STUDY OF THE LOCAL STRUCTURE IN POORLY-ORDERED PRECURSORS OF IRON OXI-HYDROXIDES

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STUDY OF THE LOCAL STRUCTURE IN POORLY-ORDERED PRECURSORS OF IRON

OXI-HYDROXIDES

J. Combes, A. Manceau, G. Calas

To cite this version:

J. Combes, A. Manceau, G. Calas. STUDY OF THE LOCAL STRUCTURE IN POORLY-ORDERED PRECURSORS OF IRON OXI-HYDROXIDES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8- 697-C8-701. �10.1051/jphyscol:19868131�. �jpa-00226031�

STUDY OF THE LOCAL STRUCTURE IN POORLY-ORDERED PRECURSORS OF IRON 0x1-HYDROXIDES

J . M . COMBES, A. MANCEAU and G. CALAS

Laboratoire de Mineralogie-Cristallographie, Universites Paris V I et VII, U A 09 CNRS, 4, Place Jussieu, F-75252 Paris Cedex 05, France

RESUME

La spectroscopie d'absorption X a ete utilisee pour etudier la structure locale d'hydroxydes ferriques tres ma1 organises. L'environnement d'oxygenes autour du fer est analogue pour les produits formes apres oxydation de fer divalent et ceux precipites a partir de fer trivalent, mais il est different de celui des composes cristallises. Les distances Fe- premier voisin Fe indiquent que les octaedres adjacents partagent des argtes. Les structures locales des deux materiaux peuvent etre distinguees au niveau des couches d'atomes de fer. Au cours de sa transformation en hematite (a-Fe203) I' evolution de la structure locale du precipite obtenu a partir de fer trivalent a ete suivie: la deshydratation engendre un raccourcissement des distances Fe-Fe anisi que la distorsion des octaedres.

ABSTRACT

X-ray absorption spectroscopy has been used to study the local structure of poorly-ordered ferric hydroxides. Materials obtained by oxidation of divalent iron and by precipitation from trivalent iron solutions present the same oxygen environment around iron atoms, but different from that of crystalline compounds. Fe- nearest Fe distances indicate that adjacent octatfedra share edges. Local structures of both materials can whereas be distinguish because of their different iron atomic shells. During its transformation into hematite (a- FepOg) the evolution of the local structure of the material precipitated from ferric solution has been followed:

the dehydration induces a shortening of Fe-Fe distances and the distortion of octahedra.

INTRODUCTION

The iron oxi-hydroxides occurring in continental environments and marine sediments form usually in two main stages: precipitation from aqueous solutions of poorly-ordered ferric hydroxides, which further transform into well-crystallized phases. Among them, hematite (a-Fe203), goethite (a-FeOOH) and lepidocrocite(yFeO0H) are by far the most abundant. Their formation processes have been extensively investigated and numerous works have stressed that the nature of the formed phases strongly depends on physico-chemical conditions ( pH, temperature, concentration of the various components ...) existing during the ageing of the Fe-hydroxides after precipitating in ground waters (1-3). The term of "ferrihydrite" was given to these poorly ordered ferric hydroxides (4) which are relatively unstable in natural conditions. A better understanding of speciation and transformation processes involves a good knowledge of the structure of these precursors. It must be pointed out that the basic structure of crystalline ferric oxi-hydroxides consists in close-packed planes of oxygen atoms containing 6-fold coordinated ~ eions (5-7 and Table 1). The structural-unit of FeOOH poiymorphs is made up ~ + by double-chains of edge-sharing octahedra (d(Fe-Fe)=3.03 and 3.05A). These double-chains are linked by corners in a-FeOOH (d(Fe-Fe)=BBOA) and by edges in yFeOOH (d(Fe-Fe)=3.O8A) . In a-FelOa, each FeO6 octahedron within the same octaedral layer shares three edges (d(~e-~e)=2.97h). Octahedra of adjacent octaedral layers are sharing one face (d(~e-~e)=2.90& and corners (d(Fe-Fe)=3.37A). This arrangement yields a structure more compact than those of FeOOH polymorphs. EXAFS has been used to study the local organization of various synthetic ferric hydroxides compared to these reference compounds. Furthermore, the evolution of local structure around Fe-atoms has been followed on various materials sampled during the transformation towards hematite.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868131

C8-698 JOURNAL DE PHYSIQUE

EXPERIMENTAL

Two freshly precipitated hydroxides were prepared by hydrolysis at pH7 of either divalent (material A) or trivalent (material B) iron solutions at room temperature. X-ray diffraction patterns of both materials exhibit only two hk broad reflections at 2.54 8, and 1.5 8,. Finally the transformation into hematite has been followed step by step on the material B aged at 90°C. EXAFS experiments were performed at L.U.R.E/DCI on powder-dry samples by the direct transmission method at Fe K-edge, using a Si(311) double-cristal. The storage ring was running at 1,85 GeV with a current of 200mA. Harmonics were suppressed by detuning. Interatomic distances have been determined on the Fourier filtered x(k) using theoritical phase shift functions tabulated by Teo and Lee.

RESULTS

The Fourier transform of the EXAFS spectrum of a-Fe203, a-FeOOH and yFeOOH are reported in Fig. 1. The first peak is relative to Fe-0 pairs. The Fe-Fe atomic pairs correspond to the second peak and to a third one present in a-Fe,O, and a-FeOOH. The iron atoms are organized in two subshells with Fe-Fe distances of respectively 3.03- 3.49 1\ and 2.95-3.408, (Table 1). On the contrary in yFeOOH the iron in second neighbour position correspond to Fe-Fe distances of 3.05A and 3.08A which are therefore analyzed as an unique shell (3.088,, Table 1). The iron atoms located beyond 3.5-3.68, give no significant contribution to the Fourier transform.

Is1 alomic sPcII 2nd nlomie sltell 3rd a l o m i e s l ~ e l l

... ...

ND N D ND

XIID' E Y A F S XRD* E X A F S XRD* EX,\I?S

...

Fe-Fe I ^{Table 1:} Interatomic distances and coordination

numbers (in parentheses) as found by E M FS compared to neutron (ND) and X-ray (XRD) dif- fraction data for all compounds investigated here.

Fig. 1: Fourier transform of the K-edge spectra of reference compounds and non-aged materials

.a,- EXPERIMENTAL SPECTRUM

- ,.' 111.11

.0?-

-a,-

Fig.2 and 3 : Fourier filteredx(k) function of the first atomic shell for a-Fe00H and material A

.I¶

- x

rc -

-.a,

First atomic shell: In a-FeOOH and in a-Fe,O, the FeO6 octahedron is strongly distorted leading to 2 oxygen sub- shells and a wave cancellation at 9-10A-l(Fig. 2), whereas in rFeOOH and in the non-aged material, Fourier filtered x(k) functions exhibit apparent single frequencies. The x(k) of the material A (Fig.3) is well fitted with oxygen neighbours at 1.96A, a value close to that of iron-hexahydrate complexes in solution . It is to be pointed out that the Fe-0 distances, found by EXAFS for rFeOOH, are not consistent with those given by neutron diffraction (6)(Table 1). According to neutron diffraction data, there are at least three separate positions of oxygen atoms yielding three distinct Fe-0 distances ranging from 1.9 to 2.1 1A. Such a difference of distance (0.24 is not evidenced by EXAFS analysis. The x(k) function exhibits no significant wave interference, and is well fitted with 6 oxygen atoms at a mean distance of 1.99A.

Second atomic shell: The inverse transform of the second structural peak in rFeOOH and in the material A are in phase ( Fig.4).

r 4 a a 10 12 ~ ( 3 ,

1

EXPERIMENTAL SPECTRA

.01

- x

*

-.a1 - YFsOOH

... ^A-produet

I

-

Fig.4: Comparison of filtered x(k) function of the iron shell for rFeOOH and the material A

EXPERIMEHTAL SPECTRUM 1.' shalt

-. ^{,-. I ,.}

v - , * .

A-p'odusl

The corresponding Fe-Fe distance is equal to 3.06A. This mean bondlength is consistent with neutron diffraction data. The amplitude of the backscattered wave is lower in the ferric hydroxide than in rFeOOH because of the greater structural disorder. Some of the iron positions could be vacant in material A generating this relative disorder.

These results clearly indicate that the local atomic environment of the material A exhibited by EXAFS, is different from that of hematite and goethite as it does not present a third atomic shell. Two structures can be considered on the basis of EXAFS data.

- The material A might represent a poorly-organized rFeOOH structure as attested by the similarities of their structural parameters. Accordingly Fe06 octahedra are joined by edges building a three-dimensional structure.

d , ^{t , 9 t -}

-The two broad bands observed by X-ray diffraction have been interpreted as arising from tetramers of edge- sharing octahedra (8). This model is consistent with the Fe-nearest Fe distance as determined by EXAFS in so far as the d-spacing (d(Fe-Fe)l2) is equal to about 1.53A. In this way poorly-ordered iron-hydroxides would possess a planar arrangement.

C8-700 JOURNAL DE PHYSIQUE

EXAFS cannot distinguish these two local organizations because of the proximity of Fe atoms in double-chains (3.05A) and in adjacent layers (3.08A) in the y-FeOOH structure. However, the experimental Fe-Fe distances found here show that, in material A, FeO, octahedra share only edges.

Material B : The Fourier transform of its EXAFS spectrum is clearly different from that observed in the material A, although both exhibit the same XRD patterns. The second peak is broadened and presents a shoulder on its right side (Fig. 1). This composite structure arises from a contribution of a Fe-next nearest Fe atomic shell. This is confirmed examining the corresponding filtered x(k) function which presents a wave cancellation in the 10-12 k range. A good fit is obtained assuming two iron shells, the first one at 3,05A and the second at 3,49A. Besides, the ~ ( k ) function of the first atomic shell is similar to that of the material A since a shell of 6 oxygen neighbours at 1,96A yielded the best fit.

The mean Fe-0 bond length is shorter than in the well-crystallized compounds and no significant distortion of the octahedron appears. The Fe- nearest Fe distance is characteristic of edges-sharing octahedra and the Fe-next nearest Fe distance (3.494 of corner-sharing octahedra. With the exception of the first atomic shell, the local structure of the material B appears to be close to that of a-FeOOH.

EVOLUTION DURING AGEING TOWARDS HEMATITE.

Ageing heat treatments at 90°C have been made to follow the evolution of the local structure around Fe atoms during the transformation of the material B into hematite (Fig.5) . The less aged materials (30 mn) show the same local structure than that synthesized at 25°C (material B). At the first stage of the ageing (up to 8 hours) the material gives the same XRD pattern as material 5. This early stage is characterized by a significant increase of the number of Fe neighbours measured by EXAFS. The positions of the surrounding atoms becomf more clearly defined and thus the local order improves. The Fe-nearest Fe and Fe-next nearest Fe distances an the same as in the material B (Fig. 5).

EXPERIMENTAL SPECTRA

n ^{n ,}

Fig.5: Fourier transforms of materials sampled

at increasing time of ageing 4

Fig.6: Comparison of filteredxfk) functions of the first Fe-shell for materials sampled after 30 minutes and 96 hours of ageing.

After 24 hours the XRD patterns clearly show the presence of increasing minor amounts of relatively well- crystallized hematite. On Fourier transforms of EXAFS spectra, the nearest Fe shell occurs at 2.96 A instead of 3.06A until 8 hours. This mean distance indicates that iron atoms are located in edge- and face-sharing octahedra . The presence of face-sharing octahedra, inducing more constraint in the structure, is supported by the a pearance of the trigonal distortion in octahedra (two dstinct Fe-0 distances found by EXAFS: 1.95A and 2.1 14. The next nearest Fe shell at 3.40A (instead of 3.50A) becomes more individualized and characterizes Fe atoms of adjacent octahedral layers located in corner-sharing octahedra. This shortening of the Fe-nearest Fe shell is evidenced on the inverse Fourier transform (Fig.6 ): the frequency of the backscattered wave of the 96 hours-aged material is lower and its amplitude is greater. Finally, the 96 hours-aged material exhibits the same EXAFS spectrum than hematite including octahedra distortion.

The transformation occurs between 8 and 24 hours at 90'C. As no sampling has been made between the two contrasted EXAFS spectra obtained, the detailed evolution of these local structures is not yet elucidated. The elimination of water in F ~ ~ + ( H ~ o ) ~ atomic groups ("internal dehydration"), could induce the shortening of the Fe-Fe bondlengths (appearance of face-sharing octahedra) and thus the trigonal distortion in the first shell .

access to an other investigation scale and permits their characterization and distinction by their local structure:

-The mean Fe-0 distance of materials A and B is shorter than in reference compounds investigated here and is very close to that of the Fe hexahydrate in solution.

-The Fe-shells are different in materials A and 8. Fe nearest Fe distances indicate that adjacent FeO6 octahedra only share edges; but the Fe-next nearest Fe shell is only present in material B.

-The material B ---> hematite transformation takes place by a structural rearrangement generating a contraction of the interatomic distances.

These non-aged materials possess distinct local organizations allowing to foresee the nature of the crystallized phase, which wuld be formed later on.

REFERENCES

1. FISCHER .W.R and SCHWERTMANN. U, Clays and clays minerals 23,33-37 (1 974) 2. SCHWERTMANN. U , CARLSON. L. and FECHTER. H, Schweiz.ZHydrol.46/2, (1 984) 3. CARLSON. Land SCHWERTMANN. U, Geo.Cosm.Acr, ~0145,421-429. (1981) 4. CHUKROV. F.V.et al, Proc.lnt.Clay Conf.Madrid, 333-341 (1 972)

5. SZYTULA. A, et al, Phys.Stat.Sol, 26,429-434 (1 968)

6. CHRISTENSEN. Hand CHRISTENSEN. A.N, Acta.Chern.Scand, A32,87-88 (1978) 7. BLAKE. R. L, HESSEWICK. R.E, ZOLTAI. T and FINGER. LW, Arn.Min. 51,123-1 29 (1 966) 8. FEITKNECHT.W, GIOVANOLI. R, MICHAELIS. W and MULLER. M ,Helv.Chim.Act,56,2847-56 (1973)