- Email: [email protected]
Mechanism and reaction kinetics in the solid phase transformation α-FeOOH → α-Fe2O3 studied by Mössbauer spectroscopy
SOLID STATE Solid State Ionics 101-103
IONICS
(1997) 591-596
Mechanism and reaction kinetics in the solid phase transformation cx-FeOOH + a-Fe,O, studied by Miissbauer spectroscopy L. Diamandescua’*,
D. Mih&E-T%%bQanu”, S. Calogerob, M. Federc
N. Popescu-Pogrion”,
“Institute of Atomic Physics, P.O. Box MG-6, Bucharest, Romania hDepartment of Physical Chemistry, University of Venice, I-30123, Italy ‘S.C. AFERO,
Calea Floreasca
169, Bucharest,
Romania
Abstract The thermal decomposition of u-FeOOH, prepared by air oxidation of Fe(OH), at room temperature, was investigated by means of MGssbauer spectroscopy and transmission electron microscopy. The measurements were performed on powder
samples previously annealed between 200 and 700°C. From MGssbauer spectral data the reaction isotherms, rate constants and the activation energy of the process were determined. The transformation into a-iron oxide takes place preserving the needle-like shape of particles. An Avrami-Erofeev of alpha oxyhydroxide to alpha iron oxide. Keywords: Reaction kinetics; Solid phase transformation; microscopy Materials: cr-FeOOH;
mechanism
was identified to characterise
Iron hydroxide;
Iron oxide; MGssbauer
the solid phase transformation
spectroscopy;
Transmission
electron
a-Fe,O,
1. Introduction The transformation a-FeOOH (goethite) + CYFe,O, (hematite) is an important step in the preparation of certain materials for magnetic recording, ferrites, pigments and catalysts. Considerable theoretical and experimental effort has been invested to understand the mechanism of this transformation in solid phase reaction [l-8]. a-FeOOH is antiferromagnetic with a NCel temperature TN = 400 K. The goethite structure consists *Corresponding author. Tel.: +40 3700; e-mail: [email protected]
1 780 3469; fax: +40 1 420
0167.2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO167-2738(97)00200-2
of double chain of [Fe(O, OH),] octahedra linked together by sharing opposite edges. Although goethite displays a multitude of shapes and sizes, only two basic morphologies occur: acicular and twinned crystals [9]. Synthetic a-FeOOH usually has an acicular habit. By our knowledge, little attention has been paid to the problem of solid phase reaction kinetics associated to the transformation a-FeOOH-+a-Fe,O,. Is the aim of the this paper to reports on the results of Massbauer spectroscopy (MS), transmission electron microscopy (TEM) and electron diffraction (ED) investigation connected with the reaction kinetics and mechanism of the solid phase transformation
592
L. Diamandescu
goethite + hematite in the temperature 700°C.
et al. / Solid State lonics
IOI-
103 (1997)
591-596
range of 200-
2. Experimental Mossbauer (M) spectra were obtained at room temperature with a constant acceleration spectrometer and a 57Co (Rh) source. The obtained M spectra were least squares fitted assuming Lorentzian line shaped sextets. The TEM and ED analyses were performed with a JEOL 200 CX apparatus. For TEM and ED measurements the powder samples were settled on a glass microscope lamella with 25% collodium in amyl acetate. The formatted films were removed by immersion in distilled water and floated off on copper grids. Finally they were fixed with carbon by vacuum evaporation. The goethite was prepared by air oxidation of Fe(OH), at room temperature. The ferrous hydroxide was obtained by ammonia precipitation in a ferrous sulphate solution. Samples of polycrystalline goethite with particle size in the range of 0.3-l 5 pm and accicular ratio 1:6 to 1: 15 were introduced in a furnace that was heated at a rate of 6°C min-‘, kept at heating temperature (in the range of 200-700°C) for different times up to 4 h and finally cooled to room temperature.
3. Results and discussion A typical TEM image of the used goethite is presented in Fig. 1. One can notice the needle-like shape of the particles. At room temperature the M spectrum of the used goethite sample exhibits magnetic sextets with nonlorentzian lines, suggesting a distribution of hyperfine fields. The line shape problem of synthetic and natural goethite has been extensively investigated [ 10-131. The spectrum of goethite can be mainly resolved up to the eighties subspectra using Lorentzian line shape. A representative M spectrum for the calcined samples is shown in Fig. 2 together with the calculated curve. One can observe the simultaneous presence of the two iron phases. In both phases the computer processing revealed a distribution of hyperfine magnetic sextets (corresponding to different
Fig. I. TEM image of goethite diffraction diagram.
05
“.lll
particles
together
with electron
-Fe,O,
.
-10
5
0
-5
Velocity
Fig. 2. MGssbauer spectrum of a-FeOOH
10
(mm/s)
calcined 5 min at 300°C.
internal magnetic fields) with large line widths. This behaviour indicates the presence of iron in different position in a not so well crystallised lattice, during transformation. In Fig. 3(a, b, c) the M computed envelopes, versus heating time r at different temperatures are presented in a surface graphic mode. At 225”C, after a thermal treatment of 5 min the spectrum reveals only the goethite phase (Fig. 3a); the iron oxide phase appears for a treatment time in the range of 20-60 min and is still in development after four hours. In the case of the thermal treatment at 250°C the transformation into alpha iron oxide is practically completed after one hour; however some goethite traces can be also observed after four hours (Fig. 3b). A faster transformation for the samples calcined at 275°C is evidenced by the evolution of M calculated spectra represented in Fig. 3c. In the temperature range of 350-700°C the transformation is very fast and the M spectra reveal essentially the
L. Diamandescu et al. I Solid State lonics 101-103
L.
225OC
\klocity (mm/ sl
2 50°C
(mm/s)
275OC
\klocity (mm/ sl Fig. 3. Miissbauer calculated envelopes versus treatment time for the samples calcined at: 225°C (a), 250°C (b) and 275°C (c).
593
typical sextet of alpha iron oxide. The M line width decreases as the temperature of calcination increases indicating a better crystallisation. Only o-FeOOH and a-Fe,O, phases were identified in the analysed spectra. The Mossbauer areas of these phases were used to calculate the weighted amount of cx-Fe,O, formed at a given time and temperature by assuming equal recoil-free fraction fgOethitcand fhematite. In Fig. 4(a-d) the TEM images together with ED patterns of samples calcined one hour at different temperatures are presented. From the ED patterns one can remark the presence of goethite (d,,, = 4.183 A) together with hematite up to 275°C. The goethite amount decreases as the temperature increases and no intermediate phases were observed, in agreement with the M data. An increase of the microporosity of iron oxide particles is evidenced by Fig. 4(e-h) for temperatures higher than 350°C. This behaviour could by attributed to the water loss during the crystallisation of a-iron oxide. Insomuch as no third phase was detected, the dehydration reaction can be represented as 2a-FeOOH
U&city
(1997) 591-596
+ o-Fe,O,
+ H,O
(1)
suggesting a direct transformation of goethite into hematite, in good agreement with Watari et al. [3]. By using the Mossbauer areas of the two phases, the degree of transformation (r) into o-Fe,O, as a function of reaction time (reaction isotherm) was obtained. Fig. 5 shows the reaction isotherms at 250, 275, and 300°C together with the best computer fitting - a sigmoidal one. The shape of the isotherms indicates a decceleratory reaction throughout; that means the reaction rate is progressively diminishing as the reactant is consumed. For slow decomposition reactions, the diagram of the degree of transformation versus time usually presents a characteristic sigmoidal shape [ 14,151. This behaviour can be interpreted as arising from the creation of nuclei at localised places in the crystal, the growth of these nuclei and after the point of inflection, the decay of the reaction as nuclei overlap and the area of the interface between reactant and product phases decreases. For the accelerated period the extent of decomposition is given by the product of two terms, one of which describes the rate of nucleation and the other the rate of growth. It is
L. Diamandescu et al. I Solid State
e
d
h
Fig. 4. TEM images and ED patterns for the samples calcined one hcmr at 225°C (a), 275°C (b), 350°C (c), 700°C (d) showing the particle shape; the micrographs (e-h) are details at 275”C, 350°C. 45O”C, 7CNYC respectively.
L. Diamandescu
et al.
I Solid
State
tonics
IOI-
103 (1997)
591-596
595
2.0
60 1’ 0
I 2000
4000
6000
8000
0.01 0
2OOu
t (s)
6CNXI
St 30
t 6)
Fig. 5. Reaction isotherms obtained from the MGssbauer data. The solid lines represent the sigmoidal fitting.
generally adequate to assume that the rate of growth of the nuclei is constant so the expression for the extent of decomposition depends mainly upon the form that is taken for the rate of nucleation. Avrami [ 161 and Erofeev [ 171 have shown that for nuclei having a constant rate of three-dimensional growth one can write: - log( 1 - r) = (k x f)”
4000
(2)
where k is the rate constant, n = 4 at the beginning of the reaction and n =3 in the decay period. A unimolecular decay law (first-order reaction) with n = 1 follows if one assumes that each molecule has the same probability of decomposition. Taking into account the r values obtained for the studied transformation the plot -log( 1 -r) versus t has been performed. The best fit with the experimental data was achieved in the case of n = 1, for each temperature. So, one has obtained straight lines whose slope determines the reaction constants k(T). Fig. 6 shows for example the plot and the computer fitting in the case of the samples calcined at 250°C for different reaction times. From this behaviour one conclude that the solid phase transformation goethitejhematite obeys first-order kinetics. Therefore, the thermal decomposition of powder goethite in the investigated temperature range seems to be controlled by a nucleation process and each individual particle in the assemblage may be nucleated with equal probability. The reaction of each individual crystal results from the formation of a single nucleus on the surface of the particular particle. The
Fig. 6. Plot of log( 1 -r) versus reaction determination of the reaction order.
time
t, at 250°C for the
TEM images in Fig. 4 show that the shape of the particles does not essentially change up to 700°C. This behaviour seems to be in good agreement with the mechanism identified by M investigation. The non-iterative fitting of the data with a straight line gives for the rate constants the values: kzz5 =7X 10-5; k,,,=17x lo-‘; k>,,=40x lo-‘; k,,o=61 X lo-5 s-‘. Using a Polany-Wigner equation for the rate constant [14,15] the activation energy E, which represents the energy barrier to reaction, can be obtained from the plot In(k) =f( 1 lT) that is shown in Fig. 7. The linear fit (continuous line in Fig. 7) leads to the evaluation of the activation energy, E= 125 kJ mol K-l. -6
,
Fig. 7. The plot In(k) versus activation energy.
(l/T)
for the estimation
of the
596
L. Diamandescu
et al. I Solid State tonics 101-103
A large hyperline magnetic fields distribution was found by computer fitting in the analysed Mossbauer spectra. The effective magnetic field at the iron nucleus changes from H,,+.=520 kOe to 410 (24 kOe) in hematite and from H,,,=400 kOe to 300 kOe (24 kOe) in goethite. The asymmetrically broadened Mossbauer lines and the decrease of hyperfine magnetic fields in goethite are generally explained by various effects such as deviation from crystalline perfection, magnetic exchange interaction between neighbouring particles (so-called ‘superferromagnetism’) [ 181, col[ 191, surface effects lective magnetic excitations [20]. Such spectra can best be described by distributions of hyperfine fields [ 10,211. The origin of the hyperfine magnetic fields distribution in hematite is similar to that of goethite. Moreover, the decrease of hyperfine magnetic fields by surface effects has been well established by Mossbauer spectroscopy on 57Fe surface enriched hematite [22,23]. In the present study, the distributive behaviour of hyperfine magnetic fields could be attributed to the imperfectly crystallised goethite and hematite as well as to surface effects, during transformation.
Acknowledgements This work is partly supported by the University of Venice, Italy. The research fellowship of one of the authors (L.D.) is gratefully acknowledged.
References [I] J. Lima de Faria, Z. Kristallogr. 119 (1963) 176. [2] I. Dezsi, M. Fodor, Phys. Stat. Sol. 15 (1966) 247.
(1997) 591-596
[31 F. Watari, J. van Landuyt, P. Delavignette, S. Amelinckx, J. Solid State Chem. 29 (1979) 137. 141 F. Watari, J. van Landuyt, P. Delavignette, S. Amelinckx, J. Solid State Chem. 29 (1979) 417. [51 F. Watari, J. van Landuyt, P Delavignette, S. Amelinckx, J. Solid State. Chem. 48 (1983) 49. [61 Y. Maeda, M. Igarashi, A. Terada, F. Yoshimura, Jap. J. Appl. Phys. 13 (1974) 381. [71 W. van Oosterhout, Acta Crystallogr. 13 (1960) 932. 181N. Yamamoto, T. Shinjo, M. Kiyama, Y. Bando, T. Takada, J. Phys. Sot. Japan 25 (1968) 1267. L91 R.J. Atkinson, A.M. Posner, J.P. Quirk. J. Inorg. Nucl. Chem. 30 (1968) 2371. I101 E. Murad, Am. Mineralogist 67 (1982) 1007. 1111U. Schwertmann, P. Cambier. E. Murad, Clay and Clay Min. 33 (1985) 369. 1121R.E. Vandenberghe, E. de Grave, C. Landuydt, L.M. Bowen, Hyp. Int. 53 (1990) 175. S.M. Irkaev, VP. Pritkov and V.G. [I31 G.N. Belozerski, Semenov, Book of Abstracts of ICAME, Vancouver, Canada, 1993, p. 105. [I41 A.W. Galwey. Chemistry of Solids, Chapman and Hall Ltd., London 1967. S.W. Benson, Fundation of Chemical Kinetics, McGraw-Hill, New York, 1960. 1161 M. Avrami, J. Chem. Phys. 9 (1941) 177. [I71 B.V. Erofeev, Compt. Rend. Acad. Sci. URSS 52 (1946) 51 I. 1181 S. Morup, M.B. Madsen, J. Frank, J. Villadsen, C.J.W. Koch, J. Magn. Magn. Mater. 40 (1983) 163. [I91 S. Morup, H. Topsoe, Appl. Phys. 1 I (1976) 63. I201 A. Govaert, C. Dauwe, J. De Sitter, E. De Grave, J. Physique Coil. C6 37 (1976) 291. 1211 C. Wivel, S. Mbrup, J. Phys. E 14 (1981) 605. 1221 A.M. van der Kraan, Phys. Status Solidi (a) 18 (1973) 215. ~231 T. Shinjo, M. Kiyama, N. Sugita, K. Watanabe, T. Takada, J. Magn. Magn. Mater. 35 (1983) 133.
IONICS
(1997) 591-596
Mechanism and reaction kinetics in the solid phase transformation cx-FeOOH + a-Fe,O, studied by Miissbauer spectroscopy L. Diamandescua’*,
D. Mih&E-T%%bQanu”, S. Calogerob, M. Federc
N. Popescu-Pogrion”,
“Institute of Atomic Physics, P.O. Box MG-6, Bucharest, Romania hDepartment of Physical Chemistry, University of Venice, I-30123, Italy ‘S.C. AFERO,
Calea Floreasca
169, Bucharest,
Romania
Abstract The thermal decomposition of u-FeOOH, prepared by air oxidation of Fe(OH), at room temperature, was investigated by means of MGssbauer spectroscopy and transmission electron microscopy. The measurements were performed on powder
samples previously annealed between 200 and 700°C. From MGssbauer spectral data the reaction isotherms, rate constants and the activation energy of the process were determined. The transformation into a-iron oxide takes place preserving the needle-like shape of particles. An Avrami-Erofeev of alpha oxyhydroxide to alpha iron oxide. Keywords: Reaction kinetics; Solid phase transformation; microscopy Materials: cr-FeOOH;
mechanism
was identified to characterise
Iron hydroxide;
Iron oxide; MGssbauer
the solid phase transformation
spectroscopy;
Transmission
electron
a-Fe,O,
1. Introduction The transformation a-FeOOH (goethite) + CYFe,O, (hematite) is an important step in the preparation of certain materials for magnetic recording, ferrites, pigments and catalysts. Considerable theoretical and experimental effort has been invested to understand the mechanism of this transformation in solid phase reaction [l-8]. a-FeOOH is antiferromagnetic with a NCel temperature TN = 400 K. The goethite structure consists *Corresponding author. Tel.: +40 3700; e-mail: [email protected]
1 780 3469; fax: +40 1 420
0167.2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO167-2738(97)00200-2
of double chain of [Fe(O, OH),] octahedra linked together by sharing opposite edges. Although goethite displays a multitude of shapes and sizes, only two basic morphologies occur: acicular and twinned crystals [9]. Synthetic a-FeOOH usually has an acicular habit. By our knowledge, little attention has been paid to the problem of solid phase reaction kinetics associated to the transformation a-FeOOH-+a-Fe,O,. Is the aim of the this paper to reports on the results of Massbauer spectroscopy (MS), transmission electron microscopy (TEM) and electron diffraction (ED) investigation connected with the reaction kinetics and mechanism of the solid phase transformation
592
L. Diamandescu
goethite + hematite in the temperature 700°C.
et al. / Solid State lonics
IOI-
103 (1997)
591-596
range of 200-
2. Experimental Mossbauer (M) spectra were obtained at room temperature with a constant acceleration spectrometer and a 57Co (Rh) source. The obtained M spectra were least squares fitted assuming Lorentzian line shaped sextets. The TEM and ED analyses were performed with a JEOL 200 CX apparatus. For TEM and ED measurements the powder samples were settled on a glass microscope lamella with 25% collodium in amyl acetate. The formatted films were removed by immersion in distilled water and floated off on copper grids. Finally they were fixed with carbon by vacuum evaporation. The goethite was prepared by air oxidation of Fe(OH), at room temperature. The ferrous hydroxide was obtained by ammonia precipitation in a ferrous sulphate solution. Samples of polycrystalline goethite with particle size in the range of 0.3-l 5 pm and accicular ratio 1:6 to 1: 15 were introduced in a furnace that was heated at a rate of 6°C min-‘, kept at heating temperature (in the range of 200-700°C) for different times up to 4 h and finally cooled to room temperature.
3. Results and discussion A typical TEM image of the used goethite is presented in Fig. 1. One can notice the needle-like shape of the particles. At room temperature the M spectrum of the used goethite sample exhibits magnetic sextets with nonlorentzian lines, suggesting a distribution of hyperfine fields. The line shape problem of synthetic and natural goethite has been extensively investigated [ 10-131. The spectrum of goethite can be mainly resolved up to the eighties subspectra using Lorentzian line shape. A representative M spectrum for the calcined samples is shown in Fig. 2 together with the calculated curve. One can observe the simultaneous presence of the two iron phases. In both phases the computer processing revealed a distribution of hyperfine magnetic sextets (corresponding to different
Fig. I. TEM image of goethite diffraction diagram.
05
“.lll
particles
together
with electron
-Fe,O,
.
-10
5
0
-5
Velocity
Fig. 2. MGssbauer spectrum of a-FeOOH
10
(mm/s)
calcined 5 min at 300°C.
internal magnetic fields) with large line widths. This behaviour indicates the presence of iron in different position in a not so well crystallised lattice, during transformation. In Fig. 3(a, b, c) the M computed envelopes, versus heating time r at different temperatures are presented in a surface graphic mode. At 225”C, after a thermal treatment of 5 min the spectrum reveals only the goethite phase (Fig. 3a); the iron oxide phase appears for a treatment time in the range of 20-60 min and is still in development after four hours. In the case of the thermal treatment at 250°C the transformation into alpha iron oxide is practically completed after one hour; however some goethite traces can be also observed after four hours (Fig. 3b). A faster transformation for the samples calcined at 275°C is evidenced by the evolution of M calculated spectra represented in Fig. 3c. In the temperature range of 350-700°C the transformation is very fast and the M spectra reveal essentially the
L. Diamandescu et al. I Solid State lonics 101-103
L.
225OC
\klocity (mm/ sl
2 50°C
(mm/s)
275OC
\klocity (mm/ sl Fig. 3. Miissbauer calculated envelopes versus treatment time for the samples calcined at: 225°C (a), 250°C (b) and 275°C (c).
593
typical sextet of alpha iron oxide. The M line width decreases as the temperature of calcination increases indicating a better crystallisation. Only o-FeOOH and a-Fe,O, phases were identified in the analysed spectra. The Mossbauer areas of these phases were used to calculate the weighted amount of cx-Fe,O, formed at a given time and temperature by assuming equal recoil-free fraction fgOethitcand fhematite. In Fig. 4(a-d) the TEM images together with ED patterns of samples calcined one hour at different temperatures are presented. From the ED patterns one can remark the presence of goethite (d,,, = 4.183 A) together with hematite up to 275°C. The goethite amount decreases as the temperature increases and no intermediate phases were observed, in agreement with the M data. An increase of the microporosity of iron oxide particles is evidenced by Fig. 4(e-h) for temperatures higher than 350°C. This behaviour could by attributed to the water loss during the crystallisation of a-iron oxide. Insomuch as no third phase was detected, the dehydration reaction can be represented as 2a-FeOOH
U&city
(1997) 591-596
+ o-Fe,O,
+ H,O
(1)
suggesting a direct transformation of goethite into hematite, in good agreement with Watari et al. [3]. By using the Mossbauer areas of the two phases, the degree of transformation (r) into o-Fe,O, as a function of reaction time (reaction isotherm) was obtained. Fig. 5 shows the reaction isotherms at 250, 275, and 300°C together with the best computer fitting - a sigmoidal one. The shape of the isotherms indicates a decceleratory reaction throughout; that means the reaction rate is progressively diminishing as the reactant is consumed. For slow decomposition reactions, the diagram of the degree of transformation versus time usually presents a characteristic sigmoidal shape [ 14,151. This behaviour can be interpreted as arising from the creation of nuclei at localised places in the crystal, the growth of these nuclei and after the point of inflection, the decay of the reaction as nuclei overlap and the area of the interface between reactant and product phases decreases. For the accelerated period the extent of decomposition is given by the product of two terms, one of which describes the rate of nucleation and the other the rate of growth. It is
L. Diamandescu et al. I Solid State
e
d
h
Fig. 4. TEM images and ED patterns for the samples calcined one hcmr at 225°C (a), 275°C (b), 350°C (c), 700°C (d) showing the particle shape; the micrographs (e-h) are details at 275”C, 350°C. 45O”C, 7CNYC respectively.
L. Diamandescu
et al.
I Solid
State
tonics
IOI-
103 (1997)
591-596
595
2.0
60 1’ 0
I 2000
4000
6000
8000
0.01 0
2OOu
t (s)
6CNXI
St 30
t 6)
Fig. 5. Reaction isotherms obtained from the MGssbauer data. The solid lines represent the sigmoidal fitting.
generally adequate to assume that the rate of growth of the nuclei is constant so the expression for the extent of decomposition depends mainly upon the form that is taken for the rate of nucleation. Avrami [ 161 and Erofeev [ 171 have shown that for nuclei having a constant rate of three-dimensional growth one can write: - log( 1 - r) = (k x f)”
4000
(2)
where k is the rate constant, n = 4 at the beginning of the reaction and n =3 in the decay period. A unimolecular decay law (first-order reaction) with n = 1 follows if one assumes that each molecule has the same probability of decomposition. Taking into account the r values obtained for the studied transformation the plot -log( 1 -r) versus t has been performed. The best fit with the experimental data was achieved in the case of n = 1, for each temperature. So, one has obtained straight lines whose slope determines the reaction constants k(T). Fig. 6 shows for example the plot and the computer fitting in the case of the samples calcined at 250°C for different reaction times. From this behaviour one conclude that the solid phase transformation goethitejhematite obeys first-order kinetics. Therefore, the thermal decomposition of powder goethite in the investigated temperature range seems to be controlled by a nucleation process and each individual particle in the assemblage may be nucleated with equal probability. The reaction of each individual crystal results from the formation of a single nucleus on the surface of the particular particle. The
Fig. 6. Plot of log( 1 -r) versus reaction determination of the reaction order.
time
t, at 250°C for the
TEM images in Fig. 4 show that the shape of the particles does not essentially change up to 700°C. This behaviour seems to be in good agreement with the mechanism identified by M investigation. The non-iterative fitting of the data with a straight line gives for the rate constants the values: kzz5 =7X 10-5; k,,,=17x lo-‘; k>,,=40x lo-‘; k,,o=61 X lo-5 s-‘. Using a Polany-Wigner equation for the rate constant [14,15] the activation energy E, which represents the energy barrier to reaction, can be obtained from the plot In(k) =f( 1 lT) that is shown in Fig. 7. The linear fit (continuous line in Fig. 7) leads to the evaluation of the activation energy, E= 125 kJ mol K-l. -6
,
Fig. 7. The plot In(k) versus activation energy.
(l/T)
for the estimation
of the
596
L. Diamandescu
et al. I Solid State tonics 101-103
A large hyperline magnetic fields distribution was found by computer fitting in the analysed Mossbauer spectra. The effective magnetic field at the iron nucleus changes from H,,+.=520 kOe to 410 (24 kOe) in hematite and from H,,,=400 kOe to 300 kOe (24 kOe) in goethite. The asymmetrically broadened Mossbauer lines and the decrease of hyperfine magnetic fields in goethite are generally explained by various effects such as deviation from crystalline perfection, magnetic exchange interaction between neighbouring particles (so-called ‘superferromagnetism’) [ 181, col[ 191, surface effects lective magnetic excitations [20]. Such spectra can best be described by distributions of hyperfine fields [ 10,211. The origin of the hyperfine magnetic fields distribution in hematite is similar to that of goethite. Moreover, the decrease of hyperfine magnetic fields by surface effects has been well established by Mossbauer spectroscopy on 57Fe surface enriched hematite [22,23]. In the present study, the distributive behaviour of hyperfine magnetic fields could be attributed to the imperfectly crystallised goethite and hematite as well as to surface effects, during transformation.
Acknowledgements This work is partly supported by the University of Venice, Italy. The research fellowship of one of the authors (L.D.) is gratefully acknowledged.
References [I] J. Lima de Faria, Z. Kristallogr. 119 (1963) 176. [2] I. Dezsi, M. Fodor, Phys. Stat. Sol. 15 (1966) 247.
(1997) 591-596
[31 F. Watari, J. van Landuyt, P. Delavignette, S. Amelinckx, J. Solid State Chem. 29 (1979) 137. 141 F. Watari, J. van Landuyt, P. Delavignette, S. Amelinckx, J. Solid State Chem. 29 (1979) 417. [51 F. Watari, J. van Landuyt, P Delavignette, S. Amelinckx, J. Solid State. Chem. 48 (1983) 49. [61 Y. Maeda, M. Igarashi, A. Terada, F. Yoshimura, Jap. J. Appl. Phys. 13 (1974) 381. [71 W. van Oosterhout, Acta Crystallogr. 13 (1960) 932. 181N. Yamamoto, T. Shinjo, M. Kiyama, Y. Bando, T. Takada, J. Phys. Sot. Japan 25 (1968) 1267. L91 R.J. Atkinson, A.M. Posner, J.P. Quirk. J. Inorg. Nucl. Chem. 30 (1968) 2371. I101 E. Murad, Am. Mineralogist 67 (1982) 1007. 1111U. Schwertmann, P. Cambier. E. Murad, Clay and Clay Min. 33 (1985) 369. 1121R.E. Vandenberghe, E. de Grave, C. Landuydt, L.M. Bowen, Hyp. Int. 53 (1990) 175. S.M. Irkaev, VP. Pritkov and V.G. [I31 G.N. Belozerski, Semenov, Book of Abstracts of ICAME, Vancouver, Canada, 1993, p. 105. [I41 A.W. Galwey. Chemistry of Solids, Chapman and Hall Ltd., London 1967. S.W. Benson, Fundation of Chemical Kinetics, McGraw-Hill, New York, 1960. 1161 M. Avrami, J. Chem. Phys. 9 (1941) 177. [I71 B.V. Erofeev, Compt. Rend. Acad. Sci. URSS 52 (1946) 51 I. 1181 S. Morup, M.B. Madsen, J. Frank, J. Villadsen, C.J.W. Koch, J. Magn. Magn. Mater. 40 (1983) 163. [I91 S. Morup, H. Topsoe, Appl. Phys. 1 I (1976) 63. I201 A. Govaert, C. Dauwe, J. De Sitter, E. De Grave, J. Physique Coil. C6 37 (1976) 291. 1211 C. Wivel, S. Mbrup, J. Phys. E 14 (1981) 605. 1221 A.M. van der Kraan, Phys. Status Solidi (a) 18 (1973) 215. ~231 T. Shinjo, M. Kiyama, N. Sugita, K. Watanabe, T. Takada, J. Magn. Magn. Mater. 35 (1983) 133.