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Improved structural, dielectric and magnetic properties of Ca2+ and Nb5+ co-substituted BiFeO3 multiferroics
Accepted Manuscript Improved structural, dielectric and magnetic properties of Ca substituted BiFeO3 multiferroics
2+
5+
and Nb
co-
Sandhaya Jangra, Sujata Sanghi, Ashish Agarwal, Manisha Rangi, Kavita Kaswan, Satish Khasa PII:
S0925-8388(17)32131-X
DOI:
10.1016/j.jallcom.2017.06.132
Reference:
JALCOM 42201
To appear in:
Journal of Alloys and Compounds
Received Date: 15 February 2017 Revised Date:
9 June 2017
Accepted Date: 12 June 2017
Please cite this article as: S. Jangra, S. Sanghi, A. Agarwal, M. Rangi, K. Kaswan, S. Khasa, Improved 2+ 5+ structural, dielectric and magnetic properties of Ca and Nb co-substituted BiFeO3 multiferroics, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.132. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Improved structural, dielectric and magnetic properties of Ca2+ and Nb5+co-substituted BiFeO3 multiferroics Sandhaya Jangra1, Sujata Sanghi1*, Ashish Agarwal1, Manisha Rangi1, Kavita Kaswan1,
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Satish Khasa2 Department of Physics, Guru Jambheshwar University of Science & Technology, Hisar-
125001, Haryana, India
Materials Research Laboratory, Department of Physics, Deenbandhu Chhotu Ram
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University of Science & Technology, Murthal-131039, Haryana, India
______________________ *
Corresponding author. Tel.: +91-1662-263385; Fax: +91-1662-276240.
E-mail: [email protected] (S. Sanghi)
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ACCEPTED MANUSCRIPT Abstract Ca2+ and Nb5+ co-doped multiferroics having composition Bi0.8Ca0.2Fe1-xNbxO3 (x = 0.00, 0.05, 0.07, and 0.10) were synthesized via solid-state reaction method. Effect of substitution
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was analyzed by structural, electrical and magnetic measurements of modified BiFeO3 multiferroics. The samples were characterized by X-ray diffraction (XRD), and results support that impurity phases, which normally occurred in BFO, were significantly diminished in modified BiFeO3. Rietveld refinement of XRD patterns demonstrates that phase
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transformation occurred in all prepared samples. Structure symmetry gets disturbed in
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presence of Nb and mixed phase (rhombohedral (R3c space group) and orthorhombic phase (Pbnm space group)) was confirmed by refinement. The studies of dielectric, impedance and electrical properties at a wide range of temperature and frequency offered us noticeable information of Nb substitution. Frequency dependence impedance confirms the non-Debye type of relaxation. Nb doping increases resistivity and samples show polarization hysteresis
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loop. All samples show ferromagnetism in modified multiferroics with maximum remanent magnetization is Mr=0.0581 emu g-1 and coercive field is Hc = 4.7505 kOe for x = 0.10 which
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is ten times of magnetization observed in parent sample. As co-substitution at A and B site is
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responsible for phase transformation and enhancing electrical and magnetic properties.
Keywords: Multiferroics; X-ray Diffraction; Rietveld Refinement; Magnetic Properties; Dielectric Properties.
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ACCEPTED MANUSCRIPT 1. Introduction Multiferroics are those materials which possess at least two of ferroics orders out of three (ferroelectricity, ferromagnetism, ferroelasticity) simultaneously in a single phase[1]. Among
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these ferroic orders, ferromagnetism and ferroelectricity are highly desired and coupling between these ferroic orders is essential in enhancing their functionalities[2], which make them useful for potential application in data–storage system, multi-state memories, and spintronics devices. Among a few encouraging multiferroics, BiFeO3 is most significant as it
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possesses magnetic ordering and ferroelectric polarization above room temperature (RT)[3].
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Magnetoelectric coupling and high ferroelectric Curie temperature of about 1100 K (i.e. ferroelectric ordering exist up TC =1100 K), and Neel temperature of about 640 K (i.e. magnetic ordering exist up to TN = 640 K) make it useful for practical applications[3, 4]. BiFeO3 possesses distorted rhombohedral perovskite structure with polar space group R3c[5]. Dipole ordering in bismuth ferrite is due to the relative shift of Bi3+, Fe3+, and O2- ions along
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the [001] hexagonal axis. Dipole order emerges due to the stoichiometric activity of 6s2 electrons pair of bismuth ion. The Bi3+ ion has the 6s2p0 valence electron configuration; the 6s2 electron of Bi3+ ion is hybridized with empty 6p0 orbital’s of Bi3+ and with electron-filled
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2p6 orbital’s of O2- anions to form the Bi-O covalent bond, which is the reason behind
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structural distortion and dipole ordering[6]. BiFeO3 exhibits G-type anti-ferromagnetic ordered spin which is tilted under Dzyaloshinskii-Moriya (D-M) interaction. This D-M interaction diminishes macroscopic ferro-magnetization, and inhibits the existence of linear ME effect [7]. Although weak ferromagnetism is always observed in a pure BiFeO3 which is due to partial destruction of spiral periodicity in grains with limited dimensions[8]. As BiFeO3 is a promising material for storages devices yet it has some drawbacks for room temperature applications, such as the large value of leakage current due to charge defects of Bi3+, the existence of impurity phase like Bi25FeO40 and Bi2Fe4O9, non-stoichiometry and 3
ACCEPTED MANUSCRIPT oxygen vacancies, high dielectric loss and unsaturated magnetic loops. Oxygen vacancies are generated during heat treatment due to volatile nature of bismuth and tendency of Fe ions to exist in multiple oxidation states (Fe3+, Fe2+, and Fe4+)[7]. Several strategies have been adopted for enhancement of ferroelectric and ferromagnetic properties and suppressing
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impurity phase and leakage current in BiFeO3 multiferroics[3, 7]. Various techniques were reported to improve properties of BiFeO3 including doping of aliovalent and isovalent ions at A and B site[7]. Khomchenko et al. [9] have reported effects of divalent ions (Ca2+, Sr2+,
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Pb2+, and Ba2+) doping at A site of BiFeO3 and suggested that there is a strong correlation between ionic radii and ferromagnetism. With increased ionic radius of dopant at A site leads
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to suppression of spiral spin structure and contribute to net magnetization. Zhai et al. [10] have reported structural transition from rhombohedral to monoclinic with La doping at A site of BiFeO3 (BLFO) and enhanced coercive field and remanent magnetization with Nb doping in BLFO. Jun et al. [11] observed an increase in electrical resistivity and ferromagnetic
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behavior in Nb5+ doped BiFeO3. Makhdoom et al. [7] have reported reduced leakage current density and superimposition of weak ferromagnetism with primary antiferromagnetism in Ba2+ and Nb5+ co-doped samples. Reetu et al. [3] have also reported a structural change from
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rhombohedral to triclinic, enhanced magnetic properties, increased resistivity and decreased
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dielectric losses in Sr2+ and Nb5+ co-doped BiFeO3. All these reports suggest that cosubstitution at A and B site suppresses the modulated spiral spin structure and improves multiferroics properties of BiFeO3. In the view of above, in present work effect of cosubstitution of Ca and Nb in BiFeO3 having composition Bi0.8Ca0.2Fe1-xNbxO3 (x = 0.00, 0.05, 0.07, and 0.10) have been investigated. A systematic study regarding the structural change, measurement of electric and magnetic properties has been carried out.
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ACCEPTED MANUSCRIPT 2. Experimental details Polycrystalline multiferroics with composition Bi0.8Ca0.2Fe1-xNbxO3 (BCFNx); x = 0.00, 0.05, 0.07, and 0.10 were synthesized via conventional solid state reaction method. High purity
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reagents (Sigma-Aldrich): Bi2O3 (98%), CaCO3 (99%), Fe2O3 (99%) and Nb2O5 (99.9%) were used as starting materials and weighted in stoichiometry proportions. These materials were mixed thoroughly and were ground in an agate mortar for the homogeneous mixture. Then calcination was performed at 673 K for 6 hrs at a rate of 5 °C/min. Calcined powders of
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all the samples were reground for 30 min and sintered at a temperature range of 1023-1073 K
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followed by immediate quenching in the air after removing samples. Sintering temperature increased with Nb concentration. Crystal structure of obtained samples was recorded using X-ray diffractometer (Rigaku Miniflex-II) with Cu-Kα radiation (λ = 1.5418 Å) with scanning speed of 2 °/min and 2θ range (10–80°) at room temperature. No major impurity phase was observed in the prepared samples. Further, crystal structure was analyzed by Rietveld
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refinement using Full Prof program. For dielectric measurements, the powder was pressed into a disk shape (1 mm thickness, 10 mm diameter) by using PVA (organic binder) by
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applying pressure of ≈ 10 ton using pellet press. Ag paste was applied on these sintered pellets for making electrodes and dielectric measurements were carried out with an
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impendence/gain-phase analyzer (Newton’s 4th Ltd.) in frequency range 100 Hz to 5 MHz and temperature range 323-573 K. Magnetic characterization was performed using Vibrating Sample Magnetometer (Lakeshore VSM 7410) up to field of 15 kOe. Polarization hysteresis measurements were carried out by the P-E analyzer (Marine India).
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ACCEPTED MANUSCRIPT 3. Results and discussions 3.1. Structural Analysis Fig.1 (a) shows XRD patterns of co-doped BCFO, BCFN5, BCFN7, and BCFN10 multiferroics. All the samples are poly-crystalline in nature. A minor impurity peak around
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28ᵒ in BCFN5 sample belongs to Bi2Fe4O9 and doesn’t affect the magnetic properties as basic nature of this Fe rich phase is non-magnetic in nature[12, 13]. Magnified view of XRD patterns (Fig. 1 (b)) revealed that there is shifting of diffraction peaks towards lower angle
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which indicates that unit cell volume increases. This shifting is due to the difference in ionic
also broadened in mixed phase sample.
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Intensity (a.u.)
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radii of ions (Bi3+= 1.17 Å, Ca2+= 1.12 Å; Fe3+= 0.645 Å, and Nb5+= 0.72 Å) and peaks are
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Figure 1 (a) XRD pattern of BCFO, BCFN5, BCFN7 and BCFN10 samples. Diffraction peak indicating impurity phase (Bi2Fe4O9) is marked by *. (b) An enlarged view of XRD data in a 2θ range of (20-40°) shows shifting of peaks.
Crystal structure was also analyzed by the Rietveld refinement by using Fullprof program, which shows good agreement between observed and calculated intensities and demonstrate that with increasing Nb content, amount of rhombohedral phase (space group R3c) decreased and that of orthorhombic (space group Pbnm) phase increased. Refined XRD data for observed, calculated and their difference intensities displayed in Fig. 2. ((a)-(d)). It is 6
ACCEPTED MANUSCRIPT observed that, 20% Ca2+ substitution in bismuth ferrite (BCFO sample) is not able to change basic rhombohedral structure with hexagonal symmetry and has R3c space group. The low value of goodness of fit parameter (χ2) and profile parameters Rwp and Rp (<10%) also agree with these results. Peaks shapes were fitted by Pseudo-Voigt function and background
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parameters were fitted by cosine Fourier series. All co-doped samples were best fitted by mixed rhombohedral (R3c space group) and orthorhombic (Pbnm space group) phase as shown in fig. 2 (a, b, c and d). The crystal symmetry, unit cell parameters and profile
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parameters for all compositions are listed in Table 1. The bond angle and bond distance parameters measured by Rietveld refinement are listed in Table 2. Therefore, Nb content
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plays a significant role in phase transformation due to the simultaneous substitution of aliovalent ions, i. e., Ca2+ and Nb5+ in place of host Bi3+ and Fe3+ ions. Phase transformation may be due to the accommodation of larger and higher valence Nb5+ ion at B site in a
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rhombohedral structure in place of smaller ion resulting in structural distortion.
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Figure 2 Rietveld refined XRD patterns for prepared samples Bi0.8Ca0.2Fe1-xNbxO3 (BCFNx): (a) x=0.00; (b) x=0.05; (c) x=0.07; (d) x=0.10.
Figure 3 The schematic view of the crystal structure for (a) BCFO and (b) BCFN10. The distortion in ABO3 type perovskite can be calculated by Goldschmidt’s tolerance factor (t) given by (1)
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ACCEPTED MANUSCRIPT where RA, RB, and RO are ionic radii of atoms at A site (Bi3+ and Ca2+), at B site (Nb5+ and Fe3+), and for oxygen anion respectively. For ideal conditions (cubic structure), t = 1 and deviation from the unity quantifies the stability of the prepared compound, e.g., for the t ≤ 1 structure will be more stable[3, 14]. Fig. 4 shows that tolerance factor decreases with
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progressive doping of Nb5+ which implies that, co-doping is favorable for the stabilization of the crystal structure.
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Tolarance Factor (t)
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Figure 4 Tolerance Factor vs concentration for all prepared samples.
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3.2. Dielectric Analysis
Figs.5 (a)-(d) show the temperature dependent behavior of real part of dielectric constant (ε') and loss tangent (in inset) for all prepared BCFN multiferroics for different frequencies (10
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kHz, 100 kHz, and 1 MHz). It shows the strong dependence of the real part of dielectric constant (ε') and loss tangent (tan δ) on temperature and frequency. Dielectric constant (ε')
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and loss tangent (tan δ) are calculated by using the formulas:
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where Z' and Z″ represent real and imaginary impedance, f is frequency and C0 is geometrical capacitance. The dielectric constant gradually increases from T > 400 K and is due to 9
ACCEPTED MANUSCRIPT hopping conduction. At low temperature, dipoles freezes due to relaxation process causing decreased polarization and hence small dielectric constant[5]. Dielectric constant is not an intrinsic property but varies due to charge defect caused by heterovalent substitutions and is further associated with the driven frequencies[15]. Dielectric constant increases with Nb
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content and shows monotonous decreasing behavior with increasing frequency for same temperature range. At low-frequency, defect related dipoles are able to follow alternating current and results in high values of dielectric constant, as the frequency increases these start
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Figure 6 The variation of imaginary part of dielectric constant (εʺ) with temperature at different frequencies
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Fig. 6 illustrates the temperature dependent behavior of dielectric constant at different frequencies. The variation in the value of εʺ is prominent at a lower frequency as compared to a higher frequency. The value of εʺ found to increase sharply with temperature for a fixed frequency due to thermally induced hopping conduction[16].
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Figure 7 Frequency dependence of imaginary part of dielectric constant (εʺ) for all prepared samples at different temperatures.
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Fig. 7 depicts the variation of εʺ with frequencies at different temperature ranges. With
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increasing temperature, εʺ increases for all samples. At lower frequency range, we observe a dispersion behavior and which attains nearly constant value at higher frequencies. Similar behavior is reported previously in BFO by several research groups[16, 17]. These plots were well explained by space charge polarization model of Maxwell-Wagner[18, 19]. Koop’s phenomenological theory also gives conformity of such behavior[20]. Polycrystalline materials exhibit conducting grains and these grains are separated by each other by insulating grain boundaries. The space charge polarization comes into play by the accumulation of grains at grain boundaries by displacement of charge carrier on the application of the field.
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ACCEPTED MANUSCRIPT This space charge polarization is controlled by available free charges. Therefore, increased value of dielectric constant increases with temperature.
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Figure 8 Frequency dependence of dielectric constant (ɛʹ) and dielectric loss (tan δ) for all
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samples at 523 K.
Fig. 8 shows variation of dielectric constant (ɛʹ) and tan δ (in inset) with frequency at 523 K for the samples x=0.0 (BCFO), x=0.05 (BCFN5), x=0.07 (BCFN7) and x=0.10 (BCFN10).
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All samples show high dielectric constant at lower frequency, as large dipole responds at a
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lower frequency and their response ceases at high frequency of applied field. The main reason of interfacial polarization (Maxwell-Wagner) occurrence at grain boundaries is segregation of impurity phases and oxygen vacancies. The large value of the dielectric constant in BCFO is attributed to oxygen vacancy created during charge compensation by Ca2+ doping. Further Nb5+ doping, is also expected to fill the oxygen vacancy or varying cation valence for balancing the charge. Therefore, Nb5+ substitution results in reduced dielectric constant and losses with frequency[7, 33].
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Figure 9 Nyquist plots ((a)-(d)) for BCFO, BCFN5, BCFN7, and BCFN10 at different temperature.
3.3 Impedance analysis
Fig. 9 depicts temperature-frequency dependence of complex impedance spectra (Nyquist plot) for selected temperatures. As complex impedance spectroscopy (CIS) is a unique technique used to estimate the contribution of grain and grain boundaries in electrical processes and how they affect the microstructure of the materials. The Nyquist plots comprise single semicircular arc, which mainly originates due to the effect of grains [21]. With
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ACCEPTED MANUSCRIPT temperature and composition, the shape of semicircular arc changes signifying a change in the resistive and capacitive behavior of materials. It is also observed that with the rise in temperature, intercept at x-axis shifts towards lower Zʹ indicating a reduction in resistance and negative temperature coefficient of resistance (NTCR) behavior. The depressed
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semicircle having center located below the real impedance axis suggests that relaxation of ions is non-Debye type in nature[22, 23]. It can be seen from plots that with increasing doping maximum impedance is observed for x=10 which will lead to low value of leakage
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3.4 Conductivity analysis
Figs. 10 ((a)-(d)) illustrate the variation of ac conductivity (σac) for all samples at different temperature in the frequency range 50-106 Hz. All plots give evidence of frequency dispersion, which shifts towards higher frequency with increase in temperature. The
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phenomenon of conductivity dispersion is an important representation as it affects the dielectric properties prominently and generally examined by Jonscher’s power law[25, 26] σac(ω) = σ0 + Aωs
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where ω is angular frequency (ω=2πf) of applied field, σ0 is frequency independent (dc) conductivity occurred due to excitation of an electron from one confined state to the
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conduction band. A is frequency independent and temperature dependent factor. The temperature dependent frequency exponent ‘s’, lies between 0 and 1[27]. At lower frequency range, σac consists of plateau region. As temperature is increased, dc conduction dominates and it results in increased flat region in conductivity plot. It is well known that variation of s with temperature shows the type of conduction mechanism. If (i) s is independent of temperature then conduction is based on quantum mechanical tunneling (QMT), if (ii) s decreases with temperature then correlated barrier hopping (CBH) is expected, if (iii) s increases with temperature, then mechanism is correlated with small polaron conduction, and 15
ACCEPTED MANUSCRIPT if (iv) s firstly decreases followed by increased value then dominant conduction mechanism is an overlapping large polaron tunneling (OLPT)[4]. The above system follows power law as confirmed by non-linear curve fitting and goodness of fitting is confirmed by experimental and fitted curve (solid line). It is very clear from the plots (fig. 10 (a)-(d)) that value of s
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increases with temperature, therefore small polaron conduction mechanism is expected. As BCFO shows minimum ‘s’ followed by increasing trend so it is expected to have OLPT mechanism. Fig. 11 depicts the variation of σac with doping and minimum conductivity is
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observed for BCFN10 among all samples which supports that electrical properties are improved in modified system. The highest conductivity is observed for BCFN7 sample may
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be due to increased oxygen vacancies and lowest conductivity for BCFN10 may be due to suppression of lattice conduction path or local lattice distortion or enhancement in barrier properties[14].
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Figure 10 (a)-(d) Frequency dependence of σac (ω) at different temperatures for all samples.
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Figure 11 Variation of exponent ‘s’-parameters with temperature for BCFO, BCFN5, BCFN7, and BCFN10.
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Figure 12 Frequency dependence of ac conductivity for Bi0.8Ca0.2Fe1-xNbxO3 samples at 490 K.
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ACCEPTED MANUSCRIPT 3.5. Ferroelectric properties Fig. 13 represents polarization hysteresis (P-E) loops for BCFOx=0.0, BCFN5x=0.05, BCFN7x=0.07 and BCFN10x=0.10 samples at room temperature. Since pure BiFeO3 having high conductivity so it is difficult to observe P-E loop[28]. As co-doped samples are more resistive
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due to less charge defect, therefore these samples show opening of loop. Although observed loops were not saturated due to the existence of leakage current and high coercive field[14, 29]. The breakdown field is different for different samples. In modified bismuth ferrite
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Bi0.8Ca0.2Fe1-xNbxO3 maximum polarization is observed for x=0.05, further addition of Nb content reduced polarization. Improved ferroelectric properties obtained in presence of Nb is
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due to diminishing charge defect (which plays a fundamental role in pinning polarization switching domain) generally originated by volatile nature of bismuth [30]. Secondly, large polarization in co-doped samples might be due to structural changes from rhombohedral to mixed phase (rhombohedral + orthorhombic). Incorporation of Nb at B site noticeably
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enhances ferroelectric properties.
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Figure 13 Polarization hysteresis loop (P-E) of BCFO, BCFN5, BCFN7, and BCFN10 samples measured at room temperature.
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ACCEPTED MANUSCRIPT 3.6. Magnetic Properties Fig. 14 represents the magnetic hysteresis loops (M-H) for all samples Bi0.8Ca0.2Fe1-xNbxO3 (x = 0.0, 0.05, 0.07 and 0.10) up to maximum applied magnetic field of 15 kOe.
Hysteresis
loops indicate non-zero remanent magnetization (Mr) and coercive field (Hc). BiFeO3
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possesses an antiferromagnetic structure with G-type spin ordering modulated cycloidal by spiral spin having a large period of λ = 62 nm. This long range cycloidal modulated spin structure vanishes the ferromagnetism and causes basic antiferromagnetism in BiFeO3[7, 31].
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The parent Bi0.8Ca0.2FeO3 (BCFO) shows a linear variation of magnetization with an applied field, which confirms the antiferromagnetic behavior. The magnetic moment of Fe3+ ion is
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coupled ferromagnetically within the pseudo-cubic plane (111) and anti-ferromagnetically between adjacent planes. Alignment of ferromagnetically ordered planes parallel to [111] plane leads to canting of antiferromagnetic sublattice which collapses the spirally modulated spin structure and results in the release of net magnetization[15]. Indeed, divalent ion doping
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at A site is not so effective in enhancing the magnetization but higher valence Nb5+ at B site is helpful in increasing the ferromagnetism as is clearly observervable in fig.14. 0.3
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Figure 14 Magnetic Hysteresis loops for BCFO, BCFN5, BCFN7 and BCFN10 samples recorded at room temperature.
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ACCEPTED MANUSCRIPT Nb substitution, which is having higher valence and ionic radius as compared to host Fe3+ ion, is responsible for structural transformation which can tilt FeO6 octahedrons[3, 7]. It might be due to the breakdown of balance between antiparallel sublattice magnetization of Fe3+ due to the presence of higher valence Nb5+[11]. As these results are in consonance with
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the structural investigations, i.e., there was the coexistence of two phases (rhombohedral with R3c and orthorhombic with Pbnm) [3, 27]. All hysteresis loops were not saturated up to maximum applied field which may be due to incomplete magnetic transition in a new
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magnetic state or uncompensated antiferromagnetic nature present in the samples[31, 32]. The non-zero value of net magnetization indicates the sign of improved magnetic properties.
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As reported earlier [9, 12, 33] and observed in the present study as that Ca doping is not able to generate the ferromagnetic character in bismuth ferrite. Thus, the enhancement in the magnetic properties in modified BCFO were due to the presence of Nb. Under an applied magnetic field of 15 kOe, the maximum value of remanent magnetization is 0.0660 emu g-1
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and coercive field is 4.7505 kOe for BCFN10. The remanent magnetization Mr and coercive field Hc for all samples are presented in Table 4. Remanent magnetization increased nearly ten times for BCFN10 as compared with parent BCFO and is attributed to the co-doping of
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Ca and Nb simultaneously in BiFeO3.
4. Conclusions
Polycrystalline samples with Ca and Nb substitution having composition Bi0.8Ca0.2Fe1-xNbxO3 (x = 0.00, 0.05, 0.07 and 0.10) were prepared by solid state reaction method. Primary structural investigation was performed by XRD at room temperature. Ca2+ and Nb5+ ions substitution is helpful in reduction of the secondary phases, slight foreign peaks like Bi2Fe4O9 were observed in BCFN5 sample. The sharpness of diffraction peaks was due to a good degree of crystallinity and shifting of diffraction peaks towards lower angle is due to 20
ACCEPTED MANUSCRIPT increased unit cell volume with Nb5+ concentration. Structural parameters investigations described the role of Nb5+ content in phase transformation from rhombohedral (R3c) to mixed phase (R3c+Pbnm). Improved dielectric properties obtained with reduced losses and enhanced resistivity. Magnetic properties investigation shows significant enhancement in
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remanent magnetization, which favors substitution of Ca2+ and Nb5+ in suppressing spiral spin structure of BFO and released locked magnetization, by transforming basic magnetic character antiferromagnetic to ferromagnetic. The results of present work provide a possible
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way in improving multiferroics properties.
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Acknowledgements
Authors are thankful to Department of Science and Technology, Government of India for providing XRD facility through FIST scheme. One of the authors (A. Agarwal) is thankful to the Science and Engineering Research Board, Department of Science and Technology,
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Government of India for providing financial assistance in the form of the research project (Grant No. SR/S2/CMP-44/2012).
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References
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materials, Nature, 442 (2006) 759–765. [2]J. Ma, J. Hu, Z. Li, C. W. Nan, Recent progress in multiferroic magnetoelectric composites: from bulk to thin films, Adv. Mater., 23 (2011) 1062–1087. [3]A. Agarwal, S. Sanghi, N. Ahlawat, Phase transformation, dielectric and magnetic properties of Nb doped Bi0.8Sr0.2FeO3 multiferroics, J. Appl. Phys. 111 (2012) 113917. [4]R. Das, T. Sarkar, K. Mandal, Multiferroic properties of Ba2+ and Gd3+ co-doped bismuth ferrite: magnetic, ferroelectric and impedance spectroscopic analysis, J. Phys. D. Appl. Phys.,
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ACCEPTED MANUSCRIPT 45 (2012) 455002. [5]R. Singh, G. D. Dwevedi, P. Shahi, D. Kumar, O. Prakash, A. K. Ghosh, S. Chatterjee, Effect of Pr- and Nd- doping on structural, dielectric, and magnetic properties of multiferroic Bi0.8La0.2Fe0.9Mn0.1O3, J. Appl. Phys., 115 (2014) 134102.
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[7]A. R. Makhdoom, M. J. Akhtar, M. A. Rafiq, M. Siddique, M. Iqbal, M. M. Hasan, Enhancement in the multiferroic properties of BiFeO3 by charge compensated aliovalent
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crystal structure and multiferroic properties of the BiFeO3 perovskite, J. Appl. Phys., 103
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Bi0.9La0.1Fe0.98Nb0.02O3 polycrystalline compound, J. Phys. D. Appl. Phys., 42 (2009) 165004.
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[11]Y. K. Jun, W. T. Moon, C. M. Chang, H. S. Kim, H. S. Ryu, J. W. Kim, S. H. Hong, Effects of Nb-doping on electric and magnetic properties in multi-ferroic BiFeO3 ceramics, Solid State Commun. 135 (2005) 133–137. [12]M. Rangi, A. Agarwal, S. Sanghi, R. Singh, S. S. Meena, A. Das, Crystal structure and magnetic properties of Bi0.8A0.2FeO3 (A=La, Ca, Sr, Ba) multiferroics using neutron diffraction and Mossbauer spectroscopy, AIP Adv., 4 (2014) 087121. [13]F. Azough, R. Freer, M. Thrall, R. Cernik, F. Tuna, D. Collison, Microstructure and properties of Co-, Ni-, Zn-, Nb- and W-modified multiferroic BiFeO3 ceramics, J. Eur.
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ACCEPTED MANUSCRIPT Ceram. Soc., 30 (2010) 727–736. [14]S. Dash, R. N. P. Choudhary, P. R. Das, A. Kumar, Effect of KNbO3 modification on structural, electrical and magnetic properties of BiFeO3, Appl. Phys. A, 118 (2015) 10231031.
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[15]M. S. Wu, Z. B. Huang, C. X. Han, S. L. Yuan, C. L. Lu, S. C. Xia, Enhanced multiferroic properties of BiFeO3 ceramics by Ba and high-valence Nb co-doping, Solid state commun. 152 (2012) 2142–2146.
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[16]P. Godara, A. Agarwal, N. Ahlawat, S. Sanghi, Crystal structure refinement, dielectric and magnetic properties of Sm modified BiFeO3 multiferroic, J. Mol. Struct., 1097 (2015)
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207–213.
[17]R. K. Mishra, D. K. Pradhan, R. N. P. Choudhary, A. Banerjee, Dipolar and magnetic ordering in Nd-modified BiFeO3 nanoceramics, J. Magn. Magn. Mater. 320 (2008) 2602– 2607.
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[21]S. Pattanayak, R. N. P. Choudhary, D. Pattanayak, A comparative study of structural, electrical and magnetic properties rare-earth (Dy and Nd)-modified BiFeO3, J. Mater. Sci. Mater. Electron. 25 (2014) 3854–3861. [22]R. Kumari, N. Ahlawat, A. Agarwal, S. Sanghi, M. Sindhu, “Structural transformation and investigation of dielectric properties of Ca substituted (Na0.5Bi0.5)0.95-xBa0.05CaxTiO3
ceramics, J. Alloys Compd. 695 (2017) 3282–3289. [23]S. Rani, S. Sanghi, N. Ahlawat, A. Agarwal, Influence of Bi2O3 on physical, electrical and thermal properties of Li2O·ZnO·Bi2O3·SiO2 glasses, J. Alloys Compd., 619 (2015) 659– 23
ACCEPTED MANUSCRIPT 666. [24]Y. P. Jiang, X. G. Tang, Q. X. Liu, D. G. Chen, C. B. Ma, Improvement of electrical conductivity and leakage current in co-precipitation derived Nd-doping BiFeO3 ceramics, J. Mater. Sci. Mater. Electron. 25 (2014) 495–499.
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[25]R. Dahiya, A. Agarwal, S. Sanghi, A. Hooda, P. Godara, Structural, magnetic and dielectric properties of Sr and V doped BiFeO3 multiferroics, J. Magn. Magn. Mater. 385 (2015)175–181.
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[26] M. Rangi, S. Sanghi, S. Jangra, K. Kaswan, A. Agarwal, Effect of Mn doping on crystal structure, dielectric and magnetic ordering of Bi0.8Ba0.2FeO3multiferroic, Ceram. Int., 42,
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(2016) 5403–5411.
[27]P. Godara, A. Agarwal, N. Ahlawat, S. Sanghi, R. Dahiya, Crystal structure transformation, dielectric and magnetic properties of Ba and Co modified BiFeO3 multiferroic, J. Alloys Compd., 594, (2014)175–181.
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[28]R. Gupta, J. Shah, S. Chaudhary, S. Singh, R. K. Kotnala, Magnetoelectric couplinginduced anisotropy in multiferroic nanocomposite (1-X)BiFeO3-xBaTiO3, J. Nanoparticle Res., 15 (2013) 1–9.
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[29]A. Sharma, R. K. Kotnala, N. S. Negi, Observation of multiferroic properties and
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magnetoelectric effect in (x)CoFe2O4-(1-x)Pb0.7Ca0.3TiO3 composites, J. Alloys Compd. 582 (2014) 628–634.
[30]A. Mukherjee, M. Banerjee S. Basu, L. A. W. Green, N. T. K. Thanh, M. Pal, Enhanced multiferroic properties of Y and Mn codoped multiferroic BiFeO3 nanoparticles, Physica B 448 (2014) 199–203. [31]H. Singh, K. L. Yadav, Effect of Nb substitution on the structural, dielectric and magnetic properties of multiferroic BiFe1−xNbxO3 ceramics, Mater. Chem. Phys. 132 (2012)17–21.
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ACCEPTED MANUSCRIPT [32]I. O. Troyanchuk, N. V. Tereshko, D. V. Karpinsky, A. L. Kholkin, M. Kopcewicz, K. Bärner, Enhanced piezoelectric and magnetic properties of Bi1− xCaxFe1−x/2Nbx/2O3 solid solutions, J. Appl. Phys., 109 (2011) 114102. [33]Reetu, A. Agarwal, S. Sanghi, Ashima, N. Ahlawat, Improved dielectric and magnetic
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properties of Ti modified BiCaFeO3 multiferroic ceramics, J. Appl. Phys., 113(2013) 023908.
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ACCEPTED MANUSCRIPT Table 1 Refined structural parameters of BCFO, BCFN5, BCFN7, and BCFN10 samples.
Sample Structural model Cell parameters
Position coordinates x
y
R-Factors
z
(%)
BCFO a=5.5496Å Bi/Ca (0,0, 0.2676) c=13.5949Å Fe/Nb(0,0, -0.003) V=362.609Å3 O (0.9927, 0.5149, 0.3234)
BCFN5 Rhombohedral (R3c 73.11%)
a=5.5658Å Bi/Ca(0,0, 0.2193) Rp =3.86 c=13.806 Å Fe/Nb( 0, 0, -0.0023) Rwp=5.03 V=370.413Å3 O(0.8484, 0.6337, 0.4296) χ2 = 3.03 a=5.5583Å Bi/Ca (-0.0111, 0.0107, 0.2500) b=5.5179Å Fe/Nb(0, 0.5, 0) c=7.8558 Å O1(0.5963, 0.0056,0.2500) V=240.093Å3 O2(0.4525, 0.1917, 0.1801)
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Orthorhombic (Pbnm 26.89%)
BCFN7 Rhombohedral (R3c 55.83%)
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a=5.5693Å Bi/ Ca(0, 0, 0.2243) Rp =3.64 c= 13.806Å Fe/Nb(0,0, 0.0006) Rwp=4.66 V=370.890Å3 O ( 0.8835, 0.6416, 0.4351) χ2=2.58 a=5.5679 Å Bi/ Ca( 0.0134, 0.0166, 0.25) b= 5.5207Å Fe/Nb(0, 0.5, 0) c=7.8644Å O1(0.5414, 0.0038, 0.25) V=241.74Å3 O2(0.7075, 0.3922, 0.0426)
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Orthorhombic (Pbnm 46.56%)
a=5.5670 Å Bi/Ca(0, 0, 0.2225) Rp= 4.53 c=13.798Å Fe/Nb(0, 0,0.0009) Rwp=5.82 3 χ2 = 3.72 V= 370.360Å O(0.7682, 0.611, 0.4321) a=5.5089 Å Bi/ Ca (0.0139, 0.0369, 0.25) b=5.5890 Å Fe/Nb (0, 0.5, 0) c=7.8607Å O1(0.5581, 0.0016, 0.25) V=242.63Å3 O2(0.3031, 0.2809, 0.0282)
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Orthorhombic (Pbnm 44.17%)
BCFN10 Rhombohedral (R3c 53.44%)
Rp=4.45 Rwp=5.86 χ²=4.60
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Rhombohedral (R3c 100%)
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ACCEPTED MANUSCRIPT Table 2 Bond length and bond angles of Bi0.8Ca0.2Fe1-xNbxO3 (BCFNx); x=0.00, 0.05, 0.07, and 0.10 samples obtained by refinement.
Bond angle ( ᵒ )
Sample x=0.0 Bi−O 2.777, 2.976, 2.227 Fe−O 2.692, 1.576
Bi−O−Bi Bi−O−Fe
Sample Bi−O2 Fe−O1 Fe−O2
Bi−O−Bi 115.80 Bi−O2−Fe 94.56, 76.850 O1−Fe−O2 149.94, 137.25
x=0.05 2.817, 1.872 2.035 1.786, 2.739
Sample x=0.07 Bi−O2 2.592, 2.154 Fe−O1 1.979 Fe−O2 1.766, 2.477
149.36, 96.846 132.19, 95.06
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Bi−O2−Bi 160.79, 97.558 Bi−O2−Fe 81.563, 91.488 O1−Fe−O2 88.477, 91.523, 91.55, 88.448
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Sample x=0.10 Bi−O2 2.819, 2.614, 2.537 Fe−O1 1.991 Fe−O2 2.082, 1.920
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Bi−O2−Bi 111.85 Bi−O2−Fe 98.64, 81.66 O1−Fe−O2 101.44, 107.01, 78.56
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ACCEPTED MANUSCRIPT Table 3 Fitting parameters of BCFNx (x=0.0, x=0.05, x=0.07 and x=0.10) at different temperatures. Samples Temperature (K) _____________________________________________________________ 483 K 503K 523K 543K 563K BCFO 4.65E-05 1.06E-06 0.37066
1.13E-04 1.35E-06 0.37336
2.93E-04 9.51E-07 0.4282
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2.06E-05 3.03E-07 0.43434
1.57E-05 2.19E-07 0.44441
4.07E-05 2.56E-07 0.47132
1.05E-04 1.58E-07 0.53988
BCFN7 σ0 A s
2.78E-05 1.73E-06 0.35488
4.16E-05 1.05E-06 0.38785
8.36E-05 7.25E-07 0.42468
BCFN10 σ0 5.70E-06 A 3.59E-07 s 0.40011
1.40E-05 4.61E-07 0.41017
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Table 4 Magnetic parameters of Bi0.8Ca0.2Fe1-xNbxO3 (x = 0.0, 0.05, 0.07, and 0.10).
x=0.00
0.8852
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0.8836
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1.4735
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0.8829
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Figure captions: Figure 1 (a) XRD pattern of BCFO, BCFN5, BCFN7 and BCFN10 samples. Diffraction peak indicating impurity phase (Bi2Fe4O9) is marked by *. (b) An enlarge view of XRD data in a 2θ range of (20-40°) shows clearly diffraction peaks shifting.
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Figure 2 The results of Rietveld Refined XRD pattern for prepared samples Bi0.8Ca0.2Fe1xNbxO3 (BCFNx): (a) x=0.00; (b) x=0.05; (c) x=0.07; (d) x=0.10. Figure 3 The schematic view of the crystal structure for (a) BCFO and (b) BCFN10. Figure 4 Tolerance Factor vs concentration for all prepared samples.
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Figure 5 Variation of the real part of dielectric constant (εʹ) and loss tangent (tan δ) (in inset) with the temperature at different frequencies.
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Figure 6 The variation of imaginary part of dielectric constant (εʺ) with temperature at different frequencies. Figure 7 Frequency dependence of imaginary part of dielectric constant (εʺ) for all prepared samples at different temperatures. Figure 8 Frequency dependence of dielectric constant (ɛʹ) and dielectric loss (tan δ) for all samples at 523 K.
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Figure 9 Nyquist plots ((a)-(d)) for BCFO, BCFN5, BCFN7, and BCFN10 at different temperature. Figure 10 (a)-(d) Frequency dependence of σac (ω) at different temperatures for all samples.
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Figure 11 Variation of exponent ‘s’-parameters with temperature for BCFO, BCFN5, BCFN7, and BCFN10.
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Figure 12 Frequency dependence of ac conductivity for Bi0.8Ca0.2Fe1-xNbxO3 samples at 490 K. Figure 13 Polarization hysteresis loop (P-E) of BCFO, BCFN5, BCFN7, and BCFN10 samples measured at room temperature. Figure 14 Magnetic Hysteresis loops for BCFO, BCFN5, BCFN7 and BCFN10 samples collected at room temperature.
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ACCEPTED MANUSCRIPT Highlights 1. Ca2+ and Nb5+ co-doped BiFeO3 shows improved magnetic structure. 2. Crystal structure has been analyzed using Rietveld refinement.
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3. Multiferroic properties are also improved.
2+
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Sandhaya Jangra, Sujata Sanghi, Ashish Agarwal, Manisha Rangi, Kavita Kaswan, Satish Khasa PII:
S0925-8388(17)32131-X
DOI:
10.1016/j.jallcom.2017.06.132
Reference:
JALCOM 42201
To appear in:
Journal of Alloys and Compounds
Received Date: 15 February 2017 Revised Date:
9 June 2017
Accepted Date: 12 June 2017
Please cite this article as: S. Jangra, S. Sanghi, A. Agarwal, M. Rangi, K. Kaswan, S. Khasa, Improved 2+ 5+ structural, dielectric and magnetic properties of Ca and Nb co-substituted BiFeO3 multiferroics, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.132. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Improved structural, dielectric and magnetic properties of Ca2+ and Nb5+co-substituted BiFeO3 multiferroics Sandhaya Jangra1, Sujata Sanghi1*, Ashish Agarwal1, Manisha Rangi1, Kavita Kaswan1,
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Satish Khasa2 Department of Physics, Guru Jambheshwar University of Science & Technology, Hisar-
125001, Haryana, India
Materials Research Laboratory, Department of Physics, Deenbandhu Chhotu Ram
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University of Science & Technology, Murthal-131039, Haryana, India
______________________ *
Corresponding author. Tel.: +91-1662-263385; Fax: +91-1662-276240.
E-mail: [email protected] (S. Sanghi)
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ACCEPTED MANUSCRIPT Abstract Ca2+ and Nb5+ co-doped multiferroics having composition Bi0.8Ca0.2Fe1-xNbxO3 (x = 0.00, 0.05, 0.07, and 0.10) were synthesized via solid-state reaction method. Effect of substitution
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was analyzed by structural, electrical and magnetic measurements of modified BiFeO3 multiferroics. The samples were characterized by X-ray diffraction (XRD), and results support that impurity phases, which normally occurred in BFO, were significantly diminished in modified BiFeO3. Rietveld refinement of XRD patterns demonstrates that phase
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transformation occurred in all prepared samples. Structure symmetry gets disturbed in
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presence of Nb and mixed phase (rhombohedral (R3c space group) and orthorhombic phase (Pbnm space group)) was confirmed by refinement. The studies of dielectric, impedance and electrical properties at a wide range of temperature and frequency offered us noticeable information of Nb substitution. Frequency dependence impedance confirms the non-Debye type of relaxation. Nb doping increases resistivity and samples show polarization hysteresis
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loop. All samples show ferromagnetism in modified multiferroics with maximum remanent magnetization is Mr=0.0581 emu g-1 and coercive field is Hc = 4.7505 kOe for x = 0.10 which
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is ten times of magnetization observed in parent sample. As co-substitution at A and B site is
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responsible for phase transformation and enhancing electrical and magnetic properties.
Keywords: Multiferroics; X-ray Diffraction; Rietveld Refinement; Magnetic Properties; Dielectric Properties.
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ACCEPTED MANUSCRIPT 1. Introduction Multiferroics are those materials which possess at least two of ferroics orders out of three (ferroelectricity, ferromagnetism, ferroelasticity) simultaneously in a single phase[1]. Among
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these ferroic orders, ferromagnetism and ferroelectricity are highly desired and coupling between these ferroic orders is essential in enhancing their functionalities[2], which make them useful for potential application in data–storage system, multi-state memories, and spintronics devices. Among a few encouraging multiferroics, BiFeO3 is most significant as it
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possesses magnetic ordering and ferroelectric polarization above room temperature (RT)[3].
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Magnetoelectric coupling and high ferroelectric Curie temperature of about 1100 K (i.e. ferroelectric ordering exist up TC =1100 K), and Neel temperature of about 640 K (i.e. magnetic ordering exist up to TN = 640 K) make it useful for practical applications[3, 4]. BiFeO3 possesses distorted rhombohedral perovskite structure with polar space group R3c[5]. Dipole ordering in bismuth ferrite is due to the relative shift of Bi3+, Fe3+, and O2- ions along
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the [001] hexagonal axis. Dipole order emerges due to the stoichiometric activity of 6s2 electrons pair of bismuth ion. The Bi3+ ion has the 6s2p0 valence electron configuration; the 6s2 electron of Bi3+ ion is hybridized with empty 6p0 orbital’s of Bi3+ and with electron-filled
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2p6 orbital’s of O2- anions to form the Bi-O covalent bond, which is the reason behind
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structural distortion and dipole ordering[6]. BiFeO3 exhibits G-type anti-ferromagnetic ordered spin which is tilted under Dzyaloshinskii-Moriya (D-M) interaction. This D-M interaction diminishes macroscopic ferro-magnetization, and inhibits the existence of linear ME effect [7]. Although weak ferromagnetism is always observed in a pure BiFeO3 which is due to partial destruction of spiral periodicity in grains with limited dimensions[8]. As BiFeO3 is a promising material for storages devices yet it has some drawbacks for room temperature applications, such as the large value of leakage current due to charge defects of Bi3+, the existence of impurity phase like Bi25FeO40 and Bi2Fe4O9, non-stoichiometry and 3
ACCEPTED MANUSCRIPT oxygen vacancies, high dielectric loss and unsaturated magnetic loops. Oxygen vacancies are generated during heat treatment due to volatile nature of bismuth and tendency of Fe ions to exist in multiple oxidation states (Fe3+, Fe2+, and Fe4+)[7]. Several strategies have been adopted for enhancement of ferroelectric and ferromagnetic properties and suppressing
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impurity phase and leakage current in BiFeO3 multiferroics[3, 7]. Various techniques were reported to improve properties of BiFeO3 including doping of aliovalent and isovalent ions at A and B site[7]. Khomchenko et al. [9] have reported effects of divalent ions (Ca2+, Sr2+,
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Pb2+, and Ba2+) doping at A site of BiFeO3 and suggested that there is a strong correlation between ionic radii and ferromagnetism. With increased ionic radius of dopant at A site leads
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to suppression of spiral spin structure and contribute to net magnetization. Zhai et al. [10] have reported structural transition from rhombohedral to monoclinic with La doping at A site of BiFeO3 (BLFO) and enhanced coercive field and remanent magnetization with Nb doping in BLFO. Jun et al. [11] observed an increase in electrical resistivity and ferromagnetic
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behavior in Nb5+ doped BiFeO3. Makhdoom et al. [7] have reported reduced leakage current density and superimposition of weak ferromagnetism with primary antiferromagnetism in Ba2+ and Nb5+ co-doped samples. Reetu et al. [3] have also reported a structural change from
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rhombohedral to triclinic, enhanced magnetic properties, increased resistivity and decreased
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dielectric losses in Sr2+ and Nb5+ co-doped BiFeO3. All these reports suggest that cosubstitution at A and B site suppresses the modulated spiral spin structure and improves multiferroics properties of BiFeO3. In the view of above, in present work effect of cosubstitution of Ca and Nb in BiFeO3 having composition Bi0.8Ca0.2Fe1-xNbxO3 (x = 0.00, 0.05, 0.07, and 0.10) have been investigated. A systematic study regarding the structural change, measurement of electric and magnetic properties has been carried out.
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ACCEPTED MANUSCRIPT 2. Experimental details Polycrystalline multiferroics with composition Bi0.8Ca0.2Fe1-xNbxO3 (BCFNx); x = 0.00, 0.05, 0.07, and 0.10 were synthesized via conventional solid state reaction method. High purity
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reagents (Sigma-Aldrich): Bi2O3 (98%), CaCO3 (99%), Fe2O3 (99%) and Nb2O5 (99.9%) were used as starting materials and weighted in stoichiometry proportions. These materials were mixed thoroughly and were ground in an agate mortar for the homogeneous mixture. Then calcination was performed at 673 K for 6 hrs at a rate of 5 °C/min. Calcined powders of
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all the samples were reground for 30 min and sintered at a temperature range of 1023-1073 K
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followed by immediate quenching in the air after removing samples. Sintering temperature increased with Nb concentration. Crystal structure of obtained samples was recorded using X-ray diffractometer (Rigaku Miniflex-II) with Cu-Kα radiation (λ = 1.5418 Å) with scanning speed of 2 °/min and 2θ range (10–80°) at room temperature. No major impurity phase was observed in the prepared samples. Further, crystal structure was analyzed by Rietveld
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refinement using Full Prof program. For dielectric measurements, the powder was pressed into a disk shape (1 mm thickness, 10 mm diameter) by using PVA (organic binder) by
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applying pressure of ≈ 10 ton using pellet press. Ag paste was applied on these sintered pellets for making electrodes and dielectric measurements were carried out with an
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impendence/gain-phase analyzer (Newton’s 4th Ltd.) in frequency range 100 Hz to 5 MHz and temperature range 323-573 K. Magnetic characterization was performed using Vibrating Sample Magnetometer (Lakeshore VSM 7410) up to field of 15 kOe. Polarization hysteresis measurements were carried out by the P-E analyzer (Marine India).
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ACCEPTED MANUSCRIPT 3. Results and discussions 3.1. Structural Analysis Fig.1 (a) shows XRD patterns of co-doped BCFO, BCFN5, BCFN7, and BCFN10 multiferroics. All the samples are poly-crystalline in nature. A minor impurity peak around
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28ᵒ in BCFN5 sample belongs to Bi2Fe4O9 and doesn’t affect the magnetic properties as basic nature of this Fe rich phase is non-magnetic in nature[12, 13]. Magnified view of XRD patterns (Fig. 1 (b)) revealed that there is shifting of diffraction peaks towards lower angle
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which indicates that unit cell volume increases. This shifting is due to the difference in ionic
also broadened in mixed phase sample.
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radii of ions (Bi3+= 1.17 Å, Ca2+= 1.12 Å; Fe3+= 0.645 Å, and Nb5+= 0.72 Å) and peaks are
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Figure 1 (a) XRD pattern of BCFO, BCFN5, BCFN7 and BCFN10 samples. Diffraction peak indicating impurity phase (Bi2Fe4O9) is marked by *. (b) An enlarged view of XRD data in a 2θ range of (20-40°) shows shifting of peaks.
Crystal structure was also analyzed by the Rietveld refinement by using Fullprof program, which shows good agreement between observed and calculated intensities and demonstrate that with increasing Nb content, amount of rhombohedral phase (space group R3c) decreased and that of orthorhombic (space group Pbnm) phase increased. Refined XRD data for observed, calculated and their difference intensities displayed in Fig. 2. ((a)-(d)). It is 6
ACCEPTED MANUSCRIPT observed that, 20% Ca2+ substitution in bismuth ferrite (BCFO sample) is not able to change basic rhombohedral structure with hexagonal symmetry and has R3c space group. The low value of goodness of fit parameter (χ2) and profile parameters Rwp and Rp (<10%) also agree with these results. Peaks shapes were fitted by Pseudo-Voigt function and background
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parameters were fitted by cosine Fourier series. All co-doped samples were best fitted by mixed rhombohedral (R3c space group) and orthorhombic (Pbnm space group) phase as shown in fig. 2 (a, b, c and d). The crystal symmetry, unit cell parameters and profile
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parameters for all compositions are listed in Table 1. The bond angle and bond distance parameters measured by Rietveld refinement are listed in Table 2. Therefore, Nb content
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plays a significant role in phase transformation due to the simultaneous substitution of aliovalent ions, i. e., Ca2+ and Nb5+ in place of host Bi3+ and Fe3+ ions. Phase transformation may be due to the accommodation of larger and higher valence Nb5+ ion at B site in a
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Figure 2 Rietveld refined XRD patterns for prepared samples Bi0.8Ca0.2Fe1-xNbxO3 (BCFNx): (a) x=0.00; (b) x=0.05; (c) x=0.07; (d) x=0.10.
Figure 3 The schematic view of the crystal structure for (a) BCFO and (b) BCFN10. The distortion in ABO3 type perovskite can be calculated by Goldschmidt’s tolerance factor (t) given by (1)
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ACCEPTED MANUSCRIPT where RA, RB, and RO are ionic radii of atoms at A site (Bi3+ and Ca2+), at B site (Nb5+ and Fe3+), and for oxygen anion respectively. For ideal conditions (cubic structure), t = 1 and deviation from the unity quantifies the stability of the prepared compound, e.g., for the t ≤ 1 structure will be more stable[3, 14]. Fig. 4 shows that tolerance factor decreases with
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progressive doping of Nb5+ which implies that, co-doping is favorable for the stabilization of the crystal structure.
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3.2. Dielectric Analysis
Figs.5 (a)-(d) show the temperature dependent behavior of real part of dielectric constant (ε') and loss tangent (in inset) for all prepared BCFN multiferroics for different frequencies (10
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kHz, 100 kHz, and 1 MHz). It shows the strong dependence of the real part of dielectric constant (ε') and loss tangent (tan δ) on temperature and frequency. Dielectric constant (ε')
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ACCEPTED MANUSCRIPT hopping conduction. At low temperature, dipoles freezes due to relaxation process causing decreased polarization and hence small dielectric constant[5]. Dielectric constant is not an intrinsic property but varies due to charge defect caused by heterovalent substitutions and is further associated with the driven frequencies[15]. Dielectric constant increases with Nb
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content and shows monotonous decreasing behavior with increasing frequency for same temperature range. At low-frequency, defect related dipoles are able to follow alternating current and results in high values of dielectric constant, as the frequency increases these start
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Figure 6 The variation of imaginary part of dielectric constant (εʺ) with temperature at different frequencies
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Fig. 6 illustrates the temperature dependent behavior of dielectric constant at different frequencies. The variation in the value of εʺ is prominent at a lower frequency as compared to a higher frequency. The value of εʺ found to increase sharply with temperature for a fixed frequency due to thermally induced hopping conduction[16].
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Figure 7 Frequency dependence of imaginary part of dielectric constant (εʺ) for all prepared samples at different temperatures.
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Fig. 7 depicts the variation of εʺ with frequencies at different temperature ranges. With
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increasing temperature, εʺ increases for all samples. At lower frequency range, we observe a dispersion behavior and which attains nearly constant value at higher frequencies. Similar behavior is reported previously in BFO by several research groups[16, 17]. These plots were well explained by space charge polarization model of Maxwell-Wagner[18, 19]. Koop’s phenomenological theory also gives conformity of such behavior[20]. Polycrystalline materials exhibit conducting grains and these grains are separated by each other by insulating grain boundaries. The space charge polarization comes into play by the accumulation of grains at grain boundaries by displacement of charge carrier on the application of the field.
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Figure 8 Frequency dependence of dielectric constant (ɛʹ) and dielectric loss (tan δ) for all
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Fig. 8 shows variation of dielectric constant (ɛʹ) and tan δ (in inset) with frequency at 523 K for the samples x=0.0 (BCFO), x=0.05 (BCFN5), x=0.07 (BCFN7) and x=0.10 (BCFN10).
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All samples show high dielectric constant at lower frequency, as large dipole responds at a
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lower frequency and their response ceases at high frequency of applied field. The main reason of interfacial polarization (Maxwell-Wagner) occurrence at grain boundaries is segregation of impurity phases and oxygen vacancies. The large value of the dielectric constant in BCFO is attributed to oxygen vacancy created during charge compensation by Ca2+ doping. Further Nb5+ doping, is also expected to fill the oxygen vacancy or varying cation valence for balancing the charge. Therefore, Nb5+ substitution results in reduced dielectric constant and losses with frequency[7, 33].
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Figure 9 Nyquist plots ((a)-(d)) for BCFO, BCFN5, BCFN7, and BCFN10 at different temperature.
3.3 Impedance analysis
Fig. 9 depicts temperature-frequency dependence of complex impedance spectra (Nyquist plot) for selected temperatures. As complex impedance spectroscopy (CIS) is a unique technique used to estimate the contribution of grain and grain boundaries in electrical processes and how they affect the microstructure of the materials. The Nyquist plots comprise single semicircular arc, which mainly originates due to the effect of grains [21]. With
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semicircle having center located below the real impedance axis suggests that relaxation of ions is non-Debye type in nature[22, 23]. It can be seen from plots that with increasing doping maximum impedance is observed for x=10 which will lead to low value of leakage
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3.4 Conductivity analysis
Figs. 10 ((a)-(d)) illustrate the variation of ac conductivity (σac) for all samples at different temperature in the frequency range 50-106 Hz. All plots give evidence of frequency dispersion, which shifts towards higher frequency with increase in temperature. The
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phenomenon of conductivity dispersion is an important representation as it affects the dielectric properties prominently and generally examined by Jonscher’s power law[25, 26] σac(ω) = σ0 + Aωs
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conduction band. A is frequency independent and temperature dependent factor. The temperature dependent frequency exponent ‘s’, lies between 0 and 1[27]. At lower frequency range, σac consists of plateau region. As temperature is increased, dc conduction dominates and it results in increased flat region in conductivity plot. It is well known that variation of s with temperature shows the type of conduction mechanism. If (i) s is independent of temperature then conduction is based on quantum mechanical tunneling (QMT), if (ii) s decreases with temperature then correlated barrier hopping (CBH) is expected, if (iii) s increases with temperature, then mechanism is correlated with small polaron conduction, and 15
ACCEPTED MANUSCRIPT if (iv) s firstly decreases followed by increased value then dominant conduction mechanism is an overlapping large polaron tunneling (OLPT)[4]. The above system follows power law as confirmed by non-linear curve fitting and goodness of fitting is confirmed by experimental and fitted curve (solid line). It is very clear from the plots (fig. 10 (a)-(d)) that value of s
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increases with temperature, therefore small polaron conduction mechanism is expected. As BCFO shows minimum ‘s’ followed by increasing trend so it is expected to have OLPT mechanism. Fig. 11 depicts the variation of σac with doping and minimum conductivity is
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observed for BCFN10 among all samples which supports that electrical properties are improved in modified system. The highest conductivity is observed for BCFN7 sample may
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be due to increased oxygen vacancies and lowest conductivity for BCFN10 may be due to suppression of lattice conduction path or local lattice distortion or enhancement in barrier properties[14].
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Figure 10 (a)-(d) Frequency dependence of σac (ω) at different temperatures for all samples.
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BCFO BCFN5 BCFN7 BCFN10
s-parameter
0.55 0.50 0.45 0.40 0.35 480
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Figure 11 Variation of exponent ‘s’-parameters with temperature for BCFO, BCFN5, BCFN7, and BCFN10.
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Log σac (Ωm)-1
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Figure 12 Frequency dependence of ac conductivity for Bi0.8Ca0.2Fe1-xNbxO3 samples at 490 K.
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ACCEPTED MANUSCRIPT 3.5. Ferroelectric properties Fig. 13 represents polarization hysteresis (P-E) loops for BCFOx=0.0, BCFN5x=0.05, BCFN7x=0.07 and BCFN10x=0.10 samples at room temperature. Since pure BiFeO3 having high conductivity so it is difficult to observe P-E loop[28]. As co-doped samples are more resistive
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due to less charge defect, therefore these samples show opening of loop. Although observed loops were not saturated due to the existence of leakage current and high coercive field[14, 29]. The breakdown field is different for different samples. In modified bismuth ferrite
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Bi0.8Ca0.2Fe1-xNbxO3 maximum polarization is observed for x=0.05, further addition of Nb content reduced polarization. Improved ferroelectric properties obtained in presence of Nb is
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due to diminishing charge defect (which plays a fundamental role in pinning polarization switching domain) generally originated by volatile nature of bismuth [30]. Secondly, large polarization in co-doped samples might be due to structural changes from rhombohedral to mixed phase (rhombohedral + orthorhombic). Incorporation of Nb at B site noticeably
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enhances ferroelectric properties.
BCFO BCFN5 BCFN7 BCFN10
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Polarization (µ µ C/cm2)
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Figure 13 Polarization hysteresis loop (P-E) of BCFO, BCFN5, BCFN7, and BCFN10 samples measured at room temperature.
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ACCEPTED MANUSCRIPT 3.6. Magnetic Properties Fig. 14 represents the magnetic hysteresis loops (M-H) for all samples Bi0.8Ca0.2Fe1-xNbxO3 (x = 0.0, 0.05, 0.07 and 0.10) up to maximum applied magnetic field of 15 kOe.
Hysteresis
loops indicate non-zero remanent magnetization (Mr) and coercive field (Hc). BiFeO3
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possesses an antiferromagnetic structure with G-type spin ordering modulated cycloidal by spiral spin having a large period of λ = 62 nm. This long range cycloidal modulated spin structure vanishes the ferromagnetism and causes basic antiferromagnetism in BiFeO3[7, 31].
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The parent Bi0.8Ca0.2FeO3 (BCFO) shows a linear variation of magnetization with an applied field, which confirms the antiferromagnetic behavior. The magnetic moment of Fe3+ ion is
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coupled ferromagnetically within the pseudo-cubic plane (111) and anti-ferromagnetically between adjacent planes. Alignment of ferromagnetically ordered planes parallel to [111] plane leads to canting of antiferromagnetic sublattice which collapses the spirally modulated spin structure and results in the release of net magnetization[15]. Indeed, divalent ion doping
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at A site is not so effective in enhancing the magnetization but higher valence Nb5+ at B site is helpful in increasing the ferromagnetism as is clearly observervable in fig.14. 0.3
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Figure 14 Magnetic Hysteresis loops for BCFO, BCFN5, BCFN7 and BCFN10 samples recorded at room temperature.
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ACCEPTED MANUSCRIPT Nb substitution, which is having higher valence and ionic radius as compared to host Fe3+ ion, is responsible for structural transformation which can tilt FeO6 octahedrons[3, 7]. It might be due to the breakdown of balance between antiparallel sublattice magnetization of Fe3+ due to the presence of higher valence Nb5+[11]. As these results are in consonance with
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the structural investigations, i.e., there was the coexistence of two phases (rhombohedral with R3c and orthorhombic with Pbnm) [3, 27]. All hysteresis loops were not saturated up to maximum applied field which may be due to incomplete magnetic transition in a new
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magnetic state or uncompensated antiferromagnetic nature present in the samples[31, 32]. The non-zero value of net magnetization indicates the sign of improved magnetic properties.
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As reported earlier [9, 12, 33] and observed in the present study as that Ca doping is not able to generate the ferromagnetic character in bismuth ferrite. Thus, the enhancement in the magnetic properties in modified BCFO were due to the presence of Nb. Under an applied magnetic field of 15 kOe, the maximum value of remanent magnetization is 0.0660 emu g-1
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and coercive field is 4.7505 kOe for BCFN10. The remanent magnetization Mr and coercive field Hc for all samples are presented in Table 4. Remanent magnetization increased nearly ten times for BCFN10 as compared with parent BCFO and is attributed to the co-doping of
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Ca and Nb simultaneously in BiFeO3.
4. Conclusions
Polycrystalline samples with Ca and Nb substitution having composition Bi0.8Ca0.2Fe1-xNbxO3 (x = 0.00, 0.05, 0.07 and 0.10) were prepared by solid state reaction method. Primary structural investigation was performed by XRD at room temperature. Ca2+ and Nb5+ ions substitution is helpful in reduction of the secondary phases, slight foreign peaks like Bi2Fe4O9 were observed in BCFN5 sample. The sharpness of diffraction peaks was due to a good degree of crystallinity and shifting of diffraction peaks towards lower angle is due to 20
ACCEPTED MANUSCRIPT increased unit cell volume with Nb5+ concentration. Structural parameters investigations described the role of Nb5+ content in phase transformation from rhombohedral (R3c) to mixed phase (R3c+Pbnm). Improved dielectric properties obtained with reduced losses and enhanced resistivity. Magnetic properties investigation shows significant enhancement in
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remanent magnetization, which favors substitution of Ca2+ and Nb5+ in suppressing spiral spin structure of BFO and released locked magnetization, by transforming basic magnetic character antiferromagnetic to ferromagnetic. The results of present work provide a possible
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way in improving multiferroics properties.
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Acknowledgements
Authors are thankful to Department of Science and Technology, Government of India for providing XRD facility through FIST scheme. One of the authors (A. Agarwal) is thankful to the Science and Engineering Research Board, Department of Science and Technology,
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Government of India for providing financial assistance in the form of the research project (Grant No. SR/S2/CMP-44/2012).
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References
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materials, Nature, 442 (2006) 759–765. [2]J. Ma, J. Hu, Z. Li, C. W. Nan, Recent progress in multiferroic magnetoelectric composites: from bulk to thin films, Adv. Mater., 23 (2011) 1062–1087. [3]A. Agarwal, S. Sanghi, N. Ahlawat, Phase transformation, dielectric and magnetic properties of Nb doped Bi0.8Sr0.2FeO3 multiferroics, J. Appl. Phys. 111 (2012) 113917. [4]R. Das, T. Sarkar, K. Mandal, Multiferroic properties of Ba2+ and Gd3+ co-doped bismuth ferrite: magnetic, ferroelectric and impedance spectroscopic analysis, J. Phys. D. Appl. Phys.,
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ACCEPTED MANUSCRIPT 45 (2012) 455002. [5]R. Singh, G. D. Dwevedi, P. Shahi, D. Kumar, O. Prakash, A. K. Ghosh, S. Chatterjee, Effect of Pr- and Nd- doping on structural, dielectric, and magnetic properties of multiferroic Bi0.8La0.2Fe0.9Mn0.1O3, J. Appl. Phys., 115 (2014) 134102.
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[7]A. R. Makhdoom, M. J. Akhtar, M. A. Rafiq, M. Siddique, M. Iqbal, M. M. Hasan, Enhancement in the multiferroic properties of BiFeO3 by charge compensated aliovalent
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[8]Z. X. Cheng, X. L. Wang, Y. Du, S. X. Dou, A way to enhance the magnetic moment of multiferroic bismuth ferrite, J. Phys. D. Appl. Phys., 43 (2010) 242001. [9]V. A. Khomchenko et al., Effect of diamagnetic Ca, Sr, Pb, and Ba substitution on the
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crystal structure and multiferroic properties of the BiFeO3 perovskite, J. Appl. Phys., 103
[10]L. Zhai, Y. G. Shi, S. L. Tang, L. Y. L. and Y. W. Du, Large magnetic coercive field in
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Bi0.9La0.1Fe0.98Nb0.02O3 polycrystalline compound, J. Phys. D. Appl. Phys., 42 (2009) 165004.
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[11]Y. K. Jun, W. T. Moon, C. M. Chang, H. S. Kim, H. S. Ryu, J. W. Kim, S. H. Hong, Effects of Nb-doping on electric and magnetic properties in multi-ferroic BiFeO3 ceramics, Solid State Commun. 135 (2005) 133–137. [12]M. Rangi, A. Agarwal, S. Sanghi, R. Singh, S. S. Meena, A. Das, Crystal structure and magnetic properties of Bi0.8A0.2FeO3 (A=La, Ca, Sr, Ba) multiferroics using neutron diffraction and Mossbauer spectroscopy, AIP Adv., 4 (2014) 087121. [13]F. Azough, R. Freer, M. Thrall, R. Cernik, F. Tuna, D. Collison, Microstructure and properties of Co-, Ni-, Zn-, Nb- and W-modified multiferroic BiFeO3 ceramics, J. Eur.
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ACCEPTED MANUSCRIPT Ceram. Soc., 30 (2010) 727–736. [14]S. Dash, R. N. P. Choudhary, P. R. Das, A. Kumar, Effect of KNbO3 modification on structural, electrical and magnetic properties of BiFeO3, Appl. Phys. A, 118 (2015) 10231031.
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[15]M. S. Wu, Z. B. Huang, C. X. Han, S. L. Yuan, C. L. Lu, S. C. Xia, Enhanced multiferroic properties of BiFeO3 ceramics by Ba and high-valence Nb co-doping, Solid state commun. 152 (2012) 2142–2146.
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[16]P. Godara, A. Agarwal, N. Ahlawat, S. Sanghi, Crystal structure refinement, dielectric and magnetic properties of Sm modified BiFeO3 multiferroic, J. Mol. Struct., 1097 (2015)
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207–213.
[17]R. K. Mishra, D. K. Pradhan, R. N. P. Choudhary, A. Banerjee, Dipolar and magnetic ordering in Nd-modified BiFeO3 nanoceramics, J. Magn. Magn. Mater. 320 (2008) 2602– 2607.
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[18]J. C. Maxwell, 1929 Electricity and Magnetism vol. 1(Oxford: Oxford University Press)
[19]K. W. Wagner 1913 Ann. Phys., Lpz. 40, 817.
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[20]C. G. Koops 1951 Phys. Rev. 83, 121.
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[21]S. Pattanayak, R. N. P. Choudhary, D. Pattanayak, A comparative study of structural, electrical and magnetic properties rare-earth (Dy and Nd)-modified BiFeO3, J. Mater. Sci. Mater. Electron. 25 (2014) 3854–3861. [22]R. Kumari, N. Ahlawat, A. Agarwal, S. Sanghi, M. Sindhu, “Structural transformation and investigation of dielectric properties of Ca substituted (Na0.5Bi0.5)0.95-xBa0.05CaxTiO3
ceramics, J. Alloys Compd. 695 (2017) 3282–3289. [23]S. Rani, S. Sanghi, N. Ahlawat, A. Agarwal, Influence of Bi2O3 on physical, electrical and thermal properties of Li2O·ZnO·Bi2O3·SiO2 glasses, J. Alloys Compd., 619 (2015) 659– 23
ACCEPTED MANUSCRIPT 666. [24]Y. P. Jiang, X. G. Tang, Q. X. Liu, D. G. Chen, C. B. Ma, Improvement of electrical conductivity and leakage current in co-precipitation derived Nd-doping BiFeO3 ceramics, J. Mater. Sci. Mater. Electron. 25 (2014) 495–499.
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[25]R. Dahiya, A. Agarwal, S. Sanghi, A. Hooda, P. Godara, Structural, magnetic and dielectric properties of Sr and V doped BiFeO3 multiferroics, J. Magn. Magn. Mater. 385 (2015)175–181.
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[26] M. Rangi, S. Sanghi, S. Jangra, K. Kaswan, A. Agarwal, Effect of Mn doping on crystal structure, dielectric and magnetic ordering of Bi0.8Ba0.2FeO3multiferroic, Ceram. Int., 42,
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(2016) 5403–5411.
[27]P. Godara, A. Agarwal, N. Ahlawat, S. Sanghi, R. Dahiya, Crystal structure transformation, dielectric and magnetic properties of Ba and Co modified BiFeO3 multiferroic, J. Alloys Compd., 594, (2014)175–181.
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[28]R. Gupta, J. Shah, S. Chaudhary, S. Singh, R. K. Kotnala, Magnetoelectric couplinginduced anisotropy in multiferroic nanocomposite (1-X)BiFeO3-xBaTiO3, J. Nanoparticle Res., 15 (2013) 1–9.
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[29]A. Sharma, R. K. Kotnala, N. S. Negi, Observation of multiferroic properties and
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magnetoelectric effect in (x)CoFe2O4-(1-x)Pb0.7Ca0.3TiO3 composites, J. Alloys Compd. 582 (2014) 628–634.
[30]A. Mukherjee, M. Banerjee S. Basu, L. A. W. Green, N. T. K. Thanh, M. Pal, Enhanced multiferroic properties of Y and Mn codoped multiferroic BiFeO3 nanoparticles, Physica B 448 (2014) 199–203. [31]H. Singh, K. L. Yadav, Effect of Nb substitution on the structural, dielectric and magnetic properties of multiferroic BiFe1−xNbxO3 ceramics, Mater. Chem. Phys. 132 (2012)17–21.
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ACCEPTED MANUSCRIPT [32]I. O. Troyanchuk, N. V. Tereshko, D. V. Karpinsky, A. L. Kholkin, M. Kopcewicz, K. Bärner, Enhanced piezoelectric and magnetic properties of Bi1− xCaxFe1−x/2Nbx/2O3 solid solutions, J. Appl. Phys., 109 (2011) 114102. [33]Reetu, A. Agarwal, S. Sanghi, Ashima, N. Ahlawat, Improved dielectric and magnetic
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properties of Ti modified BiCaFeO3 multiferroic ceramics, J. Appl. Phys., 113(2013) 023908.
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ACCEPTED MANUSCRIPT Table 1 Refined structural parameters of BCFO, BCFN5, BCFN7, and BCFN10 samples.
Sample Structural model Cell parameters
Position coordinates x
y
R-Factors
z
(%)
BCFO a=5.5496Å Bi/Ca (0,0, 0.2676) c=13.5949Å Fe/Nb(0,0, -0.003) V=362.609Å3 O (0.9927, 0.5149, 0.3234)
BCFN5 Rhombohedral (R3c 73.11%)
a=5.5658Å Bi/Ca(0,0, 0.2193) Rp =3.86 c=13.806 Å Fe/Nb( 0, 0, -0.0023) Rwp=5.03 V=370.413Å3 O(0.8484, 0.6337, 0.4296) χ2 = 3.03 a=5.5583Å Bi/Ca (-0.0111, 0.0107, 0.2500) b=5.5179Å Fe/Nb(0, 0.5, 0) c=7.8558 Å O1(0.5963, 0.0056,0.2500) V=240.093Å3 O2(0.4525, 0.1917, 0.1801)
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Orthorhombic (Pbnm 26.89%)
BCFN7 Rhombohedral (R3c 55.83%)
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a=5.5693Å Bi/ Ca(0, 0, 0.2243) Rp =3.64 c= 13.806Å Fe/Nb(0,0, 0.0006) Rwp=4.66 V=370.890Å3 O ( 0.8835, 0.6416, 0.4351) χ2=2.58 a=5.5679 Å Bi/ Ca( 0.0134, 0.0166, 0.25) b= 5.5207Å Fe/Nb(0, 0.5, 0) c=7.8644Å O1(0.5414, 0.0038, 0.25) V=241.74Å3 O2(0.7075, 0.3922, 0.0426)
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Orthorhombic (Pbnm 46.56%)
a=5.5670 Å Bi/Ca(0, 0, 0.2225) Rp= 4.53 c=13.798Å Fe/Nb(0, 0,0.0009) Rwp=5.82 3 χ2 = 3.72 V= 370.360Å O(0.7682, 0.611, 0.4321) a=5.5089 Å Bi/ Ca (0.0139, 0.0369, 0.25) b=5.5890 Å Fe/Nb (0, 0.5, 0) c=7.8607Å O1(0.5581, 0.0016, 0.25) V=242.63Å3 O2(0.3031, 0.2809, 0.0282)
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Orthorhombic (Pbnm 44.17%)
BCFN10 Rhombohedral (R3c 53.44%)
Rp=4.45 Rwp=5.86 χ²=4.60
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Rhombohedral (R3c 100%)
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ACCEPTED MANUSCRIPT Table 2 Bond length and bond angles of Bi0.8Ca0.2Fe1-xNbxO3 (BCFNx); x=0.00, 0.05, 0.07, and 0.10 samples obtained by refinement.
Bond angle ( ᵒ )
Sample x=0.0 Bi−O 2.777, 2.976, 2.227 Fe−O 2.692, 1.576
Bi−O−Bi Bi−O−Fe
Sample Bi−O2 Fe−O1 Fe−O2
Bi−O−Bi 115.80 Bi−O2−Fe 94.56, 76.850 O1−Fe−O2 149.94, 137.25
x=0.05 2.817, 1.872 2.035 1.786, 2.739
Sample x=0.07 Bi−O2 2.592, 2.154 Fe−O1 1.979 Fe−O2 1.766, 2.477
149.36, 96.846 132.19, 95.06
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Bond length (Å)
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Bi−O2−Bi 160.79, 97.558 Bi−O2−Fe 81.563, 91.488 O1−Fe−O2 88.477, 91.523, 91.55, 88.448
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Sample x=0.10 Bi−O2 2.819, 2.614, 2.537 Fe−O1 1.991 Fe−O2 2.082, 1.920
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Bi−O2−Bi 111.85 Bi−O2−Fe 98.64, 81.66 O1−Fe−O2 101.44, 107.01, 78.56
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ACCEPTED MANUSCRIPT Table 3 Fitting parameters of BCFNx (x=0.0, x=0.05, x=0.07 and x=0.10) at different temperatures. Samples Temperature (K) _____________________________________________________________ 483 K 503K 523K 543K 563K BCFO 4.65E-05 1.06E-06 0.37066
1.13E-04 1.35E-06 0.37336
2.93E-04 9.51E-07 0.4282
σ0 A s
2.06E-05 3.03E-07 0.43434
1.57E-05 2.19E-07 0.44441
4.07E-05 2.56E-07 0.47132
1.05E-04 1.58E-07 0.53988
BCFN7 σ0 A s
2.78E-05 1.73E-06 0.35488
4.16E-05 1.05E-06 0.38785
8.36E-05 7.25E-07 0.42468
BCFN10 σ0 5.70E-06 A 3.59E-07 s 0.40011
1.40E-05 4.61E-07 0.41017
7.85E-04 3.08E-07 0.5524
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2.38E-05 4.94E-07 0.40417
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3.56E-03 1.18E-06 0.55302
Sample
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Table 4 Magnetic parameters of Bi0.8Ca0.2Fe1-xNbxO3 (x = 0.0, 0.05, 0.07, and 0.10).
x=0.00
0.8852
x=0.05
0.8836
0.0255
1.4735
x=0.07
0.8829
0.0322
2.2122
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0.8819
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Fig. 8
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ACCEPTED MANUSCRIPT 700
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Z " (M Ω )
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493 K 513 K 533 K 553 K
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Z′ (kΩ)
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Z ″ (k Ω )
200
M AN U
493 K 513 K 533 K 553 K
(c)
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Z' (kΩ)
1.6 300
300
SC
0
RI PT
400
493 K 513 K 533 K 553 K
(b)
900
Z" (k Ω )
(a)
500
Z″ (kΩ )
493 K 513 K 533 K 553 K
Fig. 9
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Z'(MΩ)
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ACCEPTED MANUSCRIPT 0.01
0.01
483 K 503 K 523 K 543 K 563 K Fitted curve
1E-3
(b)
σ ac (Ω Ω m)-1
σ ac(Ω Ω m)-1
1E-3
1000
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1E-5
1000000
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ω (Hz)
(c)
1E-3
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M AN U
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483 K 503 K 523 K 543 K 563 K Fitted curve
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1E-4
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σ ac(Ω Ω m)-1
483 K 503 K 523 K 543 K 563 K Fitted curve
(a)
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Fig. 10
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ω (Hz)
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ACCEPTED MANUSCRIPT 0.60 BCFO BCFN5 BCFN7 BCFN10
0.50
0.45
0.40 0.35 500
520
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M AN U
Temperature (K)
560
SC
480
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s-param eter
0.55
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Fig. 11
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BCFO BCFN5 BCFN7 BCFN10
RI PT
-4.0
-4.5
-5.0 2
3
4
5
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Log σ ac ( Ω m ) -1
-3.5
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0.2
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-0.4 -20
0
40
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Field (kV/cm)
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0.3
M agnetization (emu/g)
0.1 0.0 -0.1 -0.2 -0.3
-10
-5
0
5
15
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Magnetic Field (kOe)
10
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0.2
AC C
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TE D
Fig. 14
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ACCEPTED MANUSCRIPT
Figure captions: Figure 1 (a) XRD pattern of BCFO, BCFN5, BCFN7 and BCFN10 samples. Diffraction peak indicating impurity phase (Bi2Fe4O9) is marked by *. (b) An enlarge view of XRD data in a 2θ range of (20-40°) shows clearly diffraction peaks shifting.
RI PT
Figure 2 The results of Rietveld Refined XRD pattern for prepared samples Bi0.8Ca0.2Fe1xNbxO3 (BCFNx): (a) x=0.00; (b) x=0.05; (c) x=0.07; (d) x=0.10. Figure 3 The schematic view of the crystal structure for (a) BCFO and (b) BCFN10. Figure 4 Tolerance Factor vs concentration for all prepared samples.
SC
Figure 5 Variation of the real part of dielectric constant (εʹ) and loss tangent (tan δ) (in inset) with the temperature at different frequencies.
M AN U
Figure 6 The variation of imaginary part of dielectric constant (εʺ) with temperature at different frequencies. Figure 7 Frequency dependence of imaginary part of dielectric constant (εʺ) for all prepared samples at different temperatures. Figure 8 Frequency dependence of dielectric constant (ɛʹ) and dielectric loss (tan δ) for all samples at 523 K.
TE D
Figure 9 Nyquist plots ((a)-(d)) for BCFO, BCFN5, BCFN7, and BCFN10 at different temperature. Figure 10 (a)-(d) Frequency dependence of σac (ω) at different temperatures for all samples.
EP
Figure 11 Variation of exponent ‘s’-parameters with temperature for BCFO, BCFN5, BCFN7, and BCFN10.
AC C
Figure 12 Frequency dependence of ac conductivity for Bi0.8Ca0.2Fe1-xNbxO3 samples at 490 K. Figure 13 Polarization hysteresis loop (P-E) of BCFO, BCFN5, BCFN7, and BCFN10 samples measured at room temperature. Figure 14 Magnetic Hysteresis loops for BCFO, BCFN5, BCFN7 and BCFN10 samples collected at room temperature.
43
ACCEPTED MANUSCRIPT Highlights 1. Ca2+ and Nb5+ co-doped BiFeO3 shows improved magnetic structure. 2. Crystal structure has been analyzed using Rietveld refinement.
AC C
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TE D
M AN U
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3. Multiferroic properties are also improved.