57Fe MÖSSBAUER INVESTIGATION OF LOCAL STRUCTURES IN MIXED-VALENCE Fe2F5.7H2O AND ITS AQUEOUS OXIDATION PRODUCTS, α AND β-FeF33.H2O

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57Fe MÖSSBAUER INVESTIGATION OF LOCAL STRUCTURES IN MIXED-VALENCE Fe2F5.7H2O

AND ITS AQUEOUS OXIDATION PRODUCTS, α AND β -FeF33.H2O

D. Gallie, R. Vogel, B. Evans

To cite this version:

D. Gallie, R. Vogel, B. Evans. 57Fe MÖSSBAUER INVESTIGATION OF LOCAL STRUC- TURES IN MIXED-VALENCE Fe2F5.7H2O AND ITS AQUEOUS OXIDATION PRODUCTS, α AND β-FeF33.H2O. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-289-C1-290.

�10.1051/jphyscol:19801102�. �jpa-00219583�

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57 Fe

~SSBAUER INVESTIGATION OF LOCAL STRUCTURES IN MIXED-VALENCE Fe2F5-7H20 AM) ITS AQUEOUS OXIDATION PRODUCTS

AND

3H20

D. Gallie, R.H. Vogel and B.J. Evans

Depelhnent of Chemistry, University o f Michigan, Ann Arbor, Michigan 48109 U.S.A.

Fe F -7H 0 is a yellow, mixed-valence solid 2 5 2

w i l i c i l can be partially dehydrated at 190°C to give

Fe F .2H 0, which is red in color. Primarily on 2 5 2

the basis of their Mtissbauer spectra, the heptahy- drate and dihydrate have been classified as Class I and Class I1 mixed-valence solids, respectively.

The structure of the dihydrate has been determined and is consistent with this classification /I/.

The structure of Fe2F5-7H 0, however, is unknown;

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and even though the Fe coordination polyhedra have been tentatively identified as F ~ F ~ ~ - and

~e ( ~ ~ 0 ) 63+, there are some unresolved questions re- garding its Mtlssbauer spectrum: One group of investigators have suggested that there are two

~ e components in the spectrum 121, but no spec- ~ + tral parameters were reported. If there is indeed more than one ~e~~ site in Fe2F5- 7H20, then the dehydration to Fe2F5-2H20 cannot be viewed simply as a condensation of coordination polyhedral since there is only one re3+ site in Fe2F5.2H20. In addition, it is not known if the ~ e and Fe ~ + 3+

coordination polyhedra in Fe2FgS7H20 are isolated from each other but with shared polyhedral elements between sites of the same oxidation state, or if there is a complete absence of shared polyhedral elements. Fortunately, a-$?eF3.3H20 and 6-FeF3- 3H 0 can be employed to shed some light on the

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structure/property correlations in Fe2Fge7H20 and Fe2Fga2H 0 since the colorless a-phase is charac-

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terized by the occurrence of completely iaolated FeF6 3- and F ~ ( H ~ O ) ~ ~ + groups and the pale-pink

0-phase by chains of trans-vertex sharing F~(H~O)~F&~- groups, linked via I- and with the equatorial 2 ~ - and 2H20 groups being disordered to

We have therefore reinvestigated the 5 7 ~ e Mtissbauer spectra of Fe2F5-7H 0 and B-FeF3.3H20

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and report the spectrum of a-FeF3.3H20 for the first time. We confirm the existence of at least two ~ e components in the spectrum of the mixed- ~ + valence heptyhydrate at 298 K. Even though the spectrum of a-FeF3.3H 0 is a broad single line, it

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can be readily fit to two components, corresponding to the two ~ e coordination polyhedra. The dis- ~ + order in the structure of 6-FeF a3H 0 is also

3 2

apparent in the spectrum; and two quadrupole doublets corresponding presumably to the two con- figurations for the eqriatorial ligands have been fitted.

The samples were prepared according to re- ported procedures /4,5/. It is noteworthy that both FeF .3H20 polymorphs can be prepared from

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dilute HF solutions of a Fe2F5-7H20 precursor.

All spectra were obtained at 298 K with a con- stant acceleration electromechanical transducer and a 25 mCi 5 7 ~ o / ~ h source. Isomer shifts are reported relative to iron metal.

The spectra are shown in Fig. 1. The inade- quacy of fitting a single ~ e ~ ' pattern to the spectrum of Fe2Fgm7H20 and 0-FeF3.3H20 is clearly demonstrated by the residual. The parameters re- sulting from the fits are given in Table 1.

From the overlap of the parameters for pattern I1 of a-FeFe3H20 with those of B-FeF

3 3H20, it is obvious that, in general, the coordi- nation of ~e~~ in unknown structures with ioni- cities as high as those in the iron fluoride- hydrates cannot be deduced from a comparison of its MHssbauer parameters with those for pe3+ in give both cis- and trans-configurations 131. a known structure. This becomes even more obvious Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801102

C 1-290 JOURNAL DE PHYSIQUE

when the MBssbauer parameters of K2[FeF5(H20)],

-

AE = 0.60 mm s and 6 = 0.42 mm s are consid-

Q

ered (cf. Table I); there is no FeF5(H20) species in the a-FeF3-3H20. If we assume by analogy with isomorphous FeSiF6.6H20 that the FeF6 3- octahedron in a-FeF3.3H 0 is less distorted than that for

2

F ~ ( H ~ o ) ~ ~ + , then pattern I would correspond most likely to F ~ F ~ ~ + and pattern I1 to Fe(H20)6 3+

.

A two quadrupole doublet fit to the spectrum of f$-FeF '3H 0, which does not meet our goodness-of-

3 2

VELOCITY MM/SEC

Figme 1. j 7 f c AItJ.~baueh Speotha

at

6i.t

6 L t

h i t

60.1 a-FeF3.3ff20, AL~owiizg RLoo q ~ a d h u p ~ l ~ doublet

6 2

TABLE I.

Relative

Phase AE 6

r

Q-

(mm s (mm s-'1 (mm scl)

fit criteria, results in parameters in excellent agreement with those reported previously /6/. In view of the relative colors of a- and 6-FeF3.3H20 and their M8ssbauer parameters, it is probable that there are some shared polyhedral elements.

From the asymmetry in the ~ e component in ~ + the spictrunl of Fe2Fga7H20, it is obvious that a single quadrupole doublet would provide an inade- quate fit. The fit of two ~ e quadrupole doublets ~ + lead to a statistically better fit and also to an improvement in the ~ e ~ + / ~ e ~ + ratio, which is 1.13 for the fit of a single Fe3+ component and 0.98 for two Fe3+ components. This interpretation of the spectrum is supported by the unambiguous observa- tion of two !?e3+ patterns in the magnetically- split speccrum at 1.3 K /6/.

References

/1/ Walton, E. G., Rrow:~, D. B., Wong, H. and Reiff, W. M., Inorg. Chem.

16

121 Sakai, T. and Tominaga, T., Bull. Chem. Soc.

Japan (1975) 3168.

/3/ Maak, I., Ecketlin, P. and Rabenau, A.

Naturwiss.

48

/4/ Nielsen, A. H., Z. Anorg. Allg. Chem.

244

(1940) 85.

151 Brauer, G. and Eichner, M., Z. Anorg. Allg.

Chem.

96

/ 6 / Imbert, P. Macheteau, Y. and Varret, J., J. de Phys.

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