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5VO4 studied by complex impedance spectroscopy
Journal of Physics and Chemistry of Solids 73 (2012) 25–29
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Frequency and temperature-dependent electrical properties of LiNi3/5Fe2/5VO4 studied by complex impedance spectroscopy Moti Ram n Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur, West Bengal 721302, India
a r t i c l e i n f o
abstract
Article history: Received 5 January 2011 Received in revised form 20 July 2011 Accepted 7 September 2011 Available online 29 September 2011
The LiNi3/5Fe2/5VO4 has been synthesized by solution-based chemical method. Grain morphology of the material is investigated using field emission scanning electron microscopy. The existence of electrical conduction due to bulk and grain boundary effects at 21, 75 and 250 1C, and grain boundary and polarization effects at 275 and 300 1C has been verified by impedance spectroscopy. DC conductivity shows electrical conduction in the material as a thermally activated process. The frequency dependence of AC conductivity is described by Jonscher’s power law. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics B. Chemical synthesis D. Microstructure D. Electrical properties
1. Introduction Lithiated transition metal oxides form an important class of materials due to their various device applications [1–4]. The performance of these devices is greatly influenced by the quality of the lithiated transition metal oxides, which itself is dependent on several factors viz. sintering temperature and time, type and amount of substitution, synthesis method, preparation conditions, etc. [5]. These factors also affect the chemical and physical (electrical, dielectric, ferroelectric, magnetic, etc.) properties of these materials, which are characterized by various techniques viz. complex impedance spectroscopy, X-ray diffraction, etc. Dielectric/ferroelectric properties of such compounds depend strongly on the polarization process [6], and they are also related to the conduction mechanism [7]. Dielectric behavior offers information about the localized electric charge carriers due to the effect of frequency, temperature and composition, which explains the responsible mechanisms for charge transport phenomena and polarization process [8]. Complex impedance spectroscopy (CIS) technique has been used extensively to characterize the electrical/dielectric properties of these materials as a function of frequency, which resolves the contributions and relative importance to electrical conduction and/or polarization of different phenomena in the studied frequency region [9]. The results are also used to interpret the impedance spectra in terms of resistance and capacitance associated with the
n
Tel.: þ91 3222 281902. E-mail address: [email protected]
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.09.003
material [9]. Many researchers have used CIS technique for electrical characterization of lithiated transition metal oxides as a function of frequency at various temperatures [10–13]. This was motivated to prepare a lithiated transition metal oxide (LiNi3/5Fe2/5VO4) and characterize its electrical properties as a function of frequency and temperature. In the present work, synthesis of LiNi3/5Fe2/5VO4 fine powder by solution-based chemical method and its frequency and temperature-dependent electrical properties using CIS technique have been analyzed.
2. Experimental procedures 2.1. Material preparation The LiNi3/5Fe2/5VO4 fine powder was synthesized using solution-based chemical method. The stoichiometric amounts of highly pure LiNO3 (98%, M/s Loba Chemie Pvt. Ltd., India), Ni(NO3)2 6H2O (98%, E. Merck (India) Ltd.), Fe(NO3)3 9H2O ( Z98%, E. Merck (India) Ltd.) and NH4VO3 (99%, M/s S.D. FineChem Pvt. Ltd., India) were dissolved in distilled water and mixed together. Thereafter triethanolamine ( 497%, E. Merck (India) Ltd.) was added maintaining a ratio of 3:1 with metal ions. HNO3 (68–72%, E. Merck (India) Ltd.) and oxalic acid ( 499%, E. Merck (India) Ltd.) were added to dissolve the precipitate and then the clear solution was evaporated at 200 1C temperature with continuous stirring. A fluffy, mesoporous and carbon-rich precursor mass was formed by complete evaporation of the solution. After grinding, the voluminous, fluffy and black carbonaceous mass was calcined at 525 1C for 4 h to produce the
26
M. Ram / Journal of Physics and Chemistry of Solids 73 (2012) 25–29
desired phase, which is confirmed by the X-ray diffraction analysis. The calcined powder was cold pressed into circular disk shaped pellets of diameter 12–13 mm and various thicknesses with polyvinyl alcohol as the binder using hydraulic press at a pressure of 7.85–9.81 MPa. These pellets were then sintered at 575 1C for 2 h followed by the slow cooling process. The heating and cooling rates during calcination and sintering processes were 3 1C/min. The binder was removed during the sintering process. Subsequently, the pellets were polished by fine emery paper to make their faces smooth and parallel. One such sintered pellet of average thickness (1.52 mm) and radius (5.59 mm) was taken for electrical measurements. This pellet was finally coated with conductive silver paint and dried at 150 1C for 3 h before carrying out electrical measurements.
LiNi3/5Fe2/5VO4 is similar to the grain size 0.1–1.6 mm of Li(Ni7/10Fe3/10)VO4 [15], and smaller than that of the grain size 1–3 mm of LiFe1/2Ni1/2VO4 [16]. This indicates a clear modification in the microstructure of LiFe1/2Ni1/2VO4. The Nyquist plot (i.e., Z0 verses Z00 graph) at some selected temperatures is shown in Fig. 2. It is characterized by depressed semicircles, which suggest the non-Debye type behavior of the material. The depressed semicircle at high, middle and low frequency regions arises due to the bulk, grain boundary and
2.2. Material characterization The surface morphology of the gold-sputtered sample was recorded with different magnifications at room temperature using a ZEISS (Model: SUPRATM 40) field emission scanning electron microscope. Electrical impedance, phase angle, tangent loss and capacitance were measured by applying a voltage of 0.701 V using a computer-controlled frequency response analyzer (HIOKI LCR HiTESTER, Model: 3532–50) with varying temperatures over the frequency range of 102–106 Hz.
3. Results and discussion The compound (LiNi3/5Fe2/5VO4) has an orthorhombic unit cell ˚ b¼15.82 A˚ and structure with lattice parameters a ¼3.78 A, ˚ which is revealed by X-ray diffraction analysis [14]. c¼5.56 A, It is clear that LiNi3/5Fe2/5VO4 prepared by solution-based chemical route results in unit cell structure that remains same with ˚ ˚ modification in lattice parameters (a ¼8.23 A, b¼7.02 A, ˚ ˚ b ¼17.75 A, ˚ c¼5.83 A) of Li(Ni7/10Fe3/10)VO4 and (a ¼3.56 A, ˚ of LiFe1/2Ni1/2VO4, which were prepared by soluc¼12.29 A) tion-based chemical route and conventional solid state route, respectively [15,16]. This modification in lattice parameters may be due to the change of ionic radii of Ni and Fe. Fig. 1 shows the field emission scanning electron micrograph at room temperature of the surface of the gold-sputtered sintered pellet sample. It indicates the polycrystalline texture of the compound with grains of different sizes 0.1–2.0 mm. These grains present an average grain size with polydisperse distribution on the surface of the sample. The grain size 0.1–2.0 mm of
Fig. 1. Field emission scanning electron micrograph of LiNi3/5Fe2/5VO4 at room temperature.
Fig. 2. Nyquist plots at some selected temperatures with electrical equivalent circuit (inset).
M. Ram / Journal of Physics and Chemistry of Solids 73 (2012) 25–29
polarization (polarization at the electrode–material interface) effects, respectively [9]. It is clear from Fig. 2 that electrical conduction in the material is due to the bulk and grain boundary effects at 21, 75 and 250 1C, and grain boundary and polarization effects at 275 and 300 1C. The corresponding electrical equivalent circuits are presented in Fig. 2(inset). But, electrical conduction in the material is due to the bulk effect at Tr50 1C for Li(Ni7/10Fe3/10)VO4 and Tr175 1C for LiFe1/2Ni1/2VO4, bulk and grain boundary effects at 75 1CrTr225 1C for Li(Ni7/10Fe3/10)VO4 and 200 1CrTr350 1C for LiFe1/2Ni1/2VO4, and bulk, grain boundary and polarization effects at 250 1CrTr350 1C for Li(Ni7/10Fe3/10)VO4 (i.e., electrical conduction in the material is due to the bulk and grain boundary effects in
Fig. 3. Variation of sdc as a function of temperature.
27
LiFe1/2Ni1/2VO4, and bulk, grain boundary and polarization effects in Li(Ni7/10Fe3/10)VO4 at studied temperatures) [15,16]. This indicates a clear modification in the electrical behavior of Li(Ni7/10Fe3/10)VO4 and LiFe1/2Ni1/2VO4. Further, the intercept of the depressed semicircle on the real axis shifts toward the origin with rise in temperature, which indicates an increase in DC conductivity (sdc) of the material. This increase in sdc with temperature is shown in Fig. 3. It obeys an Arrhenius relation [sdc ¼ so exp (Ea/kT)], where so, Ea, k and T are the pre-exponential factor corresponding to 1/T¼0, activation energy for charge transfer, Boltzmann constant and absolute temperature, respectively. This feature suggests electrical conduction in the material as a thermally activated process and it may be due to electrons movements [17,18]. The value of Ea is calculated using the Arrhenius relation and slope of the Fig. 3 as (0.4070.01 eV at 21–250 1C). It is similar to Ea (0.49 eV at 50–250 1C) of Li(Ni7/10Fe3/10)VO4 [15] and Ea ( 0.42 eV at 23–350 1C) of LiFe1/2Ni1/2VO4 [16], which indicates the same process due to hopping mechanism. Fig. 4 shows the frequency dependence of Z0 at some selected temperatures. The value of Z0 decreases with rise in both the frequency and temperature, which indicates the increase in AC conductivity (sac). A merger of the curves at high frequencies is observed due to the possible release of space charge [9,19]. These charges release by the discontinuities at material–electrode interface or grain boundaries, and also release from the electrode [20]. Similar features were shown by Li(Ni7/10Fe3/10)VO4 and LiFe1/2Ni1/2VO4 [15,16]. The frequency dependence of Z00 at some selected temperatures is shown in Fig. 5. This pattern of variation is characterized by (i) the appearance of peaks with asymmetric broadening, (ii) shifting of the peak with decrease of the peak’s height toward high frequency side on increasing temperature and (iii) merging of the curves at high frequencies. These features suggest the
Fig. 4. Frequency dependence of Z0 at some selected temperatures.
28
M. Ram / Journal of Physics and Chemistry of Solids 73 (2012) 25–29
Fig. 7. Temperature dependence of sac at some selected frequencies. Fig. 5. Frequency dependence of Z00 at some selected temperatures.
LiFe1/2Ni1/2VO4 [15,16]. The changes in the slopes indicate multiconduction/activated process in the material. The values of Ea are calculated at 50 kHz as (0.2670.01 eV at 21–180 1C) and (0.8970.03 eV at 250–375 1C). The conduction may happen due to electrons at 21–180 1C and defects/oxygen vacancies at 250–375 1C [26,27,18]. The value of Ea at 21–180 1C is less than that of calculated using Fig. 3. Furthermore, a merger of the curves at high temperatures is observed due to the intrinsic conductivity of the material [28].
4. Conclusions
Fig. 6. Frequency dependence of sac at some selected temperatures.
existence of electrical relaxation process in the material with temperature dependence of relaxation time. Also, asymmetric broadening of the peak with increase in temperature indicates multiple relaxation-timescale [21,22]. Similar response was indicated by Li(Ni7/10Fe3/10)VO4 and LiFe1/2Ni1/2VO4 [15,16]. Fig. 6 shows the frequency dependence of sac at some selected temperatures. This pattern shows (i) nearly plateau region at low frequencies corresponds to sdc (ii) dispersion at high frequencies. The frequency at which the dispersion takes place is known as hopping frequency, which shifts toward the high frequency side as temperature increases. This feature signifies the presence of hopping type mechanism for electrical conduction in the material that follows Jonscher’s power equation [sac ¼ sdc þ A(o)n], where n is the frequency exponent in the range of 0r nr1, and A is a constant that depends upon temperature [23]. Similar mechanism was shown by Li(Ni7/10Fe3/10)VO4 and LiFe1/2Ni1/2VO4 [15,16]. Furthermore, the deviation from the plateau region at low frequency side is due to the electrode polarization effect [24,25]. The temperature dependence of sac at some selected frequencies is shown in Fig. 7. The value of sac increases with temperature-rise, which indicates electrical conduction in the material as a thermally activated process from different localized states in the gap or its tails. Similar behavior was shown by Li(Ni7/10Fe3/10)VO4 and
The compound (LiNi3/5Fe2/5VO4) prepared by the solution-based chemical method has an orthorhombic unit cell structure. Microstructure study indicates polycrystalline texture of the compound with grains of unequal sizes 0.1–2.0 mm. Impedance analysis shows electrical conduction in the material is due to bulk, grain boundary and polarization effects. A detailed electrical conductivity study indicates that electrical conduction in the material is a thermally activated process. Clearly, LiNi3/5Fe2/5VO4 prepared by the solution-based chemical route results in the modification of the properties for Li(Ni7/10Fe3/10)VO4 and LiFe1/2Ni1/2VO4 that were prepared by the solution-based chemical route and the conventional solid state route, respectively.
Acknowledgments The author is grateful to the Nanomaterials Laboratory of the Department of Chemistry and Central Research Facility, Indian Institute of Technology, Kharagpur 721302 (W.B.), India, for providing facilities to conduct experiments. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
J.S. Pena, O. Crosnier, T. Brousse, Electrochim. Acta 55 (2010) 7511. Y. Sakurai, H. Arai, J. Yamaki, Solid State Ionics 113–115 (1998) 29. H. Demidzu, T. Nakamura, Y. Yamada, J. Magn. Magn. Mater. 322 (2010) 1816. J.R. Dahn, U.V. Sacken, C.A. Michal, Solid State Ionics 44 (1990) 87. M.M. Costa, G.F.M.P. Ju´nior, A.S.B. Sombra, Mater. Chem. Phys. 123 (2010) 35. R.S. Patil, S.V. Kakatkar, S.A. Patil, P.K. Maskar, S.R. Sawant, Phys. Status Solidi A 126 (1991) K185. K. Iwauchi, Jpn. J. Appl. Phys. 10 (1971) 1520. A.M.M. Farea, S. Kumar, K.M. Batoo, A. Yousef, C.G. Lee, Alimuddin, J. Alloys Compd 464 (2008) 361. J.R. MacDonald, Impedance Spectroscopy: Emphasizing Solid Materials and Systems, John Wiley & Sons, New York, 1987. S.A. Mazen, F. Metawe, S.F. Mansour, J. Phys. D: Appl. Phys 30 (1997) 1799.
M. Ram / Journal of Physics and Chemistry of Solids 73 (2012) 25–29
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
P.V. Reddy, T.S. Rao, J. Less Common Met 79 (1981) 191. A. Ahmed, J. Mater. Sci. 27 (1992) 4120. M. Ram, S. Chakrabarti, J. Phys. Chem. Solids 69 (2008) 905. M. Ram, J. Mater. Sci.: Mater. Electron., doi:10.1007/s10854-011-0446-2, in press. M. Ram, J. Alloys Compd 509 (2011) 1744. M. Ram, R.N.P. Choudhary, A.K. Thakur, Mater. Chem. Phys. 101 (2007) 455. S. Kumar, K.B.R. Varma, Solid State Commun. 146 (2008) 137. P.R. Das, R.N.P. Choudhary, B.K. Samantray, Mater. Chem. Phys. 101 (2007) 228. J. Plocharski, W. Wieczoreck, Solid State Ionics 28–30 (1988) 979. P.K. Khare, D.K. Sahu, A. Verma, R.K. Srivastava, Indian J. Pure Appl. Phys. 42 (2004) 693.
29
[21] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsca Dielectric Press, London, 1983. [22] R.S. Kumar, K. Hariharan, Mater. Chem. Phys. 60 (1999) 28. [23] A.K. Jonscher, Nature 267 (1977) 673. [24] D.K. Pradhan, R.N.P. Choudhary, B.K. Samantaray, Int. J. Electrochem. Sci. 3 (2008) 597. [25] F. Bordi, C. Cametti, T. Gili, Bioelectrochemistry 54 (2001) 53. [26] D. Fasquelle, J.C. Carru, J. Eur. Ceram. Soc. 28 (2008) 2071. [27] M. Ram, Solid State Commun. 149 (2009) 1226. [28] K.S. Rao, D.M. Prasad, P.M. Krishna, B. Tilak, K.C. Varadarajulu, Mater. Sci. Eng. B 133 (2006) 141.
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Frequency and temperature-dependent electrical properties of LiNi3/5Fe2/5VO4 studied by complex impedance spectroscopy Moti Ram n Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur, West Bengal 721302, India
a r t i c l e i n f o
abstract
Article history: Received 5 January 2011 Received in revised form 20 July 2011 Accepted 7 September 2011 Available online 29 September 2011
The LiNi3/5Fe2/5VO4 has been synthesized by solution-based chemical method. Grain morphology of the material is investigated using field emission scanning electron microscopy. The existence of electrical conduction due to bulk and grain boundary effects at 21, 75 and 250 1C, and grain boundary and polarization effects at 275 and 300 1C has been verified by impedance spectroscopy. DC conductivity shows electrical conduction in the material as a thermally activated process. The frequency dependence of AC conductivity is described by Jonscher’s power law. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics B. Chemical synthesis D. Microstructure D. Electrical properties
1. Introduction Lithiated transition metal oxides form an important class of materials due to their various device applications [1–4]. The performance of these devices is greatly influenced by the quality of the lithiated transition metal oxides, which itself is dependent on several factors viz. sintering temperature and time, type and amount of substitution, synthesis method, preparation conditions, etc. [5]. These factors also affect the chemical and physical (electrical, dielectric, ferroelectric, magnetic, etc.) properties of these materials, which are characterized by various techniques viz. complex impedance spectroscopy, X-ray diffraction, etc. Dielectric/ferroelectric properties of such compounds depend strongly on the polarization process [6], and they are also related to the conduction mechanism [7]. Dielectric behavior offers information about the localized electric charge carriers due to the effect of frequency, temperature and composition, which explains the responsible mechanisms for charge transport phenomena and polarization process [8]. Complex impedance spectroscopy (CIS) technique has been used extensively to characterize the electrical/dielectric properties of these materials as a function of frequency, which resolves the contributions and relative importance to electrical conduction and/or polarization of different phenomena in the studied frequency region [9]. The results are also used to interpret the impedance spectra in terms of resistance and capacitance associated with the
n
Tel.: þ91 3222 281902. E-mail address: [email protected]
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.09.003
material [9]. Many researchers have used CIS technique for electrical characterization of lithiated transition metal oxides as a function of frequency at various temperatures [10–13]. This was motivated to prepare a lithiated transition metal oxide (LiNi3/5Fe2/5VO4) and characterize its electrical properties as a function of frequency and temperature. In the present work, synthesis of LiNi3/5Fe2/5VO4 fine powder by solution-based chemical method and its frequency and temperature-dependent electrical properties using CIS technique have been analyzed.
2. Experimental procedures 2.1. Material preparation The LiNi3/5Fe2/5VO4 fine powder was synthesized using solution-based chemical method. The stoichiometric amounts of highly pure LiNO3 (98%, M/s Loba Chemie Pvt. Ltd., India), Ni(NO3)2 6H2O (98%, E. Merck (India) Ltd.), Fe(NO3)3 9H2O ( Z98%, E. Merck (India) Ltd.) and NH4VO3 (99%, M/s S.D. FineChem Pvt. Ltd., India) were dissolved in distilled water and mixed together. Thereafter triethanolamine ( 497%, E. Merck (India) Ltd.) was added maintaining a ratio of 3:1 with metal ions. HNO3 (68–72%, E. Merck (India) Ltd.) and oxalic acid ( 499%, E. Merck (India) Ltd.) were added to dissolve the precipitate and then the clear solution was evaporated at 200 1C temperature with continuous stirring. A fluffy, mesoporous and carbon-rich precursor mass was formed by complete evaporation of the solution. After grinding, the voluminous, fluffy and black carbonaceous mass was calcined at 525 1C for 4 h to produce the
26
M. Ram / Journal of Physics and Chemistry of Solids 73 (2012) 25–29
desired phase, which is confirmed by the X-ray diffraction analysis. The calcined powder was cold pressed into circular disk shaped pellets of diameter 12–13 mm and various thicknesses with polyvinyl alcohol as the binder using hydraulic press at a pressure of 7.85–9.81 MPa. These pellets were then sintered at 575 1C for 2 h followed by the slow cooling process. The heating and cooling rates during calcination and sintering processes were 3 1C/min. The binder was removed during the sintering process. Subsequently, the pellets were polished by fine emery paper to make their faces smooth and parallel. One such sintered pellet of average thickness (1.52 mm) and radius (5.59 mm) was taken for electrical measurements. This pellet was finally coated with conductive silver paint and dried at 150 1C for 3 h before carrying out electrical measurements.
LiNi3/5Fe2/5VO4 is similar to the grain size 0.1–1.6 mm of Li(Ni7/10Fe3/10)VO4 [15], and smaller than that of the grain size 1–3 mm of LiFe1/2Ni1/2VO4 [16]. This indicates a clear modification in the microstructure of LiFe1/2Ni1/2VO4. The Nyquist plot (i.e., Z0 verses Z00 graph) at some selected temperatures is shown in Fig. 2. It is characterized by depressed semicircles, which suggest the non-Debye type behavior of the material. The depressed semicircle at high, middle and low frequency regions arises due to the bulk, grain boundary and
2.2. Material characterization The surface morphology of the gold-sputtered sample was recorded with different magnifications at room temperature using a ZEISS (Model: SUPRATM 40) field emission scanning electron microscope. Electrical impedance, phase angle, tangent loss and capacitance were measured by applying a voltage of 0.701 V using a computer-controlled frequency response analyzer (HIOKI LCR HiTESTER, Model: 3532–50) with varying temperatures over the frequency range of 102–106 Hz.
3. Results and discussion The compound (LiNi3/5Fe2/5VO4) has an orthorhombic unit cell ˚ b¼15.82 A˚ and structure with lattice parameters a ¼3.78 A, ˚ which is revealed by X-ray diffraction analysis [14]. c¼5.56 A, It is clear that LiNi3/5Fe2/5VO4 prepared by solution-based chemical route results in unit cell structure that remains same with ˚ ˚ modification in lattice parameters (a ¼8.23 A, b¼7.02 A, ˚ ˚ b ¼17.75 A, ˚ c¼5.83 A) of Li(Ni7/10Fe3/10)VO4 and (a ¼3.56 A, ˚ of LiFe1/2Ni1/2VO4, which were prepared by soluc¼12.29 A) tion-based chemical route and conventional solid state route, respectively [15,16]. This modification in lattice parameters may be due to the change of ionic radii of Ni and Fe. Fig. 1 shows the field emission scanning electron micrograph at room temperature of the surface of the gold-sputtered sintered pellet sample. It indicates the polycrystalline texture of the compound with grains of different sizes 0.1–2.0 mm. These grains present an average grain size with polydisperse distribution on the surface of the sample. The grain size 0.1–2.0 mm of
Fig. 1. Field emission scanning electron micrograph of LiNi3/5Fe2/5VO4 at room temperature.
Fig. 2. Nyquist plots at some selected temperatures with electrical equivalent circuit (inset).
M. Ram / Journal of Physics and Chemistry of Solids 73 (2012) 25–29
polarization (polarization at the electrode–material interface) effects, respectively [9]. It is clear from Fig. 2 that electrical conduction in the material is due to the bulk and grain boundary effects at 21, 75 and 250 1C, and grain boundary and polarization effects at 275 and 300 1C. The corresponding electrical equivalent circuits are presented in Fig. 2(inset). But, electrical conduction in the material is due to the bulk effect at Tr50 1C for Li(Ni7/10Fe3/10)VO4 and Tr175 1C for LiFe1/2Ni1/2VO4, bulk and grain boundary effects at 75 1CrTr225 1C for Li(Ni7/10Fe3/10)VO4 and 200 1CrTr350 1C for LiFe1/2Ni1/2VO4, and bulk, grain boundary and polarization effects at 250 1CrTr350 1C for Li(Ni7/10Fe3/10)VO4 (i.e., electrical conduction in the material is due to the bulk and grain boundary effects in
Fig. 3. Variation of sdc as a function of temperature.
27
LiFe1/2Ni1/2VO4, and bulk, grain boundary and polarization effects in Li(Ni7/10Fe3/10)VO4 at studied temperatures) [15,16]. This indicates a clear modification in the electrical behavior of Li(Ni7/10Fe3/10)VO4 and LiFe1/2Ni1/2VO4. Further, the intercept of the depressed semicircle on the real axis shifts toward the origin with rise in temperature, which indicates an increase in DC conductivity (sdc) of the material. This increase in sdc with temperature is shown in Fig. 3. It obeys an Arrhenius relation [sdc ¼ so exp (Ea/kT)], where so, Ea, k and T are the pre-exponential factor corresponding to 1/T¼0, activation energy for charge transfer, Boltzmann constant and absolute temperature, respectively. This feature suggests electrical conduction in the material as a thermally activated process and it may be due to electrons movements [17,18]. The value of Ea is calculated using the Arrhenius relation and slope of the Fig. 3 as (0.4070.01 eV at 21–250 1C). It is similar to Ea (0.49 eV at 50–250 1C) of Li(Ni7/10Fe3/10)VO4 [15] and Ea ( 0.42 eV at 23–350 1C) of LiFe1/2Ni1/2VO4 [16], which indicates the same process due to hopping mechanism. Fig. 4 shows the frequency dependence of Z0 at some selected temperatures. The value of Z0 decreases with rise in both the frequency and temperature, which indicates the increase in AC conductivity (sac). A merger of the curves at high frequencies is observed due to the possible release of space charge [9,19]. These charges release by the discontinuities at material–electrode interface or grain boundaries, and also release from the electrode [20]. Similar features were shown by Li(Ni7/10Fe3/10)VO4 and LiFe1/2Ni1/2VO4 [15,16]. The frequency dependence of Z00 at some selected temperatures is shown in Fig. 5. This pattern of variation is characterized by (i) the appearance of peaks with asymmetric broadening, (ii) shifting of the peak with decrease of the peak’s height toward high frequency side on increasing temperature and (iii) merging of the curves at high frequencies. These features suggest the
Fig. 4. Frequency dependence of Z0 at some selected temperatures.
28
M. Ram / Journal of Physics and Chemistry of Solids 73 (2012) 25–29
Fig. 7. Temperature dependence of sac at some selected frequencies. Fig. 5. Frequency dependence of Z00 at some selected temperatures.
LiFe1/2Ni1/2VO4 [15,16]. The changes in the slopes indicate multiconduction/activated process in the material. The values of Ea are calculated at 50 kHz as (0.2670.01 eV at 21–180 1C) and (0.8970.03 eV at 250–375 1C). The conduction may happen due to electrons at 21–180 1C and defects/oxygen vacancies at 250–375 1C [26,27,18]. The value of Ea at 21–180 1C is less than that of calculated using Fig. 3. Furthermore, a merger of the curves at high temperatures is observed due to the intrinsic conductivity of the material [28].
4. Conclusions
Fig. 6. Frequency dependence of sac at some selected temperatures.
existence of electrical relaxation process in the material with temperature dependence of relaxation time. Also, asymmetric broadening of the peak with increase in temperature indicates multiple relaxation-timescale [21,22]. Similar response was indicated by Li(Ni7/10Fe3/10)VO4 and LiFe1/2Ni1/2VO4 [15,16]. Fig. 6 shows the frequency dependence of sac at some selected temperatures. This pattern shows (i) nearly plateau region at low frequencies corresponds to sdc (ii) dispersion at high frequencies. The frequency at which the dispersion takes place is known as hopping frequency, which shifts toward the high frequency side as temperature increases. This feature signifies the presence of hopping type mechanism for electrical conduction in the material that follows Jonscher’s power equation [sac ¼ sdc þ A(o)n], where n is the frequency exponent in the range of 0r nr1, and A is a constant that depends upon temperature [23]. Similar mechanism was shown by Li(Ni7/10Fe3/10)VO4 and LiFe1/2Ni1/2VO4 [15,16]. Furthermore, the deviation from the plateau region at low frequency side is due to the electrode polarization effect [24,25]. The temperature dependence of sac at some selected frequencies is shown in Fig. 7. The value of sac increases with temperature-rise, which indicates electrical conduction in the material as a thermally activated process from different localized states in the gap or its tails. Similar behavior was shown by Li(Ni7/10Fe3/10)VO4 and
The compound (LiNi3/5Fe2/5VO4) prepared by the solution-based chemical method has an orthorhombic unit cell structure. Microstructure study indicates polycrystalline texture of the compound with grains of unequal sizes 0.1–2.0 mm. Impedance analysis shows electrical conduction in the material is due to bulk, grain boundary and polarization effects. A detailed electrical conductivity study indicates that electrical conduction in the material is a thermally activated process. Clearly, LiNi3/5Fe2/5VO4 prepared by the solution-based chemical route results in the modification of the properties for Li(Ni7/10Fe3/10)VO4 and LiFe1/2Ni1/2VO4 that were prepared by the solution-based chemical route and the conventional solid state route, respectively.
Acknowledgments The author is grateful to the Nanomaterials Laboratory of the Department of Chemistry and Central Research Facility, Indian Institute of Technology, Kharagpur 721302 (W.B.), India, for providing facilities to conduct experiments. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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