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Mössbauer study of the local environments of 57Fe in FeO
Solid State Communications,
Vol. 7, Pp. 1785—1788, 1969
Pergamon Press.
Printed in Great Britain
MOSSBAUER STUDY OF THE LOCAL ENVIRONMENTS OF ~Fe IN FeO D.P. Johnson Advanced Materials Research and Development Laboratory, Pratt and Whitney Aircraft, Middletown, Connecticut 06458 (Received 4 August 1969 by J.A. Krumhansl)
Mössbauer spectra of quenched Fe~Ohave been obtained for several compositions and temperatures. The aspectra can be described bythe two 2~doublets and F&~ singlet. A change in quadrupole split Fe type of predominant Fe2~ion is observed at the same composition that the p—n transition is observed in the electrical properties.
FeO IS CONSIDERED to have a NaC1 structure with at least four atomic per cent cation vacancies, Recent measurements of the Seebeck coefficient of Fe~Oat elevated temperature indicate that wustite undergoes a p—n transition as the cation deficiency increases.’ ~ Although the composition at which this transition occurs is sensitive to impurities, recent measurements indicate that the p—n transition occurs at X 0.92.’ ,2 The nature of this transition is not understood and previous models for Wüstite do not predict this behavior. The Móssbauer study reported here was undertaken to gain additional information on the nature of this transition. Spectra were obtained on quenched samples of various compositions ranging from X = 0.935 to X = 0.905 and for various temperatures ranging from 297°K to 203°K.The Fe~Osamples were prepared from 0.001 cm thick 99.996 per cent iron foils by heating in a controlled C0 2—CO atmosphere at 1200°Cfor 2hr and then quenching to room ternperature. Theand composition was weight gain agreed with thatdetermined predicted by by Darken and Curry4 for the C0 2—CO ratio used. X-ray diffraction lines obtained from the samples are sharp and indicate only the W~istitephase is present.
In agreement with previous measurements, we observe two partially resolved peaks separated by approximately 0.6 mm/sec and a distortion in the wing near zero velocity.5—7 However, the more acute spectra reported here exhibit a definite asymmetry in the peak heights, which is independent of absorber orientation, and additional structure in the line near 0.4 mm/sec. In a preliminary study on this material, Wertheim7 identified the peak near 0.4 mm/sec with tnvalent iron and the peak near 1.0 mm/sec with di-valent iron by reasoning that the X-ray and neutron evidence that wüstite is cubic and the unequal intensity of the two peaks eliminated quadrupole splitting as a possible interpretation. Shirane, Cox and Ruby6 observed that the strength of the 0.4 mm/sec peak was nearly independent of composition and hence could not be identified with Fe3’. In addition, they pointed out that the cation vacancies could easily remove the local cubic symmetry at the iron nucleus. Since the spatial dependence of the quadrupole field from 3, the quadrupole a point charge decreases as R splitting would be most sensitive to the distribution of the nearest neighbor iron vacancies and intensitive to the long range symmetry of the crystal. These arguments and the appearance of only a minimal asymmetry in their observed peak heights led to the identification of the two2’major and peaks as arising split Fe from the distortion nearfrom zeroquadrupole velocity as arising
Móssbauer of various compositions Wüstite at room spectra temperature are shown in Fig. 1.of 1785
MOSSBAUER STUDY OF 57Fe IN FeO
1786
1• 0
1*J1.~_
Vol.7, No.24
— — ~
0.9 X=.930
A
0.8’ A”~
0.7 —
~1’
~! •1 .
A
I
~
/~
X~ .920
0.8
S/
V
0.7
I
.
I
TTh I
‘Aj
X=.910
0.8
0.7
-1.5
-1.0
—0.5
0
0.5
1.0
1.5
Doppler Velocity (min/sec)
FIG. 1. Room temperature Mössbauer absorption spectra of Fe 57Co) source. A and B indicate the two types of Fe 2 quadrupole doublets1 0and absorbers C indicates vs. athe Cu(Fe~ singlet. I
I
a small amount of Fe I
N /
60
-~
50
‘
IRON + WUSTITE
~.
However, they note that
the available data did not eliminate the possibility that up to 30 per cent of the peak near 1.0 mm/sec could be the result of an unsplit Fe 2+ line. Because of the improved resolution of the spectra
WUSTIT
_______________
~
z
,T 40 II
I
I
_________
~“
cannot by ita is single reportedbeindescribed this paper, clearquadrupole that the spectra doublet and a small Fe line near zero velocity, but must be made up of additional lines.
In order to determine whether the absorption
RATIO OF IRON TO OXYGEN (%)
FIG. 2. Fraction of Fe
(A) and Fe 2 (B) as a function of composition. The closed circles correspond to the fraction of Fe2 in A sites and the open circles correspond to the fraction of Fe2~ in B sites. The p—ti composition line is determined from the results of Hillegas and Wagner (Ref. 2) at 1200 C. 2’
spectra are made up of non quadrupole split singlets and/or quadrupole split doublets, an examination of the temperature dependence of the position of these lines was undertaken. The dominate cause of the shift in Mössbauer spectral lines with temperature for ~Fe is the second order Doppler shift, resulting from the temperature
Vol.7, No.24
MOSSBAUER STUDY OF ~Fe IN FeO
1787
Table 1. Quadrupole Splitting and Velocity Shift of Fe2’ (A) and Fe 2+ (B) Fe 2+ (A) mm/sec Fe 2+ (B) mm/sec T°K Q.S. V.S.~’ Q.S. 297 248 218 203 * Velocity
0.46 ±0.04 0.50 0.54 0.58
0.68 ±0.02 0.71 0.71 0.73
0.78 ±0.04 0.84 0.94 1.00
0.63 ±0.02 0.66 0.67 0.70
shift vs. a 297°K Cu(~Co)source.
dependence of the mean square velocity. Over the temperature range of this study, the mean square velocity can be approximated by 3kT/mFe, where k is Boltzmann constant, mFe is the mass of 87Fe, and T is the absolute temperature. Therefore, one would expect the change in position of a singlet or the bisection of a quadrupole doublet to be approximately + 0.07mm/sec upon changing the temperature of the absorber from 297°K to 203°K. A liberal estimate of the quantum corrections to the classical expression for the mean square velocity indicates that this change must be between + 0.07 and + 0.05 mm/sec. The observed change in position of the lines constituting the peak near 0.4 mm/sec is in the negative direction and the observed change in position of the lines constituting the high velocity peak is greater than + 0.15 mm/sec over this temperature interval. Consequently, there is no evidence of singlet lines contributing to the
doublets of Fe2~and a singlet of Fe A summary of the quadrupole splitting and velocity shift are given in Table 1. ~
The reasonably discrete nature of the Mössbauer spectra implies that the point defects are not randomly distributed, as has often been assumed, but in the vicinity of an Fe 2+ ion form predominantly in one of two noncubic arrays. Figure 2 shows the ratio of the two types of Fe2~ ions as a function of composition.t It is evident that there is a change in the majority local environment of the Fe2~ions at the same cornposition as the p—n transition occurs in the electrical properties. Above X = 0.92, one array is predominant, presumably associated with p type conduction, and below this composition, the second array is predominant, presumably associated with n type conduction.
observed spectra other than a Fe ~ line near zero velocity. The detailed shape and temperature
Acknowledgements
dependence of the spectra can be consistently described by assuming two quadrupole split
p—n transition and C.S. Giggins for preparing the Wustite samples.
—
The author would like to
thank R.R. Dils for pointing out the anomalous
REFERENCES 1. 2.
HILLEGAS W.J. and WAGNER J.B., Bull. Am. phys. Soc. 14, 353 (1969). HILLEGAS W.J. and WAGNER J.B., Phys. Lett. 25A, 742 (1967).
3.
TANNHAUSER D.S., J. Phys. Chem. Solids 23, 25 (1962).
4.
DARKEN L.S. and GURRY R.W., J. Am. Chem. Soc. 67, 1398 (1945).
The ratio of the two types of Fe2~ions is determined from the integrated intensities of the constituent spectra by assuming the recoil free fraction of the two sites are equal. This assumption is consistent t
with the observation that the total intensity of the high velocity peak, after correcting for variations in thickness and background, decreases by (3 ±2) per cent upon changing the composition from X = 0.935 to X = 0.905. Because of saturation effects this decrease in total intensity corresponds to (4 ±3) per
cent decrease in the number of Fe2~ions, which is in agreement with the expected conversion of 6 per cent of the Fe2~ions to Fe ~ ions.
MOSSBAUER STUDY OF 57Fe IN FeO
1788 5. 6.
SHECHTER H., HILLMAN P. and RON M., J. appi. Phys. 37, 3043 (1966). SHIRANE G., COX D.E. and RUBY S.L., Phys. Rev. 125, 1158 (1962).
7.
WERTHEIM G.K., J. appl. Phys. 32, 1105 (1961).
Mössbauer Spektren von abgeschrecktem Fe~0 wurden fuer mehrere Zusammensetzungen und Temperatu:en bestimmt. Man kann die Spektren durch zwei quadrupole aufgespaltene Fe 2+ Dubletten und em Fe ~ Singulett beschreiben. Man beobachtet eine Aenderung im
vorherrschenden Typ des Fe2~-Ionsbei derselben Zusamrnensetzung wo der p—n Uebergang in den elektrischen Eigenschaften erscheint.
Vol.7, No.24
Vol. 7, Pp. 1785—1788, 1969
Pergamon Press.
Printed in Great Britain
MOSSBAUER STUDY OF THE LOCAL ENVIRONMENTS OF ~Fe IN FeO D.P. Johnson Advanced Materials Research and Development Laboratory, Pratt and Whitney Aircraft, Middletown, Connecticut 06458 (Received 4 August 1969 by J.A. Krumhansl)
Mössbauer spectra of quenched Fe~Ohave been obtained for several compositions and temperatures. The aspectra can be described bythe two 2~doublets and F&~ singlet. A change in quadrupole split Fe type of predominant Fe2~ion is observed at the same composition that the p—n transition is observed in the electrical properties.
FeO IS CONSIDERED to have a NaC1 structure with at least four atomic per cent cation vacancies, Recent measurements of the Seebeck coefficient of Fe~Oat elevated temperature indicate that wustite undergoes a p—n transition as the cation deficiency increases.’ ~ Although the composition at which this transition occurs is sensitive to impurities, recent measurements indicate that the p—n transition occurs at X 0.92.’ ,2 The nature of this transition is not understood and previous models for Wüstite do not predict this behavior. The Móssbauer study reported here was undertaken to gain additional information on the nature of this transition. Spectra were obtained on quenched samples of various compositions ranging from X = 0.935 to X = 0.905 and for various temperatures ranging from 297°K to 203°K.The Fe~Osamples were prepared from 0.001 cm thick 99.996 per cent iron foils by heating in a controlled C0 2—CO atmosphere at 1200°Cfor 2hr and then quenching to room ternperature. Theand composition was weight gain agreed with thatdetermined predicted by by Darken and Curry4 for the C0 2—CO ratio used. X-ray diffraction lines obtained from the samples are sharp and indicate only the W~istitephase is present.
In agreement with previous measurements, we observe two partially resolved peaks separated by approximately 0.6 mm/sec and a distortion in the wing near zero velocity.5—7 However, the more acute spectra reported here exhibit a definite asymmetry in the peak heights, which is independent of absorber orientation, and additional structure in the line near 0.4 mm/sec. In a preliminary study on this material, Wertheim7 identified the peak near 0.4 mm/sec with tnvalent iron and the peak near 1.0 mm/sec with di-valent iron by reasoning that the X-ray and neutron evidence that wüstite is cubic and the unequal intensity of the two peaks eliminated quadrupole splitting as a possible interpretation. Shirane, Cox and Ruby6 observed that the strength of the 0.4 mm/sec peak was nearly independent of composition and hence could not be identified with Fe3’. In addition, they pointed out that the cation vacancies could easily remove the local cubic symmetry at the iron nucleus. Since the spatial dependence of the quadrupole field from 3, the quadrupole a point charge decreases as R splitting would be most sensitive to the distribution of the nearest neighbor iron vacancies and intensitive to the long range symmetry of the crystal. These arguments and the appearance of only a minimal asymmetry in their observed peak heights led to the identification of the two2’major and peaks as arising split Fe from the distortion nearfrom zeroquadrupole velocity as arising
Móssbauer of various compositions Wüstite at room spectra temperature are shown in Fig. 1.of 1785
MOSSBAUER STUDY OF 57Fe IN FeO
1786
1• 0
1*J1.~_
Vol.7, No.24
— — ~
0.9 X=.930
A
0.8’ A”~
0.7 —
~1’
~! •1 .
A
I
~
/~
X~ .920
0.8
S/
V
0.7
I
.
I
TTh I
‘Aj
X=.910
0.8
0.7
-1.5
-1.0
—0.5
0
0.5
1.0
1.5
Doppler Velocity (min/sec)
FIG. 1. Room temperature Mössbauer absorption spectra of Fe 57Co) source. A and B indicate the two types of Fe 2 quadrupole doublets1 0and absorbers C indicates vs. athe Cu(Fe~ singlet. I
I
a small amount of Fe I
N /
60
-~
50
‘
IRON + WUSTITE
~.
However, they note that
the available data did not eliminate the possibility that up to 30 per cent of the peak near 1.0 mm/sec could be the result of an unsplit Fe 2+ line. Because of the improved resolution of the spectra
WUSTIT
_______________
~
z
,T 40 II
I
I
_________
~“
cannot by ita is single reportedbeindescribed this paper, clearquadrupole that the spectra doublet and a small Fe line near zero velocity, but must be made up of additional lines.
In order to determine whether the absorption
RATIO OF IRON TO OXYGEN (%)
FIG. 2. Fraction of Fe
(A) and Fe 2 (B) as a function of composition. The closed circles correspond to the fraction of Fe2 in A sites and the open circles correspond to the fraction of Fe2~ in B sites. The p—ti composition line is determined from the results of Hillegas and Wagner (Ref. 2) at 1200 C. 2’
spectra are made up of non quadrupole split singlets and/or quadrupole split doublets, an examination of the temperature dependence of the position of these lines was undertaken. The dominate cause of the shift in Mössbauer spectral lines with temperature for ~Fe is the second order Doppler shift, resulting from the temperature
Vol.7, No.24
MOSSBAUER STUDY OF ~Fe IN FeO
1787
Table 1. Quadrupole Splitting and Velocity Shift of Fe2’ (A) and Fe 2+ (B) Fe 2+ (A) mm/sec Fe 2+ (B) mm/sec T°K Q.S. V.S.~’ Q.S. 297 248 218 203 * Velocity
0.46 ±0.04 0.50 0.54 0.58
0.68 ±0.02 0.71 0.71 0.73
0.78 ±0.04 0.84 0.94 1.00
0.63 ±0.02 0.66 0.67 0.70
shift vs. a 297°K Cu(~Co)source.
dependence of the mean square velocity. Over the temperature range of this study, the mean square velocity can be approximated by 3kT/mFe, where k is Boltzmann constant, mFe is the mass of 87Fe, and T is the absolute temperature. Therefore, one would expect the change in position of a singlet or the bisection of a quadrupole doublet to be approximately + 0.07mm/sec upon changing the temperature of the absorber from 297°K to 203°K. A liberal estimate of the quantum corrections to the classical expression for the mean square velocity indicates that this change must be between + 0.07 and + 0.05 mm/sec. The observed change in position of the lines constituting the peak near 0.4 mm/sec is in the negative direction and the observed change in position of the lines constituting the high velocity peak is greater than + 0.15 mm/sec over this temperature interval. Consequently, there is no evidence of singlet lines contributing to the
doublets of Fe2~and a singlet of Fe A summary of the quadrupole splitting and velocity shift are given in Table 1. ~
The reasonably discrete nature of the Mössbauer spectra implies that the point defects are not randomly distributed, as has often been assumed, but in the vicinity of an Fe 2+ ion form predominantly in one of two noncubic arrays. Figure 2 shows the ratio of the two types of Fe2~ ions as a function of composition.t It is evident that there is a change in the majority local environment of the Fe2~ions at the same cornposition as the p—n transition occurs in the electrical properties. Above X = 0.92, one array is predominant, presumably associated with p type conduction, and below this composition, the second array is predominant, presumably associated with n type conduction.
observed spectra other than a Fe ~ line near zero velocity. The detailed shape and temperature
Acknowledgements
dependence of the spectra can be consistently described by assuming two quadrupole split
p—n transition and C.S. Giggins for preparing the Wustite samples.
—
The author would like to
thank R.R. Dils for pointing out the anomalous
REFERENCES 1. 2.
HILLEGAS W.J. and WAGNER J.B., Bull. Am. phys. Soc. 14, 353 (1969). HILLEGAS W.J. and WAGNER J.B., Phys. Lett. 25A, 742 (1967).
3.
TANNHAUSER D.S., J. Phys. Chem. Solids 23, 25 (1962).
4.
DARKEN L.S. and GURRY R.W., J. Am. Chem. Soc. 67, 1398 (1945).
The ratio of the two types of Fe2~ions is determined from the integrated intensities of the constituent spectra by assuming the recoil free fraction of the two sites are equal. This assumption is consistent t
with the observation that the total intensity of the high velocity peak, after correcting for variations in thickness and background, decreases by (3 ±2) per cent upon changing the composition from X = 0.935 to X = 0.905. Because of saturation effects this decrease in total intensity corresponds to (4 ±3) per
cent decrease in the number of Fe2~ions, which is in agreement with the expected conversion of 6 per cent of the Fe2~ions to Fe ~ ions.
MOSSBAUER STUDY OF 57Fe IN FeO
1788 5. 6.
SHECHTER H., HILLMAN P. and RON M., J. appi. Phys. 37, 3043 (1966). SHIRANE G., COX D.E. and RUBY S.L., Phys. Rev. 125, 1158 (1962).
7.
WERTHEIM G.K., J. appl. Phys. 32, 1105 (1961).
Mössbauer Spektren von abgeschrecktem Fe~0 wurden fuer mehrere Zusammensetzungen und Temperatu:en bestimmt. Man kann die Spektren durch zwei quadrupole aufgespaltene Fe 2+ Dubletten und em Fe ~ Singulett beschreiben. Man beobachtet eine Aenderung im
vorherrschenden Typ des Fe2~-Ionsbei derselben Zusamrnensetzung wo der p—n Uebergang in den elektrischen Eigenschaften erscheint.
Vol.7, No.24