A MÖSSBAUER EFFECT STUDY OF THE QUADRUPOLE INTERACTION IN PARAMAGNETIC CHLORINE AND FLURINE CONTAINING β-FeOOH

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A MÖSSBAUER EFFECT STUDY OF THE QUADRUPOLE INTERACTION IN

PARAMAGNETIC CHLORINE AND FLURINE CONTAINING β -FeOOH

D. Chambaere, A. Govaert, E. de Grave, G. Harts, G. Robbrecht

To cite this version:

D. Chambaere, A. Govaert, E. de Grave, G. Harts, G. Robbrecht. A MÖSSBAUER EFFECT

STUDY OF THE QUADRUPOLE INTERACTION IN PARAMAGNETIC CHLORINE AND

FLURINE CONTAINING β-FeOOH. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-350-C2-

352. �10.1051/jphyscol:19792124�. �jpa-00218495�

JOURNAL DE PHYSIQUE Coiloque C2, suppldment au n o 3, Tome 40, mars 1979, page

~2-350

A MbSSBAUER EFFECT STUDY OF THE QUADRUPOLE INTERACTION I N PARAMAGNETIC CHLORINE AND F L U 3 R I N E CONTAINING

B-FeOOH

D. Chambaere, A. Govaert, E. de Grave, G. Harts and G. Robbrecht

Laboratory of Magnetism, University of Ghent, Proeftuinatraat 86, B-9000 Ghent, Belgium

Rdsum6.- Des ichantillons de 0-FeOOH contenant des ions de chlorure et fluorure ont 6tL dtudids par microscopic Llectronique, diffraction de rayons X et spectroscopie Mb'ssbauer. L'apparance macrosco- pique des cristaux, ainsi que leurs propridt6.s microscopiques diffsrent beaucoup d'une variitL P l'autre. On sugg2re que, les ions de chlorure n'occupant que les trous de la maille Llimentaire, les ions de fluorure peuvent aussi substituer des OH dans les octasdres O/OH.

Abstract.- Chlorine and fluorine containing 8-FeOOH samples are studied by electron microscopy, Rkgendiffraction and Mb'ssbauer spectroscopy. The macroscopic shape of the crystals, as well as their microscopic properties are found to be very different for both varieties. It is suggested that, while chlorine ions only occupy the holes exhibited by the unit cell, fluorine ions might also subs-

titute OH'S in the O/OH octahedra.

In a previous paper /I/, we reported the in- fluence of the chlorine ions on the quadrupole inte- raction in paramagnetic 8-FeOOH (further denoted 8-FeOOH(C1)). " ~ e MEssbauer effect measurements on an expanded velocity scale revealed the presence of two strongly overlapping doublets, the area ratio of which could be related to the chlorine content, de-

termined by 37~1-neutron activation analysis.

Fluorine containing 0-FeOOH (further denoted by 8-FeOOH(F)) has been prepared by hydrolysis of a 0.1 M FeF3 solution at temperatures ranging from 40Dc to 100°C. The obtained compounds were studied by electron microscopy, Riintgendiffraction and Mijss- bauer spectroscopy. The experimental set-up is des- cribed elsewhere /I/.

Electron microscopy on 8-FeOOH(C1) reveals typical cigar shaped crystals and clearly shows that these somatoids are built up of parallel tubular rods discovered by Feitknecht /2/ and further studied by e.g. Watson et al. /3/ and Gallagher 141. On the other hand electron micrographs of 0-FeOOH(F) reveal crystals compressed along the long axis (c-axis) and in which the above mentioned superstructure is not detectable.

X-ray diffraction on 0-FeOOH(C1) shows the same pattern as described by Mackay /5/, but yields slightly larger values of the tetragonal lattice parameters : a = 10.54 t 0.02 and c = 3.033

+

i.

For 8-FeOOH(F) we calculate values of a = 10.44

*

0 0

0.03 A and c = 3.02650.005 A which are consistent with the lattice parameters of both varieties, repor

ted by Keller 161.

Figure la shows a typical 8-PeOOH(C1) MSssbauer spectrum at room temperature.

Fig. 1 : Mgssbauer spectrum of 8-FeOOH(C1) (a) and 8-FeOOH(F) (b) at room temperature. Dots represent experimental data. The best fit and its constituent Lorentzian lines are drawn in full line.

The spectra can satisfactorily be described as a superposition of two Lorentzian shaped doublets, denoted the low distortion (LD) and high distortion

(HD) doublet /I/. Figure Ib shows a Mb'ssbauer spec- trum of 0-FeOOH(F). One can inrmediately notice, even without fitting the spectrum, that the absorption envelope of the 8-FeOOH(F) spectrum is broader than that of the 8-FeOOH(C1) spectrum. The only reasonable-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792124

way of fitting this spectrum with Lorentzian lines is by introducing a third doublet. This yields logi- cal variations of isomer shift and quadrupole split- ting throughout the paramagnetic temperature region.

while a two doublet fit introduces inconsistencies in the obtained results.

The quadrupole splitting of the two innermost doublets is close to the value observed in 6-FeOOH

(Cl) (table I). The existence of the third doublet is somewhat enigmatic. It exhibits a very large qua- drupole splitting, in fact beyond the usually cited range for Fe3+ in octahedral coordination 171.

Experimental values of quadrupole splitting and iso- mer shift (referred to Pd) for both varieties. The numbers in brackets denote the region in which para- meter values for different samples (7 for each va- riety) are observed, not the experimental errors.

Its linewidth decreases with decreasing hydrolysis- temperature from 0.30nrm. s-' for a sample prepared at 100°C to 0.23mm.s-' for a sample prepared at 40°C, i.e. below the experimentally observable limit.

One explanation for this doublet could be that the spectra of 6-FeOOH(F) can no longer be fitted with a sum of Lorentzian lines, this procedure being

only an approximation, but need an analysis invol- ving complete transmission integrals as described by e.g. Dunham et al. 181.

Another possibility is that the 6-FeOOH(F) spectrum originates from a more continuous distribu- tion of quadrupole interactions than that of

B-

FeOOH(Cl), because more F- than C1- configurations around an iron atom are possible. Indeed, since the ionic radius of F- is approximately equal to that of OH-, the latter ion might be substituted by the former within the OIOH octahedra themselves, while this is highly improbable for the much larger C1- ion. This would greatly increase the possible nuder of iron surroundings, each giving rise to its parti-

cular quadrupole interaction.

In this case we can write down the following formulae for both compounds :

Fe808 (OH-) (C1-, OH-)

< 2

and

F- (F-, OH-) (O<Y< 1)

< 2

The charge compensating hydrogen ions are omitted and the halogen ions between brackets are those lo- cated in the holes of 0-FeOOH's hollandite type structure /5,6/.

Another argument in favour of anion exchange in 6-FeOOH(F) are thesurprisingly high fluorine con- tents which we determined by "F-neutron activation analysis. The maximum chlorine content observed is 8.17 weight XC1. Arranging one C1-ion in each avai- lable halogen ion site (the holes exhibited by the unit cell) yields a maximum occupancy of 82% of the holes. On the other hand calculation learns that a

100% occupancy of the holes by F-' ions would lead to a fluorine content of 5.34 weight X F. Still, the minimum fluorine content measured in our samples amounts to 7.36 weight % F (corrected for excess water). This excess of F- ions possibly substitutes OH- ions in the O/OH octahedra.

Furthermore, it is impossible, contrary to the case of 6-FeOOH(Cl), to straightforwardly correlate the intensities of the Msssbauer lines to the obser- ved fluorine content. This again suggests a more continuous approach to the ~assbauer absorption en- velope, because introducing still more discrete d o w blets would increase the uncertainty on the inten- sities and make their obtained values physically irrelevant.

The problem of both B-FeOOH's varieties surely merits further investigation. From the data reported above, it follows already, be it only qualitatively, that the kind of halogen ion involved in the forrna- tion of this oxyhydroxide very much influences its crystallization and its chemical and physical pro- perties.

Acknowledgements.- Pr. Dr. J. Hoste is acknowledged for his kind permission to perform the neutron acti- vation analysis in his laboratory. We also wish to

thank Dr. R. Cornelis and Dr. P. Vanacker for their assistance at these analyses. The authors are indeb- ted to I.W.O.N.L. and F.K.F.O. for financial support.

JOURNAL DE PHYSIQUE R e f e r e n c e s

/ I / Chambaere, D., C o v a e r t , A., d e S i t t e r , J . , d e Grave, E . , S o l i d S t a t e Conrmun.

26

/ 2 / F e i t k n e c h t , W . , F o u r t h I n t e r n . Symp. R e a c t i v i t y i n S o l i d s , E l s e v i e r , Amsterdam (1960) 579.

/ 3 / Watson, W., C a r d e l l , R., H e l l e r , W . , J. Phys.

Chem.

66

/ 4 / G a l l a g h e r , K . , N a t u r e

226

151 Mackay, A . , Miner. Mag.

2

1 6 1 K e l l e r , P . , N. Jb. Xiner. Abh.

113,

/ 7 / B a n c r o f t , G., "M6ssbauer Spectroscopy", (McGraw H i l l , London) 1973, p. 157.

181 Dunham, W., Wu, C . , P o l i c h a r , R., Sands, R . , Harding. L., Nucl. I n s t r u m . Methods

145