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Nano-crystalline LaFeO3 powders synthesized by the citrate–gel method
Materials Letters 60 (2006) 3706 – 3709 www.elsevier.com/locate/matlet
Nano-crystalline LaFeO3 powders synthesized by the citrate–gel method G. Shabbir a,⁎, A.H. Qureshi b , K. Saeed b a
b
Physics Research Division, P. O. Nilore, Islamabad, Pakistan Materials Division, PINSTECH, P. O. Nilore, Islamabad, Pakistan Received 20 October 2005; accepted 26 March 2006 Available online 27 April 2006
Abstract A novel sol–gel process was developed for preparing nano-sized, perovskite-type LaFeO3 powder by the thermal decomposition of the gelcomplex of LaFe–(C6H8O7·H2O). The structural evolution has been systematically investigated by X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Perovskite powder of ∼25 nm size could be obtained at a temperature of ∼ 600 °C without formation of any secondary phases of La2O3 and Fe2O3 single oxides and no requirements of high temperature/vacuum/pH control etc. Analysis of the X-ray powder diffraction data showed a decrease in the value of lattice strains with increasing decomposition temperature, whereas the particle size increases with increasing decomposition temperature. © 2006 Elsevier B.V. All rights reserved. Keywords: Perovskite-type oxide; Thermal decomposition; LaFeO3; Sol–gel
1. Introduction Perovskite-type oxides with the general formula ABO3, where A can be an alkali, alkaline earth, or a lanthanide metal and B may be a transition metal, play an important role in the preparation of catalysts for specific applications [1]. Their composition can be varied over a wide range by partial substitution of cations in positions A and B yielding compounds of the formula (AxA′1−x)(ByB′1−y)O3. They are important for cathode and interconnection materials for solid oxide fuel cells (SOFCs) [2], gas sensors [3], and humidity sensors [4]. The microstructure of the end product plays an important role in the applications, which can be achieved by using ultrafine, homogeneously sized ceramic powders as starting materials. Traditionally, this kind of compound is prepared by the equimolar oxide mixing synthesis method and decomposition synthesis method. The former needs a high calcination temperature (1100–1300 °C) for the reaction to occur and often results in the formation of coarse aggregates, which are difficult to break down. The grain size of the product obtained by this method is relatively large making it unsuitable for some applications. Another drawback of this method is that the purity of the product is not high. The later method normally has a lower synthesis temperature (below 800 °C); thus high purity and ⁎ Corresponding author. Tel.: +92 51 9290231x4316; fax: +92 51 9290275. E-mail address: [email protected] (G. Shabbir). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.093
homogeneity of the product are obtainable [3]. Among the decomposition synthesis/sol–gel methods are the cyanide and mixed nitrate decomposition routes [5–7], mechanochemical processing [8–10] and precipitation/co-precipitation methods [11], which need proper control of pH of the solution. The decomposition/ sol–gel methods normally provide well-crystallized nano-size powders but are time consuming and even one has to deal with toxic media. Here we report experimental results to synthesize nanosized LaFeO3 powders from the amorphous citrate precursor method from the viewpoint of low cost, fast reaction time and easy handling of the materials. Moreover, this method is very simple and it does not involve any intermediate precipitation as do other sol–gel methods [5–7]. The optimum synthesis conditions of LaFeO3 fine powders are reported here, using, Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TGA), and powder X-ray diffraction (XRD) techniques. 2. Experimental Analytical grade La(NO3)3·6H2O (Merck), Fe(NO3)3·9H2O (BDH), and C6H8O7·H2O (BDH) were used as starting materials. The equimolar amounts of metal nitrates were weighed according to the nominal composition of LaFeO3 and then dissolved in deionized water. The citric acid was then added in the nitrates solution such that the molar amount of citric acid was equal to the
G. Shabbir et al. / Materials Letters 60 (2006) 3706–3709
3707
total molar amount of nitrates in the solution. The solution then was gently evaporated at 60–70 °C under continuous stirring, resulting in a dark brown LaFe–(C6H8O7·H2O) gel-complex which finally turned into a light brown highly porous foamy solid. The nano-crystals of perovskite-type LaFeO3 were obtained by decomposition of the dry gel-complex at selected temperatures. The crystallization process was monitored by Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA) by heating in air at a rate of 5 °C min− 1, employing a NETZSCH Simultaneous Thermal Analyzer STA 409, Germany. X-ray diffraction patterns were acquired from a Geigerflex D/Max-3B (M/S Rigaku Corporation, Japan) powder diffractometer with nickel filtered Cu Kα source. Powder patterns were taken in step mode at an interval of 0.03° and counting time of 5 s in the range 20–90° 2θ. Si (99.999%, Johnson Matthey) powder was used as the internal standard to correct the line position. The particle size and lattice strains/distortions were calculated using the Hall–Williamson or Gaussian squared method assuming a Gaussian–Gaussian profile. The peaks were first deconvoluted, and the full widths at half maximum (FWHM) and integral breadths were then computed by a profile fitting procedure. 3. Results and discussion
Fig. 1. The DTA/TGA/DTG curves of the gel-complex recorded in air at a heating rate of 5 °C min− 1.
Fig. 1 shows the thermal decomposition curves of the gel-complex that can be described as follows. The weight loss (∼ 35%) started slowly at about ∼75 °C, followed by a maximum at ∼ 171 °C, which was completed at ∼ 240 °C, as shown in the TG and DTG plots (Fig. 1). This was accompanied by an exothermic effect in the DTA curve at ∼173 °C. Below 240 °C, removal of the water of crystallization and decomposition of the organic substances cause the weight loss. The weight loss (∼ 21%)between 300–400 °C with maximum at 340 °C (DTG curve) and an exothermic peak in the DTA curve at ∼ 348 °C,
Fig. 2. The powder X-ray diffraction patterns of: the freshly prepared dry gel (a), and LaFeO3 powders calcined for 1 h at temperatures: 620 °C (b), 700 °C (c), 800 °C (d), and 900 °C (e), respectively. The inset showing broad bands centered around 30° and 45° in 2θ is for the gel-complex decomposed at 350 °C.
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G. Shabbir et al. / Materials Letters 60 (2006) 3706–3709
may be regarded as a result of the burning of the remaining organic matter and formation of the disordered La2O3 [7] as evident from the corresponding broad bands in the X-ray diffraction pattern (Fig. 2 inset). Further heating caused a small weight loss (∼ 5%) at ∼ 464 and ∼ 525 °C with the release of minute gaseous products in the form of CO2. In the DTA curve there is a peak at 464 °C that corresponds to the Néel Temperature of LaFeO3 [12] and the loss at 525 °C is attributed to the release of gaseous impurities during ordering of the material in the antiferromagnetic phase. There is another weight loss (∼ 4%) at ∼ 620 °C ascribed to the removal of the residual carbon during crystallization of the precursor. No further weight loss or peak appears thereafter, indicating that all the organic matter has been burnt out and there is no change in the phase of the product so formed in the higher temperature range investigated. The observed weight loss (∼ 65%) associated with these chemical changes was in reasonable agreement with the theoretical weight loss as shown in Eq. (1): LaðNO3 Þ3 d 6H2 O þ FeðNO3 Þ3 d 9H2 O þ C6 H8 O7 d H2 O→ LaFeO3 þ 6CO2 þ 2N2 þ 2NO2 þ 20H2 O
ð1Þ
Table 1 Root mean square lattice strain (rmss) and particle size calculated from X-ray diffraction line broadening using the Hall–Williamson plots with Gaussian profile for LaFeO3 nano-crystals Temperature (°C)
620
700
800
900
Crystallite size (nm) Scherrer equation Hall–Williamson plots rmss (ε2)1/2 × 10− 3
11.3 ± 1.7 – –
16.11 ± 1.28 16.12 ± 0.61 4.15 ± 0.17
25.6 ± 0.8 24.09 ± 0.24 2.97 ± 0.51
34.58 ± 2.77 29.33 ± 1.31 2.31 ± 0.32
losses below ≤200 °C does not contribute appreciably to the TG curve (Fig. 2 Ref. [5]) most probably due to prior dried nature of the gelcomplex. Furthermore, due to low temperature combustion process most of the organic matter is lost abruptly giving rise to sharp weight loss. The particle size of the synthesized powders was estimated from the broadening of the XRD lines. Different investigators [14] have worked on equations relating particle size to the measured breadth of the X-ray diffraction lines. They all take approximately the form: D ¼ ðKkÞ=ðbcoshÞ
Fig. 2 shows the XRD patterns of the powders prepared at different temperatures. For this purpose, the decomposition temperatures were selected following the thermal analysis results. The XRD plot of the complex decomposed at 350 °C in air for 1 h showed broad bands centered around 30° and 45° 2θ (Fig. 2 inset), suggesting the existence of disordered La2O3 phase [7]. No change in the XRD pattern was observed above 350 °C and below 600 °C. For the complex decomposed at 620 °C for 1 h, the XRD plot showed some peaks attributed to LaFeO3 perovskite phase. The XRD profiles of the complex decomposed at T ≥ 620 °C showed only the presence of the orthorhombic LaFeO3 phase (JCPDS file No., 37-1493) without broad bands. This confirmed the crystallization temperature of the perovskite phase around ∼ 620 °C as anticipated by the corresponding thermal decomposition curves (Fig. 1). The phase identification was done first by profile fitting in the WinPLOTR software and then using TREOR program for indexing purpose. It is worth mentioning that thermal analysis behaviour of the freshly prepared gel (Fig. 1) is in good agreement with earlier reports [4,7,13] on the perovskite oxides prepared from the sol–gel routes. While it slightly differs to that of the dried gel tendency [5] where the weight
ð2Þ
where θ is the Bragg angle, λ is the wavelength of the X-radiation, D is the average particle dimensions composing the powder, β is the pure full width in radians subtended by the half maximum intensity of the powder pattern peak. The constant K depends largely upon the particle shape, hkl, and definitions taken for β and D. K is taken as 0.9 for half maximum line breadth and if the integral line breadth is used, K increases to 1.0 or more. Determination of the pure diffraction breadth β constitutes the major effect associated with the particle size/strain analysis, which necessitates proper approximation of the profile functions. Diffraction pattern profile functions are generally approximated by the Cauchy or Gauss functions. If at least two orders of reflections of the same family of planes are known then by using the Hall–Williamson or Gaussian squared method assuming a Gaussian–Gaussian profile, crystallite size and lattice strain parameters can be calculated [15]: ðbcosh=kÞ2 ¼ ð0:89=DÞ2 þ 16e2 ðsinh=kÞ2 2
ð3Þ 2
When (βcosθ/λ) is plotted versus (4sinθ/λ) for the available multiple orders of a given reflection, a linear fit is expected. β was corrected for the instrumental broadening using Warren's method, and the resulting data are shown in Fig. 3. From the slopes of these curves, the root mean square lattice strains (ε2)1/2 were obtained whereas the intercept of the lines gave the particle size and the results are summarized in Table 1. The overall magnitude of the calculated strain is small (b 0.003) and maximum particle size thus obtained is 29.33 ± 1.31 nm. The higher value of the strain parameter may be associated to the smaller size at ∼620 °C and relatively higher free energy of the particles.
4. Conclusions
Fig. 3. The Hall–Williamson plots with Gaussian–Gaussian profile for the nanosized LaFeO3 powders obtained at different temperatures. Solid curves are mere eye guide lines.
Nano-sized LaFeO3 powder of the average particle size ∼ 25 nm was successfully synthesized by the citrate–gel method. By careful control of gelling conditions the formation of LaFeO3 perovskite phase can be achieved without passing through explosion/combustion process and pH control. XRD results confirmed that although the disordered phase of La2O3 appeared at ∼ 350 °C but it was transformed completely into the
G. Shabbir et al. / Materials Letters 60 (2006) 3706–3709
perovskite phase at/above 600 °C with final particle size of the order of ∼ 25 nm. References [1] [2] [3] [4]
L.G. Tejuca, J. Less-Common Met. 146 (1989) 251. N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. T. Arakawa, H. Kurachi, J. Shiokawa, J. Mater. Sci. 20 (1985) 1207. E. Traversa, P. Nunziante, M. Sakamoto, Y. Sadaoka, R. Montanari, Mater. Res. Bull. 33 (1998) 673. [5] X. Qi, J. Zhou, Z. Yue, Z. Gui, L. Li, Ceram. Int. 29 (2003) 347. [6] E. Traversa, P. Nunziante, L. Sangaletti, B. Allieri, L.E. Depero, H. Aono, Y. Sadaoka, J. Am. Cream.Soc. 83 (2000) 1087.
3709
[7] Y. Sadaoka, H. Aono, E. Traversa, M. Sakamoto, J. Alloys Compd. 278 (1998) 135. [8] L.M. Cukrov, P.G. McCormick, K. Galatsis, W. Wlodarski, Sens. Actuators, B, Chem. 77 (2001) 491. [9] L.M. Cukrov, T. Tsuzuki, P.G. McCormick, Scr. Mater. 44 (2001) 1787. [10] T. Tsuzuki, P.G. McCormick, Mat. Sci. Forum 343–346 (2000) 383. [11] G. Xiong, Z. Zhi, X. Yang, L. Lu, X. Wang, J. Mater. Sci. Lett. 16 (1997) 1064. [12] H. Tanaka, N. Okawa, T. Kawai, Solid State Commun. 110 (1999) 191. [13] R. Asiaie, W. Zhu, S.A. Akbar, P.K. Dutta, Chem. Mater. 8 (1996) 226. [14] L.S. Birks, H. Friedman, J. Appl. Phys. 17 (1946) 687. [15] B.D. Cullity, Elements of X-ray Diffraction, Addison–Wesley, London, 1978.
Nano-crystalline LaFeO3 powders synthesized by the citrate–gel method G. Shabbir a,⁎, A.H. Qureshi b , K. Saeed b a
b
Physics Research Division, P. O. Nilore, Islamabad, Pakistan Materials Division, PINSTECH, P. O. Nilore, Islamabad, Pakistan Received 20 October 2005; accepted 26 March 2006 Available online 27 April 2006
Abstract A novel sol–gel process was developed for preparing nano-sized, perovskite-type LaFeO3 powder by the thermal decomposition of the gelcomplex of LaFe–(C6H8O7·H2O). The structural evolution has been systematically investigated by X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Perovskite powder of ∼25 nm size could be obtained at a temperature of ∼ 600 °C without formation of any secondary phases of La2O3 and Fe2O3 single oxides and no requirements of high temperature/vacuum/pH control etc. Analysis of the X-ray powder diffraction data showed a decrease in the value of lattice strains with increasing decomposition temperature, whereas the particle size increases with increasing decomposition temperature. © 2006 Elsevier B.V. All rights reserved. Keywords: Perovskite-type oxide; Thermal decomposition; LaFeO3; Sol–gel
1. Introduction Perovskite-type oxides with the general formula ABO3, where A can be an alkali, alkaline earth, or a lanthanide metal and B may be a transition metal, play an important role in the preparation of catalysts for specific applications [1]. Their composition can be varied over a wide range by partial substitution of cations in positions A and B yielding compounds of the formula (AxA′1−x)(ByB′1−y)O3. They are important for cathode and interconnection materials for solid oxide fuel cells (SOFCs) [2], gas sensors [3], and humidity sensors [4]. The microstructure of the end product plays an important role in the applications, which can be achieved by using ultrafine, homogeneously sized ceramic powders as starting materials. Traditionally, this kind of compound is prepared by the equimolar oxide mixing synthesis method and decomposition synthesis method. The former needs a high calcination temperature (1100–1300 °C) for the reaction to occur and often results in the formation of coarse aggregates, which are difficult to break down. The grain size of the product obtained by this method is relatively large making it unsuitable for some applications. Another drawback of this method is that the purity of the product is not high. The later method normally has a lower synthesis temperature (below 800 °C); thus high purity and ⁎ Corresponding author. Tel.: +92 51 9290231x4316; fax: +92 51 9290275. E-mail address: [email protected] (G. Shabbir). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.093
homogeneity of the product are obtainable [3]. Among the decomposition synthesis/sol–gel methods are the cyanide and mixed nitrate decomposition routes [5–7], mechanochemical processing [8–10] and precipitation/co-precipitation methods [11], which need proper control of pH of the solution. The decomposition/ sol–gel methods normally provide well-crystallized nano-size powders but are time consuming and even one has to deal with toxic media. Here we report experimental results to synthesize nanosized LaFeO3 powders from the amorphous citrate precursor method from the viewpoint of low cost, fast reaction time and easy handling of the materials. Moreover, this method is very simple and it does not involve any intermediate precipitation as do other sol–gel methods [5–7]. The optimum synthesis conditions of LaFeO3 fine powders are reported here, using, Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TGA), and powder X-ray diffraction (XRD) techniques. 2. Experimental Analytical grade La(NO3)3·6H2O (Merck), Fe(NO3)3·9H2O (BDH), and C6H8O7·H2O (BDH) were used as starting materials. The equimolar amounts of metal nitrates were weighed according to the nominal composition of LaFeO3 and then dissolved in deionized water. The citric acid was then added in the nitrates solution such that the molar amount of citric acid was equal to the
G. Shabbir et al. / Materials Letters 60 (2006) 3706–3709
3707
total molar amount of nitrates in the solution. The solution then was gently evaporated at 60–70 °C under continuous stirring, resulting in a dark brown LaFe–(C6H8O7·H2O) gel-complex which finally turned into a light brown highly porous foamy solid. The nano-crystals of perovskite-type LaFeO3 were obtained by decomposition of the dry gel-complex at selected temperatures. The crystallization process was monitored by Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA) by heating in air at a rate of 5 °C min− 1, employing a NETZSCH Simultaneous Thermal Analyzer STA 409, Germany. X-ray diffraction patterns were acquired from a Geigerflex D/Max-3B (M/S Rigaku Corporation, Japan) powder diffractometer with nickel filtered Cu Kα source. Powder patterns were taken in step mode at an interval of 0.03° and counting time of 5 s in the range 20–90° 2θ. Si (99.999%, Johnson Matthey) powder was used as the internal standard to correct the line position. The particle size and lattice strains/distortions were calculated using the Hall–Williamson or Gaussian squared method assuming a Gaussian–Gaussian profile. The peaks were first deconvoluted, and the full widths at half maximum (FWHM) and integral breadths were then computed by a profile fitting procedure. 3. Results and discussion
Fig. 1. The DTA/TGA/DTG curves of the gel-complex recorded in air at a heating rate of 5 °C min− 1.
Fig. 1 shows the thermal decomposition curves of the gel-complex that can be described as follows. The weight loss (∼ 35%) started slowly at about ∼75 °C, followed by a maximum at ∼ 171 °C, which was completed at ∼ 240 °C, as shown in the TG and DTG plots (Fig. 1). This was accompanied by an exothermic effect in the DTA curve at ∼173 °C. Below 240 °C, removal of the water of crystallization and decomposition of the organic substances cause the weight loss. The weight loss (∼ 21%)between 300–400 °C with maximum at 340 °C (DTG curve) and an exothermic peak in the DTA curve at ∼ 348 °C,
Fig. 2. The powder X-ray diffraction patterns of: the freshly prepared dry gel (a), and LaFeO3 powders calcined for 1 h at temperatures: 620 °C (b), 700 °C (c), 800 °C (d), and 900 °C (e), respectively. The inset showing broad bands centered around 30° and 45° in 2θ is for the gel-complex decomposed at 350 °C.
3708
G. Shabbir et al. / Materials Letters 60 (2006) 3706–3709
may be regarded as a result of the burning of the remaining organic matter and formation of the disordered La2O3 [7] as evident from the corresponding broad bands in the X-ray diffraction pattern (Fig. 2 inset). Further heating caused a small weight loss (∼ 5%) at ∼ 464 and ∼ 525 °C with the release of minute gaseous products in the form of CO2. In the DTA curve there is a peak at 464 °C that corresponds to the Néel Temperature of LaFeO3 [12] and the loss at 525 °C is attributed to the release of gaseous impurities during ordering of the material in the antiferromagnetic phase. There is another weight loss (∼ 4%) at ∼ 620 °C ascribed to the removal of the residual carbon during crystallization of the precursor. No further weight loss or peak appears thereafter, indicating that all the organic matter has been burnt out and there is no change in the phase of the product so formed in the higher temperature range investigated. The observed weight loss (∼ 65%) associated with these chemical changes was in reasonable agreement with the theoretical weight loss as shown in Eq. (1): LaðNO3 Þ3 d 6H2 O þ FeðNO3 Þ3 d 9H2 O þ C6 H8 O7 d H2 O→ LaFeO3 þ 6CO2 þ 2N2 þ 2NO2 þ 20H2 O
ð1Þ
Table 1 Root mean square lattice strain (rmss) and particle size calculated from X-ray diffraction line broadening using the Hall–Williamson plots with Gaussian profile for LaFeO3 nano-crystals Temperature (°C)
620
700
800
900
Crystallite size (nm) Scherrer equation Hall–Williamson plots rmss (ε2)1/2 × 10− 3
11.3 ± 1.7 – –
16.11 ± 1.28 16.12 ± 0.61 4.15 ± 0.17
25.6 ± 0.8 24.09 ± 0.24 2.97 ± 0.51
34.58 ± 2.77 29.33 ± 1.31 2.31 ± 0.32
losses below ≤200 °C does not contribute appreciably to the TG curve (Fig. 2 Ref. [5]) most probably due to prior dried nature of the gelcomplex. Furthermore, due to low temperature combustion process most of the organic matter is lost abruptly giving rise to sharp weight loss. The particle size of the synthesized powders was estimated from the broadening of the XRD lines. Different investigators [14] have worked on equations relating particle size to the measured breadth of the X-ray diffraction lines. They all take approximately the form: D ¼ ðKkÞ=ðbcoshÞ
Fig. 2 shows the XRD patterns of the powders prepared at different temperatures. For this purpose, the decomposition temperatures were selected following the thermal analysis results. The XRD plot of the complex decomposed at 350 °C in air for 1 h showed broad bands centered around 30° and 45° 2θ (Fig. 2 inset), suggesting the existence of disordered La2O3 phase [7]. No change in the XRD pattern was observed above 350 °C and below 600 °C. For the complex decomposed at 620 °C for 1 h, the XRD plot showed some peaks attributed to LaFeO3 perovskite phase. The XRD profiles of the complex decomposed at T ≥ 620 °C showed only the presence of the orthorhombic LaFeO3 phase (JCPDS file No., 37-1493) without broad bands. This confirmed the crystallization temperature of the perovskite phase around ∼ 620 °C as anticipated by the corresponding thermal decomposition curves (Fig. 1). The phase identification was done first by profile fitting in the WinPLOTR software and then using TREOR program for indexing purpose. It is worth mentioning that thermal analysis behaviour of the freshly prepared gel (Fig. 1) is in good agreement with earlier reports [4,7,13] on the perovskite oxides prepared from the sol–gel routes. While it slightly differs to that of the dried gel tendency [5] where the weight
ð2Þ
where θ is the Bragg angle, λ is the wavelength of the X-radiation, D is the average particle dimensions composing the powder, β is the pure full width in radians subtended by the half maximum intensity of the powder pattern peak. The constant K depends largely upon the particle shape, hkl, and definitions taken for β and D. K is taken as 0.9 for half maximum line breadth and if the integral line breadth is used, K increases to 1.0 or more. Determination of the pure diffraction breadth β constitutes the major effect associated with the particle size/strain analysis, which necessitates proper approximation of the profile functions. Diffraction pattern profile functions are generally approximated by the Cauchy or Gauss functions. If at least two orders of reflections of the same family of planes are known then by using the Hall–Williamson or Gaussian squared method assuming a Gaussian–Gaussian profile, crystallite size and lattice strain parameters can be calculated [15]: ðbcosh=kÞ2 ¼ ð0:89=DÞ2 þ 16e2 ðsinh=kÞ2 2
ð3Þ 2
When (βcosθ/λ) is plotted versus (4sinθ/λ) for the available multiple orders of a given reflection, a linear fit is expected. β was corrected for the instrumental broadening using Warren's method, and the resulting data are shown in Fig. 3. From the slopes of these curves, the root mean square lattice strains (ε2)1/2 were obtained whereas the intercept of the lines gave the particle size and the results are summarized in Table 1. The overall magnitude of the calculated strain is small (b 0.003) and maximum particle size thus obtained is 29.33 ± 1.31 nm. The higher value of the strain parameter may be associated to the smaller size at ∼620 °C and relatively higher free energy of the particles.
4. Conclusions
Fig. 3. The Hall–Williamson plots with Gaussian–Gaussian profile for the nanosized LaFeO3 powders obtained at different temperatures. Solid curves are mere eye guide lines.
Nano-sized LaFeO3 powder of the average particle size ∼ 25 nm was successfully synthesized by the citrate–gel method. By careful control of gelling conditions the formation of LaFeO3 perovskite phase can be achieved without passing through explosion/combustion process and pH control. XRD results confirmed that although the disordered phase of La2O3 appeared at ∼ 350 °C but it was transformed completely into the
G. Shabbir et al. / Materials Letters 60 (2006) 3706–3709
perovskite phase at/above 600 °C with final particle size of the order of ∼ 25 nm. References [1] [2] [3] [4]
L.G. Tejuca, J. Less-Common Met. 146 (1989) 251. N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. T. Arakawa, H. Kurachi, J. Shiokawa, J. Mater. Sci. 20 (1985) 1207. E. Traversa, P. Nunziante, M. Sakamoto, Y. Sadaoka, R. Montanari, Mater. Res. Bull. 33 (1998) 673. [5] X. Qi, J. Zhou, Z. Yue, Z. Gui, L. Li, Ceram. Int. 29 (2003) 347. [6] E. Traversa, P. Nunziante, L. Sangaletti, B. Allieri, L.E. Depero, H. Aono, Y. Sadaoka, J. Am. Cream.Soc. 83 (2000) 1087.
3709
[7] Y. Sadaoka, H. Aono, E. Traversa, M. Sakamoto, J. Alloys Compd. 278 (1998) 135. [8] L.M. Cukrov, P.G. McCormick, K. Galatsis, W. Wlodarski, Sens. Actuators, B, Chem. 77 (2001) 491. [9] L.M. Cukrov, T. Tsuzuki, P.G. McCormick, Scr. Mater. 44 (2001) 1787. [10] T. Tsuzuki, P.G. McCormick, Mat. Sci. Forum 343–346 (2000) 383. [11] G. Xiong, Z. Zhi, X. Yang, L. Lu, X. Wang, J. Mater. Sci. Lett. 16 (1997) 1064. [12] H. Tanaka, N. Okawa, T. Kawai, Solid State Commun. 110 (1999) 191. [13] R. Asiaie, W. Zhu, S.A. Akbar, P.K. Dutta, Chem. Mater. 8 (1996) 226. [14] L.S. Birks, H. Friedman, J. Appl. Phys. 17 (1946) 687. [15] B.D. Cullity, Elements of X-ray Diffraction, Addison–Wesley, London, 1978.