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The transformation γ-FeO(OH) to α-FeO(OH)
J. inorg, nucl. Chem., 1967, Vol. 29, pp. 1235 to 1238. Pergamon Press Ltd. Printed in Northern Ireland
THE TRANSFORMATION ~,-FeO(OH) TO .-FeO(OH) G. W. VAN OOSTERHOUT Philips Research Laboratories, N.V. Philips' Gloeilampenfabrieken, Eindhoven-Netherlands (Received 22 December 1966) Abstract--The effect of u-FeO(OH) nuclei on the transformation of y-FeO(OH) to u-FeO(OH) in alkali hydroxide- or FeSO,-solutions suggests that the transformation is not a solid state transformation but proceeds by dissolution followed by crystallization. This is confirmed by a comparison of electron micrographs of the original y-FeO(OH) and the x.FeO(OH) obtained from it. The incidental formation of ~-FezOs is not due to the sequence y-FeO(OH) --~ ~-FeO(OH) --~ c~-Fe208 but to the competitive reactions ?'-FeO(OH)--* x-FeO(OH) and ~,-FeO(OH)--~ x-FesOa. No transformation occurs in Na2SO~ and CoSO~ solutions and in dilute sulphuric acid. The effect of Fe ~+is an increased dissolution rate of v-FeO(OH), which is thought to be related to the known effects of Fea+ on the dissolution rate of ferric sulphate and of Cr ~+ on the dissolution rate of CrCI,. INTRODUCTION
OF TIIE many investigations on iron oxyhydroxides, only few have been devoted to the transformation of 7-FeO(OH) to ,-FeO(OH). KaAUSE(~) observed that 7-FeO(OH) transforms into ,-FeO(OH) on heating, at 150°C, in 2N KOH. NITSCHMANN(a) found that ~,-FeO(OH) transforms very easily on storage in a 5 % FeSO 4 7 aq. solution kept at 60°C. In a Na~SO4 solution no reaction occurs. From the fact that particle shape and size, and the behaviour under the polarization microscope, changed "only very little" on transformation, NITSCHMANNconcluded that the transformation is a pseudomorphosis. He stated explicity that the reaction proceeds in the solid state, is topotactic and pseudomorphic. As these results might be of importance in research on magnetic recording materials we repeated NITSCHMANN'Sexperiments and found the same results. Hence we believed his conclusions. About 10 years ago SANGHI(3) did some work on this transformation. He used Eisenoxyd 1300, a commercial sample of v-FeO(OH) described below, and at that time easily obtainable. In spite of much effort he only found transformation to 0~-Fe20z without a trace of ~-FeO(OH). This discrepancy with the results of KRAUSEand NITSCHMANNremained unexplained till the problem was reinvestigated recently. The results are presented in this paper. EXPERIMENTAL We used Eisenoxyd 1300, obtained from Farbenfabriken Bayer, Leverkusen. It was identified as 7,-FeO(OH) (ASTM: 8-98) by the X-ray powder method. Speetroehemieal analysis showed the main impurity to be Zn ( ~ 0.3%). ~-FeO(OH) could not be detected. Magnetic measurements suggest the presence of a trace of a "ferro-magnetic" impurity, perhaps ~,-Fe~Oa. Following KRAUS~ and NrrscHMAI,~ we made two series of experiments: one by heating y-FeO(OI-I) in 2N KOH solution, the other by heating in 5 % FeSO, 7 aq. solution. The composition of the end product was determined in a semiquantitative way from X-ray powder diagrams. The results are given in Table 1. It was thought very likely that nucleation of the new phase might be difl~eult. For this reason we added to the reaction medium nuclei of ~-FeO(OH) to an amount of 10% by weight of the ~,-FeO(OH). The results are shown in Table 2. c1~A. Kv~usE, K. MORO~OWNAand E. PRZYBY/..SK1,Z. anorg, allg. Chem. 219, 203 (1934). t2~ H. Nrrsc~ANN, Helv. cMm. Acta 21, 1609 (1938). ~3) I. Sm~GHI,Unpublished (1954). 1235
1236
G.W. VAN OOSTERHOUT TABLE 1.--TRANSFORMATION OF y-FeO(OH) ON HEATING IN SOLUTIONS OF K O H AND FeSO4
No.
Time (hr)
Temp. (°C)
1 2 3 4 5
2 17 168 210 480
150 150 70 70 70
Medium
Result
2 N KOH 2 N KOH 5~o FeSO4 5~0 FeSO, 5~0 FeSO4
~t-FeO(OI-I)+ ~t-Fe~Os ),-FeO(OH) + 7-FeO(OH) + ~,-FeO(OH)+
ot-FezOa [~-FeO(OH)I* [ct-FeO(OH)] [~-FeO(OI-l)]
* IX] indicates that only a trace of X has been formed. TABLE2.~EFFECr OF ¢x-FeO(OH) NUCLEION THE TRANSFORMATION OF 7-FeO(OH) IN FeSO4 OR IN KOH SOLUTION No.
Time (hr)
Temp. (°C)
Medium
2 6 7 8 9 5 10 11 12
17 17 120 23 24 480 480 72 240
150 150 80 80 70 70 70 80 80
2 N KOH 2 N KOH 2 N KOH 2 N KOH 2 N KOH 5 ~o FeSO4 5 ~o FeSO4 5 y. FeSO4 5 ~. FeSO4
Addition none ~t-FeO(OH) ct-FeO(OH) ct-FeO(OH) ~-FeO(OH) none ~-FeO(OH) ~-FeO(OH) ~-FeO(OH)
Result ~t-FesO, ~-FeO(OH) + ct-FeO(OH)+ ~=FeO(OH) ct-FeO(OH) 7-FeO(OH) + m-FeO(OH)+ ~-FeO(OI-I)+ et-FeO(OH)+
[~t-FetOB]* [~-FesOd [~-FeO(OH)] 7-FeO(OH)t [~-FeIOa] [~t-Fe20,]
* IX] cf. Table 1. t indicates approximately equal amounts. DISCUSSION The experiments with ~-FeO(OH) nuclei added to the reaction mixture show that this addition has a very marked effect upon the results. This suggests that the reaction takes place by dissolution of the original particles, followed by crystallization on suitable nuclei, contrary to what NITSCHMA~C~qproposed. N I T S C H ~ concluded the reaction to be a pseudomorphosis from optical microscopy and "Schlieren" formation on peptization of the material before and after transformation. It should be borne in mind, however, that the natural habit of both 7- and ~-FeO(OH) under NITSCH~mq'S conditions of preparation is needle-like. Hence an apparent conservation of shape also occurs in a solution-crystallization process and thus cannot be used as evidence in support of pseudomorphosis. The last doubt is removed by the change in size and shape of the particles observed in the electron micrographs in Figs. la and b, showing the original y-FeO(OH) and the ~-FeO(OH) obtained from it in experiment no. 9. The same is observed in Figs. 2a and b. These are electron micrographs of a material, obtained with the N I T S C H ~ method, before and after transformation. In cases where only a trace of ~t-FeO(OH) is formed (no. 5) addition of 0~-FeO(OH) nuclei leads to about 50 per cent transformation. Under conditions leading to pure 0t-Fe20a (no. 2) addition of ~-FeO(OH) nuclei leads to ct-FeO(OH) containing only a
The transformation of 7-FeO(OH) to ~-FeO(OH)
1237
small amount of ct-FezO3. The latter experiment shows that at 150°C in 2N K O H the reaction is probably: o~-FeO(OH)
/
y'FeO(OH)~x,~.Fe203 and not y-FeO(OH)
~, ~-FeO(OH)
~, ~-Fe~O3
unless a reaction ~-Fe20 3 ---* ~-FeO(OH) occurs for which no indications exist so far. Samples of 7-FeO(OH) prepared along the lines given by HAHN and HERTPaCrIla) and NITSCH~IArCNtz) are found to transform to e-FeO(OH) without addition of nuclei. The exception is Eisenoxyd 1300, the preparation of which can only be guessed. It seems reasonable to conclude that this is due to the absence of e-FeO(OH) in this material. HAnN and HERTmCH'S method was believed to lead to very pure 7FeO(OH), {~}but a recent patent tr} shows that small changes in the reaction temperature may result in the formation of 0c-FeO(OH). NITSCrIMANN'S method practically always leads to mixtures of 7- and e-FeO(OH). Hence these materials practically always contain 0~-FeO(OH) nuclei. Finally we come to the mechanism of the transformation. The foregoing shows dissolution to be a necessary step. The solubility of y-FeO(OH) in K O H or NaOH and a very low solubility in Na~SO4 solution are not surprising. The necessary solubility in F e S t a solution could be understood from the solubility in H~SO4 formed on hydrolysis of FeSO4. If the H~SO4 is responsible for the transformation then C o S t 4 solution or dilute HzSO4 should lead to transformation as well. Separate experiments show, however, that neither C o S t 4 nor HzSO4 lead to a transformation. This suggests that the Fe ~+ ion might have some special function in the process. One possibility is that the transport of Fe z+ ions in solution might be easier in the presence of sufficient Fe ~+ ions by means of electron transfer. This cannot be a strong effect as the anions have still to be transported in the ordinary way. A second possibility is that Fe z+ increases the solution rate of y-FeO(OH) in the slightly acid solution. A very pronounced example of this is known from literature. Ferric sulphate is very soluble in water but its dissolution is extremely slow. Addition of a small amount of Fe z+ to the solution leads to rapid dissolution of the ferric sulphate, tTI To corroborate this idea, the effect of Fe z+ upon the solution rate of F-FeO(OH) in dilute sulphuric acid was shown in a separate experiment. Suitable temperatures and concentrations could be taken from a study of the system FezO~-SO3-HzO by POSNJAK and MERWIN.ts~ The clearest result was obtained by heating two samples of 100 mg 7-FeO(OH) in 60 ml 0.27 N H2SO4, one with 3.0 g FeSOa 7 aq. and the other without, in sealed glass tubes at 70°C. The presence of Fe z+ reduces the time necessary for dissolution to less than a half. c4~F. L. HA~ and M. HERTRtCFI,Ber. dt. chem. Ges. 26, 1729 (1923). csJ O. GLEr,lSER,Bet. dt. chem. Ges. 71, 158 (1938). tr> U.S. Patent 3.243. 375. 17~CH. BA~RESWlLL,J. Pharm. Chim., Paris 7, 431 (1845); Also: Gmelin's ttandbuch der Anorffanischen Chemie (8th Edn) Bd. 59B, 444. Verlag Chemic GmbH, Berlin (1932). tsl E. POSNJAKand H. E. MERWIN,J. Am. chem. Soc. 44, 1965 (1922).
1238
G. W. VAN OOSTERHOUT
The phenomenon has not yet been explained. It may be that surface-Fe~+ is transformed to Fe ~+ by electron transfer, which might go more easily into the solution. The transferred electrons lead to the formation of a potential difference, inside the solid, over a thin layer near the surface. This possibility relates the increased solution rate to the electrical conductivity of solid insulators at large field strengths c9). Another example is the increased solution rate of CrC13 due to traces of Cr 2+ in the solution described by PELmOTc1°~in 1844. CONCLUSION
(1) ~-FeO(OH) transforms easily to ~-FeO(OH) on heating in alkali hydroxide or FeSO4 solutions only in the presence of ~-FeO(OH) nuclei. (2) This transformation is not a solid state transformation but proceeds by dissolution and crystallization. (3) No transformation occurs in Na~SOv COSO4 and dilute H2SO4. (4) The function of the Fe ~+ ions is to increase the solution rate of ~-FeO(OH) in the acid medium.
Acknowledgements--Theauthor would like to express his thanks to Miss G. STmNSTgAand Mr. D. BEMELMANSfor making the electron mierographs and the prints, to Miss E. DENDEKKERfor making the X-ray diagrams, to Mr. J. Gowl~ for part of the chemical work and to Dr. E. J. W. VERWEY, Dr. M. D. Win,raN and Dr. J. DEJONGFfor stimulating discussions on the subject. cg) E. J. W. VERWEY,Physica 2, 1059 (1935). tl0~ E. M. P~Lmor, Annls. Chim. Phys. 12, 533 (1844).
THE TRANSFORMATION ~,-FeO(OH) TO .-FeO(OH) G. W. VAN OOSTERHOUT Philips Research Laboratories, N.V. Philips' Gloeilampenfabrieken, Eindhoven-Netherlands (Received 22 December 1966) Abstract--The effect of u-FeO(OH) nuclei on the transformation of y-FeO(OH) to u-FeO(OH) in alkali hydroxide- or FeSO,-solutions suggests that the transformation is not a solid state transformation but proceeds by dissolution followed by crystallization. This is confirmed by a comparison of electron micrographs of the original y-FeO(OH) and the x.FeO(OH) obtained from it. The incidental formation of ~-FezOs is not due to the sequence y-FeO(OH) --~ ~-FeO(OH) --~ c~-Fe208 but to the competitive reactions ?'-FeO(OH)--* x-FeO(OH) and ~,-FeO(OH)--~ x-FesOa. No transformation occurs in Na2SO~ and CoSO~ solutions and in dilute sulphuric acid. The effect of Fe ~+is an increased dissolution rate of v-FeO(OH), which is thought to be related to the known effects of Fea+ on the dissolution rate of ferric sulphate and of Cr ~+ on the dissolution rate of CrCI,. INTRODUCTION
OF TIIE many investigations on iron oxyhydroxides, only few have been devoted to the transformation of 7-FeO(OH) to ,-FeO(OH). KaAUSE(~) observed that 7-FeO(OH) transforms into ,-FeO(OH) on heating, at 150°C, in 2N KOH. NITSCHMANN(a) found that ~,-FeO(OH) transforms very easily on storage in a 5 % FeSO 4 7 aq. solution kept at 60°C. In a Na~SO4 solution no reaction occurs. From the fact that particle shape and size, and the behaviour under the polarization microscope, changed "only very little" on transformation, NITSCHMANNconcluded that the transformation is a pseudomorphosis. He stated explicity that the reaction proceeds in the solid state, is topotactic and pseudomorphic. As these results might be of importance in research on magnetic recording materials we repeated NITSCHMANN'Sexperiments and found the same results. Hence we believed his conclusions. About 10 years ago SANGHI(3) did some work on this transformation. He used Eisenoxyd 1300, a commercial sample of v-FeO(OH) described below, and at that time easily obtainable. In spite of much effort he only found transformation to 0~-Fe20z without a trace of ~-FeO(OH). This discrepancy with the results of KRAUSEand NITSCHMANNremained unexplained till the problem was reinvestigated recently. The results are presented in this paper. EXPERIMENTAL We used Eisenoxyd 1300, obtained from Farbenfabriken Bayer, Leverkusen. It was identified as 7,-FeO(OH) (ASTM: 8-98) by the X-ray powder method. Speetroehemieal analysis showed the main impurity to be Zn ( ~ 0.3%). ~-FeO(OH) could not be detected. Magnetic measurements suggest the presence of a trace of a "ferro-magnetic" impurity, perhaps ~,-Fe~Oa. Following KRAUS~ and NrrscHMAI,~ we made two series of experiments: one by heating y-FeO(OI-I) in 2N KOH solution, the other by heating in 5 % FeSO, 7 aq. solution. The composition of the end product was determined in a semiquantitative way from X-ray powder diagrams. The results are given in Table 1. It was thought very likely that nucleation of the new phase might be difl~eult. For this reason we added to the reaction medium nuclei of ~-FeO(OH) to an amount of 10% by weight of the ~,-FeO(OH). The results are shown in Table 2. c1~A. Kv~usE, K. MORO~OWNAand E. PRZYBY/..SK1,Z. anorg, allg. Chem. 219, 203 (1934). t2~ H. Nrrsc~ANN, Helv. cMm. Acta 21, 1609 (1938). ~3) I. Sm~GHI,Unpublished (1954). 1235
1236
G.W. VAN OOSTERHOUT TABLE 1.--TRANSFORMATION OF y-FeO(OH) ON HEATING IN SOLUTIONS OF K O H AND FeSO4
No.
Time (hr)
Temp. (°C)
1 2 3 4 5
2 17 168 210 480
150 150 70 70 70
Medium
Result
2 N KOH 2 N KOH 5~o FeSO4 5~0 FeSO, 5~0 FeSO4
~t-FeO(OI-I)+ ~t-Fe~Os ),-FeO(OH) + 7-FeO(OH) + ~,-FeO(OH)+
ot-FezOa [~-FeO(OH)I* [ct-FeO(OH)] [~-FeO(OI-l)]
* IX] indicates that only a trace of X has been formed. TABLE2.~EFFECr OF ¢x-FeO(OH) NUCLEION THE TRANSFORMATION OF 7-FeO(OH) IN FeSO4 OR IN KOH SOLUTION No.
Time (hr)
Temp. (°C)
Medium
2 6 7 8 9 5 10 11 12
17 17 120 23 24 480 480 72 240
150 150 80 80 70 70 70 80 80
2 N KOH 2 N KOH 2 N KOH 2 N KOH 2 N KOH 5 ~o FeSO4 5 ~o FeSO4 5 y. FeSO4 5 ~. FeSO4
Addition none ~t-FeO(OH) ct-FeO(OH) ct-FeO(OH) ~-FeO(OH) none ~-FeO(OH) ~-FeO(OH) ~-FeO(OH)
Result ~t-FesO, ~-FeO(OH) + ct-FeO(OH)+ ~=FeO(OH) ct-FeO(OH) 7-FeO(OH) + m-FeO(OH)+ ~-FeO(OI-I)+ et-FeO(OH)+
[~t-FetOB]* [~-FesOd [~-FeO(OH)] 7-FeO(OH)t [~-FeIOa] [~t-Fe20,]
* IX] cf. Table 1. t indicates approximately equal amounts. DISCUSSION The experiments with ~-FeO(OH) nuclei added to the reaction mixture show that this addition has a very marked effect upon the results. This suggests that the reaction takes place by dissolution of the original particles, followed by crystallization on suitable nuclei, contrary to what NITSCHMA~C~qproposed. N I T S C H ~ concluded the reaction to be a pseudomorphosis from optical microscopy and "Schlieren" formation on peptization of the material before and after transformation. It should be borne in mind, however, that the natural habit of both 7- and ~-FeO(OH) under NITSCH~mq'S conditions of preparation is needle-like. Hence an apparent conservation of shape also occurs in a solution-crystallization process and thus cannot be used as evidence in support of pseudomorphosis. The last doubt is removed by the change in size and shape of the particles observed in the electron micrographs in Figs. la and b, showing the original y-FeO(OH) and the ~-FeO(OH) obtained from it in experiment no. 9. The same is observed in Figs. 2a and b. These are electron micrographs of a material, obtained with the N I T S C H ~ method, before and after transformation. In cases where only a trace of ~t-FeO(OH) is formed (no. 5) addition of 0~-FeO(OH) nuclei leads to about 50 per cent transformation. Under conditions leading to pure 0t-Fe20a (no. 2) addition of ~-FeO(OH) nuclei leads to ct-FeO(OH) containing only a
The transformation of 7-FeO(OH) to ~-FeO(OH)
1237
small amount of ct-FezO3. The latter experiment shows that at 150°C in 2N K O H the reaction is probably: o~-FeO(OH)
/
y'FeO(OH)~x,~.Fe203 and not y-FeO(OH)
~, ~-FeO(OH)
~, ~-Fe~O3
unless a reaction ~-Fe20 3 ---* ~-FeO(OH) occurs for which no indications exist so far. Samples of 7-FeO(OH) prepared along the lines given by HAHN and HERTPaCrIla) and NITSCH~IArCNtz) are found to transform to e-FeO(OH) without addition of nuclei. The exception is Eisenoxyd 1300, the preparation of which can only be guessed. It seems reasonable to conclude that this is due to the absence of e-FeO(OH) in this material. HAnN and HERTmCH'S method was believed to lead to very pure 7FeO(OH), {~}but a recent patent tr} shows that small changes in the reaction temperature may result in the formation of 0c-FeO(OH). NITSCrIMANN'S method practically always leads to mixtures of 7- and e-FeO(OH). Hence these materials practically always contain 0~-FeO(OH) nuclei. Finally we come to the mechanism of the transformation. The foregoing shows dissolution to be a necessary step. The solubility of y-FeO(OH) in K O H or NaOH and a very low solubility in Na~SO4 solution are not surprising. The necessary solubility in F e S t a solution could be understood from the solubility in H~SO4 formed on hydrolysis of FeSO4. If the H~SO4 is responsible for the transformation then C o S t 4 solution or dilute HzSO4 should lead to transformation as well. Separate experiments show, however, that neither C o S t 4 nor HzSO4 lead to a transformation. This suggests that the Fe ~+ ion might have some special function in the process. One possibility is that the transport of Fe z+ ions in solution might be easier in the presence of sufficient Fe ~+ ions by means of electron transfer. This cannot be a strong effect as the anions have still to be transported in the ordinary way. A second possibility is that Fe z+ increases the solution rate of y-FeO(OH) in the slightly acid solution. A very pronounced example of this is known from literature. Ferric sulphate is very soluble in water but its dissolution is extremely slow. Addition of a small amount of Fe z+ to the solution leads to rapid dissolution of the ferric sulphate, tTI To corroborate this idea, the effect of Fe z+ upon the solution rate of F-FeO(OH) in dilute sulphuric acid was shown in a separate experiment. Suitable temperatures and concentrations could be taken from a study of the system FezO~-SO3-HzO by POSNJAK and MERWIN.ts~ The clearest result was obtained by heating two samples of 100 mg 7-FeO(OH) in 60 ml 0.27 N H2SO4, one with 3.0 g FeSOa 7 aq. and the other without, in sealed glass tubes at 70°C. The presence of Fe z+ reduces the time necessary for dissolution to less than a half. c4~F. L. HA~ and M. HERTRtCFI,Ber. dt. chem. Ges. 26, 1729 (1923). csJ O. GLEr,lSER,Bet. dt. chem. Ges. 71, 158 (1938). tr> U.S. Patent 3.243. 375. 17~CH. BA~RESWlLL,J. Pharm. Chim., Paris 7, 431 (1845); Also: Gmelin's ttandbuch der Anorffanischen Chemie (8th Edn) Bd. 59B, 444. Verlag Chemic GmbH, Berlin (1932). tsl E. POSNJAKand H. E. MERWIN,J. Am. chem. Soc. 44, 1965 (1922).
1238
G. W. VAN OOSTERHOUT
The phenomenon has not yet been explained. It may be that surface-Fe~+ is transformed to Fe ~+ by electron transfer, which might go more easily into the solution. The transferred electrons lead to the formation of a potential difference, inside the solid, over a thin layer near the surface. This possibility relates the increased solution rate to the electrical conductivity of solid insulators at large field strengths c9). Another example is the increased solution rate of CrC13 due to traces of Cr 2+ in the solution described by PELmOTc1°~in 1844. CONCLUSION
(1) ~-FeO(OH) transforms easily to ~-FeO(OH) on heating in alkali hydroxide or FeSO4 solutions only in the presence of ~-FeO(OH) nuclei. (2) This transformation is not a solid state transformation but proceeds by dissolution and crystallization. (3) No transformation occurs in Na~SOv COSO4 and dilute H2SO4. (4) The function of the Fe ~+ ions is to increase the solution rate of ~-FeO(OH) in the acid medium.
Acknowledgements--Theauthor would like to express his thanks to Miss G. STmNSTgAand Mr. D. BEMELMANSfor making the electron mierographs and the prints, to Miss E. DENDEKKERfor making the X-ray diagrams, to Mr. J. Gowl~ for part of the chemical work and to Dr. E. J. W. VERWEY, Dr. M. D. Win,raN and Dr. J. DEJONGFfor stimulating discussions on the subject. cg) E. J. W. VERWEY,Physica 2, 1059 (1935). tl0~ E. M. P~Lmor, Annls. Chim. Phys. 12, 533 (1844).