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The effect of water content on the magnetic and structural properties of goethite
Journal of Alloys and Compounds 369 (2004) 247–251
The effect of water content on the magnetic and structural properties of goethite J.D. Betancur a,∗ , C.A. Barrero a , J.M. Greneche b , G.F. Goya c b
a Grupo de Estado Sólido, Instituto de F´ısica, Universidad de Antioquia, A.A. 1226 Medell´ın, Colombia Laboratoire de Physique de l’Etat Condensé, UMR CNRS 6087, Université du Maine, 72085 Le Mans Cedex 9, France c Laboratório de Materiais Magnéticos, Instituto de F´ısica, Universidade de São Paulo, São Paulo, Brazil
Abstract We have studied the effect of water content on the magnetic and structural properties of goethite. For that purpose, four samples were prepared using two different hydrothermal methods, one of them is derived on the Fe(II) precursors and the other one from Fe(III) precursors. The samples were characterized by X-ray diffraction (XRD), TGA, BET, FTIR, Mössbauer spectrometry at RT, 77 and 4.2 K and ZFC and FC curves. The results suggest that the goethites from the Fe(II) precursors are less crystalline, have higher water contents and do not show magnetic ordered structure at RT in comparison to the goethites from the Fe(III) precursors. The goethites from the last systems exhibit good crystallinity, low water content and magnetic ordering at room temperature. Our results suggest that both structural and adsorbed water contents reduce the magnetic hyperfine field at 4.2 K. A linear correlation with regression coefficient of 0.91 between the saturation hyperfine field and both the structural hydroxyl content and the surface area could be derived. © 2003 Elsevier B.V. All rights reserved. Keywords: Goethite; Water content; Mössbauer spectrometry; Magnetic properties; Crystallinity
1. Introduction Goethite (␣-FeOOH) is one of the most common iron oxyhydroxides in nature [1]. It is also one of the most important products of the atmospheric corrosion of iron and steel [2]. Goethite is also sometimes used as a starting material to produce maghemite, which is used as magnetic pigment. Well-crystallized goethites are antiferromagnetic below its Néel temperature of about 400 K, and its magnetic structure is similar to the crystalline one [1,3]. It is well established that the magnetic properties of ␣-FeOOH are strongly affected by both the particle size and the degree of crystallinity. In relation to the first, various theoretical models like surface effects, superparamagnetism, collective magnetic excitations, and magnetic cluster ordering, among others, have been put forward in order to explain the magnetic properties, in which some of them are still controversial [3–5]. On the other hand, the degree of crystallinity is usually quantified by the mean coherence length in a certain crystallographic direction which is deter∗
Corresponding author. E-mail address: [email protected] (J.D. Betancur).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.09.046
mined from the line broadening of the corresponding X-ray reflection. However, this term is very complex because it includes mixed contributions coming from small particle size and non-stoichiometry, essentially due to the presence of H2 O and/or OH− bound into the structures, etc. To the best of our knowledge, there is a lack of work related with the effect of water content on the magnetic properties of goethite and this is the purpose of the present investigation. Water content includes both the structural OH− and the adsorbed water, the first one being considered in the general chemical formula ␣-Fe1−(y/3) O1−y (OH)1+y [6], where y parameter is directly related to the hydroxyl content.
2. Experimental Four samples were prepared following hydrothermal methods reported in the literature [1,7]. Two of the samples were synthesized from Fe(II) precursors, which consist of dissolving 9.9(1) g of FeCl2 ·4H2 O in 1 l of distilled water which was previously exposed to a flux of N2 for 30 min. The solution is prepared in a 2 l recipient while keeping the nitrogen flux. Lately, 110 ml of 1 M NaHCO3 is added
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to the solution, and the flux of nitrogen is replaced by a flux of air at a rate of 30–40 ml/min. The solution is stirred continuously. The oxidation is completed after 48 h, during which the suspension favors the change of the color from green into ochre. The pH during oxidation is kept at 7. Finally, the product is filtrated, washed several times, and dried in an oven at 40 ◦ C. The two samples obtained were named GOE24H and GOE48H; they correspond to 24 and 48 h drying time, respectively. The other two samples were prepared from Fe(III) precursors according to the procedure described in [7]. The two samples were named GONITRA and GONOSA. In the first one, the mixed solutions were kept under constant magnetic stirring and in the second without agitation. Samples were characterized by Mössbauer spectrometry (MS), using the conventional transmission method. The spectra were adjusted using the MOSF and DIST3E programs [8], which are based respectively, in lines of Lorentzian shape and a distribution of hyperfine parameters. X-ray diffraction (XRD) data in the 10–80◦ 2θ range were obtained in a BRUKER AKS D8 ADVANCE equipped with a PSD detector and Co tube. Thermogravimetric analysis (TGA) was done in a TA instrument 2950 TGA HR V6.1A. The curves were obtained in a flux of 100 ml/min of N2 UAP, using a heating rate of 10 ◦ C/min from RT till 900 ◦ C. BET analysis with N2 was performed in a ASAP 2010 V4.00 D. FTIR data in the reflectance mode were collected in a Perkin–Elmer, Model Spectrum One, with DTGS detector. Magnetization data in zero field-cooling (ZFC) and field-cooling (FC) modes were taken in a commercial SQUID magnetometer between 5 and 360 K, in fields varying from 1 to 100 mT.
3. Results and discussion The hydroxyl (OH− ) and surface water contents were determined from the TGA curves (see Fig. 1). Indeed, they show essentially two steps of weight loss, the first one occurring at about 68, 68, 53 and 48 ◦ C, for samples GOE24H, GOE48H, GONITRA and, GONOSA, respectively. This phenomenon is principally associated to the surface water content (%H2 O) [9]. The second weight loss which starts below about 400 ◦ C for all samples, is associated to the structural hydroxyl content (%OH). Above this temperature, the weight remains rather constant. The %OH can be
Fig. 1. TGA curves for all the samples.
related to the hydroxyl content in the goethite’s formula through the equation [6]: %OH =
93.56(1 + y) 9.24 − 1.83y
(1)
On the other hand, the total water content (%OH) is given by: %OH = %OH + %H2 O
(2)
From the calculated data listed in Table 1, one can conclude that the goethites prepared from the Fe(II) precursors contain more structural and surface water and hence more total water molecules than the goethites from the Fe(III) precursors. The infrared spectra of the samples, given in Fig. 2, exhibit bands at 795 and 890 cm−1 corresponding to goethite. In addition, the presence of two strong and broad bands around 3140 and 3484 cm−1 is associated with surface hydroxyl and bulk hydroxyl stretch bands, respectively [1]. The relative intensities of these bands with respect to those of goethite demonstrate that the goethites from Fe(II) precursors contain more water than the goethites from Fe(III) precursors, which is in good agreement with the TGA results. Some X-ray diffractograms are shown in Fig. 3. It is observed that the Bragg peaks of samples GOE48H and GOE24H are broader in comparison to those of GONITRA and GONOSA, which could be related to the poor degree of crystallinity of the samples coming from the
Table 1 Summary of the data obtained from molar ratio of mass lost during thermal treatment, specific area, lattice parameters and cell volume and hyperfine field by SM at 4.2 K Sample
%H2 O
%OH
%OH
GOE24H GOE48H GONITRA GONOSA
0.97 1.36 0.11 0.10
19.4 18.8 13.3 11.7
20.4 20.1 13.3 11.7
(3) (3) (3) (3)
(1) (1) (1) (1)
(1) (1) (1) (1)
y 0.76 0.63 0.25 0.13
(1) (1) (2) (1)
Number in parenthesis represents the uncertainty in the last digit.
a (Å)
b (Å)
c (Å)
V (Å3 )
S (m2 /g)
Bhf (T)
– 9.932 (1) 9.964 (1) 9.962 (1)
– 3.029 (1) 3.024 (1) 3.024 (1)
– 4.623 (1) 4.615 (1) 4.615 (1)
139.0 (1) 139.0 (1) 139.0 (1)
135.6 153.4 27.8 32.5
– 49.9 (1) 50.5 (1) 50.5 (1)
(4) (4) (1) (2)
J.D. Betancur et al. / Journal of Alloys and Compounds 369 (2004) 247–251
249
Fig. 2. FTIR spectra for GOE48H and GONITRA samples.
Fe(II) precursors. The values of the cell parameters refined by means of Rietveld method [10] are listed in Table 1: goethites from Fe(II) precursors have larger b and c values and shorter a values in comparison to the goethites from Fe(III) precursors, while the cell volume is quite unchanged. Those features could be related to the different ionic radii of the O2− and OH− . BET analysis shows that (see Table 1) the surface areas of the goethites from Fe(II) precursors are larger than others goethites. Fig. 4. ZFC and FC curves for the different samples. ZFC/FC curves for GOE48H and GOE24H are similar.
Fig. 3. XRD patterns of GOE48H and GONITRA samples.
Blocking temperatures of about 279 and 262 ± 1 K were derived for the goethites from Fe(II) precursors, according to the ZFC and FC curves (see Fig. 4), which may suggest a large distributions of particle sizes. However, this behavior could also be related to the magnetic ordering of clusters, which are created by high concentration of vacancy defects presented in these fine-particle goethites [11]. On the other hand, the curves for the goethites from Fe(III) precursors suggest a magnetic behavior perhaps like freezing of spin-cluster glass. It is evident that further experiments have to be performed on both families of goethites to draw clear conclusions. Fig. 5 shows the Mössbauer spectra for some selected samples at different temperatures. Hyperfine structures consist of either single quadrupolar doublet, or magnetic sextet, or a mixture of both. In order to correlate all these data, we have performed multiple linear regression analysis between the water content and the saturation hyperfine field (hyperfine field at 4.2 K). It is found that the variations of this hyperfine field are poorly
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Bhf = 50.7 − 0.0032S
(R2 ≈ 0.70),
(6)
Bhf = 50.6 − 0.9672y
(R2 ≈ 0.62),
(7)
Bhf = 50.8 − 0.6433y − 0.0023S
(R2 ≈ 0.91),
Bhf = 51.4 − 0.073%OH + 0.00009S
(8)
(R2 ≈ 0.87). (9)
In addition, broad and asymmetrical lines are presented, which can be assigned to the presence of disordered and ultra fine crystalline grains, especially in the case of GOE48H and GOE24H samples. The evolutions versus temperature are in good agreement with the ZFC and FC curves. The calculated maximum hyperfine fields at 4.2 K are summarized in Table 1, in which we have considered 13 data points, and R are the linear correlation coefficients. To calculate those equations, we have also considered the data reported by Schwertmann and coworkers [9]. It is worth mentioning that they did neither report any equation similar to the ones presented here, nor they calculated the y values. As expected, the saturation magnetization decreases with increasing the water content, mainly the structural water content. This is due to the fact that the replacement of the Fe ions (magnetic ions) by hydroxyl groups (non-magnetic ions) and hence the presence of more vacancies [6] weaken the magnetic interactions. The equation with the largest correlation coefficient was that in which it is considered both the parameter y and the surface area (Eq. (8)). These two quantities are commonly referred to as the degree of crystallinity. It is probable that the effect of surface water content on Bhf may be reflected on the surface area, because both are directly correlated, i.e. larger the surface is the higher the surface water content.
4. Conclusions Fig. 5. Mössbauer spectrum for some samples at given temperatures on GOE48H (top) and GONITRA (bottom).
described when only one of the structural properties is taken into account. For example, the variation of hyperfine field is explained only about 12% by %H2 O alone, 70% by S alone, 87% by OH content alone, etc. However, the linear correlations are noticeably improved when two physical properties are taken into account. These findings are reflected in the following equations: Bhf = 50.5 − 0.039%H2 O Bhf = 51.4 − 0.072%OH Bhf = 51.3 − 0.071%OH
(R2 ≈ 0.12), (R2 ≈ 0.87), (R2 ≈ 0.64),
(3) (4) (5)
We have presented several phenomenological linear equations, which try to quantify the effect of the water content on the magnetic properties of goethite. It is found that equation, which considers both the structural water content and the surface area, describes best the tendency of reduction of the saturation magnetic field with their increment. It is evident that surface water and excess hydroxyl all having some influence on the lattice parameters will also play a substantial role in the magnitude of the hyperfine field. Of course, these additional structural parameters are mainly determined by goethite formation factors such as crystallization, rates, temperature, etc. In situ Mössbauer experiments are currently in progress to follow the evolution of hyperfine structure of goethites annealed at different temperatures, comprised between 360 and 400 K.
J.D. Betancur et al. / Journal of Alloys and Compounds 369 (2004) 247–251
Acknowledgements The financial support of CODI (University of Antioquia) and COLCIENCIAS, under contracts IN378CE and 1115-05-10113 respectively. The authors are also grateful to the ECOS NORD–French–Colombian exchange program under project CF99P04. References [1] R.M. Cornell, U. Schwertmann, 1996, The Iron Oxides, VCH, Weinheim, Germany.
251
[2] J.Y. Lee, S.J. Oh, J.G. Sohn, S.J. Kwon, Corros. Sci. 43 (2001) 803. [3] E. Murad, Phys. Chem. Miner. 23 (1996) 248. [4] C.A. Barrero, R.E. Vandenberghe, E. De Grave, Hyperfine Interact. 122 (1999) 39. [5] E. De Grave, C.A. Barrero, G.M. da Costa, R.E. Vandenberghe, E. Van San, Clay Miner. 37 (2002) 591. [6] E. Wolska, U. Schwertmann, N. Jb. Mh. H. 5 (1993) 217. [7] U. Schwertmann, R.M. Cornell, 2000, Iron Oxides in the Laboratory, VCH, Weinheim, Germany. [8] R.E. Vandenberghe, E. De Grave, P.M.A. de Bakker, Hyperfine Interact. 83 (1994) 29. [9] U. Schwertmann, P. Cambier, E. Murad, Clays Clays Miner. 33 (1995) 369. [10] http://www.ing.unitn.it/∼luttero/maud. [11] S. Bocquet, R.J. Pollard, J.D. Cashion, Phys. Rev. B 46 (1992) 11657.
The effect of water content on the magnetic and structural properties of goethite J.D. Betancur a,∗ , C.A. Barrero a , J.M. Greneche b , G.F. Goya c b
a Grupo de Estado Sólido, Instituto de F´ısica, Universidad de Antioquia, A.A. 1226 Medell´ın, Colombia Laboratoire de Physique de l’Etat Condensé, UMR CNRS 6087, Université du Maine, 72085 Le Mans Cedex 9, France c Laboratório de Materiais Magnéticos, Instituto de F´ısica, Universidade de São Paulo, São Paulo, Brazil
Abstract We have studied the effect of water content on the magnetic and structural properties of goethite. For that purpose, four samples were prepared using two different hydrothermal methods, one of them is derived on the Fe(II) precursors and the other one from Fe(III) precursors. The samples were characterized by X-ray diffraction (XRD), TGA, BET, FTIR, Mössbauer spectrometry at RT, 77 and 4.2 K and ZFC and FC curves. The results suggest that the goethites from the Fe(II) precursors are less crystalline, have higher water contents and do not show magnetic ordered structure at RT in comparison to the goethites from the Fe(III) precursors. The goethites from the last systems exhibit good crystallinity, low water content and magnetic ordering at room temperature. Our results suggest that both structural and adsorbed water contents reduce the magnetic hyperfine field at 4.2 K. A linear correlation with regression coefficient of 0.91 between the saturation hyperfine field and both the structural hydroxyl content and the surface area could be derived. © 2003 Elsevier B.V. All rights reserved. Keywords: Goethite; Water content; Mössbauer spectrometry; Magnetic properties; Crystallinity
1. Introduction Goethite (␣-FeOOH) is one of the most common iron oxyhydroxides in nature [1]. It is also one of the most important products of the atmospheric corrosion of iron and steel [2]. Goethite is also sometimes used as a starting material to produce maghemite, which is used as magnetic pigment. Well-crystallized goethites are antiferromagnetic below its Néel temperature of about 400 K, and its magnetic structure is similar to the crystalline one [1,3]. It is well established that the magnetic properties of ␣-FeOOH are strongly affected by both the particle size and the degree of crystallinity. In relation to the first, various theoretical models like surface effects, superparamagnetism, collective magnetic excitations, and magnetic cluster ordering, among others, have been put forward in order to explain the magnetic properties, in which some of them are still controversial [3–5]. On the other hand, the degree of crystallinity is usually quantified by the mean coherence length in a certain crystallographic direction which is deter∗
Corresponding author. E-mail address: [email protected] (J.D. Betancur).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.09.046
mined from the line broadening of the corresponding X-ray reflection. However, this term is very complex because it includes mixed contributions coming from small particle size and non-stoichiometry, essentially due to the presence of H2 O and/or OH− bound into the structures, etc. To the best of our knowledge, there is a lack of work related with the effect of water content on the magnetic properties of goethite and this is the purpose of the present investigation. Water content includes both the structural OH− and the adsorbed water, the first one being considered in the general chemical formula ␣-Fe1−(y/3) O1−y (OH)1+y [6], where y parameter is directly related to the hydroxyl content.
2. Experimental Four samples were prepared following hydrothermal methods reported in the literature [1,7]. Two of the samples were synthesized from Fe(II) precursors, which consist of dissolving 9.9(1) g of FeCl2 ·4H2 O in 1 l of distilled water which was previously exposed to a flux of N2 for 30 min. The solution is prepared in a 2 l recipient while keeping the nitrogen flux. Lately, 110 ml of 1 M NaHCO3 is added
248
J.D. Betancur et al. / Journal of Alloys and Compounds 369 (2004) 247–251
to the solution, and the flux of nitrogen is replaced by a flux of air at a rate of 30–40 ml/min. The solution is stirred continuously. The oxidation is completed after 48 h, during which the suspension favors the change of the color from green into ochre. The pH during oxidation is kept at 7. Finally, the product is filtrated, washed several times, and dried in an oven at 40 ◦ C. The two samples obtained were named GOE24H and GOE48H; they correspond to 24 and 48 h drying time, respectively. The other two samples were prepared from Fe(III) precursors according to the procedure described in [7]. The two samples were named GONITRA and GONOSA. In the first one, the mixed solutions were kept under constant magnetic stirring and in the second without agitation. Samples were characterized by Mössbauer spectrometry (MS), using the conventional transmission method. The spectra were adjusted using the MOSF and DIST3E programs [8], which are based respectively, in lines of Lorentzian shape and a distribution of hyperfine parameters. X-ray diffraction (XRD) data in the 10–80◦ 2θ range were obtained in a BRUKER AKS D8 ADVANCE equipped with a PSD detector and Co tube. Thermogravimetric analysis (TGA) was done in a TA instrument 2950 TGA HR V6.1A. The curves were obtained in a flux of 100 ml/min of N2 UAP, using a heating rate of 10 ◦ C/min from RT till 900 ◦ C. BET analysis with N2 was performed in a ASAP 2010 V4.00 D. FTIR data in the reflectance mode were collected in a Perkin–Elmer, Model Spectrum One, with DTGS detector. Magnetization data in zero field-cooling (ZFC) and field-cooling (FC) modes were taken in a commercial SQUID magnetometer between 5 and 360 K, in fields varying from 1 to 100 mT.
3. Results and discussion The hydroxyl (OH− ) and surface water contents were determined from the TGA curves (see Fig. 1). Indeed, they show essentially two steps of weight loss, the first one occurring at about 68, 68, 53 and 48 ◦ C, for samples GOE24H, GOE48H, GONITRA and, GONOSA, respectively. This phenomenon is principally associated to the surface water content (%H2 O) [9]. The second weight loss which starts below about 400 ◦ C for all samples, is associated to the structural hydroxyl content (%OH). Above this temperature, the weight remains rather constant. The %OH can be
Fig. 1. TGA curves for all the samples.
related to the hydroxyl content in the goethite’s formula through the equation [6]: %OH =
93.56(1 + y) 9.24 − 1.83y
(1)
On the other hand, the total water content (%OH) is given by: %OH = %OH + %H2 O
(2)
From the calculated data listed in Table 1, one can conclude that the goethites prepared from the Fe(II) precursors contain more structural and surface water and hence more total water molecules than the goethites from the Fe(III) precursors. The infrared spectra of the samples, given in Fig. 2, exhibit bands at 795 and 890 cm−1 corresponding to goethite. In addition, the presence of two strong and broad bands around 3140 and 3484 cm−1 is associated with surface hydroxyl and bulk hydroxyl stretch bands, respectively [1]. The relative intensities of these bands with respect to those of goethite demonstrate that the goethites from Fe(II) precursors contain more water than the goethites from Fe(III) precursors, which is in good agreement with the TGA results. Some X-ray diffractograms are shown in Fig. 3. It is observed that the Bragg peaks of samples GOE48H and GOE24H are broader in comparison to those of GONITRA and GONOSA, which could be related to the poor degree of crystallinity of the samples coming from the
Table 1 Summary of the data obtained from molar ratio of mass lost during thermal treatment, specific area, lattice parameters and cell volume and hyperfine field by SM at 4.2 K Sample
%H2 O
%OH
%OH
GOE24H GOE48H GONITRA GONOSA
0.97 1.36 0.11 0.10
19.4 18.8 13.3 11.7
20.4 20.1 13.3 11.7
(3) (3) (3) (3)
(1) (1) (1) (1)
(1) (1) (1) (1)
y 0.76 0.63 0.25 0.13
(1) (1) (2) (1)
Number in parenthesis represents the uncertainty in the last digit.
a (Å)
b (Å)
c (Å)
V (Å3 )
S (m2 /g)
Bhf (T)
– 9.932 (1) 9.964 (1) 9.962 (1)
– 3.029 (1) 3.024 (1) 3.024 (1)
– 4.623 (1) 4.615 (1) 4.615 (1)
139.0 (1) 139.0 (1) 139.0 (1)
135.6 153.4 27.8 32.5
– 49.9 (1) 50.5 (1) 50.5 (1)
(4) (4) (1) (2)
J.D. Betancur et al. / Journal of Alloys and Compounds 369 (2004) 247–251
249
Fig. 2. FTIR spectra for GOE48H and GONITRA samples.
Fe(II) precursors. The values of the cell parameters refined by means of Rietveld method [10] are listed in Table 1: goethites from Fe(II) precursors have larger b and c values and shorter a values in comparison to the goethites from Fe(III) precursors, while the cell volume is quite unchanged. Those features could be related to the different ionic radii of the O2− and OH− . BET analysis shows that (see Table 1) the surface areas of the goethites from Fe(II) precursors are larger than others goethites. Fig. 4. ZFC and FC curves for the different samples. ZFC/FC curves for GOE48H and GOE24H are similar.
Fig. 3. XRD patterns of GOE48H and GONITRA samples.
Blocking temperatures of about 279 and 262 ± 1 K were derived for the goethites from Fe(II) precursors, according to the ZFC and FC curves (see Fig. 4), which may suggest a large distributions of particle sizes. However, this behavior could also be related to the magnetic ordering of clusters, which are created by high concentration of vacancy defects presented in these fine-particle goethites [11]. On the other hand, the curves for the goethites from Fe(III) precursors suggest a magnetic behavior perhaps like freezing of spin-cluster glass. It is evident that further experiments have to be performed on both families of goethites to draw clear conclusions. Fig. 5 shows the Mössbauer spectra for some selected samples at different temperatures. Hyperfine structures consist of either single quadrupolar doublet, or magnetic sextet, or a mixture of both. In order to correlate all these data, we have performed multiple linear regression analysis between the water content and the saturation hyperfine field (hyperfine field at 4.2 K). It is found that the variations of this hyperfine field are poorly
250
J.D. Betancur et al. / Journal of Alloys and Compounds 369 (2004) 247–251
Bhf = 50.7 − 0.0032S
(R2 ≈ 0.70),
(6)
Bhf = 50.6 − 0.9672y
(R2 ≈ 0.62),
(7)
Bhf = 50.8 − 0.6433y − 0.0023S
(R2 ≈ 0.91),
Bhf = 51.4 − 0.073%OH + 0.00009S
(8)
(R2 ≈ 0.87). (9)
In addition, broad and asymmetrical lines are presented, which can be assigned to the presence of disordered and ultra fine crystalline grains, especially in the case of GOE48H and GOE24H samples. The evolutions versus temperature are in good agreement with the ZFC and FC curves. The calculated maximum hyperfine fields at 4.2 K are summarized in Table 1, in which we have considered 13 data points, and R are the linear correlation coefficients. To calculate those equations, we have also considered the data reported by Schwertmann and coworkers [9]. It is worth mentioning that they did neither report any equation similar to the ones presented here, nor they calculated the y values. As expected, the saturation magnetization decreases with increasing the water content, mainly the structural water content. This is due to the fact that the replacement of the Fe ions (magnetic ions) by hydroxyl groups (non-magnetic ions) and hence the presence of more vacancies [6] weaken the magnetic interactions. The equation with the largest correlation coefficient was that in which it is considered both the parameter y and the surface area (Eq. (8)). These two quantities are commonly referred to as the degree of crystallinity. It is probable that the effect of surface water content on Bhf may be reflected on the surface area, because both are directly correlated, i.e. larger the surface is the higher the surface water content.
4. Conclusions Fig. 5. Mössbauer spectrum for some samples at given temperatures on GOE48H (top) and GONITRA (bottom).
described when only one of the structural properties is taken into account. For example, the variation of hyperfine field is explained only about 12% by %H2 O alone, 70% by S alone, 87% by OH content alone, etc. However, the linear correlations are noticeably improved when two physical properties are taken into account. These findings are reflected in the following equations: Bhf = 50.5 − 0.039%H2 O Bhf = 51.4 − 0.072%OH Bhf = 51.3 − 0.071%OH
(R2 ≈ 0.12), (R2 ≈ 0.87), (R2 ≈ 0.64),
(3) (4) (5)
We have presented several phenomenological linear equations, which try to quantify the effect of the water content on the magnetic properties of goethite. It is found that equation, which considers both the structural water content and the surface area, describes best the tendency of reduction of the saturation magnetic field with their increment. It is evident that surface water and excess hydroxyl all having some influence on the lattice parameters will also play a substantial role in the magnitude of the hyperfine field. Of course, these additional structural parameters are mainly determined by goethite formation factors such as crystallization, rates, temperature, etc. In situ Mössbauer experiments are currently in progress to follow the evolution of hyperfine structure of goethites annealed at different temperatures, comprised between 360 and 400 K.
J.D. Betancur et al. / Journal of Alloys and Compounds 369 (2004) 247–251
Acknowledgements The financial support of CODI (University of Antioquia) and COLCIENCIAS, under contracts IN378CE and 1115-05-10113 respectively. The authors are also grateful to the ECOS NORD–French–Colombian exchange program under project CF99P04. References [1] R.M. Cornell, U. Schwertmann, 1996, The Iron Oxides, VCH, Weinheim, Germany.
251
[2] J.Y. Lee, S.J. Oh, J.G. Sohn, S.J. Kwon, Corros. Sci. 43 (2001) 803. [3] E. Murad, Phys. Chem. Miner. 23 (1996) 248. [4] C.A. Barrero, R.E. Vandenberghe, E. De Grave, Hyperfine Interact. 122 (1999) 39. [5] E. De Grave, C.A. Barrero, G.M. da Costa, R.E. Vandenberghe, E. Van San, Clay Miner. 37 (2002) 591. [6] E. Wolska, U. Schwertmann, N. Jb. Mh. H. 5 (1993) 217. [7] U. Schwertmann, R.M. Cornell, 2000, Iron Oxides in the Laboratory, VCH, Weinheim, Germany. [8] R.E. Vandenberghe, E. De Grave, P.M.A. de Bakker, Hyperfine Interact. 83 (1994) 29. [9] U. Schwertmann, P. Cambier, E. Murad, Clays Clays Miner. 33 (1995) 369. [10] http://www.ing.unitn.it/∼luttero/maud. [11] S. Bocquet, R.J. Pollard, J.D. Cashion, Phys. Rev. B 46 (1992) 11657.