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Magnetic and crystal structure refinement in akaganeite nanoparticle
ARTICLE IN PRESS
Physica B 354 (2004) 187–190 www.elsevier.com/locate/physb
Magnetic and crystal structure refinement in akaganeite nanoparticle K.E. Garcı´ aa, A.L. Moralesa,b, C.A. Barreroa,b,, C.E. Arroyavea, J.M. Grenechec a
Grupo de Corrosio´n y Proteccio´n, Facultad de Ingenierı´as, Universidad de Antioquia. A.A. 1226, Medellı´n, Colombia b Grupo de Estado So´lido, Instituto de Fı´sica, Universidad de Antioquia. A.A. 1226, Medellı´n, Colombia c Laboratoire de Physique de l’Etat Condense´-UMR 6087, Universite´ du Maine, 72085, Le Mans Cedex 9, France
Abstract Preliminary assignments of the low-temperature Mo¨ssbauer signals obtained by taking into account a monoclinic structure for the akaganeite are presented. A powder sample was prepared by thermal hydrolysis of 0.1 M FeCl3 solutions at 70 1C during 48 h according to the literature. X-ray diffraction demonstrates the purity of the synthetic sample. The X-ray pattern was adequately adjusted by using the monoclinic space group (C2/m:b3) with a=10.5422(6) A˚, b=3.0349(1) A˚, c=10.5259(6) A˚ and b=90.1133(5)1. The average grain size is estimated to be about 46(6) nm. The monoclinic symmetry requires the existence of two distinct iron octahedral sites, which is also confirmed by the Mo¨ssbauer spectra in the paramagnetic state. However, detailed computer analysis of Mo¨ssbauer spectra in the magnetic state suggests the presence of four non-equivalent iron sites. The physical origin of these different components in the magnetic region is discussed based upon the monoclinic structure. r 2004 Elsevier B.V. All rights reserved. PACS: 73.63.Bd; 91.60.Pn; 76.80.+y; 61.10.Nz Keywords: Akaganeite; Magnetism; Mo¨ssbauer spectrometry; X-ray diffraction
1. Introduction In spite of the great number of investigations, the physical, crystallographic and chemical propCorresponding author. Grupo de Estado So´lido, Instituto
de Fı´ sica, Universidad de Antioquia, A.A. 1226, Medellı´ n, Colombia. Tel.: +057 4 210 56 30; fax: +057 4 233 01 20. E-mail address: cbarrero@fisica.udea.edu.co (C.A. Barrero).
erties of akaganeite are still a matter of controversy. For example, there is no agreement in its space group. Previous X-ray diffraction studies by Mackay [1] and Murad [2] points to the tetragonal I4/m symmetry. However, the first crystal structure refinement by Post and Buchwald [3] showed that the monoclinic I2/m symmetry produced better fit to the observed X-ray powder diffraction data than the tetragonal one. Additionally, recent synchrotron X-ray and neutron diffraction
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.09.045
ARTICLE IN PRESS 188
K.E. Garcı´a et al. / Physica B 354 (2004) 187–190
investigations supported this finding [4,5]. In this way, it seems that the monoclinic should be the space group of choice. Contradicting this result, recent detailed infrared (IR) spectroscopic studies by Weckler and Lutz [6] suggested that the I4/m symmetry is more reliable than the I2/m one, because the latter implies that more translational modes of vibration are IR active than the observed ones. Another point that needs to be solved is whether akaganeite can be stable without chloride ions in the tunnels. Most investigations support the idea that the presence of Cl ions is of critical importance for the akaganeite formation and stabilization [3–5]. However, other studies have suggested that in exceptional circumstances, like weathering of some meteorites, akaganeite may form with OH alone, instead of Cl , occupying the tunnels [7]. Additionally, Re´zel and Ge´nin [8] have suggested that Cl ions can be substituted by OH ions without loosing stability. The physical origin of the different components observed in the Mo¨ssbauer spectra at the paramagnetic and magnetic states of akaganeite is also controversial. It is well established that its room temperature Mo¨ssbauer spectrum must be fitted with two doublets. However, there is little agreement about the origin of these two components. Most investigations, based on the tetragonal structure, agree that the presence of Cl ions in the channels are the cause of the non-equivalent iron sites in the structure. Murad [2] proposed that the components arise because of the increasing distortion of mid-wall to channel near iron sites. Chambaere and De Grave [9] attributed the sites to two irons, one with O3(OH)3 coordination and O2(OH)4 coordination. Re´zel and Ge´nin [8] have suggested that replacement of Cl ions by OH ions in the structure is an important factor for the presence of different iron sites. It is seen that all previously reported Mo¨ssbauer results are interpreted in terms of the tetragonal structure. However, to the best of our knowledge none of them are based on the monoclinic symmetry. The purpose of the present study is restricted to an interpretation based upon this structure, both in the paramagnetic and magnetic states. More details will be given in a forthcoming
paper, especially the estimate of both chlorine and water contents: indeed, their presence aims to stabilize the structure of akaganeite but their contents are a matter of controversy in the literature.
2. Experimental Akaganeite sample was prepared by thermal hydrolysis of 0.1 M FeCl3 solution at 70 1C during 48 h according to the procedure described by Schwertmann and Cornell [10]. The samples obtained were pure as the X-ray diffraction (XRD) analysis showed (see below), i.e., only akaganeite peaks were present, of course the amount of chlorine and water present to stabilize the structure are a matter of controversy in the literature. XRD pattern was obtained using a D500 Siemens diffractometer equipped with a Cu (Ka) radiation; data were collected in the 15–1201 2y range with a 0.021 step and a counting time of 10 s per point. XRD pattern was analyzed using a program called MAUD which combines the Rietveld method and a Fourier transform analysis, well adapted especially in the presence of broadened Bragg peaks [11]: this program thus allows to derive crystalline cell parameters, average crystallite size as well as microstrains. The structural model used was that reported by Post and Buchwald [3]. The scale factor, sample displacement, unit cell, five-order polynomial background, and the average crystallite size were refined. The average crystallite size and the texture were assumed to be isotropic and arbitrary, respectively, in the present study. Room temperature and 4.2 K Mo¨ssbauer spectra were collected in a time-mode spectrometer working in the transmission geometry using a constant acceleration drive with triangular reference signal. Calibration was achieved from a standard a-iron foil at 300 K. The spectra were analyzed using a program called MOSFIT which is based on non-linear least squares fitting procedures assuming lorentzian Mo¨ssbauer lines while the isomer shift value is quoted to that of a-Fe at 300 K.
ARTICLE IN PRESS K.E. Garcı´a et al. / Physica B 354 (2004) 187–190
189
Fig. 1 shows the XRD pattern of the powdered sample. The X-ray pattern is adjusted by introducing only a single phase, akaganeite (PDF: 801770) [12]. The pattern was adequately adjusted with the C2/m:b3 monoclinic space group, giving the unit cell values of a=10.5422(6) A˚, b= 3.0349(1) A˚, c=10.5259(6) A˚ and b=90.1133(5)1, while the average crystallite grain size is estimated at about 46(6) nm. Statistical factors of Ro=13.80%, Rb=10.05% and Rexp=11.21% and wgt’d ssq=6373.29 were obtained. Room temperature Mo¨ssbauer spectrum (not shown) was adjusted introducing two doublets with d1=0.37 mm/s, DEQ,1=0.54 mm/s, area (A1) of 65% and d2=0.37 mm/s, DEQ,2=0.97 mm/s, area (A2) of 35%. These data are in agreement with those reported in the literature. However, the origin of these two sites is related to the two symmetrically different iron sites required by the monoclinic structure [3]. 4.2 K Mo¨ssbauer spectrum can be adjusted by introducing three or four sextets. The derived hyperfine parameters for the three sextets model are very similar to the ones reported in the literature [2,8,9,13,14]. In the case of the four sextets model, we have found that there are different ways of fitting the spectrum. However, it is believed that some constraints should be introduced, for example, the center shifts are of
similar magnitude. Fig. 2 shows the spectrum with an example fit. The magnetic hyperfine fields range from 46 to 49 T. Both fitting models seem to reproduce adequately the experimental data. However, as will be shown in another coming work, the external field Mo¨ssbauer spectrum at this temperature is in better agreement with the four sextets model than with the three sextets one. Additionally, the physical origin of these four sites is in better line with the monoclinic structure as it is shown below. Thus we will concentrate now upon the origin of these four components. First, TEM micrographs (not shown) have revealed elongated single crystallite grains of somatoidal shape with length of about 200 nm and diameter of 30 nm. This morphology suggests possible surface effects in opposition to spherical grains of 30 nm diameter. In addition, a BET surface area value of 25 m2/g was obtained. In this way, one could expect a relatively important influence on the Mo¨ssbauer spectra of the iron sites located at the surface. In this way, the two of the four sextets arise from the named Fe1 and Fe2 sites [3–5] in the bulk and the other two from the same sites but located on the surface of the particles. Yamamoto et al. [15] have shown that Fe3+ ions located at the surface layers are involved in magnetic order and that the surface hyperfine fields rapidly decrease with increasing temperature. It could also occur that surface sites do not strictly relate to the monoclinic structure, which pertains
Fig. 1. X-ray diffraction pattern of the synthetic akaganeite. The dots represent the experimental data and the solid line the best fit.
Fig. 2. Mo¨ssbauer spectra of the synthetic akaganeite at 4.2 K showing a fit with four sextet components.
3. Results and discussion
ARTICLE IN PRESS 190
K.E. Garcı´a et al. / Physica B 354 (2004) 187–190
more to a crystalline system but of course may explain the presence of four iron sites. The surface sites do give rise to different magnetic behavior. Another possibility for the physical origin of the four components may arise by checking carefully the crystal structure, as reported earlier by Post and coworkers [5]. One can see, that there are Fe1 and Fe2 ions located closer to the chlorine ions and other Fe1 and Fe2 ions located closer to the vacant chlorine sites. Because both Cl and OH are located in channels, their presence should affect the hyperfine field at Fe sites. It is thus quite important to have a good estimate of their contents, in order to attribute carefully the different magnetic components. More measurements are in progress to perform a better physical assignment of the four sites.
4. Conclusions The present results support the idea of a monoclinic space group to adequately describe the crystallographic structure of akaganeite. This symmetry requires the existence of two distinct iron octahedral sites, which is in line with the Mo¨ssbauer spectra in the paramagnetic state. Low-temperature Mo¨ssbauer spectrometry suggests the presence of four non-equivalent iron sites. The physical interpretation of these sites is very complex. These sites may arise from one of the two following sources: (i) Fe3+ and Fe3+ 1 2 located close to the Cl ions and the other Fe3+ 1 and Fe3+ located near the vacant Cl sites, or (ii) Fe3+ 2 1 and Fe3+ located at the surface sites and the other 2 two at the inner part of the particles. It is also possible that the real situation could require the combination of both sources. External field Mo¨ssbauer and other measurements are in progress to perform a better assignment of these sites.
Acknowledgments The financial support given by ECOS-NORD/ COLCIENCIAS/ICFES/ICETEX exchange program (C03P01), COLCIENCIAS (1115-05-13661), and CODI (sustainability program for Corrosion and Protection Group 2003-2004) Universidad de Antioquia, are greatly acknowledged. We are very grateful to A.M. Mercier and S. Kodjikian from Laboratoire des Fluorures of Universite´ du Maine UMR CNRS 6010 for performing XRD and TEM measurements.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11] [12] [13] [14] [15]
A.L. Mackay, Mineral. Mag. 32 (1960) 545. E. Murad, Clay Miner. 14 (1979) 273. J.E. Post, V.F. Buchwald, Am. Mineral. 76 (1991) 272. K. Stahl, K. Nielsen, J. Jiang, B. Lebech, J.C. Hanson, P. Norby, J. van Lanschot, Corros. Sci. 45 (2003) 2563. J.E. Post, P.J. Heaney, R.B. Von Dreele, J.C. Hanson, Am. Mineral. 88 (2003) 782. B. Weckler, H.D. Lutz, Eur. J. Solid State Inorg. Chem. 35 (1998) 531. P.A. Bland, S.P. Kelley, F.J. Berry, J.M. Cadogan, C.T. Pillinger, Am. Mineral. 82 (1997) 1187. D. Re´zel, J.M.R. Ge´nin, Hyperfine Interact. 57 (1990) 2067. D.G. Chambaere, E. De Grave, Hyperfine Interact. 20 (1984) 249. U. Schwertmann, R.M. Cornell, Iron Oxides in the Laboratory, Second ed., VCH, Weinheim, Germany, 2000. L. Lutteroti, MAUD, Materials Analysis using Diffraction, Version: 1.84 (2002). JCPDS, X-ray diffraction data cards of the joint committee on powder diffraction standars, (2001). D.G. Chambaere, E. De Grave, J. Magn. Magn. Mater. 42 (1984) 263. P. Refait, J.M.R. Ge´nin, Corros. Sci. 39 (1997) 539. A. Yamamoto, T. Honmyo, M. Kiyama, T. Shinjo, J. Phys. Soc. Japan 63 (1994) 176.
Physica B 354 (2004) 187–190 www.elsevier.com/locate/physb
Magnetic and crystal structure refinement in akaganeite nanoparticle K.E. Garcı´ aa, A.L. Moralesa,b, C.A. Barreroa,b,, C.E. Arroyavea, J.M. Grenechec a
Grupo de Corrosio´n y Proteccio´n, Facultad de Ingenierı´as, Universidad de Antioquia. A.A. 1226, Medellı´n, Colombia b Grupo de Estado So´lido, Instituto de Fı´sica, Universidad de Antioquia. A.A. 1226, Medellı´n, Colombia c Laboratoire de Physique de l’Etat Condense´-UMR 6087, Universite´ du Maine, 72085, Le Mans Cedex 9, France
Abstract Preliminary assignments of the low-temperature Mo¨ssbauer signals obtained by taking into account a monoclinic structure for the akaganeite are presented. A powder sample was prepared by thermal hydrolysis of 0.1 M FeCl3 solutions at 70 1C during 48 h according to the literature. X-ray diffraction demonstrates the purity of the synthetic sample. The X-ray pattern was adequately adjusted by using the monoclinic space group (C2/m:b3) with a=10.5422(6) A˚, b=3.0349(1) A˚, c=10.5259(6) A˚ and b=90.1133(5)1. The average grain size is estimated to be about 46(6) nm. The monoclinic symmetry requires the existence of two distinct iron octahedral sites, which is also confirmed by the Mo¨ssbauer spectra in the paramagnetic state. However, detailed computer analysis of Mo¨ssbauer spectra in the magnetic state suggests the presence of four non-equivalent iron sites. The physical origin of these different components in the magnetic region is discussed based upon the monoclinic structure. r 2004 Elsevier B.V. All rights reserved. PACS: 73.63.Bd; 91.60.Pn; 76.80.+y; 61.10.Nz Keywords: Akaganeite; Magnetism; Mo¨ssbauer spectrometry; X-ray diffraction
1. Introduction In spite of the great number of investigations, the physical, crystallographic and chemical propCorresponding author. Grupo de Estado So´lido, Instituto
de Fı´ sica, Universidad de Antioquia, A.A. 1226, Medellı´ n, Colombia. Tel.: +057 4 210 56 30; fax: +057 4 233 01 20. E-mail address: cbarrero@fisica.udea.edu.co (C.A. Barrero).
erties of akaganeite are still a matter of controversy. For example, there is no agreement in its space group. Previous X-ray diffraction studies by Mackay [1] and Murad [2] points to the tetragonal I4/m symmetry. However, the first crystal structure refinement by Post and Buchwald [3] showed that the monoclinic I2/m symmetry produced better fit to the observed X-ray powder diffraction data than the tetragonal one. Additionally, recent synchrotron X-ray and neutron diffraction
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.09.045
ARTICLE IN PRESS 188
K.E. Garcı´a et al. / Physica B 354 (2004) 187–190
investigations supported this finding [4,5]. In this way, it seems that the monoclinic should be the space group of choice. Contradicting this result, recent detailed infrared (IR) spectroscopic studies by Weckler and Lutz [6] suggested that the I4/m symmetry is more reliable than the I2/m one, because the latter implies that more translational modes of vibration are IR active than the observed ones. Another point that needs to be solved is whether akaganeite can be stable without chloride ions in the tunnels. Most investigations support the idea that the presence of Cl ions is of critical importance for the akaganeite formation and stabilization [3–5]. However, other studies have suggested that in exceptional circumstances, like weathering of some meteorites, akaganeite may form with OH alone, instead of Cl , occupying the tunnels [7]. Additionally, Re´zel and Ge´nin [8] have suggested that Cl ions can be substituted by OH ions without loosing stability. The physical origin of the different components observed in the Mo¨ssbauer spectra at the paramagnetic and magnetic states of akaganeite is also controversial. It is well established that its room temperature Mo¨ssbauer spectrum must be fitted with two doublets. However, there is little agreement about the origin of these two components. Most investigations, based on the tetragonal structure, agree that the presence of Cl ions in the channels are the cause of the non-equivalent iron sites in the structure. Murad [2] proposed that the components arise because of the increasing distortion of mid-wall to channel near iron sites. Chambaere and De Grave [9] attributed the sites to two irons, one with O3(OH)3 coordination and O2(OH)4 coordination. Re´zel and Ge´nin [8] have suggested that replacement of Cl ions by OH ions in the structure is an important factor for the presence of different iron sites. It is seen that all previously reported Mo¨ssbauer results are interpreted in terms of the tetragonal structure. However, to the best of our knowledge none of them are based on the monoclinic symmetry. The purpose of the present study is restricted to an interpretation based upon this structure, both in the paramagnetic and magnetic states. More details will be given in a forthcoming
paper, especially the estimate of both chlorine and water contents: indeed, their presence aims to stabilize the structure of akaganeite but their contents are a matter of controversy in the literature.
2. Experimental Akaganeite sample was prepared by thermal hydrolysis of 0.1 M FeCl3 solution at 70 1C during 48 h according to the procedure described by Schwertmann and Cornell [10]. The samples obtained were pure as the X-ray diffraction (XRD) analysis showed (see below), i.e., only akaganeite peaks were present, of course the amount of chlorine and water present to stabilize the structure are a matter of controversy in the literature. XRD pattern was obtained using a D500 Siemens diffractometer equipped with a Cu (Ka) radiation; data were collected in the 15–1201 2y range with a 0.021 step and a counting time of 10 s per point. XRD pattern was analyzed using a program called MAUD which combines the Rietveld method and a Fourier transform analysis, well adapted especially in the presence of broadened Bragg peaks [11]: this program thus allows to derive crystalline cell parameters, average crystallite size as well as microstrains. The structural model used was that reported by Post and Buchwald [3]. The scale factor, sample displacement, unit cell, five-order polynomial background, and the average crystallite size were refined. The average crystallite size and the texture were assumed to be isotropic and arbitrary, respectively, in the present study. Room temperature and 4.2 K Mo¨ssbauer spectra were collected in a time-mode spectrometer working in the transmission geometry using a constant acceleration drive with triangular reference signal. Calibration was achieved from a standard a-iron foil at 300 K. The spectra were analyzed using a program called MOSFIT which is based on non-linear least squares fitting procedures assuming lorentzian Mo¨ssbauer lines while the isomer shift value is quoted to that of a-Fe at 300 K.
ARTICLE IN PRESS K.E. Garcı´a et al. / Physica B 354 (2004) 187–190
189
Fig. 1 shows the XRD pattern of the powdered sample. The X-ray pattern is adjusted by introducing only a single phase, akaganeite (PDF: 801770) [12]. The pattern was adequately adjusted with the C2/m:b3 monoclinic space group, giving the unit cell values of a=10.5422(6) A˚, b= 3.0349(1) A˚, c=10.5259(6) A˚ and b=90.1133(5)1, while the average crystallite grain size is estimated at about 46(6) nm. Statistical factors of Ro=13.80%, Rb=10.05% and Rexp=11.21% and wgt’d ssq=6373.29 were obtained. Room temperature Mo¨ssbauer spectrum (not shown) was adjusted introducing two doublets with d1=0.37 mm/s, DEQ,1=0.54 mm/s, area (A1) of 65% and d2=0.37 mm/s, DEQ,2=0.97 mm/s, area (A2) of 35%. These data are in agreement with those reported in the literature. However, the origin of these two sites is related to the two symmetrically different iron sites required by the monoclinic structure [3]. 4.2 K Mo¨ssbauer spectrum can be adjusted by introducing three or four sextets. The derived hyperfine parameters for the three sextets model are very similar to the ones reported in the literature [2,8,9,13,14]. In the case of the four sextets model, we have found that there are different ways of fitting the spectrum. However, it is believed that some constraints should be introduced, for example, the center shifts are of
similar magnitude. Fig. 2 shows the spectrum with an example fit. The magnetic hyperfine fields range from 46 to 49 T. Both fitting models seem to reproduce adequately the experimental data. However, as will be shown in another coming work, the external field Mo¨ssbauer spectrum at this temperature is in better agreement with the four sextets model than with the three sextets one. Additionally, the physical origin of these four sites is in better line with the monoclinic structure as it is shown below. Thus we will concentrate now upon the origin of these four components. First, TEM micrographs (not shown) have revealed elongated single crystallite grains of somatoidal shape with length of about 200 nm and diameter of 30 nm. This morphology suggests possible surface effects in opposition to spherical grains of 30 nm diameter. In addition, a BET surface area value of 25 m2/g was obtained. In this way, one could expect a relatively important influence on the Mo¨ssbauer spectra of the iron sites located at the surface. In this way, the two of the four sextets arise from the named Fe1 and Fe2 sites [3–5] in the bulk and the other two from the same sites but located on the surface of the particles. Yamamoto et al. [15] have shown that Fe3+ ions located at the surface layers are involved in magnetic order and that the surface hyperfine fields rapidly decrease with increasing temperature. It could also occur that surface sites do not strictly relate to the monoclinic structure, which pertains
Fig. 1. X-ray diffraction pattern of the synthetic akaganeite. The dots represent the experimental data and the solid line the best fit.
Fig. 2. Mo¨ssbauer spectra of the synthetic akaganeite at 4.2 K showing a fit with four sextet components.
3. Results and discussion
ARTICLE IN PRESS 190
K.E. Garcı´a et al. / Physica B 354 (2004) 187–190
more to a crystalline system but of course may explain the presence of four iron sites. The surface sites do give rise to different magnetic behavior. Another possibility for the physical origin of the four components may arise by checking carefully the crystal structure, as reported earlier by Post and coworkers [5]. One can see, that there are Fe1 and Fe2 ions located closer to the chlorine ions and other Fe1 and Fe2 ions located closer to the vacant chlorine sites. Because both Cl and OH are located in channels, their presence should affect the hyperfine field at Fe sites. It is thus quite important to have a good estimate of their contents, in order to attribute carefully the different magnetic components. More measurements are in progress to perform a better physical assignment of the four sites.
4. Conclusions The present results support the idea of a monoclinic space group to adequately describe the crystallographic structure of akaganeite. This symmetry requires the existence of two distinct iron octahedral sites, which is in line with the Mo¨ssbauer spectra in the paramagnetic state. Low-temperature Mo¨ssbauer spectrometry suggests the presence of four non-equivalent iron sites. The physical interpretation of these sites is very complex. These sites may arise from one of the two following sources: (i) Fe3+ and Fe3+ 1 2 located close to the Cl ions and the other Fe3+ 1 and Fe3+ located near the vacant Cl sites, or (ii) Fe3+ 2 1 and Fe3+ located at the surface sites and the other 2 two at the inner part of the particles. It is also possible that the real situation could require the combination of both sources. External field Mo¨ssbauer and other measurements are in progress to perform a better assignment of these sites.
Acknowledgments The financial support given by ECOS-NORD/ COLCIENCIAS/ICFES/ICETEX exchange program (C03P01), COLCIENCIAS (1115-05-13661), and CODI (sustainability program for Corrosion and Protection Group 2003-2004) Universidad de Antioquia, are greatly acknowledged. We are very grateful to A.M. Mercier and S. Kodjikian from Laboratoire des Fluorures of Universite´ du Maine UMR CNRS 6010 for performing XRD and TEM measurements.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11] [12] [13] [14] [15]
A.L. Mackay, Mineral. Mag. 32 (1960) 545. E. Murad, Clay Miner. 14 (1979) 273. J.E. Post, V.F. Buchwald, Am. Mineral. 76 (1991) 272. K. Stahl, K. Nielsen, J. Jiang, B. Lebech, J.C. Hanson, P. Norby, J. van Lanschot, Corros. Sci. 45 (2003) 2563. J.E. Post, P.J. Heaney, R.B. Von Dreele, J.C. Hanson, Am. Mineral. 88 (2003) 782. B. Weckler, H.D. Lutz, Eur. J. Solid State Inorg. Chem. 35 (1998) 531. P.A. Bland, S.P. Kelley, F.J. Berry, J.M. Cadogan, C.T. Pillinger, Am. Mineral. 82 (1997) 1187. D. Re´zel, J.M.R. Ge´nin, Hyperfine Interact. 57 (1990) 2067. D.G. Chambaere, E. De Grave, Hyperfine Interact. 20 (1984) 249. U. Schwertmann, R.M. Cornell, Iron Oxides in the Laboratory, Second ed., VCH, Weinheim, Germany, 2000. L. Lutteroti, MAUD, Materials Analysis using Diffraction, Version: 1.84 (2002). JCPDS, X-ray diffraction data cards of the joint committee on powder diffraction standars, (2001). D.G. Chambaere, E. De Grave, J. Magn. Magn. Mater. 42 (1984) 263. P. Refait, J.M.R. Ge´nin, Corros. Sci. 39 (1997) 539. A. Yamamoto, T. Honmyo, M. Kiyama, T. Shinjo, J. Phys. Soc. Japan 63 (1994) 176.