- Email: [email protected]
Structural and impedance analysis of Bi0.5Na0.5Ti0.80Mn0.20O3 ceramics
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Structural and impedance analysis of Bi0.5Na0.5Ti0.80Mn0.20O3 ceramics Neeha Pradhani∗, P.K. Mahapatra, R.N.P. Choudhary Department of Physics, Multiferroic Advanced Materials Laboratory, Siksha ‘O' Anusandhan (Deemed to be University), Bhubaneswar, 751030, India
A R T I C LE I N FO
A B S T R A C T
Keywords: XRD FTIR Impedance spectroscopy Conduction mechanism
The Bi0.5Na0.5Ti0.80Mn0.20O3 ceramic was synthesized by a conventional solid-state reaction technique. Rietveld refinement of X-ray diffraction data confirms the rhombohedral crystal structure of the compound with R3c space group. The optical band gap energy of the compound is found to be 1.93 eV. The substitution of 20% Mn ions at the Ti sites results in the improved dielectric characteristics and a shift in the ferroelectric to paraelectric electric phase transition peak from 330 °C to 370 °C in the material. The frequency dispersion of dielectric constant and its footprint in the Nyquist and Cole-Cole plots have been analyzed. The analysis of complex impedance and modulus spectroscopy confirms the non-Debye type of relaxation mechanisms in the material with contributions from both the grain and grain boundary to the electrical properties. The frequency dependence of AC conductivity data exhibits overlapping large polaron tunneling conduction mechanism in the compound.
1. Introduction Bismuth sodium titanate, Bi0.5Na0.5TiO3 (BNT) a complex A-site (Bi3+ and Na+) perovskite ferroelectric of a general formula ABO3, is a step forward for phasing out the use of hazardous lead-based ferroelectric materials. The similar electronic configuration (4f145d106s2) of Bi3+ and Pb2+ with a stereochemically active lone pair of 6s electrons is accountable for high polarization in the ferroelectric compounds containing these ions [1,2]. The chemically distorted ferroelectric perovskites generally possess relaxor nature distinguished by broad maxima around the transition temperature. The chemical distortion in the compound produces local random fields, which induce polar nano regions [3]. Relaxor properties make the compound more technologically attractive for applications like transducers and actuators [4,5]. The BNT exhibits two diffused phase transitions corresponding to the (i) ferroelectric to the relaxor-ferroelectric and (ii) relaxor-ferroelectric to paraelectric at around 190 °C and 330 °C respectively [6]. The technological applications of BNT have some limitations due to the relatively high coercive field, remanent polarization and conductivity [7,8]. Manganese (Mn), with various stable oxidation states of + 4, + 3 and + 2, is one of the most useful additives at the B-site of the perovskite because of the smaller ionic radius of Mn4+ (0.53 Å) than that of Ti4+ (0.605 Å) in octahedral coordination and almost same electronegativity (Mn (1.55) and Ti (1.54)) [9,10]. The multivalent state of Mn is expected to accommodate oxygen vacancy created during sintering process. The manganese in oxide form is characterized by a high
∗
dielectric constant, being non-toxic and environmentally friendly is considered as one of the promising metal oxides for pseudocapacitors. These characteristics are also suitable for electrode material in electrochemical supercapacitors. The doping of Mn ions in some perovskites has been reported to reduce the temperature of sintering and promote the growth of the grain [11,12]. The substitution of Mn4+ ion in perovskite materials is utilized for effecting an increase in Curie temperature, a broad range of frequency response and higher anisotropy in coupling factors [13,14]. The substitution/doping mechanism establishes a new molecular orbital and introduces an impurity level to narrow down the energy band gap [15,16]. Looking at the promising improvements in the material properties with the substitution of Mn in BNT, we were motivated to fabricate Bi0.5Na0.5Ti0.80Mn0.20O3 by a low cost standard ceramic technology (high-temperature solid-state reaction method) and investigate the structural, optical, dielectric and electrical characteristics of the synthesized compound. 2. Experimental and characterization techniques The manganese modified BNT i.e. Bi0.5Na0.5Ti0.80Mn0.20O3 was fabricated by a solid-state reaction method. The stoichiometric quantity of Bi2O3 (99.0%, Himedia), Na2CO3 (99.5%, CDH), TiO2 (99.5%, Loba Chemie) and MnO2 (99.9%, Loba Chemie) were thoroughly mixed using a mortar pestle in the dry mode as well as in the wet mode taking methanol for 3 h. The fine homogeneous powder was calcined at 900 °C for 4 h using a high-temperature furnace. The structural characteristics
Corresponding author. E-mail address: [email protected] (N. Pradhani).
https://doi.org/10.1016/j.ceramint.2019.10.128 Received 31 August 2019; Received in revised form 14 October 2019; Accepted 14 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Neeha Pradhani, P.K. Mahapatra and R.N.P. Choudhary, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.128
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
of the calcined powder were analyzed by an X-ray diffractometer (Rigaku Ultima IV) using Cu-Kα radiation (λ = 1.5406 Å). The diffraction pattern was obtained in a range of the Bragg diffracted angle 20°80° with a step scan of 0.02°. The surface microstructures of the sample were studied by field emission scanning electron microscope (FESEM) (ZEISS). The compositional analysis of the compound was analyzed using energy-dispersive X-ray spectroscopy (EDX). To corroborate the structural analysis, the presence of various vibration bonds in the sample was identified by Fourier transform infrared (FTIR) spectroscopy carried out at room temperature. The optical energy gap of the specimen was determined by UV–Visible optical spectroscopy (PerkinElmer Lambda 1050). The pellets were sintered at 1000 °C for 4 h. The dielectric and electrical properties were measured through LCR meter (N4L, Impedance Analysis Interface, Model PSM-1735), in a wide range of temperature. Marine India PE loop tracer was used to measure the ferroelectric properties of the compound. 3. Results and discussions
Fig. 1b. (b) Rietveld refinement of Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
3.1. Analysis of the structure
corresponding to the most intense diffraction peak (2θ = 32.520) in the Scherrer relation, the crystallite size amounts to 53.01 nm. The surface micrograph of the sintered sample, recorded using FESEM, as shown in Fig. 1(c), exhibits uniformly distributed well-defined grains with small voids on the surface of the sample. The theoretical (calculated using the values of the lattice parameters) density and measured density (using the Archimedes principle) of the compound amounts to 6.014 g/cc and 5.352 g/cc respectively. The measured bulk density of the ceramic compound is 89% of the theoretical density. The compositional (elemental) analysis of the compound obtained from the EDX spectroscopy is shown in Fig. 1(d). The EDX spectrum confirms the presence of all the constituent element Bi, Na, Ti, Mn and O and absence of any other impurity. The Weight % and the atomic % of the elements present in the compound (as provided by the EDX spectroscopy) together with the atoms per formula unit is calculated as the ratio of the atoms of each element to that of the sum of all cations multiplied by a weight factor 2 and the expected atomic ratio on the basis of the stoichiometry of the compound is provided in the Table 1. The atoms per formula unit were calculated from the EDX spectrum for the cations are nearly equal to those expected on the basis of the formula unit of the compound. The elemental color map presented in Fig. 1(e) indicates the uniform distribution of all the elements in the compound. The FTIR spectrum of the compound is recorded to identify the chemical bonds of the constituent elements and to corroborate the structural analyses. The FTIR spectrum in the wavenumber range of 400 cm−1–2000 cm−1 together with the blown-up image of the spectrum in the range of 400 cm−1–700 cm−1 as the inset, is presented in Fig. 1(f). In the BNTM compound, the broad absorption band from 441 cm−1 to 457 cm−1 is being assigned to the Bi–O vibration bonds in the distorted BiO6 unit [19–21]. The absorption peaks observed between 480 cm−1-518 cm−1, correspond to the Mn/Ti-O vibration bond at octahedral sites [22]. The Mn-O and Ti–O stretching vibrational bands in BNTM are found at 483 cm−1 and 518 cm−1 respectively
The XRD spectrum of the calcined sample is shown in Fig. 1(a). The Rietveld refinement of the specimen using MAUD software is presented in Fig. 1(b). The refinement of the observed XRD spectrum was carried out by keeping the structural factors, (i.e., preferred orientation, profile, background, Rietveld profile factors, and unit cell) similar to those of the reference structural model. The refinement of the XRD spectrum is carried out by using the crystallographic information file of BNT i.e. CIF: NIMS_MatNavi_4295517178_1_2, with rhombohedral structure and R3c space group. The values of the structural refined factors like Rwp (weighted profile factor), Rb (Bragg R factor), Rexp (expected weighted profile factor), and the goodness of fit (GoF) which is equal to the ratio Rwp/Rexp are inserted in Fig. 1 (b). The observed and calculated XRD spectrum is represented by the blue and black color lines respectively. The black line in the bottom box represents the difference between the observed and the calculated intensity. The red symbol shows the position of the Bragg reflection peaks. The matching of the experimental spectrum with that of the reference model and the values of the refinement parameters indicate the compound formation in a singlephase rhombohedral (R3c) structure. The refined cell parameters are obtained as, a = b = 5.498 Å and c = 13.494 Å. The crystallite size (t) 0.94λ of the compound is calculated using the Scherrer relation, t = βcosθ , where, 2θ , β and λ are the diffraction angle, full width at half maxima and the wavelength of radiation respectively [17,18]. Using the data
Table 1 Elemental Weight and Atomic percentage of Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
Fig. 1a. (a) XRD spectrum. 2
Element
Weight%
Atomic %
Atoms per formula unit
Expected atoms per formula unit
Bi Na Ti Mn O
51.15 5.54 17.75 5.07 20.49
10.98 10.80 16.63 4.14 57.45
0.516 0.508 0.782 0.195 2.700
0.5 0.5 0.80 0.20 3
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
expressing the values of K and c in cgs units and μ in atomic mass units (u) the expression of wave number in cm−1 corresponding to the vibration bond reduces to
ν¯ = 1303
K μ
(3) 1 105) 2 /2πc ,
Na being the wherein the numerical constant 1303 = (Na × Avogadro's number. Generally the force constant, K is related to bond length, r, through Badger's rule expressed as, K = a (r-d) −3. Where, d and a, are the parameters that depend on the period number in the periodic table in which the elements corresponding to the cation of the bond is located. The equations of force constant for second, third and fourth period compounds appear as K = 3.0 (r-0.61)−3, K = 2.7 (r0.87)−3 and K = 2.6 (r-0.96)−3 respectively. In our perovskite compound, the A site is shared equally between the Na and Bi ions while the B site is shared by Ti and Mn in the 4:1 ratios. In the event of a lattice site is being shared by two ions (1 and 2) with an occupation ratio of x and (1-x) respectively, the effective mass of the cation at that site is taken as, mc = xm1 + (1 − x ) m2. On the basis of these relations, we have calculated the force constant and the bond length of the Bi/Na-O and Ti/Mn-O bonds. The bond lengths presented in Table- 2 matches well with the corresponding reported values [28–30]. 3.2. Optical properties
Fig. 1c. (c) FESEM micrograph.
The UV–Visible absorbance spectra and the Tauc plot ((αh ν)2 vs. Eg ) for the ceramic compound is shown in Fig. 2. Wood and Tauc equation is used for the calculation of band gap energy [31,32], (αh ν)2 = B (hν − Eg ) , where, the symbols have the usual definition [33]. The direct energy gap of the BNTM compound as obtained from Tauc Plot amounts to 1.93 eV which is smaller than that of pure BNT. It is pertinent to note here that the dissociation energy of the Mn-O bond (402 kJ/mol) is smaller than that of the Ti-O bond (662 kJ/mol). Further, the bond length of the Mn-O bond is higher to that of the Ti-O bond as predicted in Table- 2. Consequently, the lattice parameters of the Mn doped BNT are found to be higher to that of pure BNT [34]. The increased lattice parameters results in a smaller Brillouin zone and likely to contribute to the decrease in the energy gap. Further, the weaker Mn-O bond might also result in a smaller lattice potential effecting a decrease in the energy gap. The theoretical calculations based on the density functional theory also suggest a decrease in the energy gap of Mn doped perovskite compounds [35]. 3.3. Impedance characteristics Fig. 1d. (d) EDX spectrum.
The impedance measurement of the ceramic is analyzed to identify the grain (bulk), the grain boundary and the electrode interface contributions to the electrical properties of the materials. The effect of each of these contributions is being represented through an electrical circuit consisting of capacitance (C) and resistance (R0) connected in parallel. The equivalent electrical circuit of the specimen is represented through the series connection of the parallel R0C circuits with respect to each of these effects. In homogeneous systems, like single crystals where the capacitance is assumed to be the ideal impedance corresponding to each of these effects is represented by:
[23–26]. The absorption peak at 878 cm−1 is assigned to the Na-O banding of the Na-O-Ti bond [27]. To further ascertain the observed transmission dips with the identified bond vibrations, the force constant and the bond length associated with the Na/Bi-O and the Ti/Mn-O bonds are calculated and compared with those reported for the similar structure under similar coordination. In vibrational spectroscopy, wave number, ν¯ , for a diatomic molecule is expressed as
ν¯ =
1 2πc
K μ
Z= (1)
m c ma m c + ma
(4)
where, the relaxation time, τ = R 0 C and ω being the angular frequency of the applied field. The complex impedance (Z''∼Z′) plot which is also known as a Nyquist plot for each of the contributions, in such a situation, form a semicircular plot of radius R0/2 with the center on the real (Z′) axis at (R0/2, 0) and intersections at (0,0) and (R0, 0) corresponding to impedance values at infinite and zero field frequency respectively. The peak of the complex impedance plot (Nyquist plot)
where K is the force constant, c is the speed of light and μ is the reduced mass of the pair of atoms in the bond. The reduced mass of the bond containing the cation and anion can be obtained through the relation:
μ=
R 0 − jR 0 ωτ 1 + (ωτ )2
(2)
mc and ma being the mass of the cation and the anion of the bond. In 3
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Fig. 1e. (e) Elemental color mapping of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. Tauc plot of Bi0.5Na0.5Ti0.80Mn0.20O3 at room temperature.
However, the inhomogeneity in the size of the grains and electromagnetic diffusion in the ceramic sample results in the distribution in the relaxation time and the Nyquist plot for each of the three effects manifest through tilted semicircles where the center of the circle is located below the real (Z′) axis. The depression angle made by the line joining the center of the semicircle with its high-frequency intersection on the real axis is a measure of the distribution of relaxation time in the sample. In such systems, the capacitance is complex and is being expressed as, C = C e−jθ , where θ is the angle made by C with the real component C '. The impedance in such a case appears as,
Fig. 1f. (f) FTIR spectrum of Bi0.5Na0.5Ti0.80Mn0.20O3 at room temperature. Table 2 Vibrational Parameters from FTIR spectrum of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. Vibration Bond
Bi/Na-O Bi-O Ti/Mn-O Ti-O Mn-O
Observed wave number (cm−1) 453 441 505 518 483
Force constant (N cm−1) 1.699 1.702 1.833 1.895 1.702
Bond length (Å)
Z=
R 0 (1 + ωτ sinθ) − jR 0 ωτ cosθ 1 + (ωτ )2 + 2ωτsinθ
(5)
The Nyquist plot forms a tilted semicircle of radius
2.112 2.111 2.084 2.071 2.111
(
R
−R sinθ
R0 2cosθ
with center
)
0 at 20 , 2cosθ and intersections at (0, 0) and (R0, 0) corresponding to impedance values at infinite and zero field frequency respectively. Here, the relaxation time, τ = R 0 C , where C = (C ′2 + C ′′2 ) . C′ and C″ are associated with the electrical energy storage and dissipation in the equivalent capacitor of the compound. The peak position of the R (1 − sinθ) R , and corresemicircle has the coordinates Z′ = 0 ; Z′ ′ = 0
occurs at (R0/2, R0/2) corresponding to the condition ωτ = 1. Such perfect semicircular plots indicate ideal Debye relaxation with mono relaxation time.
(
2
2cosθ
)
sponds to the condition, ωτ = 1. The positions of the intersection and the peak confirms to the condition that the depression angle made by 4
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Table 3 Rg, Cg, Rgb, and Cgb at various temperatures of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. Temperature (oC)
Rg (Ω)
Cg (F)
Rgb (Ω)
Cgb (F)
50 100 150 200 250 300
1.073E+006 1.646E+005 3.744E+004 8.778E+003 3.063E+003 9.899E+002
1.663E-010 1.836E-010 1.835E-010 1.814E-010 1.644E-010 1.295E-010
– 5.352E+004 6.022E+003 3.753E+002 1.430E+001 1.519E+001
– 3.199E-009 5.816E-009 2.108E-009 3.971E-005 1.032E-005
temperature range of 50oC–300 °C. The plot is fitted to the equivalent circuit, i.e., RQC for the temperature at 50 °C and (RQC) (RC) for 100 °C–300 °C using the ZSIMPWIN software. The single RQC fitting confirms the grain contributions at 50 °C and the series circuit, confirming the grain and the grain boundary contribution to the electrical properties of the compound at higher temperatures. The obtained parameters, i.e. grain resistance (Rg), capacitance (Cg), grain boundary resistance (Rgb) and capacitance (Cgb), are presented in Table 3. To have a clear picture on the depression angle of the semicircular arcs in the Nyquist plot, semicircular curves for the temperature 50 °C and 100 °C are plotted in Fig. 4 (c) and (d), by considering three pairs of experimental data Z′ and Z′ ′ from each of the grain and the grain boundary region, and fitted with the circle equation. The depression angle at the temperature of 100 °C for the grain mechanism (θ g) and grain boundary mechanism (θ gb) is found to be 13.31° and 23.54° respectively. It is worth reiterating that the depression angle measures the relaxation
Fig. 3. The equivalence of the depression angle of the Nyquist plot with the phase angle of complex capacitance.
the semicircle in the Nyquist plot is equal to the angle made by the complex capacitance with its real component as shown in Fig. 3. The tilted semicircles in the Nyquist plot can be explained by RQC network, i.e., a parallel connection of R and C with a constant phase element Q (see Table 2). Fig. 4 (a) and (b) represents the Nyquist plot of BNTM in the
Fig. 4. (a) and (b): Z′ ′ vs. Z′ plot with the fitting of an equivalent circuit, (c) and (d) Nyquist plot of Bi0.5Na0.5Ti0.80Mn0.20O3 compound at 50 °C and 100 °C. Inset of Fig. 4 (a) complex admittance plot of the sample for the temperature 50 °C. 5
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Fig. 5. (a): Variation of Rg and Rgb with 1000/T, (b) lnRg and lnRgb with 1000/T to calculate the activation energy for Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
the presence of the temperature dependent relaxation mechanism in the material. The relaxation peak at the frequency, ωmax = 2πfmax, shift to high-frequency with an increase in temperature. The shifting of the peak in Z″ indicates the relaxor behavior in the compound. The asymmetric nature of the peak broadening suggests the relaxation time distribution and indicate a non-Debye type of relaxation mechanism [37,38] in the compound.
distribution parameter and is equal to the phase angle made by the corresponding capacitance. The complex admittance plot of the sample for the temperature 50 °C is shown as the inset of Fig. 4 (a). The point of intersection of the graph on the real axis Y′ correspond to the DC conductance (G0) which amounts to around 1.1 μS and is in conformity with the values of DC resistance obtained from the Nyquist plot. It is worth pointing here that for a pure capacitance, the complex admittance graph appears as a straight line parallel to the Y″ axis at the point Y' = G0. In the inset Fig. 4 (a), the graph appears to deviate slightly from a tilted line which makes an angle (θ) with the Y″ axis where θ is the depression angle in the corresponding Nyquist plot. The angle of tilt is the consequence of complex capacitance and a small deviation from the straight line is due to the frequency dispersion of C′ and C″ in the specimen. The grain and grain boundary resistances, as obtained from the ZSIMPWIN software fitting, decrease with an increase in temperature. The variation of the resistance with temperature is represented in Fig. 5 (a) and (b). It has been found that these resistances obey the equation,
R = R 0 + Aexp
3.4. Dielectric characteristics 3.4.1. Temperature effect on dielectric studies The variation of the relative dielectric constant (ε′r) and tangent loss (tanδ) with a temperature of the compound at some selected frequencies are shown in Fig. 7 (a) and (b). The dielectric constant gradually increases and attains a maximum value at around 370 °C and then smoothly decreases suggesting the presence of a phase transition in the compound. The mobility of the space charge ions increases with increasing temperature, which increases the space charge polarization based on ion movement to the surface. As a result, the dielectric constant increases with temperature up to the transition temperature, above which the dielectric constant decreases with the disappearance of domains [39] on account of the thermal motion of domain walls. With the addition of 20% Mn4+ions in BNT, the phase transition peak of the pure BNT compound at 330 °C [40] is being shifted to a higher temperature of 370 °C. The dielectric anomaly in the literature is being assigned to the ferroelectric to paraelectric phase transitions, and the transition temperature is being referred to as the Curie temperature (Tc). The ε′r value increases significantly with the substitution of Mn in the high-temperature and the low-frequency region. The BNTM compound has a maximum dielectric constant of 1131 at a frequency of
( ) [36]. The activation energy (E ) calculated from −Ea KB T
a
the plot of lnR vs. 1000/T (depicted in the inset of Fig. 5 (a) and (b)) amounts to 0.2 eV and 0.38 eV for the grain and grain boundary effects respectively. In Fig. 6 (a) and (b), we depict the Z′ and Z′ ′ variation with the frequency. The real component, Z′ decrease with the rise in frequency and becomes almost temperature independent after 100 kHz. Further, Z′ also varies with the rise in temperature indicating the NTCR (negative temperature coefficient of resistance) behavior in the compound. The rate of change of Z′ with frequency also decreases with an increase in the temperature. In Fig. 6 (b), the existence of a peak in the variation of imaginary component ( Z′ ′) with frequency and temperature explains
Fig. 6. (a): Variation of Zʹ, (b) Zʹʹ with the frequency of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. 6
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Fig. 7. (a): Variation of dielectric constant (ε′r), (b) tangent loss (tanδ) with temperature at selected frequencies of Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
εr'' with angular frequency ω in the log-log scale and the Cole-Cole plot in the complex permittivity plane in the inset of Fig. 8 (c) corresponding to the temperature of 50 °C. The dispersion of εr' with ω is much weaker than that of εr''. The variation of εr' and εr'', for that matter the variation of C′ and C″ indicates a leaky capacitor behavior in the specimen. The Cole-Cole plot for a specimen, whose real and imaginary parts of the capacitor are frequency independent and whose electrical behavior can be analyzed through a parallel RC network, is expected to be a straight line inclined to the real axis by the angle θ same as that of the depression angle in the Nyquist plot. However, both the real and the imaginary component of relative permittivity in our specimen exhibit frequency dispersion resulting in a deviation from the slanted line behavior in the Cole-Cole plot as represented in the inset of Fig. 8(c). It is worth mentioning here that the Cole-Cole plot makes semicircular arcs in specimens whose electrical behavior is represented through a series RC circuit. The infinite frequency dielectric constant at 50 °C as found from the Cole-Cole plot is 420. While the frequency dispersion of the real component of relative dielectric constant is found to follow a relation εr′ = ε∞ + Aω−0.456 that of the imaginary component follows the relation, εr′ ′ = Bω−0.454 .
100 kHz, as compared to that of 615 in pure BNT. The value of the tangent loss is low up to 100 °C in BNTM compound for the entire experimental range of the frequency. The loss tangent of the compound exhibits relaxation peaks similar to the peak of the temperature-dependent dielectric constant. At the higher temperature and lower frequency, the loss tangent is higher due to the conductivity factor owing to the presence of the alkali metal ion Na+ and the existence of various oxidation states of the Mn ions. At higher frequency and lower temperature, the tangent loss is small in the BNTM compound. As evident from Fig. 7 (a) and (b), the sample possesses a dielectric constant of 430 with a tangent loss of 0.03 at 25 °C and a maximum dielectric constant of 812 with the tangent loss of 1.6 at the transition temperature, Tc = 372 °C at the measured frequency of 1 MHz. These parameters stand comparable with many of the popular dielectric materials for device applications at high frequency. 3.4.2. Frequency effect on dielectric studies The frequency effect on the dielectric parameters, i.e. ε′r and tanδ of the compound in the frequency range 100 Hz–1 MHz at selective temperatures between 50 °C to 300 °C for the BNTM compound is represented in Fig. 8 (a) and (b). The slope of the frequency dispersion of ε′r decreases with a rise in frequency, but increase with the increase in temperature. While, the relative dielectric constant at 100 Hz decreases from 19892 at 300 °C to 1483 at 50 °C, that at 1 MHz decrease from 749 to 436 for the same temperature range. In the selected experimental range of the frequency, the value of the dielectric constant varies due to changes in the contribution from space charge and dipolar polarization. With increasing frequency, the space charge polarization and some modes of the dipolar polarization cease due to the inertial nature of the dipoles and do not respond to the fast field variations resulting in the decrease of the ε′r. In Fig. 8 (a), the high value of dielectric constant at low-frequency and high-temperature regions can be explained by Maxwell and Wagner's mechanism (Koop's phenomenological theory) [41]. This model suggests that the material exhibits a relatively conducting grain with insulating grain boundary and thereby creates a barrier layer capacitance. In the high-frequency region, the charge carriers do not move to the grain boundary due to inertia and as a result, only grain effect contributes to the polarization process. While the charge carriers require more energy for their movement in the lower frequency region due to the high resistance of the grain boundaries, the energy requirement in the higher frequency region is less due to the small resistance of the grains. Thus, tanδ possess high value at a low frequency as compared to the higher frequency region. The tanδ is high in the lowfrequency and the high-temperature, due to the dominant contribution from the DC conductivity (σdc / ωε0 εr′). For better comprehension of the variation of dielectric constant with angular frequency in Fig. 8 (c), we have plotted the variation of εr' and
3.5. AC conductivity studies Fig. 9 (a) represents the temperature dependence of AC conductivity (σac ) with a frequency range of 100 Hz–1 MHz. The σac of the compound is generally derived from the dielectric measurement using the relation, σac = ωε0 εr′ tanδ . The temperature dependence of AC conductivity is studied by using the Arrhenius equation, σac = σ0 exp
( ). In this −Ea KB T
equation, σ0, Ea and kB are the pre-exponential factor, activation energy and Boltzmann constant respectively. The Ea is determined from the log σac Vs. 1000/T plot by the best linear fitting in the high-temperature region. The calculated Ea value reduces from 0.44 eV at 100 Hz to 0.39 eV at 1 MHz. In the conduction mechanism, Ea is the energy required for the activation of the charge carriers, and their hopping over long distances. The value of Ea decreases due to the number of hopping of the electron between localized states increases with rise in frequency of the applied electric field [42]. The various oxidation states of Mn ions at the Ti sites in the material, create oxygen vacancies to maintain the charge neutrality. The conduction is based on the hopping of electrons between the multivalent Mn ions and oxygen vacancies. The frequency dependence of AC conductivity of the compound in a temperature range of 50oC–300 °C is presented in Fig. 9 (b). The variation of AC conductivity with frequency is being analyzed by using Jonscher's power law, σ (ω) = σdc + Aω s where A is the polarizability factor and the exponent is s (≤1). The fitting of the AC conductivity with frequency gives the values of the exponent, s. The variation of s with temperature is accepted as an indicator for the conduction process 7
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Fig. 8. (a): Variation of ε′r, (b) tanδ with the frequency of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. (c) Variation of ε′r and ε′′r with angular frequency and the inset figure shows the Cole-Cole plot for the temperature 50 °C.
present in the dielectric materials [43]. In Fig. 9 (c), the value of s is found to decrease with the rise in temperature, attains a minimum value, and then increases again towards unity value. This feature
indicates that the overlapping large polaron tunneling (OLPT) mechanism is the dominant conduction process in the compound. The OLPT conduction mechanism was proposed by Long [44],
Fig. 9. (a) Variation of σac with 1000/T of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. Variation of (b) σac with frequency, (c) Frequency exponent with temperature, (d) lnσdc with inverse the of the absolute temperature of Bi0.5Na0.5Ti0.95Mn0.05O3 compound. 8
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Fig. 10. (a) Variation of Mʹ, (b) Mʹʹ, (c) Mʹʹ and Zʹʹ with frequency of Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
where the large polaron wells of two sites overlap resulting in the reduction in the polaron hopping energy
almost linearly with increasing temperature and it signifies the semiconducting nature of the compound.
rp WH = WHo ⎛1 − ⎞ R⎠ ⎝
3.6. Modulus properties
(6)
where, rp and R are the polaron radius and intersite separation respectively and the constant, WHo =
e2 , 4εp rp
The variation of M′ and M″ with frequency in the temperature range of 50oC–300 °C is shown in Fig. 10 (a) and (b). The analysis of the modulus properties identifies the grain, grain boundary and electrode interface effects to the sample. The M′ value approaches zero in the lower frequency region indicating the short-range mobility of charge carriers in the presence of an electric field. Further M′ increases with increasing frequency and tends to coincide at high-frequency, suggesting the absence of the electrode polarization in the material [45]. For temperature greater than 100 °C, two distinctive variations are observed, i.e. a horizontal region with almost zero value in lower frequency and a linearly increasing nature in higher frequency. The horizontal region in the high temperature and the lower frequency region indicates to the grain boundary effect and the high-frequency region for the grain mechanism. With the increase in temperature, M″max peak moves to the high-frequency region. This behavior of dielectric relaxation indicates that the hopping mechanism of charge carriers dominates intrinsically at the high-temperature regions in the thermally activated process. The broadening of the asymmetric peak increases with an increase in temperature and decreases the relaxation time of charge carriers. This type of characteristic is observed only in the nonDebye type of relaxation mechanism. The peak value of M″ occurs at frequency ωmax, such as ωmax τ = 1. For perfect Debye type nature, the relaxor peak of both M″ and Z″ occur at the same frequency at a given temperature. As the material is inhomogeneous in nature and the combination of more than one RC element with a constant phase element (Q), the resulting M″ and Z″ spectra occur at different frequencies as shown in Fig. 10 (c) for the data corresponding to 50 °C.
εp being the effective permit-
tivity. The AC conductivity for the OLPT model is given by
σac (ω) =
ωRω4 π4 2 e (KB T )2 [N (Ef )]2 ∗ 12 2αKB T + WHo rp + Rω2
(7)
Here, Rω is the hopping length at a frequency ω and α is the spatial decay parameter and takes a constant value. The frequency exponent, s, for this model can be represented as
s=1−
8αR ω + 6βWHo rp/ Rw (2αR ω + βWHo rp/ Rw )2
(8)
where β = 1/ KB T . The OLPT mechanism suggests that s should be temperature dependent. The equation shows that the s decreases from unity with an increase in temperature and for a small value of rp, s attains a minimum at a certain temperature and consequently rises with an increase in temperature. In Fig. 9 (b), the plateau response was noticed in lower frequency and higher temperature, and thus, σdc is calculated for all the temperatures from the use of Jonscher's power law to the AC conductivity data. The temperature dependence DC conductivity obeys the Arrhenius equation, σdc = σ0 exp
( ), where, the physical parameters −Ea KB T
have their usual meaning. Fig. 9 (d) shows the graph of lnσdc vs. 1000/T and the activation energy estimated from the graph amounts to 0.43 eV. It is worth mentioning here that the activation energy, determined for the lower frequency grain boundary contribution using the Zsimpwin fitting of the Nyquist plot was 0.38 eV. The DC conductivity increases 9
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
References [1] X. He, K.j. Jin, Persistence of polar distortion with electron doping in lone-pair driven ferroelectrics, Phys. Rev. B 94 (1–8) (2016) 224107. [2] A.K. Jena, S. Satapathy, J. Mohanty, Magnetic properties and oxygen migration induced resistive switching effect in Y substituted multiferroic bismuth ferrite, Phys. Chem. Chem. Phys. 21 (2019) 15854–15860. [3] M. Gröting, K. Albe, Comparative study of A-site order in the lead-free bismuth titanates M1/2Bi1/2TiO3 (M=Li, Na, K, Rb, Cs, Ag, Tl) from first-principles, J. Solid State Chem. 213 (2014) 138–144. [4] S.-E. Park, T.R. Shrout, Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals, J. Appl. Phys. 82 (4) (1997) 1804–1811. [5] A. Safari, E.K. Akdoğan (Eds.), Piezoelectric and Acoustic Materials for Transducer Applications, Springer, New York, 2008. [6] C. Kornpom, T. Udeye, T. Bongkarn, The preparation of lead-free bismuth sodium titanate ceramics via the solid state combustion technique, Integr. Ferroelectr. 177 (2017) 59–68. [7] C.Y. Kim, T. Sekino, K. Niihara, Synthesis of bismuth sodium titanate nanosized powders by solution/sol–gel process, J. Am. Ceram. Soc. 86 (9) (2003) 1464–1467. [8] M. Li, M. J. Pietrowski, R. A. De Souza, H. Zhang, I.M. Reaney, S.N. Cook, J.A. Kilner, D.C. Sinclair, A family of oxide ion conductors based on the ferroelectric perovskite Na0.5Bi0.5TiO3, Nat. Mater., doi:10.1038/NMAT3782. [9] Y. Chen, K. Uchino, D. Viehland, Substituent effects in 0.65Pb(Mg1/3Nb2/3)O30.35PbTiO3 piezoelectric ceramics, J. Electroceram. 6 (1) (2001) 13–19. [10] X. Wang, M. Gu, B. Yang, S.N. Zhu, W.W. Cao, Hall effect and dielectric properties of Mn-doped barium titanate, Microelectron. Eng. 66 (2003) 855–859. [11] Q. Zhang, R.R. Whatmore, Hysteretic properties of Mn-doped Pb(Zr, Ti)O3 thin films, J. Eur. Ceram. Soc. 24 (2004) 277–282. [12] H. Liu, W.W. Ge, X.P. Jiang, X.Y. Zhao, H.S. Luo, Growth and characterization of Mn-doped Na1/2Bi1/2TiO3 lead-free ferroelectric single crystal, Mater. Lett. 62 (2008) 2721–2724. [13] T. Takenaka, K. Sakata, Grain orientation effects on the electrical properties of bismuth layer structured ferroelectric Pb (1−x) (NaCe) x/2 Bi4Ti4O15 solid solution, J. Appl. Phys. 55 (4) (1984) 1092–1099. [14] M.B. Suresh, E.V. Ramana, S.N. Babu, S.V. Suryanarayana, Comparison of electrical and dielectric properties of BLSF materials in Bi─Fe─Ti─O and Bi─Mn─Ti─O systems, Ferroelectrics 332 (2006) 57–63. [15] Z. Hu, X. Xiao, C. Chen, T. Li, L. Huang, C. Zhang, J. Su, L. Miao, J. Jiang, Y. Zhang, J. Zhou, Al-doped α-MnO2 for high mass- loading pseudocapacitor with excellent cycling stability, Nano Energy 11 (2015) 226–234. [16] T. Li, J. Wu, X. Xiao, B. Zhang, Z. Hu, J. Zhou, P. Yang, X. Chen, B. Wang, L. Huang, Band gap engineering of MnO2 through in situ Al-doping for applicable pseudocapacitors, RSC Adv. 6 (2016) 13914–13919. [17] V. Purohit, R. Padhee, R.N.P. Choudhary, Dielectric and impedance spectroscopy of Bi(Ca0.5Ti0.5)O3 ceramic, Ceram. Int. 44 (2018) 3993–3999. [18] R. Das, R.N. P Choudhary, Studies of structural, dielectric relaxor and electrical characteristics of lead-free double Perovskite:Gd2NiMnO6, Solid State Sci. 87 (2019) 1–8. [19] P. Pascuta, E. Culea, FTIR spectroscopic study of some bismuth germanate glasses containing gadolinium ions, Mater. Lett. 62 (2008) 4127–4129. [20] V. Dimitrov, Y. Dimitriev, A. Montenero, IR spectra and structure of V2O5-GeO2Bi2O3 glasses, J. Non-Cryst. Solids 180 (1994) 51–57. [21] I. Ardelean, S. Cora, D. Rusu, EPR and FT-IR spectroscopic studies of Bi2O3–B2O3– CuO glasses, Physica B 403 (2008) 3682–3685. [22] L. Kang, M. Zhang, Z.H. Liu, K. Ooi, IR spectra of manganese oxides with either layered or tunnel structures, Spectrochim. Acta, Part A 67 (2007) 864–869. [23] A. Azama, A.S. Ahmed, S.S. Habib, A.H. Naqvi, Effect of Mn doping on the structural and optical properties of SnO2 nanoparticles, J. Alloy. Comp. 523 (2012) 83–87. [24] A.N. Murashkevich, A.S. Lavitskaya, T.I. Barannikova, I.M. Zharskii, Infrared absorption spectra and structure of TiO2-Sio2 composites, J. Appl. Spectrosc. 75 (5) (2008) 730–734. [25] M.J. Alam, D.C. Cameron, Preparation and characterization of TiO2 thin films by sol-gel method, J. Sol. Gel Sci. Technol. 25 (2002) 137–145. [26] A.M. Bobrova, I.G. Zhigun, M.I. Bragina, A.A. Fotiev, Infrared absorption spectra of various titanium compounds, J. Appl. Spectrosc. 8 (1) (1968) 96–101. [27] D. Tsiourvas, A. Tsetsekou, M. Arkas, S. Diplas, E. Mastrogianni, Covalent attachment of a bioactive hyperbranched polymeric layer to titanium surface for the biomimetic growth of calcium phosphates, J. Mater. Sci. Mater. Med. 22 (2011) 85–96. [28] K. Liu, H. Fan, P. Ren, C. Yang, Structural, electronic and optical properties of BiFeO3 studied by first-principles, J. Alloy. Comp. 509 (2011) 1901–1905. [29] J.R. Sambrano, E. Orhan, M.F.C. Gurgel, A.B. Campos, M.S. Goes, C.O. PaivaSantos, J.A. Varela, E. Longo, Theoretical analysis of the structural deformation in Mn-doped BaTiO3, Chem. Phys. Lett. 402 (2005) 491–496. [30] J.J. Adkin, M.A. Hayward, BaMnO3-x revisited: a structural and magnetic study, Chem. Mater. 19 (2007) 755–762. [31] A. K Jena, J. Mohanty, Enhancing ferromagnetic properties in bismuth ferrites with non magnetic Y and Sc Co-doping, J. Mater. Sci. Mater. Electron. 29 (2018) 5150–5156. [32] S. Thakur, N.K. Sahoo, M. Senthilkumar, R.B. Tokas, Optical properties and morphological changes in gadolinia films deposited under ambient substrate temperature conditions, Opt. Mater. 27 (2005) 1402–1409. [33] N.K. Sahoo, S. Thakur, M. Senthilkumar, D. Bhattacharyya, N.C. Das, Reactive electron beam evaporation of gadolinium oxide optical thin films for ultraviolet and
Fig. 11. P-E ferroelectric loop of Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
3.7. Ferroelectric property The ferroelectric loop (polarization vs. electric field) of the DC poled sintered pellet is shown in Fig. 11. In the polling process, the sintered pellet was placed in silicone oil in the applied DC field of 3 kV/mm at 50 °C for 1 h. The P-E hysteresis measurement is carried out at room temperature with a frequency of 10 Hz. The observed hysteresis loop suggests the ferroelectric nature of the compound. The hysteresis loop exhibit a remnant polarization (Pr) of 3.372 μC/cm2 and a coercive field (Ec) of 4.651 kV/cm. The observed value of Pr is less than that of pure BNT (10.44 μC/cm2) prepared under similar conditions [46]. The decrease in the remanent polarization is being attributed to the smaller activation energy (as determined in the conduction process) resulting in an increase in the number of mobile ions.
4. Summary and conclusion The Bi0.5Na0.5Ti0.80Mn0.20O3 compound was fabricated by a conventional solid-state reaction technique. XRD spectrum supported by the Rietveld refinement reveals that the compound crystallizes in rhombohedral symmetry. The compound formation is supported by the presence of Bi-O, Na-O, Ti-O, and Mn-O stretching vibration band in the FTIR spectrum. The UV–Visible optical spectroscopy indicates a decrease in the optical band gap energy of the compound in comparison to BNT. The BNTM sample exhibits an enhanced dielectric characteristic in comparison to the pure BNT compound. The Nyquist plot exhibit double depressed semicircles confirming both grain and grain boundary contributions to the polarization process, and also the existence of a modified Debye relaxation process. The frequency dispersion of the complex dielectric constant is analyzed on the basis of the Nyquist plot and the Cole-Cole plot. The substitution of 20% Mn ions at the Ti sites results in a shift in the ferro-paraelectric phase transition peak from 330 °C to 370 °C in the material. The frequency dependence impedance and modulus suggest non-Debye nature of relaxation in the material with some sort of relaxor behavior. Activation energy calculated from the Nyquist plot data and conductivity data broadly agrees and points at the excitation of charge carriers on the basis of charge transfer between multivalent Mn ions and the oxygen vacancy created to maintain charge balance. The frequency-dependent AC conductivity analyzed using Jonscher's power law suggests an OLPT model of the conduction mechanism in the studied compound.
Acknowledgment The authors would like to thank Mr. Tridib Das of ZEISS Group and Central Research facility of the S'O'A (Deemed to be University) for their kind help in some experimental work. 10
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
J. Eur. Ceram. Soc. 33 (2013) 2141–2153. [41] K. Parida, S. Das, P.K. Mahapatra, R.N.P. Choudhary, Relaxor behavior and impedance spectroscopic studies of chemically synthesized SrCu3Ti4O12 ceramic, Mater. Res. Bull. 111 (2019) 7–16. [42] S. Halder, K. Parida, S.N. Das, S.K. Pradhan, S. Bhuyan, R.N.P. Choudhary, Dielectric and impedance properties of Bi(Zn2/3V1/3)O3 electronic material, Phys. Lett. A 382 (2018) 716–722. [43] A. Ghosh, Frequency-dependent conductivity in bismuth-vanadate glassy semiconductors, Phys. Rev. B 41 (3) (1990) 1479–1488. [44] A.R. Long, Frequency-dependent loss in amorphous semiconductors, Adv. Phys. 31 (5) (1982) 553–637. [45] A.K. Jena, S. Satapathy, J. Mohanty, Magnetic and dielectric response in yttrium (Y)-manganese (Mn) substituted multiferroic Bi1-xYxFe1-yMnyO3 (x = y = 0, x = 0.03, 0.06, 0.12, y = 0.05) ceramics, J. Appl. Phys. 124 (2018) 174103. [46] N. Pradhani, P.K. Mahapatra, R.N.P. Choudhary, A.K. Jena, J. Mohanty, Investigation on the effect of Mn substitution on the structural, electrical and ferroelectric characteristics of Bi0.5Na0.5TiO3 ceramic, Mater. Res. Bull. 119 (2019) 110566.
deep ultraviolet laser wavelengths, Thin Solid Films 440 (2005) 155–168. [34] N. Pradhani, P.K. Mahapatra, R.N.P. Choudhary, Effect of cerium oxide addition on optical, electrical and dielectric characteristics of (Bi0.5Na0.5)TiO3 ceramics, J. Phys.: Materials 1 (2018) 015007. [35] F. Yang, S. Lin, L. Yang, J. Liao, Y. Chen, C.Z. Wang, First-principles investigation of metal-doped cubic BaTiO3, Mater. Res. Bull. 96 (2017) 372–378. [36] S. Sahoo, P.K. Mahapatra, R.N.P. Choudhary, Influence of Compositional Variation on Structural, Electrical and Magnetic Characteristics of (Ba1-xGdx) (Ti1-xFex) O3 (0.2≤ X ≤0.5), MRX-105576.R2, (2017) 1-29. [37] C.K. Suman, K. Prasad, R.N.P. Choudhary, Complex impedance studies on tungstenbronze electroceramic: Pb2Bi3LaTi5O18, J. Mater. Sci. 41 (2006) 369–375. [38] S. Hajra, N. Pradhani, R.N.P. Choudhary, S. S Sahoo, Fabrication, Dielectric and electrical characteristics of 0.94 (Bi0.5Na0.5)TiO3-0.06 BaTiO3 ceramics, Process. Appl. Ceramics 13 (1) (2019) 24–31. [39] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, Effects of heat-treatment temperature on the microstructure, electrical and dielectric properties of M-type hexaferrites, J. Electron. Mater. 43 (2014) 512–521. [40] S.Y. Cheng, J. Shieh, H.Y. Lu, C.Y. Shen, Y.C. Tang, N.J. Ho, Structure analysis of bismuth sodium titanate-based A-site relaxor ferroelectrics by electron diffraction,
11
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Structural and impedance analysis of Bi0.5Na0.5Ti0.80Mn0.20O3 ceramics Neeha Pradhani∗, P.K. Mahapatra, R.N.P. Choudhary Department of Physics, Multiferroic Advanced Materials Laboratory, Siksha ‘O' Anusandhan (Deemed to be University), Bhubaneswar, 751030, India
A R T I C LE I N FO
A B S T R A C T
Keywords: XRD FTIR Impedance spectroscopy Conduction mechanism
The Bi0.5Na0.5Ti0.80Mn0.20O3 ceramic was synthesized by a conventional solid-state reaction technique. Rietveld refinement of X-ray diffraction data confirms the rhombohedral crystal structure of the compound with R3c space group. The optical band gap energy of the compound is found to be 1.93 eV. The substitution of 20% Mn ions at the Ti sites results in the improved dielectric characteristics and a shift in the ferroelectric to paraelectric electric phase transition peak from 330 °C to 370 °C in the material. The frequency dispersion of dielectric constant and its footprint in the Nyquist and Cole-Cole plots have been analyzed. The analysis of complex impedance and modulus spectroscopy confirms the non-Debye type of relaxation mechanisms in the material with contributions from both the grain and grain boundary to the electrical properties. The frequency dependence of AC conductivity data exhibits overlapping large polaron tunneling conduction mechanism in the compound.
1. Introduction Bismuth sodium titanate, Bi0.5Na0.5TiO3 (BNT) a complex A-site (Bi3+ and Na+) perovskite ferroelectric of a general formula ABO3, is a step forward for phasing out the use of hazardous lead-based ferroelectric materials. The similar electronic configuration (4f145d106s2) of Bi3+ and Pb2+ with a stereochemically active lone pair of 6s electrons is accountable for high polarization in the ferroelectric compounds containing these ions [1,2]. The chemically distorted ferroelectric perovskites generally possess relaxor nature distinguished by broad maxima around the transition temperature. The chemical distortion in the compound produces local random fields, which induce polar nano regions [3]. Relaxor properties make the compound more technologically attractive for applications like transducers and actuators [4,5]. The BNT exhibits two diffused phase transitions corresponding to the (i) ferroelectric to the relaxor-ferroelectric and (ii) relaxor-ferroelectric to paraelectric at around 190 °C and 330 °C respectively [6]. The technological applications of BNT have some limitations due to the relatively high coercive field, remanent polarization and conductivity [7,8]. Manganese (Mn), with various stable oxidation states of + 4, + 3 and + 2, is one of the most useful additives at the B-site of the perovskite because of the smaller ionic radius of Mn4+ (0.53 Å) than that of Ti4+ (0.605 Å) in octahedral coordination and almost same electronegativity (Mn (1.55) and Ti (1.54)) [9,10]. The multivalent state of Mn is expected to accommodate oxygen vacancy created during sintering process. The manganese in oxide form is characterized by a high
∗
dielectric constant, being non-toxic and environmentally friendly is considered as one of the promising metal oxides for pseudocapacitors. These characteristics are also suitable for electrode material in electrochemical supercapacitors. The doping of Mn ions in some perovskites has been reported to reduce the temperature of sintering and promote the growth of the grain [11,12]. The substitution of Mn4+ ion in perovskite materials is utilized for effecting an increase in Curie temperature, a broad range of frequency response and higher anisotropy in coupling factors [13,14]. The substitution/doping mechanism establishes a new molecular orbital and introduces an impurity level to narrow down the energy band gap [15,16]. Looking at the promising improvements in the material properties with the substitution of Mn in BNT, we were motivated to fabricate Bi0.5Na0.5Ti0.80Mn0.20O3 by a low cost standard ceramic technology (high-temperature solid-state reaction method) and investigate the structural, optical, dielectric and electrical characteristics of the synthesized compound. 2. Experimental and characterization techniques The manganese modified BNT i.e. Bi0.5Na0.5Ti0.80Mn0.20O3 was fabricated by a solid-state reaction method. The stoichiometric quantity of Bi2O3 (99.0%, Himedia), Na2CO3 (99.5%, CDH), TiO2 (99.5%, Loba Chemie) and MnO2 (99.9%, Loba Chemie) were thoroughly mixed using a mortar pestle in the dry mode as well as in the wet mode taking methanol for 3 h. The fine homogeneous powder was calcined at 900 °C for 4 h using a high-temperature furnace. The structural characteristics
Corresponding author. E-mail address: [email protected] (N. Pradhani).
https://doi.org/10.1016/j.ceramint.2019.10.128 Received 31 August 2019; Received in revised form 14 October 2019; Accepted 14 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Neeha Pradhani, P.K. Mahapatra and R.N.P. Choudhary, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.128
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
of the calcined powder were analyzed by an X-ray diffractometer (Rigaku Ultima IV) using Cu-Kα radiation (λ = 1.5406 Å). The diffraction pattern was obtained in a range of the Bragg diffracted angle 20°80° with a step scan of 0.02°. The surface microstructures of the sample were studied by field emission scanning electron microscope (FESEM) (ZEISS). The compositional analysis of the compound was analyzed using energy-dispersive X-ray spectroscopy (EDX). To corroborate the structural analysis, the presence of various vibration bonds in the sample was identified by Fourier transform infrared (FTIR) spectroscopy carried out at room temperature. The optical energy gap of the specimen was determined by UV–Visible optical spectroscopy (PerkinElmer Lambda 1050). The pellets were sintered at 1000 °C for 4 h. The dielectric and electrical properties were measured through LCR meter (N4L, Impedance Analysis Interface, Model PSM-1735), in a wide range of temperature. Marine India PE loop tracer was used to measure the ferroelectric properties of the compound. 3. Results and discussions
Fig. 1b. (b) Rietveld refinement of Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
3.1. Analysis of the structure
corresponding to the most intense diffraction peak (2θ = 32.520) in the Scherrer relation, the crystallite size amounts to 53.01 nm. The surface micrograph of the sintered sample, recorded using FESEM, as shown in Fig. 1(c), exhibits uniformly distributed well-defined grains with small voids on the surface of the sample. The theoretical (calculated using the values of the lattice parameters) density and measured density (using the Archimedes principle) of the compound amounts to 6.014 g/cc and 5.352 g/cc respectively. The measured bulk density of the ceramic compound is 89% of the theoretical density. The compositional (elemental) analysis of the compound obtained from the EDX spectroscopy is shown in Fig. 1(d). The EDX spectrum confirms the presence of all the constituent element Bi, Na, Ti, Mn and O and absence of any other impurity. The Weight % and the atomic % of the elements present in the compound (as provided by the EDX spectroscopy) together with the atoms per formula unit is calculated as the ratio of the atoms of each element to that of the sum of all cations multiplied by a weight factor 2 and the expected atomic ratio on the basis of the stoichiometry of the compound is provided in the Table 1. The atoms per formula unit were calculated from the EDX spectrum for the cations are nearly equal to those expected on the basis of the formula unit of the compound. The elemental color map presented in Fig. 1(e) indicates the uniform distribution of all the elements in the compound. The FTIR spectrum of the compound is recorded to identify the chemical bonds of the constituent elements and to corroborate the structural analyses. The FTIR spectrum in the wavenumber range of 400 cm−1–2000 cm−1 together with the blown-up image of the spectrum in the range of 400 cm−1–700 cm−1 as the inset, is presented in Fig. 1(f). In the BNTM compound, the broad absorption band from 441 cm−1 to 457 cm−1 is being assigned to the Bi–O vibration bonds in the distorted BiO6 unit [19–21]. The absorption peaks observed between 480 cm−1-518 cm−1, correspond to the Mn/Ti-O vibration bond at octahedral sites [22]. The Mn-O and Ti–O stretching vibrational bands in BNTM are found at 483 cm−1 and 518 cm−1 respectively
The XRD spectrum of the calcined sample is shown in Fig. 1(a). The Rietveld refinement of the specimen using MAUD software is presented in Fig. 1(b). The refinement of the observed XRD spectrum was carried out by keeping the structural factors, (i.e., preferred orientation, profile, background, Rietveld profile factors, and unit cell) similar to those of the reference structural model. The refinement of the XRD spectrum is carried out by using the crystallographic information file of BNT i.e. CIF: NIMS_MatNavi_4295517178_1_2, with rhombohedral structure and R3c space group. The values of the structural refined factors like Rwp (weighted profile factor), Rb (Bragg R factor), Rexp (expected weighted profile factor), and the goodness of fit (GoF) which is equal to the ratio Rwp/Rexp are inserted in Fig. 1 (b). The observed and calculated XRD spectrum is represented by the blue and black color lines respectively. The black line in the bottom box represents the difference between the observed and the calculated intensity. The red symbol shows the position of the Bragg reflection peaks. The matching of the experimental spectrum with that of the reference model and the values of the refinement parameters indicate the compound formation in a singlephase rhombohedral (R3c) structure. The refined cell parameters are obtained as, a = b = 5.498 Å and c = 13.494 Å. The crystallite size (t) 0.94λ of the compound is calculated using the Scherrer relation, t = βcosθ , where, 2θ , β and λ are the diffraction angle, full width at half maxima and the wavelength of radiation respectively [17,18]. Using the data
Table 1 Elemental Weight and Atomic percentage of Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
Fig. 1a. (a) XRD spectrum. 2
Element
Weight%
Atomic %
Atoms per formula unit
Expected atoms per formula unit
Bi Na Ti Mn O
51.15 5.54 17.75 5.07 20.49
10.98 10.80 16.63 4.14 57.45
0.516 0.508 0.782 0.195 2.700
0.5 0.5 0.80 0.20 3
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
expressing the values of K and c in cgs units and μ in atomic mass units (u) the expression of wave number in cm−1 corresponding to the vibration bond reduces to
ν¯ = 1303
K μ
(3) 1 105) 2 /2πc ,
Na being the wherein the numerical constant 1303 = (Na × Avogadro's number. Generally the force constant, K is related to bond length, r, through Badger's rule expressed as, K = a (r-d) −3. Where, d and a, are the parameters that depend on the period number in the periodic table in which the elements corresponding to the cation of the bond is located. The equations of force constant for second, third and fourth period compounds appear as K = 3.0 (r-0.61)−3, K = 2.7 (r0.87)−3 and K = 2.6 (r-0.96)−3 respectively. In our perovskite compound, the A site is shared equally between the Na and Bi ions while the B site is shared by Ti and Mn in the 4:1 ratios. In the event of a lattice site is being shared by two ions (1 and 2) with an occupation ratio of x and (1-x) respectively, the effective mass of the cation at that site is taken as, mc = xm1 + (1 − x ) m2. On the basis of these relations, we have calculated the force constant and the bond length of the Bi/Na-O and Ti/Mn-O bonds. The bond lengths presented in Table- 2 matches well with the corresponding reported values [28–30]. 3.2. Optical properties
Fig. 1c. (c) FESEM micrograph.
The UV–Visible absorbance spectra and the Tauc plot ((αh ν)2 vs. Eg ) for the ceramic compound is shown in Fig. 2. Wood and Tauc equation is used for the calculation of band gap energy [31,32], (αh ν)2 = B (hν − Eg ) , where, the symbols have the usual definition [33]. The direct energy gap of the BNTM compound as obtained from Tauc Plot amounts to 1.93 eV which is smaller than that of pure BNT. It is pertinent to note here that the dissociation energy of the Mn-O bond (402 kJ/mol) is smaller than that of the Ti-O bond (662 kJ/mol). Further, the bond length of the Mn-O bond is higher to that of the Ti-O bond as predicted in Table- 2. Consequently, the lattice parameters of the Mn doped BNT are found to be higher to that of pure BNT [34]. The increased lattice parameters results in a smaller Brillouin zone and likely to contribute to the decrease in the energy gap. Further, the weaker Mn-O bond might also result in a smaller lattice potential effecting a decrease in the energy gap. The theoretical calculations based on the density functional theory also suggest a decrease in the energy gap of Mn doped perovskite compounds [35]. 3.3. Impedance characteristics Fig. 1d. (d) EDX spectrum.
The impedance measurement of the ceramic is analyzed to identify the grain (bulk), the grain boundary and the electrode interface contributions to the electrical properties of the materials. The effect of each of these contributions is being represented through an electrical circuit consisting of capacitance (C) and resistance (R0) connected in parallel. The equivalent electrical circuit of the specimen is represented through the series connection of the parallel R0C circuits with respect to each of these effects. In homogeneous systems, like single crystals where the capacitance is assumed to be the ideal impedance corresponding to each of these effects is represented by:
[23–26]. The absorption peak at 878 cm−1 is assigned to the Na-O banding of the Na-O-Ti bond [27]. To further ascertain the observed transmission dips with the identified bond vibrations, the force constant and the bond length associated with the Na/Bi-O and the Ti/Mn-O bonds are calculated and compared with those reported for the similar structure under similar coordination. In vibrational spectroscopy, wave number, ν¯ , for a diatomic molecule is expressed as
ν¯ =
1 2πc
K μ
Z= (1)
m c ma m c + ma
(4)
where, the relaxation time, τ = R 0 C and ω being the angular frequency of the applied field. The complex impedance (Z''∼Z′) plot which is also known as a Nyquist plot for each of the contributions, in such a situation, form a semicircular plot of radius R0/2 with the center on the real (Z′) axis at (R0/2, 0) and intersections at (0,0) and (R0, 0) corresponding to impedance values at infinite and zero field frequency respectively. The peak of the complex impedance plot (Nyquist plot)
where K is the force constant, c is the speed of light and μ is the reduced mass of the pair of atoms in the bond. The reduced mass of the bond containing the cation and anion can be obtained through the relation:
μ=
R 0 − jR 0 ωτ 1 + (ωτ )2
(2)
mc and ma being the mass of the cation and the anion of the bond. In 3
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Fig. 1e. (e) Elemental color mapping of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. Tauc plot of Bi0.5Na0.5Ti0.80Mn0.20O3 at room temperature.
However, the inhomogeneity in the size of the grains and electromagnetic diffusion in the ceramic sample results in the distribution in the relaxation time and the Nyquist plot for each of the three effects manifest through tilted semicircles where the center of the circle is located below the real (Z′) axis. The depression angle made by the line joining the center of the semicircle with its high-frequency intersection on the real axis is a measure of the distribution of relaxation time in the sample. In such systems, the capacitance is complex and is being expressed as, C = C e−jθ , where θ is the angle made by C with the real component C '. The impedance in such a case appears as,
Fig. 1f. (f) FTIR spectrum of Bi0.5Na0.5Ti0.80Mn0.20O3 at room temperature. Table 2 Vibrational Parameters from FTIR spectrum of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. Vibration Bond
Bi/Na-O Bi-O Ti/Mn-O Ti-O Mn-O
Observed wave number (cm−1) 453 441 505 518 483
Force constant (N cm−1) 1.699 1.702 1.833 1.895 1.702
Bond length (Å)
Z=
R 0 (1 + ωτ sinθ) − jR 0 ωτ cosθ 1 + (ωτ )2 + 2ωτsinθ
(5)
The Nyquist plot forms a tilted semicircle of radius
2.112 2.111 2.084 2.071 2.111
(
R
−R sinθ
R0 2cosθ
with center
)
0 at 20 , 2cosθ and intersections at (0, 0) and (R0, 0) corresponding to impedance values at infinite and zero field frequency respectively. Here, the relaxation time, τ = R 0 C , where C = (C ′2 + C ′′2 ) . C′ and C″ are associated with the electrical energy storage and dissipation in the equivalent capacitor of the compound. The peak position of the R (1 − sinθ) R , and corresemicircle has the coordinates Z′ = 0 ; Z′ ′ = 0
occurs at (R0/2, R0/2) corresponding to the condition ωτ = 1. Such perfect semicircular plots indicate ideal Debye relaxation with mono relaxation time.
(
2
2cosθ
)
sponds to the condition, ωτ = 1. The positions of the intersection and the peak confirms to the condition that the depression angle made by 4
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Table 3 Rg, Cg, Rgb, and Cgb at various temperatures of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. Temperature (oC)
Rg (Ω)
Cg (F)
Rgb (Ω)
Cgb (F)
50 100 150 200 250 300
1.073E+006 1.646E+005 3.744E+004 8.778E+003 3.063E+003 9.899E+002
1.663E-010 1.836E-010 1.835E-010 1.814E-010 1.644E-010 1.295E-010
– 5.352E+004 6.022E+003 3.753E+002 1.430E+001 1.519E+001
– 3.199E-009 5.816E-009 2.108E-009 3.971E-005 1.032E-005
temperature range of 50oC–300 °C. The plot is fitted to the equivalent circuit, i.e., RQC for the temperature at 50 °C and (RQC) (RC) for 100 °C–300 °C using the ZSIMPWIN software. The single RQC fitting confirms the grain contributions at 50 °C and the series circuit, confirming the grain and the grain boundary contribution to the electrical properties of the compound at higher temperatures. The obtained parameters, i.e. grain resistance (Rg), capacitance (Cg), grain boundary resistance (Rgb) and capacitance (Cgb), are presented in Table 3. To have a clear picture on the depression angle of the semicircular arcs in the Nyquist plot, semicircular curves for the temperature 50 °C and 100 °C are plotted in Fig. 4 (c) and (d), by considering three pairs of experimental data Z′ and Z′ ′ from each of the grain and the grain boundary region, and fitted with the circle equation. The depression angle at the temperature of 100 °C for the grain mechanism (θ g) and grain boundary mechanism (θ gb) is found to be 13.31° and 23.54° respectively. It is worth reiterating that the depression angle measures the relaxation
Fig. 3. The equivalence of the depression angle of the Nyquist plot with the phase angle of complex capacitance.
the semicircle in the Nyquist plot is equal to the angle made by the complex capacitance with its real component as shown in Fig. 3. The tilted semicircles in the Nyquist plot can be explained by RQC network, i.e., a parallel connection of R and C with a constant phase element Q (see Table 2). Fig. 4 (a) and (b) represents the Nyquist plot of BNTM in the
Fig. 4. (a) and (b): Z′ ′ vs. Z′ plot with the fitting of an equivalent circuit, (c) and (d) Nyquist plot of Bi0.5Na0.5Ti0.80Mn0.20O3 compound at 50 °C and 100 °C. Inset of Fig. 4 (a) complex admittance plot of the sample for the temperature 50 °C. 5
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Fig. 5. (a): Variation of Rg and Rgb with 1000/T, (b) lnRg and lnRgb with 1000/T to calculate the activation energy for Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
the presence of the temperature dependent relaxation mechanism in the material. The relaxation peak at the frequency, ωmax = 2πfmax, shift to high-frequency with an increase in temperature. The shifting of the peak in Z″ indicates the relaxor behavior in the compound. The asymmetric nature of the peak broadening suggests the relaxation time distribution and indicate a non-Debye type of relaxation mechanism [37,38] in the compound.
distribution parameter and is equal to the phase angle made by the corresponding capacitance. The complex admittance plot of the sample for the temperature 50 °C is shown as the inset of Fig. 4 (a). The point of intersection of the graph on the real axis Y′ correspond to the DC conductance (G0) which amounts to around 1.1 μS and is in conformity with the values of DC resistance obtained from the Nyquist plot. It is worth pointing here that for a pure capacitance, the complex admittance graph appears as a straight line parallel to the Y″ axis at the point Y' = G0. In the inset Fig. 4 (a), the graph appears to deviate slightly from a tilted line which makes an angle (θ) with the Y″ axis where θ is the depression angle in the corresponding Nyquist plot. The angle of tilt is the consequence of complex capacitance and a small deviation from the straight line is due to the frequency dispersion of C′ and C″ in the specimen. The grain and grain boundary resistances, as obtained from the ZSIMPWIN software fitting, decrease with an increase in temperature. The variation of the resistance with temperature is represented in Fig. 5 (a) and (b). It has been found that these resistances obey the equation,
R = R 0 + Aexp
3.4. Dielectric characteristics 3.4.1. Temperature effect on dielectric studies The variation of the relative dielectric constant (ε′r) and tangent loss (tanδ) with a temperature of the compound at some selected frequencies are shown in Fig. 7 (a) and (b). The dielectric constant gradually increases and attains a maximum value at around 370 °C and then smoothly decreases suggesting the presence of a phase transition in the compound. The mobility of the space charge ions increases with increasing temperature, which increases the space charge polarization based on ion movement to the surface. As a result, the dielectric constant increases with temperature up to the transition temperature, above which the dielectric constant decreases with the disappearance of domains [39] on account of the thermal motion of domain walls. With the addition of 20% Mn4+ions in BNT, the phase transition peak of the pure BNT compound at 330 °C [40] is being shifted to a higher temperature of 370 °C. The dielectric anomaly in the literature is being assigned to the ferroelectric to paraelectric phase transitions, and the transition temperature is being referred to as the Curie temperature (Tc). The ε′r value increases significantly with the substitution of Mn in the high-temperature and the low-frequency region. The BNTM compound has a maximum dielectric constant of 1131 at a frequency of
( ) [36]. The activation energy (E ) calculated from −Ea KB T
a
the plot of lnR vs. 1000/T (depicted in the inset of Fig. 5 (a) and (b)) amounts to 0.2 eV and 0.38 eV for the grain and grain boundary effects respectively. In Fig. 6 (a) and (b), we depict the Z′ and Z′ ′ variation with the frequency. The real component, Z′ decrease with the rise in frequency and becomes almost temperature independent after 100 kHz. Further, Z′ also varies with the rise in temperature indicating the NTCR (negative temperature coefficient of resistance) behavior in the compound. The rate of change of Z′ with frequency also decreases with an increase in the temperature. In Fig. 6 (b), the existence of a peak in the variation of imaginary component ( Z′ ′) with frequency and temperature explains
Fig. 6. (a): Variation of Zʹ, (b) Zʹʹ with the frequency of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. 6
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Fig. 7. (a): Variation of dielectric constant (ε′r), (b) tangent loss (tanδ) with temperature at selected frequencies of Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
εr'' with angular frequency ω in the log-log scale and the Cole-Cole plot in the complex permittivity plane in the inset of Fig. 8 (c) corresponding to the temperature of 50 °C. The dispersion of εr' with ω is much weaker than that of εr''. The variation of εr' and εr'', for that matter the variation of C′ and C″ indicates a leaky capacitor behavior in the specimen. The Cole-Cole plot for a specimen, whose real and imaginary parts of the capacitor are frequency independent and whose electrical behavior can be analyzed through a parallel RC network, is expected to be a straight line inclined to the real axis by the angle θ same as that of the depression angle in the Nyquist plot. However, both the real and the imaginary component of relative permittivity in our specimen exhibit frequency dispersion resulting in a deviation from the slanted line behavior in the Cole-Cole plot as represented in the inset of Fig. 8(c). It is worth mentioning here that the Cole-Cole plot makes semicircular arcs in specimens whose electrical behavior is represented through a series RC circuit. The infinite frequency dielectric constant at 50 °C as found from the Cole-Cole plot is 420. While the frequency dispersion of the real component of relative dielectric constant is found to follow a relation εr′ = ε∞ + Aω−0.456 that of the imaginary component follows the relation, εr′ ′ = Bω−0.454 .
100 kHz, as compared to that of 615 in pure BNT. The value of the tangent loss is low up to 100 °C in BNTM compound for the entire experimental range of the frequency. The loss tangent of the compound exhibits relaxation peaks similar to the peak of the temperature-dependent dielectric constant. At the higher temperature and lower frequency, the loss tangent is higher due to the conductivity factor owing to the presence of the alkali metal ion Na+ and the existence of various oxidation states of the Mn ions. At higher frequency and lower temperature, the tangent loss is small in the BNTM compound. As evident from Fig. 7 (a) and (b), the sample possesses a dielectric constant of 430 with a tangent loss of 0.03 at 25 °C and a maximum dielectric constant of 812 with the tangent loss of 1.6 at the transition temperature, Tc = 372 °C at the measured frequency of 1 MHz. These parameters stand comparable with many of the popular dielectric materials for device applications at high frequency. 3.4.2. Frequency effect on dielectric studies The frequency effect on the dielectric parameters, i.e. ε′r and tanδ of the compound in the frequency range 100 Hz–1 MHz at selective temperatures between 50 °C to 300 °C for the BNTM compound is represented in Fig. 8 (a) and (b). The slope of the frequency dispersion of ε′r decreases with a rise in frequency, but increase with the increase in temperature. While, the relative dielectric constant at 100 Hz decreases from 19892 at 300 °C to 1483 at 50 °C, that at 1 MHz decrease from 749 to 436 for the same temperature range. In the selected experimental range of the frequency, the value of the dielectric constant varies due to changes in the contribution from space charge and dipolar polarization. With increasing frequency, the space charge polarization and some modes of the dipolar polarization cease due to the inertial nature of the dipoles and do not respond to the fast field variations resulting in the decrease of the ε′r. In Fig. 8 (a), the high value of dielectric constant at low-frequency and high-temperature regions can be explained by Maxwell and Wagner's mechanism (Koop's phenomenological theory) [41]. This model suggests that the material exhibits a relatively conducting grain with insulating grain boundary and thereby creates a barrier layer capacitance. In the high-frequency region, the charge carriers do not move to the grain boundary due to inertia and as a result, only grain effect contributes to the polarization process. While the charge carriers require more energy for their movement in the lower frequency region due to the high resistance of the grain boundaries, the energy requirement in the higher frequency region is less due to the small resistance of the grains. Thus, tanδ possess high value at a low frequency as compared to the higher frequency region. The tanδ is high in the lowfrequency and the high-temperature, due to the dominant contribution from the DC conductivity (σdc / ωε0 εr′). For better comprehension of the variation of dielectric constant with angular frequency in Fig. 8 (c), we have plotted the variation of εr' and
3.5. AC conductivity studies Fig. 9 (a) represents the temperature dependence of AC conductivity (σac ) with a frequency range of 100 Hz–1 MHz. The σac of the compound is generally derived from the dielectric measurement using the relation, σac = ωε0 εr′ tanδ . The temperature dependence of AC conductivity is studied by using the Arrhenius equation, σac = σ0 exp
( ). In this −Ea KB T
equation, σ0, Ea and kB are the pre-exponential factor, activation energy and Boltzmann constant respectively. The Ea is determined from the log σac Vs. 1000/T plot by the best linear fitting in the high-temperature region. The calculated Ea value reduces from 0.44 eV at 100 Hz to 0.39 eV at 1 MHz. In the conduction mechanism, Ea is the energy required for the activation of the charge carriers, and their hopping over long distances. The value of Ea decreases due to the number of hopping of the electron between localized states increases with rise in frequency of the applied electric field [42]. The various oxidation states of Mn ions at the Ti sites in the material, create oxygen vacancies to maintain the charge neutrality. The conduction is based on the hopping of electrons between the multivalent Mn ions and oxygen vacancies. The frequency dependence of AC conductivity of the compound in a temperature range of 50oC–300 °C is presented in Fig. 9 (b). The variation of AC conductivity with frequency is being analyzed by using Jonscher's power law, σ (ω) = σdc + Aω s where A is the polarizability factor and the exponent is s (≤1). The fitting of the AC conductivity with frequency gives the values of the exponent, s. The variation of s with temperature is accepted as an indicator for the conduction process 7
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Fig. 8. (a): Variation of ε′r, (b) tanδ with the frequency of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. (c) Variation of ε′r and ε′′r with angular frequency and the inset figure shows the Cole-Cole plot for the temperature 50 °C.
present in the dielectric materials [43]. In Fig. 9 (c), the value of s is found to decrease with the rise in temperature, attains a minimum value, and then increases again towards unity value. This feature
indicates that the overlapping large polaron tunneling (OLPT) mechanism is the dominant conduction process in the compound. The OLPT conduction mechanism was proposed by Long [44],
Fig. 9. (a) Variation of σac with 1000/T of Bi0.5Na0.5Ti0.80Mn0.20O3 compound. Variation of (b) σac with frequency, (c) Frequency exponent with temperature, (d) lnσdc with inverse the of the absolute temperature of Bi0.5Na0.5Ti0.95Mn0.05O3 compound. 8
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
Fig. 10. (a) Variation of Mʹ, (b) Mʹʹ, (c) Mʹʹ and Zʹʹ with frequency of Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
where the large polaron wells of two sites overlap resulting in the reduction in the polaron hopping energy
almost linearly with increasing temperature and it signifies the semiconducting nature of the compound.
rp WH = WHo ⎛1 − ⎞ R⎠ ⎝
3.6. Modulus properties
(6)
where, rp and R are the polaron radius and intersite separation respectively and the constant, WHo =
e2 , 4εp rp
The variation of M′ and M″ with frequency in the temperature range of 50oC–300 °C is shown in Fig. 10 (a) and (b). The analysis of the modulus properties identifies the grain, grain boundary and electrode interface effects to the sample. The M′ value approaches zero in the lower frequency region indicating the short-range mobility of charge carriers in the presence of an electric field. Further M′ increases with increasing frequency and tends to coincide at high-frequency, suggesting the absence of the electrode polarization in the material [45]. For temperature greater than 100 °C, two distinctive variations are observed, i.e. a horizontal region with almost zero value in lower frequency and a linearly increasing nature in higher frequency. The horizontal region in the high temperature and the lower frequency region indicates to the grain boundary effect and the high-frequency region for the grain mechanism. With the increase in temperature, M″max peak moves to the high-frequency region. This behavior of dielectric relaxation indicates that the hopping mechanism of charge carriers dominates intrinsically at the high-temperature regions in the thermally activated process. The broadening of the asymmetric peak increases with an increase in temperature and decreases the relaxation time of charge carriers. This type of characteristic is observed only in the nonDebye type of relaxation mechanism. The peak value of M″ occurs at frequency ωmax, such as ωmax τ = 1. For perfect Debye type nature, the relaxor peak of both M″ and Z″ occur at the same frequency at a given temperature. As the material is inhomogeneous in nature and the combination of more than one RC element with a constant phase element (Q), the resulting M″ and Z″ spectra occur at different frequencies as shown in Fig. 10 (c) for the data corresponding to 50 °C.
εp being the effective permit-
tivity. The AC conductivity for the OLPT model is given by
σac (ω) =
ωRω4 π4 2 e (KB T )2 [N (Ef )]2 ∗ 12 2αKB T + WHo rp + Rω2
(7)
Here, Rω is the hopping length at a frequency ω and α is the spatial decay parameter and takes a constant value. The frequency exponent, s, for this model can be represented as
s=1−
8αR ω + 6βWHo rp/ Rw (2αR ω + βWHo rp/ Rw )2
(8)
where β = 1/ KB T . The OLPT mechanism suggests that s should be temperature dependent. The equation shows that the s decreases from unity with an increase in temperature and for a small value of rp, s attains a minimum at a certain temperature and consequently rises with an increase in temperature. In Fig. 9 (b), the plateau response was noticed in lower frequency and higher temperature, and thus, σdc is calculated for all the temperatures from the use of Jonscher's power law to the AC conductivity data. The temperature dependence DC conductivity obeys the Arrhenius equation, σdc = σ0 exp
( ), where, the physical parameters −Ea KB T
have their usual meaning. Fig. 9 (d) shows the graph of lnσdc vs. 1000/T and the activation energy estimated from the graph amounts to 0.43 eV. It is worth mentioning here that the activation energy, determined for the lower frequency grain boundary contribution using the Zsimpwin fitting of the Nyquist plot was 0.38 eV. The DC conductivity increases 9
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
References [1] X. He, K.j. Jin, Persistence of polar distortion with electron doping in lone-pair driven ferroelectrics, Phys. Rev. B 94 (1–8) (2016) 224107. [2] A.K. Jena, S. Satapathy, J. Mohanty, Magnetic properties and oxygen migration induced resistive switching effect in Y substituted multiferroic bismuth ferrite, Phys. Chem. Chem. Phys. 21 (2019) 15854–15860. [3] M. Gröting, K. Albe, Comparative study of A-site order in the lead-free bismuth titanates M1/2Bi1/2TiO3 (M=Li, Na, K, Rb, Cs, Ag, Tl) from first-principles, J. Solid State Chem. 213 (2014) 138–144. [4] S.-E. Park, T.R. Shrout, Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals, J. Appl. Phys. 82 (4) (1997) 1804–1811. [5] A. Safari, E.K. Akdoğan (Eds.), Piezoelectric and Acoustic Materials for Transducer Applications, Springer, New York, 2008. [6] C. Kornpom, T. Udeye, T. Bongkarn, The preparation of lead-free bismuth sodium titanate ceramics via the solid state combustion technique, Integr. Ferroelectr. 177 (2017) 59–68. [7] C.Y. Kim, T. Sekino, K. Niihara, Synthesis of bismuth sodium titanate nanosized powders by solution/sol–gel process, J. Am. Ceram. Soc. 86 (9) (2003) 1464–1467. [8] M. Li, M. J. Pietrowski, R. A. De Souza, H. Zhang, I.M. Reaney, S.N. Cook, J.A. Kilner, D.C. Sinclair, A family of oxide ion conductors based on the ferroelectric perovskite Na0.5Bi0.5TiO3, Nat. Mater., doi:10.1038/NMAT3782. [9] Y. Chen, K. Uchino, D. Viehland, Substituent effects in 0.65Pb(Mg1/3Nb2/3)O30.35PbTiO3 piezoelectric ceramics, J. Electroceram. 6 (1) (2001) 13–19. [10] X. Wang, M. Gu, B. Yang, S.N. Zhu, W.W. Cao, Hall effect and dielectric properties of Mn-doped barium titanate, Microelectron. Eng. 66 (2003) 855–859. [11] Q. Zhang, R.R. Whatmore, Hysteretic properties of Mn-doped Pb(Zr, Ti)O3 thin films, J. Eur. Ceram. Soc. 24 (2004) 277–282. [12] H. Liu, W.W. Ge, X.P. Jiang, X.Y. Zhao, H.S. Luo, Growth and characterization of Mn-doped Na1/2Bi1/2TiO3 lead-free ferroelectric single crystal, Mater. Lett. 62 (2008) 2721–2724. [13] T. Takenaka, K. Sakata, Grain orientation effects on the electrical properties of bismuth layer structured ferroelectric Pb (1−x) (NaCe) x/2 Bi4Ti4O15 solid solution, J. Appl. Phys. 55 (4) (1984) 1092–1099. [14] M.B. Suresh, E.V. Ramana, S.N. Babu, S.V. Suryanarayana, Comparison of electrical and dielectric properties of BLSF materials in Bi─Fe─Ti─O and Bi─Mn─Ti─O systems, Ferroelectrics 332 (2006) 57–63. [15] Z. Hu, X. Xiao, C. Chen, T. Li, L. Huang, C. Zhang, J. Su, L. Miao, J. Jiang, Y. Zhang, J. Zhou, Al-doped α-MnO2 for high mass- loading pseudocapacitor with excellent cycling stability, Nano Energy 11 (2015) 226–234. [16] T. Li, J. Wu, X. Xiao, B. Zhang, Z. Hu, J. Zhou, P. Yang, X. Chen, B. Wang, L. Huang, Band gap engineering of MnO2 through in situ Al-doping for applicable pseudocapacitors, RSC Adv. 6 (2016) 13914–13919. [17] V. Purohit, R. Padhee, R.N.P. Choudhary, Dielectric and impedance spectroscopy of Bi(Ca0.5Ti0.5)O3 ceramic, Ceram. Int. 44 (2018) 3993–3999. [18] R. Das, R.N. P Choudhary, Studies of structural, dielectric relaxor and electrical characteristics of lead-free double Perovskite:Gd2NiMnO6, Solid State Sci. 87 (2019) 1–8. [19] P. Pascuta, E. Culea, FTIR spectroscopic study of some bismuth germanate glasses containing gadolinium ions, Mater. Lett. 62 (2008) 4127–4129. [20] V. Dimitrov, Y. Dimitriev, A. Montenero, IR spectra and structure of V2O5-GeO2Bi2O3 glasses, J. Non-Cryst. Solids 180 (1994) 51–57. [21] I. Ardelean, S. Cora, D. Rusu, EPR and FT-IR spectroscopic studies of Bi2O3–B2O3– CuO glasses, Physica B 403 (2008) 3682–3685. [22] L. Kang, M. Zhang, Z.H. Liu, K. Ooi, IR spectra of manganese oxides with either layered or tunnel structures, Spectrochim. Acta, Part A 67 (2007) 864–869. [23] A. Azama, A.S. Ahmed, S.S. Habib, A.H. Naqvi, Effect of Mn doping on the structural and optical properties of SnO2 nanoparticles, J. Alloy. Comp. 523 (2012) 83–87. [24] A.N. Murashkevich, A.S. Lavitskaya, T.I. Barannikova, I.M. Zharskii, Infrared absorption spectra and structure of TiO2-Sio2 composites, J. Appl. Spectrosc. 75 (5) (2008) 730–734. [25] M.J. Alam, D.C. Cameron, Preparation and characterization of TiO2 thin films by sol-gel method, J. Sol. Gel Sci. Technol. 25 (2002) 137–145. [26] A.M. Bobrova, I.G. Zhigun, M.I. Bragina, A.A. Fotiev, Infrared absorption spectra of various titanium compounds, J. Appl. Spectrosc. 8 (1) (1968) 96–101. [27] D. Tsiourvas, A. Tsetsekou, M. Arkas, S. Diplas, E. Mastrogianni, Covalent attachment of a bioactive hyperbranched polymeric layer to titanium surface for the biomimetic growth of calcium phosphates, J. Mater. Sci. Mater. Med. 22 (2011) 85–96. [28] K. Liu, H. Fan, P. Ren, C. Yang, Structural, electronic and optical properties of BiFeO3 studied by first-principles, J. Alloy. Comp. 509 (2011) 1901–1905. [29] J.R. Sambrano, E. Orhan, M.F.C. Gurgel, A.B. Campos, M.S. Goes, C.O. PaivaSantos, J.A. Varela, E. Longo, Theoretical analysis of the structural deformation in Mn-doped BaTiO3, Chem. Phys. Lett. 402 (2005) 491–496. [30] J.J. Adkin, M.A. Hayward, BaMnO3-x revisited: a structural and magnetic study, Chem. Mater. 19 (2007) 755–762. [31] A. K Jena, J. Mohanty, Enhancing ferromagnetic properties in bismuth ferrites with non magnetic Y and Sc Co-doping, J. Mater. Sci. Mater. Electron. 29 (2018) 5150–5156. [32] S. Thakur, N.K. Sahoo, M. Senthilkumar, R.B. Tokas, Optical properties and morphological changes in gadolinia films deposited under ambient substrate temperature conditions, Opt. Mater. 27 (2005) 1402–1409. [33] N.K. Sahoo, S. Thakur, M. Senthilkumar, D. Bhattacharyya, N.C. Das, Reactive electron beam evaporation of gadolinium oxide optical thin films for ultraviolet and
Fig. 11. P-E ferroelectric loop of Bi0.5Na0.5Ti0.80Mn0.20O3 compound.
3.7. Ferroelectric property The ferroelectric loop (polarization vs. electric field) of the DC poled sintered pellet is shown in Fig. 11. In the polling process, the sintered pellet was placed in silicone oil in the applied DC field of 3 kV/mm at 50 °C for 1 h. The P-E hysteresis measurement is carried out at room temperature with a frequency of 10 Hz. The observed hysteresis loop suggests the ferroelectric nature of the compound. The hysteresis loop exhibit a remnant polarization (Pr) of 3.372 μC/cm2 and a coercive field (Ec) of 4.651 kV/cm. The observed value of Pr is less than that of pure BNT (10.44 μC/cm2) prepared under similar conditions [46]. The decrease in the remanent polarization is being attributed to the smaller activation energy (as determined in the conduction process) resulting in an increase in the number of mobile ions.
4. Summary and conclusion The Bi0.5Na0.5Ti0.80Mn0.20O3 compound was fabricated by a conventional solid-state reaction technique. XRD spectrum supported by the Rietveld refinement reveals that the compound crystallizes in rhombohedral symmetry. The compound formation is supported by the presence of Bi-O, Na-O, Ti-O, and Mn-O stretching vibration band in the FTIR spectrum. The UV–Visible optical spectroscopy indicates a decrease in the optical band gap energy of the compound in comparison to BNT. The BNTM sample exhibits an enhanced dielectric characteristic in comparison to the pure BNT compound. The Nyquist plot exhibit double depressed semicircles confirming both grain and grain boundary contributions to the polarization process, and also the existence of a modified Debye relaxation process. The frequency dispersion of the complex dielectric constant is analyzed on the basis of the Nyquist plot and the Cole-Cole plot. The substitution of 20% Mn ions at the Ti sites results in a shift in the ferro-paraelectric phase transition peak from 330 °C to 370 °C in the material. The frequency dependence impedance and modulus suggest non-Debye nature of relaxation in the material with some sort of relaxor behavior. Activation energy calculated from the Nyquist plot data and conductivity data broadly agrees and points at the excitation of charge carriers on the basis of charge transfer between multivalent Mn ions and the oxygen vacancy created to maintain charge balance. The frequency-dependent AC conductivity analyzed using Jonscher's power law suggests an OLPT model of the conduction mechanism in the studied compound.
Acknowledgment The authors would like to thank Mr. Tridib Das of ZEISS Group and Central Research facility of the S'O'A (Deemed to be University) for their kind help in some experimental work. 10
Ceramics International xxx (xxxx) xxx–xxx
N. Pradhani, et al.
J. Eur. Ceram. Soc. 33 (2013) 2141–2153. [41] K. Parida, S. Das, P.K. Mahapatra, R.N.P. Choudhary, Relaxor behavior and impedance spectroscopic studies of chemically synthesized SrCu3Ti4O12 ceramic, Mater. Res. Bull. 111 (2019) 7–16. [42] S. Halder, K. Parida, S.N. Das, S.K. Pradhan, S. Bhuyan, R.N.P. Choudhary, Dielectric and impedance properties of Bi(Zn2/3V1/3)O3 electronic material, Phys. Lett. A 382 (2018) 716–722. [43] A. Ghosh, Frequency-dependent conductivity in bismuth-vanadate glassy semiconductors, Phys. Rev. B 41 (3) (1990) 1479–1488. [44] A.R. Long, Frequency-dependent loss in amorphous semiconductors, Adv. Phys. 31 (5) (1982) 553–637. [45] A.K. Jena, S. Satapathy, J. Mohanty, Magnetic and dielectric response in yttrium (Y)-manganese (Mn) substituted multiferroic Bi1-xYxFe1-yMnyO3 (x = y = 0, x = 0.03, 0.06, 0.12, y = 0.05) ceramics, J. Appl. Phys. 124 (2018) 174103. [46] N. Pradhani, P.K. Mahapatra, R.N.P. Choudhary, A.K. Jena, J. Mohanty, Investigation on the effect of Mn substitution on the structural, electrical and ferroelectric characteristics of Bi0.5Na0.5TiO3 ceramic, Mater. Res. Bull. 119 (2019) 110566.
deep ultraviolet laser wavelengths, Thin Solid Films 440 (2005) 155–168. [34] N. Pradhani, P.K. Mahapatra, R.N.P. Choudhary, Effect of cerium oxide addition on optical, electrical and dielectric characteristics of (Bi0.5Na0.5)TiO3 ceramics, J. Phys.: Materials 1 (2018) 015007. [35] F. Yang, S. Lin, L. Yang, J. Liao, Y. Chen, C.Z. Wang, First-principles investigation of metal-doped cubic BaTiO3, Mater. Res. Bull. 96 (2017) 372–378. [36] S. Sahoo, P.K. Mahapatra, R.N.P. Choudhary, Influence of Compositional Variation on Structural, Electrical and Magnetic Characteristics of (Ba1-xGdx) (Ti1-xFex) O3 (0.2≤ X ≤0.5), MRX-105576.R2, (2017) 1-29. [37] C.K. Suman, K. Prasad, R.N.P. Choudhary, Complex impedance studies on tungstenbronze electroceramic: Pb2Bi3LaTi5O18, J. Mater. Sci. 41 (2006) 369–375. [38] S. Hajra, N. Pradhani, R.N.P. Choudhary, S. S Sahoo, Fabrication, Dielectric and electrical characteristics of 0.94 (Bi0.5Na0.5)TiO3-0.06 BaTiO3 ceramics, Process. Appl. Ceramics 13 (1) (2019) 24–31. [39] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, Effects of heat-treatment temperature on the microstructure, electrical and dielectric properties of M-type hexaferrites, J. Electron. Mater. 43 (2014) 512–521. [40] S.Y. Cheng, J. Shieh, H.Y. Lu, C.Y. Shen, Y.C. Tang, N.J. Ho, Structure analysis of bismuth sodium titanate-based A-site relaxor ferroelectrics by electron diffraction,
11