Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4

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Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4 S. Santhosh, N. Lakshminarasimhann Functional Materials Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi 630006, Tamil Nadu, India Received 7 January 2014; received in revised form 9 April 2014; accepted 9 April 2014

Abstract Rutile type chromium niobate, CrNbO4 (CN), was synthesized by solid state reaction (CN-SSR) and sol–gel (CN-SG900) methods. The relative density of CN-SG900 was further improved by calcination at 1150 1C for 24 h (CN-SG1150). Scanning electron microscopic analysis showed the differences in the microstructures of these samples. The measured dielectric constants (@100 Hz) are 381, 1323 and 2973 for CN-SG900, CN-SG1150 and CN-SSR, respectively. A decrease in dielectric constant with increasing frequency in these samples revealed the presence of Maxwell–Wagner type interfacial polarization. The different dielectric behaviors of CN-SG900, CN-SG1150 and CN-SSR are related to the microstructural differences among conducting grains and resistive grain boundaries that contribute to the dielectric relaxation in these materials. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Sol–gel processing; B. Electron microscopy; C. Dielectric properties; C. Impedance; D. Niobates

1. Introduction Dielectric materials are key components in transformers, chokes, ICs, capacitors, wireless communications, etc. [1]. The efficiency of passive components depends on the magnitude of dielectric constant of the material used. A good capacitor requires a material with high dielectric constant and low dissipation factor [2]. High dielectric constant in semiconductor dielectrics originates from their microstructures comprising conducting grains with insulating grain boundaries [3]. Microstructure of a dielectric material plays a crucial role in determining its properties and it can be modified by adopting different synthesis routes and processing conditions. For instance, the internal domains were found to be responsible for the observed colossal dielectric constant in perovskites such as CaCu3Ti4O12 (CCTO) and Ba2BiSbO6 [4–6]. Colossal dielectric constant of Ba2CoNbO6 originated from a combination of polaron relaxation and Maxwell–Wagner relaxation involving the grain boundary response [7]. The internal barrier layer capacitance (IBLC) n

Corresponding author. Tel.: þ91 4565 241292; fax: þ 91 4565 227713. E-mail addresses: [email protected], [email protected] (N. Lakshminarasimhan).

associated with semiconducting grains and insulating grain boundaries, and electrode polarization effect were responsible for dielectric relaxations in NaCu3Ti3SbO12 ceramic sintered at different temperatures [8]. As compared to complex perovskite ceramics, binary columbite niobates (M2 þ Nb2O6; M¼ Ca, Mg, Mn, Co, Ni, Zn, etc.) are promising high Q dielectric microwave ceramics due to their simple chemistry, cation ordering and lower sintering temperatures (1000–1200 1C) [9]. Such simple binary oxides are advantageous over complex oxides due to facile phase formation and easy processing at lower temperatures. For example, the grain and grain boundary contributions were responsible for the dielectric constant (εr) of inverse spinel NiFe2O4 and the variation in εr was related to the difference in microstructures of samples obtained by adopting different synthesis routes [10–13]. Similarly, the orientational and space charge polarization resulted in high dielectric constant of CrNbO4 that increased with temperature due to the formation of a barrier layer between the grain boundaries [14]. CrNbO4, a representative of disordered AA0 O4 tetragonal rutile type transition metal oxide, is known for its magnetic, dielectric and sensing properties [14–16]. Chromium volatilization occurred

http://dx.doi.org/10.1016/j.ceramint.2014.04.053 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: S. Santhosh, N. Lakshminarasimhan, Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.053

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when the sample was sintered at high temperatures in the range 1250–1400 1C [14]. It is of interest to understand the grain and grain boundary contributions to dielectric properties of CrNbO4. In this work, we compared the dielectric properties of CrNbO4 synthesized by two different methods, namely, solid state reaction and sol–gel methods. The difference in the dielectric behaviors of these samples is related to their unique microstructures. The variation in the grain and grain boundary contributions to dielectric properties of CrNbO4 is understood using impedance spectroscopic studies.

These pellets were sintered in air at 1000 1C for 24 h. Thus sintered pellets were polished to get a uniform surface and applied with silver paste on both sides for electrical contact. The applied paste was dried at 400 1C for 10 min. Impedance measurements were carried out at room temperature by a computer controlled impedance analyzer (HIOKI LCR Hi Tester, Model-3532) operating in the frequency range between 100 Hz and 5 MHz. Room temperature frequency dependent dielectric constant (εr), ac conductivity (sac) and dc resistivity (ρdc) were calculated using the following relations:

2. Experimental

εr ¼

2.1. Synthesis

where C is the measured capacitance, d is the thickness of the pellet and A (=πr2) is the area of the pellet, r is the radius of the pellet and ε0 is the permittivity of free space (8.854  10  12 F m  1)

CrNbO4 was synthesized by a conventional high temperature solid state reaction method and sol–gel synthesis. In the solid state reaction method, stoichiometric amounts of Cr2O3 (Sigma Aldrich 99.9%) and Nb2O5 (Sigma Aldrich 99.9%) were thoroughly ground and heated at 1150 1C for 24 h. For sol–gel synthesis, stoichiometric amounts of Cr(NO3)3  9H2O (Sigma Aldrich 99%) and NbCl5 (Aldrich 99%) were dissolved in separate solutions containing 15 ml 1-butanol and 3 ml glacial acetic acid. The clear solutions obtained were subjected to ultrasonication for 5 min and then chromium precursor solution was added dropwise into the solution containing niobium precursor under constant stirring that continued for 2 h at room temperature. The solution was gradually heated to 80 1C under stirring to evaporate the solvent and a viscous gel was formed which on further heating to  150 1C transformed into a black mass. Portions of this mass were heated at different temperatures between 600 and 900 1C for 6 h to find the phase formation. The samples obtained by the solid state reaction method and sol–gel synthesis followed by heating at 900 1C are designated as CN-SSR and CN-SG900, respectively. CN-SG900 was further calcined at 1150 1C for 24 h to improve the density of the sample which is designated as CNSG1150. 2.2. Characterization Phase formation and purity of CN-SSR, CN-SG900 and CN-SG1150 were examined by powder X-ray diffraction (XRD) technique (D8 Advance, Bruker) using Cu Kα radiation (1.5418 Å). Rietveld refinement was performed for CN-SSR using FullProf program [17,18]. Thermogravimetric analysis (TGA) of dried precursor obtained by sol–gel synthesis was carried out in air (PerkinElmer, DTA 1700). Electron paramagnetic resonance spectra of CN-SSR and CN-SG900 powder samples were recorded at room temperature (Bruker EMX Plus, 9.781 GHz). The relative densities of CN-SG900 and CN-SG1150 with respect to CN-SSR were measured by pycnometry. Field emission scanning electron microscopy (FE-SEM; GEMINI, SUPRA 55VP Zeiss) was used to analyze the microstructures of sintered CN pellets. For impedance measurements, the obtained CN powders were pressed into pellets of dimension 10 mm diameter and 1 mm thickness.

Cd ε0 A

tan δ ¼

ð1Þ

ε″ ε0

ð2Þ

sac ¼ 2πf ε0 ε″

ð3Þ

sac ¼ 2πf ε0 ε0 tan δ

ð4Þ

ρdc ¼

R UA d

ð5Þ

where ε0 and ε″ are the real and imaginary parts of dielectric constant, respectively, δ is the phase angle, f is frequency, R is the resistance of the sample and d is the thickness of pellet [19]. 3. Results and discussion 3.1. Sample characterization CrNbO4 crystallizes in the rutile structure with space group P42/mnm [15]. Both chromium and niobium atoms distribute randomly at 2a Wyckoff position and oxygen atoms occupy the 4f position. Rietveld refinement of CN-SSR sample was carried out (S.G. P42/mnm) and the profile is shown in Fig. 1 along with the unit cell structure (inset of Fig. 1). The refined atomic coordinates, isotropic displacement parameters and lattice constants are listed in Table 1. The obtained tetragonal lattice parameters of 4.6443 and 3.0121 Å are in close agreement with the values (4.6443 and 3.0125 Å) of the standard pattern of CrNbO4 available in ICDD database (No. 034-0366). The TGA trace of dried precursor obtained in the sol–gel synthesis is shown in Fig. 2. The weight loss up to  150 1C can be attributed to the loss of adsorbed water. Further, a drastic weight loss (30%) observed between 270 and 375 1C is due to the removal of organic residues from the precursors. The insignificant weight loss above 375 1C reveals the complete decomposition of organic precursors resulting in the formation of the oxide. The powder XRD patterns of materials obtained by heating the dried precursor in portions at different temperatures in the range between 600 and 900 1C are shown in Fig. 3. It is clear that CrNbO4 phase starts evolving

Please cite this article as: S. Santhosh, N. Lakshminarasimhan, Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.053

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Fig. 1. Rietveld profiles of CN-SSR showing the observed, calculated and difference profiles, and Bragg positions. The unit cell structure of CrNbO4 is shown as inset in which small red spheres represent Cr (or Nb) and large green spheres correspond to O atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3

Fig. 3. Powder XRD patterns of samples obtained by heating the sol-gel precursor at different temperatures from 600 to 900 1C. The phases are identified and marked inside.

Table 1 Atomic coordinates and isotropic atomic displacement parameters (Å2) for CN-SSR. Atom

x

y

z

Biso

Cr (Nb) O

0 0.303(3)

0 0.303(3)

0 0

0.64(1) 0.9(1)

a¼ b¼4.6443 Å; c¼ 3.0121 Å; Rp : 10.3; Rwp ¼ 9.13; Rexp ¼6.17.

Fig. 4. Powder XRD pattern of CrNbO4 synthesized by (a) the solid state reaction method (CN-SSR). (b, c) Patterns of samples obtained after heating the sol–gel precursor at 900 1C (CN-SG900) and 1150 1C (CN-SG1150), respectively. The standard pattern is also shown for reference.

the phase purity of CN-SG900 and CN-SG1150. The lines are broader in the XRD pattern of CN-SG900 when compared to the lines of CN-SSR and CN-SG1150 and this can be due to the smaller crystallite size of the former. The crystallite sizes of all three samples were calculated from the line broadening using Scherrer's formula [20] Fig. 2. Thermogravimetric (TG) analysis trace of dried precursor obtained by the sol–gel method.

only at 800 1C below which the oxides Nb2O5 and Cr2O3 remain unreactive under the present synthesis conditions. Single phase formation was observed for CrNbO4 when the precursor was heated at 900 1C. Powder XRD patterns of CN samples obtained by the sol–gel method are compared with those of CN-SSR along with the standard pattern in Fig. 4. All the reflections were indexed and the absence of any additional reflection confirmed

t¼ B¼

0:9 λ B cos θB qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi B2M  B2S

ð6Þ ð7Þ

where λ is the wavelength of X-ray used (1.5418 Å), θB is Bragg's angle, B is the full width at half-maximum (FWHM) that can be measured using Eq. (7) in which BM is the measured FWHM in radians for the sample and BS is the FWHM of the standard used (Alumina, Korundprobe). The calculated average crystallite sizes are 207, 28 and 280 nm for CN-SSR, CN-SG900 and

Please cite this article as: S. Santhosh, N. Lakshminarasimhan, Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.053

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Fig. 5. Electron paramagnetic resonance spectra of CN-SSR and CN-SG900 powder samples obtained at room temperature.

CN-SG1150, respectively. The relative densities of CN-SG900 and CN-SG1150 with respect to CN-SSR are 78% and 87%, respectively. CrNbO4 is known to exhibit spin-glass behavior with spin only magnetic moment of 3.85 μB conforming to Cr3 þ ion [15]. CN-SSR and CN-SG900 samples were characterized by electron paramagnetic resonance (EPR) spectroscopic technique and the spectra are shown in Fig. 5. The obtained ‘g’ value for both samples is 1.97 which is similar to the values reported for Cr3 þ ion in Cr2(MoO4)3, CrVO4 and MgCr2O4 [21]. The same ‘g’ value in both CN-SSR and CN-SG900 reveals the presence of Cr3 þ ion in similar environments in these samples obtained by different methods. 3.2. Microstructural analysis Microstructures of sintered pellets of CN-SSR, CN-SG900 and CN-SG1150 were examined using FE-SEM and the images are shown in Fig. 6. Grains of CN-SSR (Fig. 6a and b) are larger when compared to those of CN-SG900 (Fig. 6c and d). When CN-SG900 is heated at 1150 1C, grain growth occurs which is witnessed in the FE-SEM image of this sample (Fig. 6e and f). FE-SEM images show that CN-SSR and CN-SG1150 possess larger grains (4400 nm in size) with well defined continuous grain boundaries, and less number of voids and pores. On the other hand, CN-SG900 has smaller grains ( 100 nm in size) and aggregates of nanoparticles resulting in less clear and discontinuous grain boundaries. These microstructural differences among CN-SSR, CN-SG900 and CN-SG1150 can be attributed to the different methods adopted for the synthesis of these samples. The differences in the microstructures of CN samples are expected to influence the dielectric properties of these samples as grain and grain boundary structures play an important role in determining the impedance. 3.3. Dielectric properties Fig. 7 shows the variation in dielectric constant of CN-SSR, CN-SG900 and CN-SG1150 as a function of frequency. CN-SSR

exhibited the highest dielectric constant at low frequencies when compared to CN-SG900 and CN-SG1150. The dielectric constant of CN-SG1150 exceeded that of CN-SG900. At low frequencies, the dielectric constant arises due to electronic, ionic, dipolar and space charge polarizations, especially with predominant contribution from space charge polarization. Maxwell–Wagner theory and Koop's model predict that the space charge polarization arises by the drop in applied voltage in a material having well conducting grains separated by insulating boundaries [22]. Materials possessing continuous grain boundaries exhibit prominent space charge polarization that leads to high dielectric constant at low frequencies. Well interconnected grains can accommodate large number of dipoles. Continuous grain boundaries between larger grains present in CN-SSR sample as seen in the FE-SEM image (Fig. 6) result in the highest dielectric constant at low frequencies among the CN samples studied in this work. The presence of noncontinuous grain boundaries and more voids in CN-SG900 results in low dielectric constant. Grain and grain boundary contributions to resistance are discussed in later section with Cole–Cole plots. At higher frequencies, the dielectric constant of both CN-SSR and CN-SG900 (Fig. 7) decreases and at frequencies beyond 104 Hz the dipole or charge carriers do not follow the frequency of applied alternating field [23]. A constant non-zero value attained at high frequencies is due to the combination of dipolar, ionic, and electronic polarizations [23]. Since our measurements are taken up to 5 MHz, we are unable to separate these contributions. The dielectric constant of CN can be estimated from the ionic polarizability values of constituent ions using Clausius– Mossotti equation [24]. The molecular polarizability of CrNbO4 can be calculated using the ionic dielectric polarizability (α) values of Cr3 þ (1.45 Å3), Nb5 þ (3.97 Å3) and O2  (2.01 Å3) with the following formula: αTDðCrNbO4 Þ ¼ αCr þ αNb þ 4αO

ð8Þ

The dielectric constant κ0 can be estimated using Clausius– Mossotti equation κ0 ¼

3V m þ 8παTD 3V m  4παTD

ð9Þ

where Vm is the molar volume in Å3 and αTD is the total dielectric polarizability. The calculated dielectric constant for CrNbO4 using the above equation is found to be 21. The experimentally measured values at a frequency of 1 MHz are 66, 19 and 47 for CN-SSR, CN-SG900 and CN-SG1150, respectively. The calculated and measured dielectric constant values are found to be in close agreement with each other in CN-SG900. On the other hand, the experimental dielectric constants of CN-SSR and CN-SG1150 exhibit a large deviation from the calculated values. The large deviation from the calculated dielectric constant can be attributed to ionic or electronic conductivity, or rattling/compressed cations in the structure [25]. However, the effect of particle size and microstructure on the deviations in the experimental dielectric constants from the calculated values is not known. We could attribute the difference between calculated and measured dielectric constants of CN-SSR, CN-SG900 and CN-SG1150 to the microstructural differences in addition to other effects.

Please cite this article as: S. Santhosh, N. Lakshminarasimhan, Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.053

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Fig. 6. FE-SEM images of sintered disks of CN-SSR (a, b), CN-SG900 (c, d) and CN-SG1150 (e, f) at different scales of magnification.

Table 2 lists the measured dielectric constant and loss factors (@ 100 Hz), ac and dc conductivities (@ 10 kHz) of CN-SSR, CN-SG900 and CN-SG1150 samples. Both ac and dc conductivities of CN-SG900 are lower when compared to those of CN-SSR. The loss coefficient of a dielectric material is the dissipation of electrical energy due to leakage of current and a perfect dielectric material must possess a low dissipation

factor (tanδ). Fig. 8 shows the variation in dissipation factor of CN-SSR, CN-SG900 and CN-SG1150 as a function of frequency. The lowest dissipation can be found for CNSG1150. At frequencies below 1 kHz, CN-SG900 exhibited a higher and constant dissipation factor ( 10) than that of CNSSR. With increase in frequency, the dissipation factor of CNSG900 decreased drastically before it approached a minimum.

Please cite this article as: S. Santhosh, N. Lakshminarasimhan, Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.053

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CN-SSR exhibited a gradual decrease in dissipation factor with increasing frequency and attained a constant minimum value at frequencies 4 100 kHz. The difference observed in the variation in dissipation factor among CN samples with frequency is due to the inhomogeneities in the microstructures such as grain, grain boundary, pores, etc. [23] The dissipation factors (@ 100 Hz) are 5.0641, 9.99, and 0.71772 for CN-SSR, CN-SG900 and CN-SG1150, respectively. Dissipation factor represents the lagging of polarization with the applied electric field and its value is expected to be large for materials with high conductivity [9]. Despite its higher conductivity, CN-SSR exhibited a lower dissipation factor when compared to that of CN-SG900. From this, it is inferred that the dissipation factor depends not only on the conductivity, but also on other extrinsic parameters like high grain boundary resistance, high dipole storage capacity of grains, etc. In CN-SSR, the accumulation of space charges at continuous grain boundaries results in the high dielectric constant at low frequencies. The absence of any peak in the variation in dissipation factor with frequency (Fig. 8) is related to the polarization by slowly mobile hopping charge carriers [25]. Low dielectric loss and enhanced dielectric constant of CN can be obtained by optimizing the microstructure in suitable manner. Fig. 9 shows the variation in real and imaginary parts of impedance of CN-SG900, CN-SSR and CN-SG1150 with applied frequency and this variation shows blunt peaks. This is due to the inhomogeneities in grain, grain boundaries and at the electrode–material interface. The blunt peak shows the

presence of a distribution of relaxation times giving rise to a constant phase element (CPE) that will be discussed in the following section. It is observed that the Debye peak of CNSSR is relatively asymmetric and the peak maximum lies at a lower frequency when compared to that of CN-SG900. With larger grains, CN-SSR can hold more number of dipoles resulting in a strong dipole–dipole interaction due to which the dipole orientation becomes difficult [26]. On the other hand, change in orientation of dipoles is possible in CN-SG900 with smaller grains that can hold less number of dipoles. The frequency response of CN-SG1150 showed the Debye peak lying at a similar frequency (104 Hz) as that of CN-SSR. Microstructure of dielectric materials is composed of conducting grains divided by resistive grain boundaries. Bulk and grain boundary effects can be distinguished for a dielectric material using complex impedance plane or Cole–Cole plot which is a plot between the imaginary part ( Z″) and real part (Z0 ) of impedance as a function of frequency [27]. Fig. 10 shows the Cole–Cole plots of CN-SSR, CN-SG900 and CNSG1150 at room temperature. All samples exhibited depressed semicircles revealing the existence of constant phase element (CPE) [22,28]. The CPE can be related to impedance as

Fig. 7. Frequency response of dielectric constant of CN-SSR, CN-SG900 and CN-SG1150 samples at room temperature.

Fig. 8. Frequency dependent dissipation factor (tanδ) of CN-SSR, CN-SG900 and CN-SG1150 revealing the different behaviors.

1 ð10Þ Q ðiωÞn pffiffiffiffiffiffiffiffi where i ¼  1, n lies between 0 and 1 and Q is a constant having dimension Fsn  1 [9]. Cole–Cole plots of CN-SSR, ZðωÞ ¼

Table 2 Calculated ac and dc conductivities of CN samples. Sample

CN-SG900 CN-SG1150 CN-SSR

Dielectric constant, εr (@ 100 Hz)

Dissipation factor, tanδ (@ 100 Hz)

Frequency at 10 kHz sac (10  5 Ω  1 m  1)

ρac (kΩ m)

sdc (10  5 Ω  1 m  1)

ρdc (kΩ m)

381 1323 2973

9.99 0.72 5.06

8.31 4.21 18.43

12.04 23.74 5.43

7.64 8.85 11.12

13.10 11.30 8.99

Please cite this article as: S. Santhosh, N. Lakshminarasimhan, Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.053

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Fig. 9. Variation in the real and imaginary parts of impedance with frequency of (a) CN-SSR, (b) CN-SG900 and (c) CN-SG1150.

CN-SG900 and CN-SG1150 are fitted using a PAS-IM6 Electrochemical Impedance Analyzer–Thales 4.18 USP software, and the insets in Fig. 10a–c show the corresponding equivalent circuit models comprising a resistor in parallel with CPE and this combination is connected in series with another similar combination. Both these combinations represent the contributions of grains and grain boundaries to the observed impedance at high and low frequencies. Table 3 lists the calculated values of grain and grain boundary resistances along with CPE values obtained from the Cole–Cole plot. Fig. 10a shows the Cole–Cole plot of CN-SSR that consists of two semicircles: one at high frequency and another incomplete one extending to a low frequency region. The dominant low frequency arc in CN-SSR can be attributed to the resistance arising from continuous grain boundaries. In this curve, the diameter of the low frequency semicircle is larger than that of the one observed at high frequencies revealing a higher grain boundary contribution to the resistance [22]. The highly resistive grain boundaries (Rgb ¼ 44.7 MΩ) of CN-SSR act as barriers for charge carriers to flow and thus lead to space charge polarization which contributes to the high dielectric constant observed at low frequencies [9,22]. The space charge polarization does not follow the frequency of the applied field at high frequencies ( 41 kHz) and undergoes relaxation. On the other hand, polarization due to dipole orientation occurs

within the grains at high frequencies. Also, the conductivity of the material increases at high frequencies. These effects led to a decrease in the dielectric constant of CN-SSR at high frequencies. Fig. 10b shows the Cole–Cole plot of CN-SG900 that is composed of two overlapping depressed semicircles. The low frequency semicircle is complete in this case when compared to that of the curve of CN-SSR (Fig. 10a). Similar to CN-SSR, the diameter of the low frequency semicircle is larger than that of the high frequency semicircle revealing higher resistance of grain boundaries (417 kΩ) than grains (49 kΩ). However, the resistance of both the grains and grain boundaries of CN-SG900 are lower than the respective values of CN-SSR. Fig. 10c shows the Cole–Cole plot of CN-SG1150 that shows one depressed semicircle and another incomplete semicircle. In this case, an increase in the resistance of grains and grain boundaries (1700 kΩ and 23.2 MΩ, respectively) enhanced its dielectric constant as compared to that of CN-SG900. Despite higher resistance of grains, the observed lower dielectric constant of CN-SG1150 than that of CN-SSR can be attributed to the low resistance of grain boundaries that hinders space charge polarization. Our results demonstrate the importance of well grown grains with continuous grain boundaries in enhancing the dielectric properties of CrNbO4. There is scope for further optimization of microstructure of CrNbO4 and other binary niobates by modifying the synthesis

Please cite this article as: S. Santhosh, N. Lakshminarasimhan, Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.053

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Fig. 10. Cole–Cole plots of (a) CN-SSR (b) CN-SG900 and (c) CN-SG1150 with equivalent circuit models used for fitting the data.

Table 3 The calculated resistance and CPE values from the fitted Cole–Cole plots of CN samples as shown in Fig. 10. Sample

CN-SG900 CN-SG1150 CN-SSR

Bulk region

Grain boundary

CPE1

CPE2

Rg(kΩ)

Qg (10  12 F)

ng

Rgb (MΩ)

Qgb (10  12 F)

ngb

49 1700 218

1400 133 21

0.6 0.7 0.9

0.4 23 45

19 665 1967

0.9 0.8 0.7

and processing conditions to achieve dielectric properties of practical interest. 4. Conclusions Microstructural dependence of dielectric properties has been demonstrated in binary rutile type CrNbO4 by comparing the impedance spectroscopic results of samples synthesized by solid state reaction and sol–gel methods. CrNbO4 synthesized by the solid state reaction method exhibited a higher dielectric constant of 2973 (@ 100 Hz) when compared to those of samples obtained by the sol–gel method (381 and 1323). Large grains with continuous and insulating grain boundaries are essential to obtain a high dielectric constant

arising from space charge polarization in CrNbO4. Optimization of synthesis and processing conditions will facilitate the fine tuning of dielectric properties of niobates through microstructural control. Acknowledgments The author SS acknowledges the University Grants Commission (UGC), New Delhi, India for a research fellowship. Central Instrumentation Facility (CIF) division of CSIR– CECRI is acknowledged for characterization facilities. MULTIFUN (CSC0101) project of CSIR New Delhi is acknowledged. Department of Chemical Engineering, POSTECH, Republic of Korea is acknowledged for TGA result.

Please cite this article as: S. Santhosh, N. Lakshminarasimhan, Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.053

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Please cite this article as: S. Santhosh, N. Lakshminarasimhan, Impedance spectroscopic studies, dielectric properties and microstructure of rutile type chromium niobate CrNbO4, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.053