Mössbauer frozen solution studies of Fe(III) nitrate and Fe(III) perchlorate

J. inorg, nucl. Chem., 1975,Vol. 37, pp. 2283-2285. Pergamon Press. Printed in Great Britain

MOSSBAUER FROZEN SOLUTION STUDIES OF Fe(III) NITRATE AND Fe(III) PERCHLORATE F. A. B. CHAVES and V. K. GARG Departamentode Ffsica,Universidadede Brasflia,Brasdia,Brazil (First received 9 October 1974; in revisedform 26 December 1974)

Abstract--Frozen solutionsof 0.67 M Fe (No3)3and Fe (C10,)3have been studiedby M6ssbauerresonance. Addition of hydroxylion induces quadrupole splitting.A phase transition(from Q.S. = 1.67mm/sec to Q.S. = 0.38 mm/sec) is observed when the temperature is raised; after the disappearenceof the absorption.This change is associatedwith the changeof structurefrom amorphousto a more orderedone. It is suggestedthat speciewith Q.S. = 1.67mm/sec is associatedwith a iron-hydroxybridge.In the case of ferric nitrate frozen solutionthe Q.S. = 0.38 mm/sec is probably associated with Fe (NO3)3.nH20 where n is still not determined.

INTRODUCTION THE chemical bonding of adsorbed surface states can be studied using radioactive doping source experiments or scattering experiments. Although non-viscous solutions do not give Mrssbauer resonance, frozen solutions usually do. Some initial reports f l] on frozen solutions of iron(II) salts showed iron species in each matrix were identical and unaffected by the anion. Very unusual behaviour was detected[2] at about 190K due to phase transition in ice, and this was later confirmed in a detailed study[3] of the frozen solutions of iron(II) chloride. Quenching from liquid state to 78K produces IFe (H20)62+1 ions trapped in a cubic ice lattice rather that in the stable hexagonal ice form. Warming to 193 K induced an irreversible transformation to hexagonal ice which is clearly detected by the discontinuous change in the quadrupole splitting. In the intermediate region of the transition temperature the Mrssbauer spectrum disappears completely because the increase in diffusion reduces the f-factor. Subsequent slow cooling does not regenerate the cubic form although quenching does. In some cases there was no dependence of anion which have been presumed therefore not to participate in formation of the inner hydrate layer. Non aquous solvent frozen solution studies[4], of FeC12 dissolved in mixtures of methanol and formanide, appear to form at least two distinct solvated species. The study of frozen aquous solutions of iron(III) compounds is only of limited value because of the tendency to hydrolyse and the relaxation broadening [1, 5]. The increase in quadrupole splitting of frozen aquous or alcoholic solution to that of solid in soduim nitroprusside has been interpreted [6] to the change in polarisation effect of the cations on the cyanide ligands. In iron carbonyl, (Fe(CO)5, frozen solutions in tetrachlroethene the principal contributions have been interpreted[7] solely from the bonding of the molecular unit rather than from intermolecular or lattice interactions. Samples of frozen solutions of oxidised cytochorome C have also been studied[8] without 57Fe enrichment. The difference between solid and frozen solution Mrssbauer spectra has been reported to result from hydration of the various protein chains in the viscinity of the haeme. The spectrum of frozen aquous solutions of 0.2 M SnCI, shows similar anomalous changes in the line intensity with temperature to those observed in solution of Fe salts. Mrssbauer frozen solutions studies for informations on

hydrolysis, electron exchange, magnetic ordering, identification of ions, glass transition, diffusion, pH dependence, rate of cooling, and spin-spin relaxation have been also reported. The present report is a study of frozen solutions of 0.67 M Fe(NO3)3 and Fe(ClO4)3. EXPERIMENTAL The Mrssbauer spectra were recorded with a constant acceleration velocity transducer coupled to ~7Co in Cu matrix source of 25 mCi initial activity in the standard transmission geometry. The velocity calibration was done with the enriched iron foil. The zero of all the depicted spectras is with respect to iron. The rate of cooling was measured by introducing a thermocouple (copper-constantan)in the sample and recording the time that elapseduntil the temperatureof the samplehad reached the temperature of the liquid nitrogen. The cooling rate of 450°/rain corresponds to placing the liquid sample in the sample-holderand dipping it directly into the liquid nitrogen. ANALYSISAND RESULTS M6ssbauer spectra of frozen solutions of Fe(NO3)3.9H20 and Fe(C104)3.9H20 as a function of temperature are depicted in Figs. 1 and 2. These spectras clearly indicate the variation of Q.S. with temperature. At nearly 183 K, the resonance, in the case Fe(III) nitrate disappears and reappears at 176 K (Fig. 1) while in the case of Fe(III) perchlorate (Fig. 2) the resonance disappears at 173 K and reappears at 152.5 K. It is suggested that the disappearence of absorption is due to increase in diffusion which reduces the f-factor. At 77 K the Fe(III) nitrate frozen solution given a spectra with two doublets. One with Q.S. = 1.67-+0.05 mm/sec. (specie I) and the other with Q.S. = 0.36-+ 0.05 mm/sec (specie II). On increasing the temperature upto 193 K and recooling to 77K the I.S. of specie II changes from 0.55 to 0.45 mm/sec. For Fe(III) perchlorate frozen solution at 77 K only specie I with Q.S. = 1.67-+0.05 mm/sec, and 8/Fe = 0.55 +-0.05 ram/see is observed but after warming upto 173 K and on recooling to 77 K we get specie II with Q.S. = 0.39 - 0.05 mm/sec and 8/Fe = 0.46-+ 0.05 mm/sec. These I.S. values are characteristic of Fe(III) high-spin compounds. The recooled to 77 K spectra of both Fe(III) nitrate and Fe(III) perchlorate frozen solutions are different. In Fe(III) perchlorate we have both the species whereas in Fe(III) nitrate we have specie II only. In ferric nitrate and ferric perchlorate the temperature

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of crystalisation, above the temperature where the resonance disappears, Could not be observed. The specie II is associated with the ordered structure. The solution of ferric nitrate with the cooling rate of 5°/min gives the same Q.S. as that of ferric perchlorate with 1.2°/min[9] cooling rate (Fig. 3), indicating that with the rate of cooling the spectra modify. Specie I is associated with glass structure; alcohol solution[10] or absolute alcohol spectras in both Fe(III) nitrate and perchlorate give the same order of Q.S. as specie I and it is known that alcohol form glass structure compounds. Addition of hydroxyl ion to both ferric nitrate and ferric perchlorate solutions, on freezing give the same type of modification in M6ssbauer spectra (Fig. 4 and 5). The change in Q.S. is associated with the region where the Fe ion changes structure from amorphous to a more ordered one. There exists many reports [l l-13] where iron-hydroxy bridge is known in Fe(III) compounds and they have the

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characteristic Q.S. of the order of 1.6 mm/sec. The slight difference in the Q.S. is because of the different ligand contributions. In Fig. 4 and 5 spectras of Fe: 0.0 OH show relaxation broadening and has been discussed in literature[14, 15]. With the addition of hydroxyl ion first the Q.S. of ~1.6 mm/sec starts appearing and at 0.5 OH and onwards the central peaks with Q.S. =0.4mm/sec appear and have line broadening probably because of the superposition of two species with Q.S. of 0.4 ram/see and 0.6 mm/sec. This is more clear at 2.0 OH. The peak with Q.S. = 0.6 mm/sec is associated with the particles that precipitate[15] with time. These particles shows the

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Acknowledgements--F.A.B.C. expresses his thankfulness to J. M. Knudsen, J. E. Morieria and A. Dufresne for some helpful suggestions. Partial financial support for this work from Project BNDE FUNTEC 104 and grant in aid by CNPq, contract No V.K.G.T.C. 17.174, is thankfully acknowledged.

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Fig. 5. M6ssbauer spectra of frozen solution of Fe(C104)3.9HzO at 77 K with various hydroxyl ion. characteristics of superparamagnetism[15].Thus we suggest 2[Fe(H20)~] 3+.+ OH--~ [(H20)5 Fe-O--Fe (HzO)d '+ + H + + 2HzO. Keeping Fe (NO3)3.9H20 at 10 -4 torr pressure for 12 hr 30 per cent of the mass is lost (water of crystalisation). The isomer shift of this substance is 8/Fe = 0.46 mm/sec. Which coincide with specie II. It is therefore probable that specie II is associated with n molecule of water.

JINC VOL. 37 N O II--E

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