Mössbauer spectra of some iron compounds formed from alkaline solutions

Corrosion Science, 1971, Vol. i !, pp. 1 to 9. Pergamon Press. Printed in Great Britain

MtJSSBAUER SPECTRA OF SOME FORMED FROM ALKALINE

IRON COMPOUNDS SOLUTIONS*

A. M. PRITCHARDand B. T. MOULD Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, England Abstract--The M6ssbauer effect has been studied in Fe(Otl)~ and Fe304 precipitated from aqueous solution under various conditions. Freshly precipitated samples gave no spectra at 25~ unless digested for a period, Oxidation of Fe(OH)~ leads initially to the appearance of two Fe ~§ sites which is interpreted in terms of lattice distortion and local field gradients from neighbouring Fe s+, and finally to ")'-FeOOH. Thermal decomposition in nitrogen leads to the production of stoichiomctric Fe~O4 of very small particle size (ca. 100 A). The M6ssbauer spectrum of FeaO4 samples prepared is determined largely by the particle size. The smallest particles have the highest water contents, and exhibit low hyperfine splittings for the Fe 2~ at 95~ R6stun6---On a 6tudi6 l'effet M6ssbauer dans Fe(OH)z et FcaO4 pr6eipit6s en solutions aqueuses dans diverses circonstances. Les (~chantillons fraichement pr&ipit(~s ne fournissent de spectre ~. 25~ que moyennant un certain vieillissement. L'oxydation de Fe(OH)z conduit d'abord tt l'apparition de deux sites Fc ~ interpr6t6s en fonetion d'une distorsion r6ticulaire et de gradients de champ locanx de Fe ~ voisin, et enfin ~ FeOOH.'t. La d&omposition thermique sous azote m~ne ~ Fe30~ stoechiometrique ~t granulom6trie tr6s find ( ~ I00 A). Le spectre de M6ssbauer d'&hantillons de FeaO4 est largement d6termin6 par la granulom6trie de celui-ei. Les particules les plus fines ont la plus grande teneur en eau et donnent de has d6doublements superfind pour Fc a~- 5 95~ Zusammenfassung--An Fe(OH)2 und FeaO4, welches aus w~.ssriger L6sung unter verschiedenen Bedingungen ausgeftillt wurde, wurde der M6ssbauer-Effekt gemessen. Frisch niedergeschlagene Proben gaben bet 25 ~ nur naeh l~ngerem Sieden ein Spektrum. Die Oxydation des Fe(OH)2 fiihrt anftinglich zum Auftreten yon zwei Fe ~247was damit gedeutet werden kann, dab sich zun3,chst eine Gitterdeformation mit 6rtlieben Feldgradienten der b~.nachbarten Fe s§ und schli.~Blich ~'-FeOOH ausbildet. Die thermisehe Zersetzung in Stickstoff ftihrt zur Bildung yon st6ekiometrischem FesO4 yon sehr geringer Kristallgr6Be. Die M6ssbauer-Spektra von FeaO4-Proben werden sehr stark yon der Teilchengr6Be bestimmt. Die kleinsten Teilchen haben den gr6Bten Wassergehalt und ergaben eine geringe Hyperfeinaufspaltung des Fe 2+ bet 95~ INTRODUCTION

DURING the corrosion of Fe in alkaline aqueous solution oxidation products appear in suspension, or at the surface of the metal. In the absence of sufficient air for complete oxidation either Fe(OH)2 or magnetite (F%O4) may be formed ~ in addition to any film which adheres to the metal surface. Since the presence of these compounds may lead to problems in the operation of mild steel steam-generating equipment, we have used the M6ssbauer effect2 in 57Fe to investigate the properties of these compounds form'ed in an aqueous environment, and their reactions at higher temperatures. PREVIOUS

WORK

Fe(OH)~ It is now generally agreed that Fe(OH)2 is stabIe below 100~ in the absence of oxidizing agents, certain transition metal ions such as Ni z+ and Cu 2§ and finely *Manuscript received 9 June 1970. 1

2

A . M . PRrrCHARD and B. T. MouLD

divided metals, notably Pt and Pd, which catalyse the liberation of hydrogen. 3 The quantity evolved is only some 20 per cent of that predicted by the equation usually written for the reaction :4

3Fe(OH)~ = Fe~O4 + H2 + 2H~O. From an X-ray powder photograph Schikorr4 identified the product of decomposition at 150-200~ as FeaO4, but because of the ease with which their product oxidized Hazell and Irving 5 thought that they had obtained FeO, though this is not thermodynamically stable below 570~ Mackay 6 found evidence for the presence of Fe, FeO, and FgaO, in the powder photographs of dry Fe(OH)~ decomposed above 200~ Under hydrothermal conditions F%O4 appears to be the only product, 4.5,7 though some workers found small amounts ofa-Fe above 178~ 5 Pritchard, Haddon, and Walton found that the particle size produced under hydrothermal conditions was small enough for no magnetic hyperfine structure to be seen in the Mfssbauer spectrum of the FeaO4.7 If Fe itself is heated in water at 300~ magnetite is the only product, z The activation energy for the decomposition appears to be less for the dry solid (22.5"kcal. 5) than in aqueous suspension (30 kcal.3), though in the latter case the presence of catalysts reduces it substantially.3 The MOssbauer spectrum of Fe(OH)2 has been reported at 90~ s and below, a At 90~ a two-line spectrum characteristic of high-spin Fe ~247 was observed, and at 4"2~ four lines which were interpreted in terms of an antiferromagnetie interaction, with the spins in the c-plane of the CdI2 hexagonal structure.

Magnetite StoichJometric magnetite has the formula Fe304 and at room temperature has a cubic structure, with 8 Fe 3§ in the tetrahedral "A" sites, and 8 Fe 3+ and 8 Fe 2+ in octahedral "B" sites Within the unit cell which contains 32 O ~ ions. Mtissbauer spectra have been reported by a number of workers, 7,1~ and interpreted in terms of two overlapping six-line patterns at room temperature, that with the larger splitting arising from the ions in the "A" sites, and the other from those in the "B" sites. The electron hopping between Fe 3§ and Fe 2§ ions in the "B" sites which gives magnetite its conductivity has been shown from the broadening of the Mrssbauer spectral lines to have a relaxation time of 1.1 -4- 0.2 nanosec at room temperature, z5 Below 100119OK, the exact temperature depending on the purity,z7 the lattice changes to an orthorhombic structure, the electron hopping ceases, and the MSssbauer spectrum may be considered to consist of three overlapping 6-line patterns. If there is a deficiency of Fe ~+ the electron hopping between ions in the "B" sites becomes restricted to local FeZ+-Fe 3+ pairs, the conductivity is reduced t7 and the relative intensity of the Mrssbauer pattern with the larger splitting at room temperature increases, application of an external magnetic field broadens the lines of the outer pattern, showing that they arise from Fe z§ both in the " A " sites and in the "B" sites where there are no Fe2§ neigh~ bours for electron hopping to take place. ~a In small enough particles of FeaOa the anisotropy energy is low enough for the total moment of the particle to fluctuate between the easy directions of magnetization in a time comparable to the lifetime of the excited state of the 57Fe nucleus (10 -7 s)

MSssbauer spectra of some iron compounds

~

3

causing collapse of the magnetic hyperfine structure (h.f.s.) so that a single MSssbauer absorption is seen. a3 This phenomenon is known as superparamagnefism, the individual particles having moments which behave like normal paramagnetio moments when the temperature is high enough to permit fluctuations between the easy directions of magnetization, the co-operative effects being restored at lower temperatures. However, on attempting to observe the MSssbauer spectrum of FeaO4 precipitated at room temperature from a solution containing the correct ratio of Fe z§ : Fe 3§ Kakabadse et aL~6 were unable to obtain any absorpti6n at 25~ though the expected spectrum was observed at 80~ The material produced by this method has long been known to differ from ordinary Fe304, being more reactive, having a lower density and a variable water content, but giving the same X-ray powder photograph lines; on account of the water content it is often known as hydrated magnetite or Fe (II, III) hydroxide,as EXPERIMENTAL

Because of the very high affinity of the materials studied for 0.2, all operations were carried out in an atmosphere of O.2-free N2 which had been further scrubbed with alkaline pyr0gallol; this gas was also passed through all solutions before use for at least 30 min to remove dissolved O~. Since glass absorbs the 14.4 keV. gamma-ray from 57mFe quite strongly a special filtrations unit was made with perspex windows and a brass body which could be connected to a Cu rod to act as a cold finger so that spectra could be obtained at low temperatures. It was found necessary to support the filter paper on a piece of nylon cloth when filtering alkaline solutions. A similar unit was used for experiments in which the precipitate was heated, the perspex being replaced by AI windows covered with a thin film of silicone grease to protect them from alkali; temperatures up to 400~ could be produced and maintained to 4- 3~ A piece of filter paper charred between brass plates at 600~ supported on glass cloth was found to be an effective filter; ordinary paper filters charred above 200~ and glass fibre filters, although chemically inert, led to oxidation of the precipitate, presumably by the desorption of oxygen from the fibres. The Mibssbauer spectra were recorded using a 100-channel pulse height analyser (Harwell type 1524) operating in the time mode as desct'ibed earlierx9 and a 10 millicurie source of 57Co in Pd. Fe foils and so~lium nitroprusside absorbers were used to calibrate the spectrometer, and all isomer shifts are referred to the latter material as zero. The spectra were fitted to Lorentzian lineshapes by a least-squares method using the Oxford University KDF 9 computer. All reagents used were of analytical grade, and the Fe z+ : Fe 3§ ratio of the samples was determined by dissolving in HCI under N2, titrating portions of this solution with N/50 KMnO4, in the presence of Mn z§ and H3PO4 to prevent oxidation of CI-, to determine the Fe z+, and then titrating the resulting mixture with HgNOa, previously standardized against the KMnO4, in order to determine the total Fe content. X-ray powder photographs were obtained for some of the samples which were stable in air using Co Ka radiation and a standard 19 cm camera. RESULTS AND DISCUSSION

Ferrous hydroxide Preparation and oxidation. The gelatinous white precipitate formed by mixing

4

A.M. PRII'CHARDand B. T. MOULD

deoxygenated solutions of FeSO4 and K O H was difficult to filter and gave no M6ssbauer spectrum at room temperature, though on freezing to 195~ or 95~ a strong absorption was recorded. Digestion of the precipitate in the mother liquor for periods longer than 1 h at 100~ produced a more granular material which gave a strong absorption at room temperature. The isomer shifts and quadrupole splittings observed are presented in Table 1, and are characteristic of high-spin Fe ~ in a nearly symmetrical electric field, s0 Since the parameters observed for both the digested and undigested precipitates are the same at 195~ and 95~ we conclude that the low TABLE I.

Material Fe(OI-I)z(ref. 8) This work Nemalite Fes~"impurity Fe(OH)27%Fe ~+ : Fe2~-

Totally-oxidized Fe(OH)z (Fe3+) ),-FeOOH (ref. 22)

MOSSBAUERPARAMETERSFOR Fe(OH)~

Isomer shift (ram/s)

Quadrupole splitting (mm/s)

1'49 4- 0.05 1'62 4- 0.05 1"55 1.44 1-44 0.45 1.65 1.68 1"60 1"55 0'64 0"65

3'02 4- 0-1 3"13 4- 0.1 3-06 2.92 3.12 0.70 3-02 2.18 2"98 1"78 0"60 0"59

Temperature (deg. K) 88 95 195 298 298 95 195 295 295

recoilless fraction observed for the undigested precipitate at room temperature must be caused by motions of the H20 molecules adsorbed on the precipitate, but which do not affect the local electric field gradient on freezing, in contrast to frozen solutions of Fe 2§ salts, in which the structure of the frozen solvent plays a considerable part. -~ Any oxidation of the precipitate, controlled or otherwise, was immediately noticeable from a broadening of the M~Sssbauer lines. Computer analysis of the spectra of slightly oxidized samples suggested that there were two Fe 2+ sites, that with the larger quadrupole Splitting having similar parameters to those observed for pure Fe(OH)2 up to a Fe z+ : Fe 3§ ratio of about 8 91 after which the quadrupole splitting decreased from this value, and the other showing a similar isomer shift but reduced quadrupole splitting which decreased rapidly with increasing degree o f oxidation. The absorption from Fe z~-overlapped with the lower velocity Fe z§ lines, and was difficult to distinguish from them in mildly oxidized samples. The spectrum of a sample having a 2 : 1 Fe z+ : Fe z+ ratio is shown in Fig. 1. Complete oxidation of a sample gave a strong absorption which was very similar to that observed by other workers for T-FeOOH, 2z though the Fe z-~ ion quadrupole splitting was somewhat lower than this value in partially oxidized samples. It is known 2z that the structure of Fe(OH)2 changes below a Fe z§ : Fe z+ ratio of 9 : 1 from the CdI2 hexagonal layer structure to a primitive hexagonal structure, and it seems that this lowering of the symmetry is reflected in the decrease o f the quadrupole splitting of the outer lines? ~ which we suggest arise from Fe z+ ions without any Fe z§ near neighb0urs. The rapid decrease in the splitting of the

MBssbauer spectra of some iron compounds

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MSssbauer spectrum of partially-oxidizedFe(OH)=.

inner pair with increasing oxidation may be attributed to the effect of neighbouring Fe 3-~ ions which provide both a distortion of the field from the charges in the lattice, and an irregular octahedron of ligands, as in 7-FeOOH where the Fe 3§ are surrounded by a distorted octahedron of oxide and hydroxide ions. This same effect may explain the iow quadrupole splitting observed for Fe 3+ in mildly oxidized samples. Mackay n has described stable green compounds of somewhat variable composition isolated after partial aerial oxidation of Fe(OH)z with a layer spacing four times that of the CdI2 lattice of the pure material. His suggestion of a layer sequence ABAC would allow easy transformation on further oxidation to the cubic y-FeOOH structure, and would explain'our observations. We prepared samples of his "green complex 11"6 but found no significant difference in the MSssbauer spectrum from other samples of oxidized Fe(OH)2. For comparison purposes we obtained samples of nemalite, a fibrous form of the mineral brucite, Mg(OH)2, which may contain Fe both as Fe304 and as Fe(OH)2Ya The total Fe content of our samples was 7 per cent, and the MSssbauer spectrum showed both Fe2+ and Fe 3§ present; the parameters are compared in Table 1 with those for other samples of Fe(OH)2. Samples with a darker colour in general showed a reduced quadrupole splitting, agreeing with previous results if the colour is due to FemO4or similar oxidation product. The spectrum of a sample taken with the gamma-radiation transmitted parallel to the fibres is shown in Fig. 2; the ratio of the low velocity to the high velocity peak areas is 0"59. Since the fibres elongate along the a-axis, 21 and the c-axis is the axis of symmetry, our results confirm the earlier report 9 that the sixth d-electron is in the c-plane, i.e. the d~ orbital.

Thermal decomposition. MSssbauer spectra of the products confirm that Fe(OH), is stable in aqueous suspension at-100~ but that the presence ofNi 2§ ions in solution3

6

A.M. PRIICHARDand B. T. MOULD

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MSssbauer spectrum of nernalite; radiation parallel to fibre axis.

caused oxidation to take place at lower temperatures, the spectra being similar to those observed after aerial oxidation. It thus appears that the Ni 2§ ions act purely as a catalyst and do not change the nature of the product. Thermal decomposition of samples, dried in a current of N2 at 90~ and then analysed by their MSssbauer spectra to ensure that no oxidation had taken place, showed a number of different behaviours. In the absence of any oxidation it was found that the samples could be heated at 200~ for 3 h without further change occurring unless the FeSO4 solution used had been kept over Fe filings to keep it from oxidation, in which case the spectrum had changed completely, showing two lines which had an isomer shift and quadrupole splitting in good agreement with those observed for 7-FeOOH, and a broad background absorption showing traces of magnetic hyperfine structure which became more pronounced at 95~ Chemical analysis showed 30 per cent of the total Fe to be present as Fe 2+, and we interpret the spectrum as a mixture of T-FeOOH formed by catalytic oxidation due to some impurity in the Fe filings and very small particles o f Fe3Oa. In the absence of this impurity reaction proceeded smoothly at 300~ the final product giving a broad spectrum which still showed some Fe(OH)2 lines after 1 h. In some cases well-defined magnetic hyperfine structure was seen, indicating that the particle size of the Fe3Oa was larger; results obtained under hydrothermal conditions7 suggest that such samples contained alkali which had not been removed by washing, thus permitting recrystallization of the initially-formed small particles. In each of these last two cases the Fe ~§ : Fe 3~ ratio was 1 : 2, as expected for FeaO4. Further heating of samples which showed some oxidation after the initial drying gave spectra showing lines from 7-FeOOH, 7-Fe203, and superparamagnetie Fe304. We therefore conclude that in the absence of catalysts or oxidizing agents the

M6ssbauer spectra of some iron compounds

7

initial product of decomposition of Fe(OH)z is Fe304 of stoichiometric composition but very small particle size (ca. 100/~). At no stage did we see any trace of the M/Sssbauer spectrum of metallic Fe, and we conclude that that observed by other workers 5 may have been produced by reduction with H2 evolved during the decomposition but not removed from the reaction vessel. Thermal decomposition of some nemalite samples proceeded in a similar manner to that described above with two differences: after heating at 200~ there was a noticeable decrease in the quadrupole splitting which we attribute to partial oxidation by 02 adsorbed on the fibres, and the spectrum obtained after decomposition at 380~ consisted of a broad structureless line with an isomer shift of 0.67 mm/s and width of 1-5 mm/s at 95~ suggesting that the Fez+ in the nemalite are well distributed throughout the material and so the product of the decomposition contains only very small clusters of Fe atoms which still show superparamagnetism at this temperature. Clearly the temperature of decomposition is too low to see the products observed by Large et al. z~ after heating above 1000~ h[agnetite

This was prepared from aqueous solution by three methods 1. Precipitation from a solution containing the correct Fe z§ : Fe z§ ratio by alkali; the order o f addition was immaterial, identical products being obtained in each case.

2. Aerial oxidation of alkaline Fe2§ solutions. 3. Oxidation of Fe(OH)2 in aqueous ammonia by the stoichiometric quantity of KNO3. The product of method one was difficult to filter, and gave no M~Sssbauer spectrum at room temperature, but a normal one with magnetic h.f.s, at 95~ in agreement with earlier work. 16 Digestion of the precipitate before filtration or drying in the filtration unit enabled a spectrum to be observed at 25~ a single non-Lorentzian line of width 1.6 mm/s with an isomer shift of 0.65 ram/s, suggesting that the material was superparamagnetic, as would have been expected for a particle size of 100/~ which was observed by Egger and Feitknecht using this method of preparation.~6 Heating of the precipitate at 400~ for several hours produced no change in the spectrum, showing that no recrystallization of the very small particles had occurred. Exposure to the atmosphere caused a rapid decrease in the Fe z§ content from 33 to 20 per cent, at which level it remained constant. That this easy oxidation was not due to physically adsorbed Fe z+ ions was shown firstly by the absence of any characteristic absorption at 95~ and secondly by the appearance of such an absorption when the Fe z+ : Fe 3~-ratio of the solution used in the preparation was increased above 1 : 2. In this latter case the intensity of the Fe(OH)2 peaks was proportional to the excess Fe ~+ ion present, and the isomer shift and quadrupole splitting were similar to those observed for the pure material, showing that the precipitate was a mixture of the two phases. On heating above 200~ the Fe(OH)z decomposed to give superparamagnetic magnetite, showing that the presence of some Fe304 particles lowered the activation energy for decomposition which otherwise took place at a higher temperature. We consider that a more probable explanation for the easy oxidation of the small particles is their size: a considerable proportion of the Fe ~§ ions will be

8

A.M. PRITCItAROand B. T. I~|OULD

within a few angstroms of the surface where they are easily oxidized, giving each particle an outer layer of, presumably, 7-Fe.oO3. The product using method two varied according to the pH of the solution: the more alkaline the solution the further oxidation proceeded, dull-brown T-FeOOH being produced at pH 14. In an aqueous ammonia-NH~Cl buffer solution a black product was obtained which had a Fe 2§ content of 20 per cent and showed full h.f.s, in the MSssbauer spectrum at 25~ together with a small amount of superparamagnetic absorption, showing that a range of particle sizes was present. In agreement with earlier work H on partially-oxidized FeaOa the " A " pattern had a larger intensity than the " B " pattern. The particle sizes for the preparations by method three at 40~ and 100~ are 1500 A and 750 A respectively,2G so that in all cases normal hyperfine structure was expected and found for the Fe304. The reaction at 25~ was slow, not being complete after one week; the product was exposed to the air before the MSssbauer spectrum was obtained, and lines from tt-FeOOH, in contrast to the 7-FeOOH produced in the absence of KNO3, were seen in the spectrum. TABLE 2,

PROPERTIF~SOF PRECIPITATED MAGNETITE SAMPLES

Method of preparation (see tex0

Percentage of total Fe as Fe~§

HzO per oxide ion

Magnetic splitting at 95~K in kOe

Particle size, (reL 26)

1 2 3, 100~

21-5 20 33

0-21 0-I0 0.10

493, 451 505, 470 510, 490

100 450 700

In Table 2 are listed data for the samples prepared by all three methods, the Ire~~ content being determined after exposure to the air. The ratio ofHzO : O ~'- was calculated from the weight change found after heating a sample to 700~ for 15 11 in a Pt boat in a stream of air, the initial formula of the material being assumed to be Fe30~. x H20, so that the ratio is x[4. X-ray powder photographs of the product showed that it consisted entirely o f ct-FezOa. Our experimental data was inadequate to permit a resolution o f the three hyperfine fields expected in the spectra observed at 95~ and we take our data to be representative of the Fe 3§ and Fe 2+ ions, the former having the larger splitting. X-ray powder photographs of the different Fe~O.l samples showed good correlation with the particle size, diffuse lines being observed for the particles with the smallest size; the lattice constant a was the same in all cases (8.36A). Differential thermal analysis and thermogravimetric analysis showed that the water was lost continuously on heating, and this made detection of the exothermic oxidation very difficult. The decrease of the hyperfine splitting with particle size at 95~ may in part be due to some residual superparamagnetism; however, the splitting for the Fe 2§ is affected to a much greater extent than for the Fe z§ and in view of the increase o f water content it seems possible that either some octahedral sites are occupied by water molecules, or else that some of the oxide ions have protons associated with them, as suggested for 7-FezO8 by David and Welch. 27 Since the smaller particles contain the most water, and are formed under conditions o f highest supersaturation, it is

MiSssbauer spectra of some iron compounds

9

easy to see how molecules could become trapped in the lattice. Because octahedral holes in the almost close-packed oxide lattice are larger than the tetrahedral ones, this would explain why the effect on the h.f.s, is greatest for the Fe ~ which are found exclusively in the octahedral sites. It is less clear why this should be so if the water is present as hydroxide ions. CONCLUSION

It is clear that M6ssbauer effect measurements on corrosion products, as well as being useful in identifying a particular pure Fe compound, ~ can also give valuable information about the more detailed'environment of the Fe atoms, and also may yield evidence which can indicate the conditions under which that particular compound was formed and thus permit a choice to be made between different possible corrosion mechanisms. Acknowledgements--We thank Chemistry Division, A.E.R.E., Harwell, for the loan of equipment. REFERENCES 1. V. J. LINNENBOM,J. electrochem. Sue. 105, 322 (1958), 2. V. I. GOL'DANSKIIand R. H. t lrgBrR, Chemical dpplications of 31ossbatwr Spectroscopy. Academic Press, New York (1968). 3. F. J. SmPKO and D. L. DOUGLAS,J. phys. Chem. 60, 1519 (1956). 4. G. SCI-IIKORR,~7. attorg, allg. Chem. 212, 33 (1933). 5. I. F. HAZELLand R. J. IRVING,d. chem. Soc. (A), 669 (1966). 6. A. L. MACKAY, Croat. chenh .4eta 31, 67 (1959). 7. A. hi. PgI'rCHARO,J. R. HADDONand G. N. ~,VAL'rON,Corros. Sci. 11, 11-23 (1971). 8. J. L. MACKEYand R. L. COLLINS,d. htorg, nucL Chem. 29, 655 (1967). 9. If. MIYAMOTO,T. SmNJO, Y. BANDOand T. TAKADA,J. phys. Soc. Japan 23, 1421 (1967). 10. R. BAtJMINGER,S. G. COHEN, A. MAmr~ov, S. OFER and E. SEGAL, Phys. Bey. 122, 1447 (1961). I 1. A. ITO, K. ONO and Y. ISmKAWA,J. phys. Soc. Japan 18, 1465 (1963). 12. F. VAN DER WOUDE, G. A. SAWATZKYand A. H. MORRISH,Phys. Rev. 167, 533 (1967). 13. T. K. MCNAB, R. A. FOX and A. J. F. BOYLE,J. appl. Phys. 39, 5703 (1968). 14. J. M. DANIEt.Sand A. R.OSENCWAIG,J. Phys. & Chem. SolMs 30, 1561 (1969). 15. W. KONDIG and R. S. HARGROVE,Solid State Commttlt. 7, 223 (1969). 16. G. J. KAKABADSE,J. R1DDOCHand D. St. P. BUNntrRY, J. chent. Soc. (A), 576 (1967). 17. E. J. W. VERWEYand P. W. HAAYrqAN,Physica 8, 979 (1941). 18. Gmelins llandb~ch der attorganischen Chemic. Eisen. Teil B. No. 59, p. 120. Verlag Chemic G.M.B.H., Berlin (1932). 19. M. J. HALSEYand A. M. PRIICrlARD, J. chem. Soc. (A), 2878 (1968). 20. R. INGALLS,Phys. Rev. 133, A787 (1964). 21. A. J. N o z m and M. KAPLAN,J. chem. Phys. 47, 2960 (1967). 22. J. H. TERRELLand J. J. SPI~KERMAN,Appl. Phys. Lett. 13, 11 (1968). 23. W. FEIIKNECHTand G. KELLER,Z. anorg, allg. Chem. 262, 61 (1950). 24. H. BERMAr,r,Am. Miner. 17, 313 (1932). 25. N. R. LARGE, R. W. WILKINSONand R. J. BULLOCK, A.E.R.E. Rep. 5580 (1968). 26. K. EGGErt and W. FEIIKNECm', Heir. chhn. Acta 45, 2042 (1962). 27. I. DAVID and A. J. E. WELCH, Trans. Faraday Soc. 52, 1642 (1956).