Journal of Alloys and Compounds 576 (2013) 154–160
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Effect of isovalent ion substitution on electrical and dielectric properties of LaCrO3 Pravin Kumar a, Rajesh Kumar Singh a, A.S.K. Sinha b, Prabhakar Singh a,⇑ a b
Department of Applied Physics, Indian Institute of Technology (Banaras Hindu University), Varanasi 221 005, India Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi 221 005, India
a r t i c l e
i n f o
Article history: Received 12 December 2012 Received in revised form 17 April 2013 Accepted 17 April 2013 Available online 24 April 2013 Keywords: Anode materials SOFC Oxide materials Dielectric response
a b s t r a c t In the present investigation, the effect of isovalent doping on the electrical and the dielectric properties of the system, La1xGdxCrO3 was studied. A few compositions of Gd doped lanthanum chromite, La1xGdxCrO3 (x = 0.00, 0.01, 0.05, 0.10, 0.15 and 0.20) were synthesised via auto-combustion route. The XRD patterns of all the compositions showed single phase formation having crystal structure similar to that of undoped LaCrO3. The Rietveld refinement of the XRD data of all the compositions indicated orthorhombic structure with space group Pnma at room temperature. The SEM micrographs revealed spherical, oval and irregular morphologies of all compositions having low porosity. The FTIR spectra of the investigated compositions showed various structural modes of vibrations. Real part of electrical conductivity exhibited frequency independent behaviour in the frequency range 20 Hz–1 MHz and in the temperature range 40–400 °C. The DC conductivity of the system was found to show Arrhenius behaviour up to 300 °C. The DC conductivity increased with x up to x = 0.01, the trend was reversed in the range 0.01 6 x 6 0.10 and thereafter it again increased with x up to x = 0.20. The temperature and composition dependent conduction mechanism was explained in terms of defects created due to change of concentration of dopant ion and the size difference between the host and dopant ions. The XPS analysis showed that the concentration of Cr6+ sites increased with the increase in Gd concentration, supporting the proposed conduction mechanism. The orientational and the space charge polarisation processes have been attributed to the total dielectric behaviour of the system in the measured frequency and temperature ranges. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Lanthanum chromite (LaCrO3) possesses perovskite structure having general formula ABO3. Suitably doped LaCrO3 has been proposed as a promising and a stable nickel-free anode material for solid oxide fuel cells (SOFCs) [1–4]. Among various existing anode materials, nickel/YSZ and Fe/Co perovskite are found to show low stability in very low oxygen partial pressure [1]. Ce doped LaCrO3 has been reported as a potential anode material for SOFC [5]. Zhu et al. [6] have mentioned that due to more concentration of Cr3+ ions in Ce0.9Sr0.1Cr0.5F0.5O3+d, this material is suitable as anode materials in SOFC. It has been reported in the literature [7–10] that lanthanum chromite, doped with alkaline earth substituent, has high p-type conductivity and are found be stable over vide range of oxygen partial pressure. Due to its stability in low partial pressure, LaCrO3 based materials also find application as interconnects for SOFC [11,12]. Ca doped lanthanum chromite (La0.7Ca0.3Cr0.97O3), system has been reported as low cost-effective interconnect material for SOFC application [13,14]. Recently, Heidarpour et al. ⇑ Corresponding author. Tel.: +91 542 6701916; fax: +91 542 2368428. E-mail address:
[email protected] (P. Singh). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.118
[15] have also shown that lanthanum chromite can be processed by screen printing/co-sintering as interconnect material on NiYSZ substrate for SOFC applications. Doped lanthanum chromites have been synthesised by various techniques such as sol gel [16], hydrothermal [17], citrate gel combustion [18] and solid state routes [19]. In our earlier work [20] we have synthesised pure lanthanum chromite by solid state route and combustion method and measured its electrical and structural properties. We found that combustion route is better for structural homogeneity however the conduction mechanism was found to be independent of synthesis route. Electrical properties of this system have been studied extensively and reported in the literature in pure and doped forms. Sr doped lanthanum chromite, La1xSrxCrO3, showed thermally activated temperature dependence of electrical conductivity over a vast temperature range [7]. This system was found to display temperature dependent carrier mobility rather than carrier concentration. The La0.7Sr0.3Mn1xCrxO3 system is reported to exhibit a change from semi-insulating to metallic behaviour at high concentration of Cr [21]. Liu et al. [22] have also reported the effect of Cr6+ ions on the electrical conductivity of La1xSrxCrO3. Ca doped LaCrO3 was found to show thermally activated small hole-polaron hopping conduction and was found to depict decreasing electrical
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conductivity with increasing concentration of Ca [23]. However, Jiang et al. [24] have reported that for La1xMxCrO3 (with M = Mg, Ca, Ba, Sr), Ca and Sr doped compositions exhibited highest conductivities. It has been reported that Ca doped lanthanum chromite and Gd doped ceria composite can potentially serve as an electrode for symmetric SOFC operation [25]. Ong and Wu [26] have also reported that electrical conductivity of lanthanum chromite increased by doping of Ca and Ba. The effect was found to be more pronounced for the Ba doped system. Zhang et al. [16] have measured electrical conductivity of mixed doped La0.9Ca0.1Cr0.5Ni0.5O3 system and showed a phase transition at 192 °C. The doping of divalent elements (like Ca, Sr, and Ba) at trivalent sites (La-site) in lanthanum chromite produced more holes in the valence band, which reduced the energy band gap resulting into an increase in conductivity of this system [27]. The heterovalent substitution in the lanthanum chromite system has been studied by various research groups, however, there are only a few studies reported on the isovalent substituted lanthanum chromite system. The structural characterisation of Sm doped lanthanum chromite, forming an isovalent system, has been studied by ELSheikh and Rashad [28]. In present investigation, we report synthesis, characterisation and electrical/dielectric properties of isovalent doped lanthanum chromite, La1xGdxCrO3, system. The ionic radius of dopant ion, Gd3+ (0.94 Å) is smaller than the ionic radius of the host ion, La3+ (1.061 Å), so it was considered worthwhile to investigate the effect of size and concentration of dopant ions on the structural, electrical and dielectric properties of LaCrO3 system. 2. Experimental A few compositions of Gd doped lanthanum chromite, La1xGdxCrO3 (x = 0.00, 0.01, 0.05, 0.10, 0.15, 0.20) were synthesised via auto-combustion synthesis route. Among the various available synthesis techniques, auto combustion route is relatively simple, fast and effective method. La1xGdxCrO3 system was prepared by using La2O3, Cr2O3 and Gd2O3 as starting materials from Alfa Aesar having purity more than 99.9%. An appropriate amount of La2O3, Cr2O3 and Gd2O3 were weighed, fully dissolved in HNO3 solution and denitrified up to five times by distilled water to maintain the pH value of the solution. 4 g of urea was added to the solution for precipitation purpose as well as for homogeneous mixing of constituents. After continuous stirring for about an hour between 80 and 150 °C, a transparent solution was obtained which was heated up 300 °C for 30 min in oven. Auto combustion took place inside the oven at around 300 °C and polycrystalline powders were obtained as ash. The dried mixed powders were calcined in an alumina crucible at 1000 °C for 5 h in air and then furnace cooled to room temperature. A few drops of 2% solution of polyvinyl alcohol were added to the powder as a binder. The calcined powders were ground, mixed and pelletized at a pressure of 10 tons. The pellets were sintered at 1300 °C for 6 h. The pellets were initially heated at a rate of 2 °C per minute to 500 °C, kept at that temperature for 1 h to burn out the binder, thereafter, the pellets were heated at a rate of 4 °C per minute to 1300 °C. Subsequently, the samples were furnace cooled to the room temperature. The phase-purity, structure, and lattice parameters of the samples were determined using X-ray powder diffraction (Rigaku Miniflex II desktop) with Cu Ka radiation (k 1.54 Å) at room temperature. Sintered pellets were polished and coated with a high temperature Ag-paint on both sides. The coating was matured by firing the samples at 700 °C for 20 min. The conductance and the dielectric loss factor of the silver coated samples were measured by Wayne Kerr (6500 P Series) LCR meter in the frequency range 20 Hz–1 MHz and temperature range 40–520 °C using the two-probe method. For this, a polished and silver coated pellet was placed in a sample holder in between the two electrodes and, conductance G and tangent loss D were measured for all compositions. The grain, grain boundary morphology were studied using a scanning electron microscopy (LEO-1455). The FT-IR spectra were recorded in the 4000– 400 cm1 range with a Shimadzu spectrometer using the KBr dilution technique. The density of each composition was measured by Archimedes Principle employing Mettler-Toledo density measurement kit attached to weighing balance. XPS spectra of a few compositions were measured on a KRATOS (Amicus model) high performance analytical instrument using Mg target under 1.0 106 Pa pressure.
3. Results and discussion The phases which were formed and the structural parameters of the powders of various compositions were investigated using XRD data. Peaks in the XRD data, of all the samples, showed high inten-
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sities indicating high crystalline nature of the samples with single phase formation. The single phase formation of LaCrO3 was confirmed by the absence of characteristic lines of constituent oxides or any other phases. Fig. 1a–f represents the Rietveld refinement of XRD data for pure and doped lanthanum chromite. The peak profiles were modelled by using pseudo-voigt function and back ground has been described in terms of twelve coefficient polynomial. The Rwp (weighted-pattern factor) and S (goodness-of-fit) parameters were used as numerical criterion of quality of the fit of calculated to experimental diffraction data. The structural parameters of the powders were investigated by the Rietveld analysis of XRD data applying the FullProf software. Results of the analysis confirmed the orthorhombic structure with space group Pnma at room temperature in all compositions. The refined lattice parameters and unit cell volume, obtained from structural refinements of all studied samples, are shown in Table 1. The values of lattice parameters corroborate with the values published earlier for pure and doped lanthanum chromite [7,18,26,27]. The table indicates a slight variation in the lattice parameters and cell volume of different samples. The lattice parameters (a, b, c) and the unit cell volume slightly decreased with an increase in the concentration of substituted Gd3+ on La3+ site in LaCrO3. This is attributed to the smaller ionic radii of dopant (Gd3+) ions in comparison to the ionic radii of host (La3+) ions. The composition with x = 0.20 shows a slight increase in its a-parameter, however, at the same time a decrease in c-parameter is also observed. It seems that in this composition, the increase in a value is compensated by decrease in c value so that the total volume remains almost constant. The percentage density of each composition was calculated from the experimental value and is mentioned in Table 1. Our density data corroborate with the data published earlier [24,30]. Average crystallite size of all the samples were calculated by analysing the XRD data and using Williamson–Hall equation b Cos h = Kk/t + 2e Sin h, where b (in radians) is the full width at half maximum (FWHM) intensity of a Bragg reflection excluding the instrumental broadening, h is a diffraction Bragg angle, K is Scherrer’s constant, k is the wavelength of X-ray radiation and e be the lattice strain. All the XRD peaks were fitted with pseudo-voigt function. The crystallite size was calculated from the Williamson–Hall plot using MDI Jade software and is given in Table 1. The FTIR spectrum for the system La0.99Gd0.01CrO3 is shown in Fig. 2 as a representative plot of the system La1xGdxCrO3. The absorption bands at 449, 594, and 636 cm1 are assigned to O–Cr–O, Cr–O, Cr–O stretching modes of vibrations, respectively and the weak intensity peak at 920 cm1 was assigned to the La–O stretching vibrations. Similar result has been reported by EL-Sheikh and Rashad [28]. The usual peaks appearing at 1650 cm1 and 3600 cm1 are assigned to the symmetric stretching and asymmetric stretching modes of vibrations, respectively of absorbed water molecules at the sample surface. No absorption peak corresponding to Gd associated bond is observed in the IR spectra of the investigated system. SEM micrographs of all the compositions of La1xGdxCrO3 are depicted in Fig. 3a–f. It is observed from these micrographs that the samples have relatively high density with spherical, oval and irregular morphology. The SEM images also indicate the formation of cluster and agglomeration of particles. The size of agglomerate Gd3+ doped LaCrO3 samples were slightly less compared to pure LaCrO3. It is also estimated from the figure that the average agglomerate sizes of the samples lie between 0.5 and 1.0 lm. From the variation of microstructures of all the samples, it is concluded that the substitution of Gd3+ on La3+ site in lanthanum chromite slightly affects the agglomerate size. The average agglomerate sizes of microscopic particles first decreases and then gradually increases with increase in dopant concentrations. In Fig. 4a and b, we show frequency dependent real part of electrical conductivity of La1xGdxCrO3 system for x = 0.01 and 0.10 at a
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Fig. 1. X-ray diffraction pattern of La1xGdxCrO3 system. Here Yobs, Ycal, Yobs-Ycal and Bragg_position represent the experimental data, calculated data, the difference of experimental and calculated data and Bragg’s positions, respectively.
Table 1 Various parameters obtained from Rietveld refinement analysis of the XRD data and the density measurements of La1xGdxCrO3 system. Sample La1xGdxCrO3
Lattice parameters a (Å)
b (Å)
c (Å)
X = 0.00 X = 0.01 X = 0.05 X = 0.10 X = 0.15 X = 0.20
5.47878 5.47755 5.47705 5.47635 5.47373 5.488
7.75956 7.75726 7.75378 7.74649 7.74425 7.74534
5.51496 5.51417 5.50834 5.50199 5.49794 5.47369
Cell volume (Å)3
Crystallite size (nm)
v2
Percent of theoretical density
234.457 234.301 233.927 233.408 233.057 232.669
51 97 74 55 72 60
5.283 3.088 0.5751 0.4544 0.2576 0.1978
84.71 84.39 83.81 82.96 82.93 82.60
Fig. 2. FTIR spectrum of La0.99Gd0.01CrO3.
few selected temperatures. This figure clearly indicates a frequency independent conductivity in almost entire frequency range. Similar behaviour has been observed for all other compositions. Generally, any ceramic, glass or polymer materials display frequency independent conductivity (DC conductivity) followed by a dispersion regime (frequency dependent part i.e. AC conductivity) in their
conductivity spectra. In present case, it seems that the dispersion regime may appear at still high frequencies. Nithya et al. [27] have reported similar behaviour for LaCr0.5Cu0.5O3 and LaCr0.5Fe0.5O3 system in their conductivity spectra. These systems were found to show dispersive conductivity after about 1 MHz. Nevertheless, DC conductivity, rDC, can be evaluated at the measured temperatures by extrapolating the low frequency plateau of the conductivity spectra. The Arrhenius plot of the DC conductivity, obtained from the low frequency plateau, for all compositions is shown in Fig. 5. Our DC conductivity values are superior to the value reported by Tripathi and Lal [29]. Some workers [24,26] have reported the DC conductivity of pure lanthanum chromite higher than that reported. This may be due to the fact that in present study, DC conductivities were evaluated from the low frequency plateau of the conductivity spectra, which shows relatively low value due to electrode polarisation dominating at low frequencies. The polarisation at low frequencies might have resulted into lowering of DC conductivity. Fig. 5 reveals that the DC conductivity, for all compositions, exhibits linear behaviour at lower temperatures, however, a nonlinear behaviour is discerned for the compositions with x P 0.10 above about 300 °C. For compositions with x P 0.10 the DC conductivity is found to increase rapidly above 300 °C. The linear region represents that DC conductivity is Arrhenius activated and follows the Arrhenius equation rDCT = r0 exp(Ea/kT), where, r0 is pre-exponential factor, Ea be the activation energy for conduction, k is the Boltzmann constant and T is the absolute temperature. The activation energy for each composition
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Fig. 3. SEM micrograph of La1xGdxCrO3 system.
has been calculated from the slope of the linear region and reported in Table. 2. The compositional dependent DC conductivities at a few representative temperatures are shown in Fig. 6. From this figure, it is clear that it first increases with x up to x = 0.01 and then between 0.01 and 0.10 it decreases and thereafter again an increasing trend in conductivity is observed up to x = 0.15. Between x = 0.15 and 0.20 again a decrease in conductivity is observed at 100 °C and 200 °C. The temperature and the compositional dependent electrical conduction behaviour of the La1xGdxCrO3 system can be understood in the following way. Pure lanthanum chromite is a p-type conductor [30,31] and the doped LaCrO3 have been reported to show a polaronic conduction [2,7,32,24]. In LaCrO3, presence of a large number of Cr4+ centres due to native defects in this system has been reported [33]. Moreover, based on the analysis of the 151Eu Mossbauer spectra of isovalent Eu doped LaCrO3, the presence of Cr6+ ions have been reported [34]. Due to the presence of these higher valent cations, holes are generated as per the following defect reaction [29]. 0
Cr3þ þ h ! Cr4þ 0
Cr3þ þ 3h ! Cr6þ
) ð1Þ
The holes, generated from the above equation, along with the lattice interaction form small polarons. In the present study, for very low
concentrations of Gd3+ (x 6 0.01), the isovalent dopant ions, Gd3+, do not create any additional defect in the system La1xGdxCrO3, and therefore the conductivity is due to the polarons generated from Eq. (1). Thus the conductivity of La1xGdxCrO3 system increases till x = 0.01. However, when x P 0.01, the ‘‘size effect’’ begins to plays an important role in the conduction process [35]. In the case of isovalent substitution, creation of additional oxygen vacancies due to considerable size difference between the constituent (host and dopant) ions has been reported [36,37]. In present case, the ionic radii of Gd3+ and La3+ ions are 0.94 Å and 1.061 Å, respectively. Consequently this size difference could results into the generation of oxygen vacancies. The same could be expressed using the Kroeger–Vink notation by:
Gd2 O3 ! 2GdLa þ 3V00O þ 3O00i
ð2Þ
The creation of oxygen vacancies has been attributed to the decrease in conductivity of La1xGdxCrO3 system as concentration of dopant ion becomes larger than 0.01. Furthermore, Fig. 6 indicates that the conductivity of the investigated system again starts to increase for the compositions with x P 0.15. Li et al. [34] have reported that the concentration of Cr6+ ions increased with increasing concentration of isovalent Eu3+ that substituted isovalent La ion in LaCrO3 system. XPS analysis was carried out to confirm the presence of Cr6+ ions in our samples of La1xGdxCrO3. The
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P. Kumar et al. / Journal of Alloys and Compounds 576 (2013) 154–160 Table 2 Activation energy for conduction for various compositions of La1xGdxCrO3 system. La1xGdxCrO3
Temperature range
Activation energy (eV)
X = 0.00 X = 0.01 X = 0.05 X = 0.10 X = 0.15 X = 0.20
40–340 °C 40–300 °C 40–220 °C 60–280 °C 60–320 °C 40–280 °C
0.31 0.21 0.26 0.25 0.30 0.39
Fig. 6. Variation of DC conductivity with dopant concentration x in the system La1xGdxCrO3. Fig. 4. Representative conductivity spectra of La1xGdxCrO3 for (a) x = 0.01 and (b) x = 0.10, at a few temperatures.
Fig. 7. XPS spectra of the Cr2p level in the system La1xGdxCrO3.
Fig. 5. Arrhenius representation of DC conductivity of La1xGdxCrO3. The symbols denote the experimental (calculated) data points and the solid lines represent the linear fit to the data points.
Cr2p level spectra of La1xGdxCrO3 system for x = 0.00, 0.01 and 0.20 are shown in Fig. 7. Leiro and Minni [38] have measured XPS spectra of metallic chromium. They assigned a peak at about 576 eV to Cr-2p3/2 (Cr3+ state) and a peak at about 582 eV to Cr2p1/2 (Cr6+ state). Similar results have been reported for other doped lanthanum chromite systems [19,34,39]. From Fig. 7, it is clear that the undoped La1xGdxCrO3 system (for x = 0.00) depicts
peak at about 576 eV only and no peak at 582 eV. Therefore, the absence of Cr6+ in undoped lanthanum chromite is confirmed. However, when the concentration of Gd increases (x = 0.01), a peak appears at about 582 eV and its intensity increases to maximum value for x = 0.20. This clearly indicates that Cr6+ concentration increases with increasing Gd concentration. Therefore, it is concluded that with increasing isovalent Gd3+ concentration, the number of Cr6+ ions also increases leading to generation of more holes as per Eq. (1) causing an increase in the conductivity of La1xGdxCrO3 when x P 0.15 as has been observed and reported in Fig. 6. Fig. 6 also depicts that, below 200 °C, the conductivity of the compositions with for x P 0.15 is almost constant, however, it shows an increasing trend at 300 °C. This could be explained in
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terms of increasing number of holes (small polarons) with increasing temperature as reported by Akashi et al. [40]. The trend of variation of activation energy, mentioned in Table 2, for different compositions of La1xGdxCrO3 indicates that activation energy first decreases up to x = 0.01 and thereafter it continuously increases with increasing x. This variation also supports variation of conductivity of the investigated system. The DC conductivity for the compositions x P 0.10 is found to increase very rapidly above 300 °C (in Fig. 5). This rapid increase may be due to the increase in mobility of the small polarons above this temperature. Thus above 300 °C, the conductivity in compositions x P 0.10 seems to be caused not only by the increase in number density of mobile charge carriers but also due to increase in their mobility. Nevertheless, for compositions with x < 0.10, above 300 °C, no considerable increase in conductivity is observed. This could be because as reported in literature [41] the mobility of oxygen vacancies, which dominate conduction mechanism for the compositions x < 0.10, do not change significantly above 300 °C and below 520 °C. Frequency dependent real part of permittivity, 0 of various compositions of La1xGdxCrO3 at 40 °C (just above the room temperature) is shown in Fig. 8. This figure depicts that, for all compositions, 0 decreases very sharply below 100 Hz and it attains a constant static value above this frequency. However, a close investigation of permittivity spectra, as depicted in the inset of Fig. 8, indicates a relaxation peak between 50 Hz and 1 kHz for all compositions except for the undoped LaCrO3. To confirm this, we also show variation of loss tangent with frequency (tan d vs log m) in Fig. 9. The relaxation peaks are more prominent in this figure. In this figure too no peak is observed for the undoped lanthanum chromite (x = 0.0). The observed dielectric behaviour of La1xGdxCrO3 can be understood in the following way. The sharp increase in 0 in the low frequency (below 100 Hz) regime corresponds to space charge polarisation in the system. The observed relaxation peaks for Gd doped compositions reflects the orientational polarisation process in the system due to formation of electric dipoles. As mentioned earlier, in our system both Cr4+ and Cr6+ centres are present. The presence of Cr4+ at Cr6+ sites will carry two effective 00 negative charges as Cr4þ . Similarly Cr3+ at Cr4+ sites will carry Cr6þ
Fig. 9. Variation of loss tangent with frequency in the system La1xGdxCrO3.
one effective negative charge as
0 Cr3þ . Many ceramics have Cr4þ
been reported to possess singly ionised oxygen vacancies (V0O ) during high temperature processing, which may be retained in the system at room temperature [42]. Moreover, the oxygen vacancies (V00O ) are already present in our Gd doped system as per Eq. (2). The negatively charged sites may form electric dipoles with oxygen 00 0 vacancies as V00O Cr4þ and V0O Cr3þ . These dipoles may Cr6þ Cr4þ change their orientation with applied varying electric field. In present system, for all Gd doped compositions, these electric dipoles seem to relax between 50 Hz and 1 kHz causing relaxation peaks in their permittivity and loss tangent spectra (Fig. 9 and inset of Fig. 8). However, due to absence of Gd ions in pure LaCrO3, the formation of oxygen vacancies is less likely. Therefore, the number of oxygen vacancies in undoped LaCrO3 is very small and hence no relaxation peak is observed for this composition (x = 0.0). 4. Conclusions
Fig. 8. Frequency dependence of real part of permittivity of La1xGdxCrO3 at 40 °C.
The isovalent Gd3+ substituted nanocrystalline La1xGdxCrO3 (0 6 x 6 0.20) system has been synthesised by auto-combustion synthesis route. All synthesised compositions show solid solution formation having orthorhombic crystal structure similar to pure LaCrO3. The lattice parameters are found to decrease with increasing dopant concentration due to smaller size of Gd3+ ions in compare to La3+ ions. The average grain size of microscopic particles first decreases and then gradually increases with increasing dopant concentration. The DC conductivity for the compositions with x 6 0.01 has been attributed to holes generated polarons. For compositions with 0.01 6 x 6 0.10, the DC conductivity is primarily governed by the oxygen ion vacancies generated due to size difference between dopant and host ions. Whereas conductivity for compositions with x P 0.10 is dominantly due to the holes (polarons) created by the enhanced number of Cr6+ centres in the system. The presence of Cr6+ sites have been confirmed in XPS measurements in La1xGdxCrO3 system for x > 0.00. The compositional dependent activation energy also supports the proposed conduction mechanism in La1xGdxCrO3 system. The frequency dependent permittivity of the investigated system reveals that at low frequencies (below 100 Hz) the space charge polarisation contributes dominantly. However, for doped samples the orientational polarisation caused by the electric dipoles formed by oxygen vacancies and negatively charged centres in the system is also obtained.
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Acknowledgement We acknowledge DST-SERC for funding this work through it Project sanction letter No. SR/FTP/ETA-0005/2010. Authors PK and RKS are thankful to UGC, New Delhi for providing financial support. We acknowledge S.S. Bhoga of RTM University, Nagpur, India for providing density measurement facility. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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