relation, ( ) =
()
(3)
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R is the reflectance, hv is the energy of UV-visible radiation in eV, A is a constant and n=1/2 for direct band gap. (F(R)hν)2 versus hν graphs are plotted for all the samples as shown in figure 9. The direct band gap values of pristine and Co doped samples as determined from these plots are depicted in table 2. The values of bandgap show red shift with the increase in Co concentration [24,25].That may be due to the nature of the excitation. The orthorhombic distortion and defects could also be one of the reasons
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for this redshift[26]. S. Singh et al.[27] and A. Mir et al.[28] have reported the energy band gap of NdFeO3 as 3.8 eV and 4.3 eV respectively. These values are much higher as compared to our energy
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band gap values (3.35 eV) and that further decreases to 3.04eV for 40% Co doping. This suggests that
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cobalt doping increases the conductivity and the system is approaching towards metallic state.
Fig. 9(()ℎ) versus energy plots of NdFe1-xCoxO3 (x=0.0, 0.1, 0.2, 0.3 and 0.4).
Urbach tail or Urbach energy is another important parameter that can be determined using UV/Vis. spectra. The higher value of Urbach energy indicates low crystallinity and the disorder in the materials. The disorder in the material develops localized states that extend in the bandgap as schematically shown in figure 10. It can be seen that these defect band states create energy tail measured between the valence band and conduction band edges. In the low photon energy range, the dependence of absorption
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Table 2 Energy band gap and Urbach energy of NdFe1-xCoxO3 (0
Composition
Energy band
≤ x ≤ 0.4).
Urbach Energy(eV)
gap (eV) 3.35
0.910
NdFe0.9Co0.1O3
3.26
1.009
NdFe0.8Co0.2O3
3.20
NdFe0.7Co0.3O3
3.09
NdFe0.6Co0.4O3
3.04
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NdFeO3
1.027 1.403
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1.456
coefficient (α) and photon energy (hν) is called Urbach empirical relation[29] and given by
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=
(4)
Here α0 is a constant and is the Urbach energy. Equation (4) can also be expressed in the following way [30]
ln() = ln( ) !
(5)
Since Kubelka-Munk function F(R) is directly proportional to absorbance coefficient (),
lnF( ) = ln( ) !
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Fig. 10 Schematic diagram of Urbach tail formation.
(6)
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A graph between lnF() and photon energy (ℎ) was plotted and the slope of the fitted straight line is used to estimate Urbach energy (figure 11). The calculated values of Urbach energy for all the samples are tabulated in the table 2. Urbach energy of the NdFeO3 is found to increase with Co concentration that shows the typical inverse dependence with energy band gap as shown in figure 12. The increase in band
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tail width with the doping of Co may be attributed to the number of defect levels produced below the
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conduction band edge of the samples [31].
Fig. 11Plots of ln(F(R)) versus hν to determine the Urbach energy (Eu) for NdFe1-xCoxO3 (0 ≤ x ≤ 0.4).
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Fig. 12 The variation of bandgap and Urbach energy forpristine and Co dopedNdFeO3samples.
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3.4 Fourier transform infrared (FTIR) spectroscopy
In order to determine the presence of various functional groups associated with NdFe1-xCoxO3 (0 ≤ x ≤ 0.4), we have recorded the FTIR spectra of our samples as displayed in figure 13. FTIR spectroscopy is useful to determine the presence of certain functional groups and bonds that absorb
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energy at particular frequencies, thus, it predicts the structure of the material. The broad absorption peaks near 3448 and 3018 cm-1 indicate the presence of O-H bonding. The strong absorption bands near
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1627 and 1402 cm-1 can be attributed to the presence of C=O stretching mode. The band near 492-414 cm-1 is observed due to Fe/Co-O (metal-oxide) stretching vibrations [32].The appearance of absorption band around 680 to 486 cm−1 indicates the presence of neodymium iron oxide phase. The strong absorption peak arising at 563 cm−1 is assigned to the bonding between Nd-O-Fe [33,34]. FTIR spectra of the samples doped with higher concentrations of Co3+ions are shifted towards the higher wavenumbers. As evident from the inset of figure 13, the intense band at 556 cm-1 for NdFeO3 shifts progressively to 561, 572, 578 and 581 cm-1 with the increase in Co concentration. This shift may be explained on the basis of the fact that the increase in concentration of Co+3 ions with smaller ionic
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radius causes tthe shrinkage in unit cell. This shrinkage of the unit cell increases the force constants of
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the vibrating bonds, and hence absorption bands are shifted towards the higher wave-numbers [27, 32].
Fig. 13 FTIR spectra of NdFe1-xCoxO3(0 ≤ x ≤ 0.4) samples.
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3.5 Thermal Analysis
Differential thermal analysis (DTA) of NdFe1-xCoxO3 (x= 0, 0.1, 0.2, 0.3 and 0.4) samples are shown in figure 14. The experiment was performed in a controlled nitrogen atmosphere for the temperature range of 30 to 1000 oC. Figure 14 represents the specific heat and heat flow as a function of sample
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temperature. The heat flow plot clearly indicates an endothermic peak around 246 oC for NdFeO3 and at 277, 297, 300, 303 oC for the Co doped samples. The exothermic peaks are found at the temperature around 785, 808, 479, 392, 374 oC for the pristine and 10, 20, 30, and 40% Co doped samples
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respectively. The endothermic peaks indicate the antiferromagnetic to paramagnetic phase transition that is also known as Néel temperature[35]. The reported Néel temperature of NdFeO3 is 690 K.[28]In our case, the Néel temperature for the pristine and Co substituted samples is found to be 519, 550, 570, 573 and 576 K. This increase in Néel temperature with Co doping may be correlated with the crystallite size calculated by Scherrer formula, as crystallite size increases with the increase in Co doping. Specific heat capacity at constant pressure (Cp) of any system can be calculated by the formula, C$ = ∆Q/(mβ) Here ∆Q is the heat flow in the system, m is the mass of the sample and β is the heating rate.
(7)
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Specific heat capacity at constant pressure and its variation with temperature is shown in figure 14. The values of Cp at the Néel temperature for pristine and Co doped samples are found to be 6.67, 45.85,
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37.79, 3.24 and 13.38 J/g K respectively.
Fig.14 Plots of specific heat and heat flow as a function of sample temperature for NdFe1-xCoxO3 (0 ≤ x ≤ 0.4).
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3.6 Dielectric properties The variation of dielectric constant (ε′) with frequency for NdFe1-xCoxO3 (0 ≤ x ≤ 0.4) has been studied in the frequency range of 75 kHz to 5 MHz. The complex dielectric constant(ε*)can bewritten as the
dissipated energy in the following way [36,37]: * ∗ = * , − -* " The real part of dielectric constant was evaluated by the equation, ε′=Cd/ε0A
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combination of real part ε′that corresponds to the stored energy and imaginary part ε′′ represents the
(8)
(9)
* " = * , /01δ
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The imaginary part of the dielectric constant (* " ) can be calculated by the relation,
(10)
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Here, C is the capacitance, d is the thickness, A represents the area of the pallet.The absolute
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permittivity of free space (ε0) to 8.85×10-12 F/m and tanδ is the loss tangent.
Fig. 15(a) Dielectric constant versus frequency plots for NdFe1-xCoxO3(0 ≤ x ≤ 0.4).
The frequency dependence of dielectric constant can be explained with the help of Maxwell-Wagner two-layer Model or the heterogeneous model of the polycrystalline structure given by Koops (1951)[38,39]. As it is clearly evident from figure 15(a) that the trend of dielectric constant decreases with the increase in frequency of applied field rapidly at lower frequencies that may be due to the charge carriers trapped at an interfacial region that is caused due tothe inhomogeneous structure of
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dielectric but the value of dielectric constant becomes almost constant at higher frequencies. This is due to the fact that in the high-frequency region the hopping mechanism is responsible for the dielectric constant and the frequency of the hopping in between ions fails to keep pace with the frequency of applied a.c. field, therefore the value of the dielectric constant decreases at the higher frequencies
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[24,37].
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Fig. 15(b) Dielectric lossversus frequency plots for NdFe1-xCoxO3(0 ≤ x ≤ 0.4)
At the same time the value of dielectric constant shows its dependence on Co concentration too. We have observed an increase in the value of dielectric constant with the increase in Co concentration.
place:
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When we doped Co ions in the NdFeO3 systemthen to conserve charge balance following reaction takes
(11)
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Fe34 + Co4 → Fe4 + Co34
In this reaction, electron hops in between Fe34 → Fe4 and hole in between Co4 → Co34 are responsible for this charge exchange. Generally, Fe2+ ions are considered dominatingto produce polarization as compared to Fe34 ions. This enhancement of Fe2+ ions is one of the reason for the enhancement of dielectric constant[15]. Figure 15(b) represents the dependence of dielectric loss (tanδ) on frequency. Dielectric loss is higher for all the samples at lower frequencies and decreases continuously with the increase in frequency. Dielectric losses at the lower frequencies are due to the space charge polarization, this phenomenon can be explained by Shockley-Read mechanism[40]. According to this mechanism, the
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space charge polarization materializes at lower frequencies due to the capture of surface electrons by the impurities and defects present in the system. While the low dielectric losses at higher frequencies
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are attributed to the emergence of defects in dipoles with the change in valence state of cations[41].
Fig. 16 Cole-Cole plots of NdFe1-xCoxO3 (0 ≤ x ≤ 0.4) at room temperature.
Figure 16 display the Cole-Cole plots between ε′ and ε′′. These plots are used to understand the Debye relaxation behavior for materials. The ideal Debye relaxation behavior suggests the semicircle variation between the parameters ε′ and ε′′. In the present study, there is no semicircular variation is observed in
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the Cole-Cole plots. This implies that our synthesized samples follow the non-Debye behavior in the
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applied frequency range [42].
Fig. 17.Plots of log (ε’×f) versus log (f) with linear fit for NdFe1-xCoxO3 (0≤x≤0.4)
In order to study the response of dielectric constant,we have used universal dielectric response
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(UDR) model [36]. According to this model, the localized charge carriers hopping between spatially fluctuating lattice potentials may give rise to the dipolar effects in addition to conductivity. Thereby,
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the plot between log(ε’×f) and log(f) should be linear. We have plotted log(ε’×f) versus log(f) as shown in figure 17. All the samples follow the UDR model in the high frequency range but show deviation at lower frequencies.The nonlinear behavior at lower frequencies for pristine and doped samples confirms the absence of any other contribution (grain boundary, electrode, or Maxwell-Wagner effect) to the dielectric response [43]. 4. Conclusions NdFe1-xCoxO3(0 ≤ x ≤ 0.4) samples are synthesized by sol-gel method. The Rietveld refinement of observed XRD patterns confirms the formation of single phase orthorhombic crystal structure with
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Pbnm space group. The lattice parameters decreases and unit cell volume contracts with the increase of Co doping on account of smaller ionic radius of Co3+ ion. TEM images confirm the formation of nanoparticles with the average particle size of 34 nm. The direct band gap as determined with Tauc’s relation using from UV-visible reflectance data for theNdFeO3sample is much less than the earlier
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reported values and it further decreases with the increase in Co concentration. The presence of characteristic bands in FTIR spectra confirms the formation of the desired phase. Néel temperature of our samples shifts towards higher values on Co doping.Cole-Cole plots indicate towards non-Debye
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type of behavior and deviation from UDR model is observed at lower frequencies for all the samples.
Acknowledgments
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One of the authors Anand Somvanshi is grateful to the UGC-DAE Consortium for Scientific Research, Mumbai for providing financial support under the project CRS-M-271. Authors are thankful to the University Sophisticated Instrument Facility (USIF), AMU for SEM and TEM facilities. Dr. Mohammad Jane Alam, Spectroscopy lab, Department of Physics, AMU is thankfully acknowledged for optical characterizations.
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The lattice parameters decrease with the increase in Co doping.
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TEM images confirm the formation of nanoparticles with average size of 34 nm.
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The energy band gap of NdFeO3 decreases with the increase in Co concentration.
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Neel temperature shifts towards higher values on Co doping.
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Cole-Cole plots reveal non-Debye type of behaviour for pristine and doped samples.
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