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Characterization of La1−xBaxCoO3 synthesized at low temperature
Journal of Alloys and Compounds 325 (2001) L17–L19
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Letter
Characterization of La 12x Ba x CoO 3 synthesized at low temperature a, a b K.D. Mandal *, L. Behera , K. Ismail a
Department of Chemistry, North Eastern Regional Institute of Science and Technology, Nirjuli, Itanagar — 791109, Arunachal Pradesh, India b Department of Chemistry, North Eastern Hill University, Shillong, India Received 31 October 2000; accepted 3 April 2001
Abstract The perovskite oxides La 12x Bax CoO 3 (x50.00, 0.10, 0.20) have been synthesized at low temperature using the chemical route. The single phase formation of the prepared samples was confirmed by X-ray diffraction studies. The electrical conductivity of the doped samples showed little variation in comparison to undoped LaCoO 3. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Oxide materials; Chemical synthesis; Electrical transport; Dielectric response
1. Introduction The perovskite oxides LaMO 3 (M5Fe, Co, Mn, Cr) have been receiving great attention as a catalyst, as well as interconnection materials for solid oxide fuel cells, which are major candidates for future energy-generating systems [1–3]. Recently, the system La 12x R x MnO 3 (R5Ba, Ca, Sr) has received great interest from different groups [4–6] for detailed studies. For a broad range of doping, these materials have a paramagnetic to ferromagnetic transition upon cooling, which is accompanied by a sharp drop in resistivity. In view of the interesting properties of LaCoO 3 , it is considered worthwhile to study the possibility of doping of barium in the lanthanum position by the chemical route at low temperature and to study the electrical and dielectric properties.
then dissolved in distilled water. Solutions having stoichiometric amounts of those elements were mixed on a hot plate magnetic stirrer and warmed up to 60–708C. Freshly prepared oxalic acid solution was added slowly for complete co-precipitation as oxalate and filtered. The precipitates were dried and then calcined at 5008C for 6 h. The calcined powders were ground, mixed and pelletized in cylindrical form. The pellets were sintered at 6008C for 6 h. Powder X-ray diffraction patterns of the resulting product were recorded using Cu-Ka radiation. The DC electrical resistance of the synthesized materials was measured with variation of temperature by a Keithley electrometer. The capacitance was measured using a HP 4192A LF Impedance Analyzer as a function of frequency at room temperature (300 K).
2. Experiment
3. Results and discussion
Lanthanum cobaltate and barium-doped lanthanum cobaltate, La 12x Ba x CoO 3 (x50.00, 0.10, 0.20) have been synthesized by the co-precipitation chemical route. Chemicals were used as barium nitrate, cobalt (II) nitrate and lanthanum oxalate which had purity of 99.5% or better. Standard solutions of barium nitrate and cobalt nitrate were prepared in distilled water. A known amount of lanthanum oxalate was converted into lanthanum nitrate by repeated boiling in conc. nitric acid and evaporated to dryness and
Powdered XRD patterns of the synthesized samples in the system La 12x Ba x CoO 3 (x50.00, 0.10, 0.20) were recorded and are shown in Fig. 1. The recorded peaks were sharp and strong enough to confirm high crystallinity of the synthesized samples. No reflections other then those belonging to tetragonal structure were observed in the XRD patterns. XRD peaks were coincident with the three compositions. This indicates the formation of single phases of the synthesized samples and were indexed as singlephase tetragonal structure. The values of lattice constants, determined from XRD data are shown in Table 1. The lattice constant, c, increases with increase in Ba 21 con-
*Corresponding author. E-mail address: [email protected] (K.D. Mandal).
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01386-X
K.D. Mandal et al. / Journal of Alloys and Compounds 325 (2001) L17 –L19
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Fig. 2. Variation of resistivity of the sample with temperature (1000 /T K 21 ) in the system La 12x Ba x CoO 3 .
Fig. 1. XRD pattern of (a) x50.00; (b) x50.10 and (c) x50.20 for the system La 12x Ba x CoO 3 .
centration. This may be due to the large ionic radius [7] of Ba 21 ion, which when substituted occupies the tetrahedral (A) site and displaces the smaller La 31 ion from tetrahedral (A) site to the octahedral (B) site. This suggests strong occupancy of Ba 21 ion on the A-site. The plot of log r vs. 1000 /T of the three sample is shown in Fig. 2. The DC resistivity of the sample x50.00 is almost constant with the variation of temperature in the range 300–500 K. When Ba 21 is doped in the LaCoO 3 , the conductivity of the sample x50.10 is lowered whereas it increases for the composition x50.20. In the doped compounds, there is a sudden increase in the resistivity. This jump occurs at lower temperature for the sample x50.10 and at higher temperature for the composition
x50.20. The sudden jump in resistivity in doped samples may be due to the formation of Co 21 and Co 41 at high temperature. The formation of ionic cobalt in two different states is due to electron transfer from high spin Co 31 to low spin Co 31 ion [8]. With partial doping of Ba 21 in place of La 31 , there is loss of oxygen for maintaining electrical neutrality of the doped material, La 12x Ba x CoO 32x / 2 . These resulting materials are oxygen deficient. The resistivity of the doped materials (x50.10 and 0.20) increases at high temperature (around 400 K) due to oxygen deficiency showing the metallic nature of the materials. The resistivity of LaCoO 3 is found to be much higher than reported earlier [9]. The differences may be due to different methods of preparation of the materials. The plot of dielectric constant (e ) versus log ( f ) is shown in Fig. 3. The dielectric constant of the compounds decreases sharply and thereafter remains constant up to the frequency 10 6 Hz. This shows that space charge polarization contributes significantly to the observed dielectric
Table 1 Composition, structure, lattice parameters and DC conductivity (s ) at 400 K for the system La 12x Ba x CoO 3 Composition
Structure
Lattice parameter
x50.00
Tetragonal
x50.10
Tetragonal
x50.20
Tetragonal
a5b56.9284 ˚ c57.6214 A a5b56.8298 ˚ c57.7859 A a5b56.7519 ˚ c58.0052 A
DC conductivity (s ) at 400 K (V 21 cm 21 )
˚ A
2.48310
29
˚ A
1.30310
29
˚ A
4.22310 29
Fig. 3. Variation of dielectric constant of the samples at room temperature (300 K) in the system La 12x Ba x CoO 3 .
K.D. Mandal et al. / Journal of Alloys and Compounds 325 (2001) L17 –L19
properties. Space charge polarization arises due to the presence of chemical heterogeneities at the microlevel in these materials.
Acknowledgements The authors are grateful to the Council of Scientific and Industrial Research, New Delhi for financial assistance under the project 01(1459) / 97 EMR-II. We also thank Professor A.L. Verma, Director, NERIST for providing the research infrastructure.
References [1] T. Yao, A. Ariyoshi, T. Inui, J. Am. Ceram. Soc. 80 (1997) 2441. [2] M.W. Murphy, T.R. Armstrong, P.A. Smith, J. Am. Ceram. Soc. 80 (1997) 165.
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[3] S. Sugunan, V. Meera, Indian J. Chem. 34A (1995) 984. [4] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [5] H. Takahashi, F. Munakata, M. Yamanaka, Phys. Rev. B 57 (1998) 15211. [6] H.L. Ju et al., Phys. Rev. B 51 (1995) 6143. [7] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B26 (1970) 1046. [8] V.G. Ghide, D.S. Rajoria, G. Rama Rao, C.N.R. Rao, Phys. Rev. B 6 (1972) 1021. [9] P. Raccah, J.B. Goodennough, Phys. Rev. 155 (1967) 932.
L
www.elsevier.com / locate / jallcom
Letter
Characterization of La 12x Ba x CoO 3 synthesized at low temperature a, a b K.D. Mandal *, L. Behera , K. Ismail a
Department of Chemistry, North Eastern Regional Institute of Science and Technology, Nirjuli, Itanagar — 791109, Arunachal Pradesh, India b Department of Chemistry, North Eastern Hill University, Shillong, India Received 31 October 2000; accepted 3 April 2001
Abstract The perovskite oxides La 12x Bax CoO 3 (x50.00, 0.10, 0.20) have been synthesized at low temperature using the chemical route. The single phase formation of the prepared samples was confirmed by X-ray diffraction studies. The electrical conductivity of the doped samples showed little variation in comparison to undoped LaCoO 3. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Oxide materials; Chemical synthesis; Electrical transport; Dielectric response
1. Introduction The perovskite oxides LaMO 3 (M5Fe, Co, Mn, Cr) have been receiving great attention as a catalyst, as well as interconnection materials for solid oxide fuel cells, which are major candidates for future energy-generating systems [1–3]. Recently, the system La 12x R x MnO 3 (R5Ba, Ca, Sr) has received great interest from different groups [4–6] for detailed studies. For a broad range of doping, these materials have a paramagnetic to ferromagnetic transition upon cooling, which is accompanied by a sharp drop in resistivity. In view of the interesting properties of LaCoO 3 , it is considered worthwhile to study the possibility of doping of barium in the lanthanum position by the chemical route at low temperature and to study the electrical and dielectric properties.
then dissolved in distilled water. Solutions having stoichiometric amounts of those elements were mixed on a hot plate magnetic stirrer and warmed up to 60–708C. Freshly prepared oxalic acid solution was added slowly for complete co-precipitation as oxalate and filtered. The precipitates were dried and then calcined at 5008C for 6 h. The calcined powders were ground, mixed and pelletized in cylindrical form. The pellets were sintered at 6008C for 6 h. Powder X-ray diffraction patterns of the resulting product were recorded using Cu-Ka radiation. The DC electrical resistance of the synthesized materials was measured with variation of temperature by a Keithley electrometer. The capacitance was measured using a HP 4192A LF Impedance Analyzer as a function of frequency at room temperature (300 K).
2. Experiment
3. Results and discussion
Lanthanum cobaltate and barium-doped lanthanum cobaltate, La 12x Ba x CoO 3 (x50.00, 0.10, 0.20) have been synthesized by the co-precipitation chemical route. Chemicals were used as barium nitrate, cobalt (II) nitrate and lanthanum oxalate which had purity of 99.5% or better. Standard solutions of barium nitrate and cobalt nitrate were prepared in distilled water. A known amount of lanthanum oxalate was converted into lanthanum nitrate by repeated boiling in conc. nitric acid and evaporated to dryness and
Powdered XRD patterns of the synthesized samples in the system La 12x Ba x CoO 3 (x50.00, 0.10, 0.20) were recorded and are shown in Fig. 1. The recorded peaks were sharp and strong enough to confirm high crystallinity of the synthesized samples. No reflections other then those belonging to tetragonal structure were observed in the XRD patterns. XRD peaks were coincident with the three compositions. This indicates the formation of single phases of the synthesized samples and were indexed as singlephase tetragonal structure. The values of lattice constants, determined from XRD data are shown in Table 1. The lattice constant, c, increases with increase in Ba 21 con-
*Corresponding author. E-mail address: [email protected] (K.D. Mandal).
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01386-X
K.D. Mandal et al. / Journal of Alloys and Compounds 325 (2001) L17 –L19
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Fig. 2. Variation of resistivity of the sample with temperature (1000 /T K 21 ) in the system La 12x Ba x CoO 3 .
Fig. 1. XRD pattern of (a) x50.00; (b) x50.10 and (c) x50.20 for the system La 12x Ba x CoO 3 .
centration. This may be due to the large ionic radius [7] of Ba 21 ion, which when substituted occupies the tetrahedral (A) site and displaces the smaller La 31 ion from tetrahedral (A) site to the octahedral (B) site. This suggests strong occupancy of Ba 21 ion on the A-site. The plot of log r vs. 1000 /T of the three sample is shown in Fig. 2. The DC resistivity of the sample x50.00 is almost constant with the variation of temperature in the range 300–500 K. When Ba 21 is doped in the LaCoO 3 , the conductivity of the sample x50.10 is lowered whereas it increases for the composition x50.20. In the doped compounds, there is a sudden increase in the resistivity. This jump occurs at lower temperature for the sample x50.10 and at higher temperature for the composition
x50.20. The sudden jump in resistivity in doped samples may be due to the formation of Co 21 and Co 41 at high temperature. The formation of ionic cobalt in two different states is due to electron transfer from high spin Co 31 to low spin Co 31 ion [8]. With partial doping of Ba 21 in place of La 31 , there is loss of oxygen for maintaining electrical neutrality of the doped material, La 12x Ba x CoO 32x / 2 . These resulting materials are oxygen deficient. The resistivity of the doped materials (x50.10 and 0.20) increases at high temperature (around 400 K) due to oxygen deficiency showing the metallic nature of the materials. The resistivity of LaCoO 3 is found to be much higher than reported earlier [9]. The differences may be due to different methods of preparation of the materials. The plot of dielectric constant (e ) versus log ( f ) is shown in Fig. 3. The dielectric constant of the compounds decreases sharply and thereafter remains constant up to the frequency 10 6 Hz. This shows that space charge polarization contributes significantly to the observed dielectric
Table 1 Composition, structure, lattice parameters and DC conductivity (s ) at 400 K for the system La 12x Ba x CoO 3 Composition
Structure
Lattice parameter
x50.00
Tetragonal
x50.10
Tetragonal
x50.20
Tetragonal
a5b56.9284 ˚ c57.6214 A a5b56.8298 ˚ c57.7859 A a5b56.7519 ˚ c58.0052 A
DC conductivity (s ) at 400 K (V 21 cm 21 )
˚ A
2.48310
29
˚ A
1.30310
29
˚ A
4.22310 29
Fig. 3. Variation of dielectric constant of the samples at room temperature (300 K) in the system La 12x Ba x CoO 3 .
K.D. Mandal et al. / Journal of Alloys and Compounds 325 (2001) L17 –L19
properties. Space charge polarization arises due to the presence of chemical heterogeneities at the microlevel in these materials.
Acknowledgements The authors are grateful to the Council of Scientific and Industrial Research, New Delhi for financial assistance under the project 01(1459) / 97 EMR-II. We also thank Professor A.L. Verma, Director, NERIST for providing the research infrastructure.
References [1] T. Yao, A. Ariyoshi, T. Inui, J. Am. Ceram. Soc. 80 (1997) 2441. [2] M.W. Murphy, T.R. Armstrong, P.A. Smith, J. Am. Ceram. Soc. 80 (1997) 165.
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[3] S. Sugunan, V. Meera, Indian J. Chem. 34A (1995) 984. [4] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [5] H. Takahashi, F. Munakata, M. Yamanaka, Phys. Rev. B 57 (1998) 15211. [6] H.L. Ju et al., Phys. Rev. B 51 (1995) 6143. [7] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B26 (1970) 1046. [8] V.G. Ghide, D.S. Rajoria, G. Rama Rao, C.N.R. Rao, Phys. Rev. B 6 (1972) 1021. [9] P. Raccah, J.B. Goodennough, Phys. Rev. 155 (1967) 932.