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Mössbauer study of α-Fe2O3 under ultra-high pressure
Physica 139 & 140B (1986) 495-498 North-Holland, Amsterdam
MOSSBAUER STUDY OF a-Fe203 UNDER ULTRA-HIGH PRESSURE K. KURIMOTO, 1 S. NASU, 2 S. NAGATOMO, 1 S. ENDO 1 and F.E. FUJITA 1 1High Pressure Research Laboratory and 2Department of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan Utilizing the diamond anvil cell, the 57Fe M6ssbauer measurements under high pressure have been performed for a-Fe20 3 up to 72 GPa at room temperature. M6ssbauer spectra above 50 GPa indicated the transition from the usual spin-flip hematite phase to the high pressure phase, which is characterized by the superimposition of the doublet and sextet in the M6ssbauer spectrum, and coincided with the spectrum reported recently by Syono et al. The critical pressure of transformation to this phase was determined to be 52 GPa at room temperature by pressure distribution measurements using the ruby fine powder and laser microscopic systems. At the high pressure phase above 52 GPa the iron has at least two different kinds of electronic states at room temperature, which are shown as the doublet and sextet and a gradual transformation from the magnetically ordered state (sextet) to the paramagnetic state (doublet) occurred with an increase in pressure.
The high pressure phase transition of ct-Fe20 3 (hematite) above 50 GPa was observed in shock compression measurements, X-ray diffraction study and 57Fe M6ssbauer measurements [1-3]. Syono and his co-workers [3] reported recently the 57Fe M6ssbauer spectra of a - F e 2 0 3 under high pressure up to 60 GPa using a diamond anvil cell and found that the spectra of the high pressure phase consisted of at least two components, which were not expected from the simple spin-pairing transition model [9]. 57 In this paper we present the Fe M6ssbauer measurements of a - F e 2 0 3 up to 72 GPa using a diamond anvil cell. The present results agree well with the results reported previously and the critical pressure was determined to be 52 GPa for the transition from the usual spin-flip phase to the new high pressure phase. The pressure dependences of the high pressure phase have been discussed above 60 GPa and it was found that the gradual transformation of the iron states occurred from magnetic (6-line splitting) to paramagnetic (doublet) iron states. 57Fe M6ssbauer measurements were performed in transmission geometry using 57Fe enriched ORNL a - F e 2 0 3 absorbers placed in a diamond anvil cell and a y-ray source of 25 mCi 57Co in Rh matrix. The diamond anvil cell was the Bassett type [4], as shown in fig. 1. Specimen powders
C.
I
I
I cm a
Tungsten carbide half cylinder
b. Diamond anvits c. Adjusting screws d, Piston e. Driving nut
Fig. 1. A modified Bassett-type diamond anvil cell. Specimen powders are placed in the hole of a Waspalloy gasket between the flat parallel faces of two opposing diamond anvils. The path and direction of the "),-ray in M6ssbauer measurements are also shown.
and the mixture of methanol, ethanol and water (16: 3 : 1) were placed in a 0.15-mm diameter hole of a Waspalloy gasket between the flat parallel faces of two opposing diamond anvils. Ruby powders were also inserted into the hole in order to determine the pressure distributions. The Waspalloy gasket contained small amounts of iron as one of the alloying elements (0.35%), but did not
0378-4363/86/$03.50 ~) Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
K. Kurimoto et al. / Study o f c~-Fe20 ~ under ultra-high pressure
496
show any contributions to the M6ssbauer spectrum. The pressure distributions inside the hole were determined using the place scanning method by the ruby fluorescence utilizing the laser microscopic system [5]. Typical size of the ruby powders was about 1 I~m and the diameter of the laser light, which corresponds to the area of pressure measurement, was about 10 t~m. Fig. 2 shows typical results from measuring the distribution of the pressure. The upper part shows a sketch of the specimen and the gasket observed in microscopy indicating the distribution of the pressure. The dotted area shows the Waspalloy gasket and the cross points show the places excited by the laser light. Numerical values indicate the magnitude of the pressure and the solid lines show the contour of the resultant pressure obtained. A histogram of the pressure determined is shown in
,i
/
\
~
iVlax 73.2 GPo
68B 59,;X 7r 1
701N
/,%/ L/
-.4
,./ //
Xx \
gasket
// / < 0.1 mm
\ \\,
AVERAGEPRESSU E -I 130 30 < nn 20 O 10 nO_ 0
692-*25 GPo
~"
65
70
75
PRESSURE (G Pa) Fig. 2. Distribution of the pressure inside a hole of the gasket. Cross points show the positions to measure the pressure by ruby fluorescence in laser microscopic system. Numerical values show the resultant pressure, and solid lines show the contour map of the pressure. The lower figure shows the histogram obtained by the analysis shown in the upper figure. The average pressure was determined to be (69.2 ± 2.5) GPa.
the lower part of fig. 2 and the average pressure was determined to be 69 GPa. All of the M6ssbauer measurements was performed at room t e m p e r a t u r e using a combination of a conventional spectrometer and a multichannel analyser operated by time mode. The pressure dependences on the M6ssbauer spectrum of ~ - F e 2 0 3 were measured from 50 G P a up to 72 GPa. The values were averaged by using the histogram analysis as shown in fig. 1. The spectrum under 5 0 G P a is shown as a superimposition of magnetically split 6-line absorption lines and the paramagnetic absorption at the center portion of the spectrum. The hyperfine field of the 6-line splitting was about 50 T at room temperature and suggested the existence of a pressure induced spin-flip hematite phase [6], which was extensively studied recently by Bruzzone and Ingalls [7]. The contribution of the central paramagnetic c o m p o n e n t to the total absorption was found to be 23% by the least-squares fitting procedure, assuming the existence of the two components. As discussed by Syono et al., the superimposition of the spin-flip hematite spectrum and the paramagnetic central absorption line is due to the pressure gradient within the cell. The pressure distribution analysis using the histogram method showed that 22% of the specimens are under a pressure between 50 and 55 GPa. The spectrum under 56 G P a showed the superimposition of three different spectra, namely the spin-flip hematite, the paramagnetic component and an additional magnetic component. The additional magnetic c o m p o n e n t showed a rather small hyperfine field which was 31.5 T at 56 GPa. At 61 G P a the spectrum consisted of only two components, namely the paramagnetic doublet and the magnetically split 6-line absorption (sextet) whose hyperfine field was 31.0T. The area contribution of the two components at 56 G P a was 87% in total absorption and corresponds to the iron at above 52 G P a in the pressure histogram. At 61 G P a the minimum pressure in the cell was 5 3 G P a and all of the iron transformed into the high pressure phase. The above results suggest that the critical pressure to transform into the high pressure phase was determined
K. Kurimoto et al. / Study of a-Fe203 under ultra-high pressure
497 ! i
1.00
O'3
co
0.98
z
0.96
Q2 C~
LtJ >
69.2 • 2.5 G Pa q
O.94
0.£2 J LJ OC
0.90
-8
I -8
E -4
I -2
VEL~3CITT
I 0 IN
t 2
I 4
t G
~ 8
I
J
MM/SEC
Fig. 3. Typical 57Fe M 6 s s b a u e r s p e c t r u m o b t a i n e d f r o m the high p r e s s u r e p h a s e at 69 G P a of h e m a t i t e . T h e s p e c t r u m was d e c o m p o s e d into two c o m p o n e n t s : one is the magnetically split 6-line absorption and the o t h e r is the p a r a m a g n e t i c doublet.
to be 52GPa at room temperature. From the M6ssbauer spectra the transformation occurred abruptly at this pressure, like a first-order transition. The spectra obtained under 69 and 72 GPa showed only the spectrum of the high pressure phase which is characterized by the superimposition of magnetically split 6-line and paramagnetic doublet. The magnitude of the hyperfine fields of 6-line splitting were about 32 T and did not show any appreciable pressure dependence up to 72 GPa. The magnitudes of the quadrupole splitting observed from the paramagnetic doublet ranged from 0.83 to 1.07 mm/s and did not show any appreciable pressure dependence up to 72 GPa. The results mentioned above suggest that there are two iron states in the high pressure phase. Unfortunately, however, there is no clear evidence for the existence of the structural change in hematite above 50 GPa. A typical M6ssbauer spectrum obtained from the high pressure phase at 69 GPa is shown in fig. 3. The solid line shows the result from the least-squares fit assuming the existence of two components. The dotted lines show the partial components assuming the magnetically split 6-line (sextet) and the paramagnetic doublet, respectively. The central doublet was tried to obtain a fit using a magnetically split 6-line (sextet) having an
extremely small hyperfine field, but a reasonable fit was not obtained. The full widths at halfmaximum of each absorption line are extremely large, ranging from 0.8 to 1.4mm/s, and the reduced chi-square values was also rather large. These broad lines may be due to the distributions of the hyperfine fields and of the isomer shift values, but they did not show any appreciable change by the pressure up to 72 GPa. The area contribution of the sextet to the total absorption was 35% at 69 GPa and gradually decreased from 45% to 25% with an increase in the pressure from 61 GPa to 72 GPa. The iron states at the high pressure phase transformed gradually from the magnetically ordered state to the paramagnetic state, which is quite contrary to the transformation observed abruptly at 52 GPa. One can expect that the gradual transformation is due to a change in the magnetically ordered temperature (decrease of the N6el temperature) with pressure. However, the isomer shift values of the sextet and doublet were 0.31-0.40 mm/s and 0.00-0.14 mm/ s relative to a-Fe, and appreciably different from each other to suggest the difference in charge states for the iron atoms. From the isomer shift values the sextet may be due to the ferrous Fe, as suggested by Syono et al. [3]. Two reasons for the creation of ferrous Fe from
498
K. Kurimoto et al. / Study of a-Fe203 under ultra-high pressure
ferric Fe with pressure may be considered: one is the reduction of the ferric Fe by the electron transfer from ligands to the Fe 3d orbital, and the other is the disproportionate reaction of the ferric Fe to Fe2+Fe4+O3, as proposed by Ringwood et al. [8]. However, the existence of Fe 4÷ is not acceptable, since the area ratio of the sextet and doublet deviated largely from unity and depends on the pressure, something which cannot be explained by the difference in recoil-free fraction. M6ssbauer parameters are not always an unequivocal identification of the state, but the present results suggest that the existence of the complex iron spin and charge states in the high pressure phase above 52 GPa, which might be ferrous, were also changed by a pressure above 60 GPa. In conclusion, we performed the 57Fe M6sbauer measurements of a - F e 2 0 3 (hematite) up to 72 GPa and the spectra of the high pressure phase were obtained above 52 GPa. The spectrum of the high pressure phase showed at least two different kinds of iron state, charcterized by the sextet and doublet. A gradual transformation
from the sextet to the doublet occurred above 60GPa with an increase in pressure, which is contrary to the transformation at 52 GPa observed as an abrupt change in the spectrum.
References [1] R.G. McQueen and S.P. Marsh, in: Handbook of Physical Constants, S.P. Clark Jr., ed. (Geol. Soc. Amer. Memoir 97, New York, 1966) p. 153. [2] T. Yagi and S. Akimoto, in: High-Pressure Research in Geophysics, S. Akimoto and M.H. Manghnani, eds. (Center Acad. Publ. Jpn., Tokyo, 1982) p. 81. [3] Y. Syono, A. Ito, S. Morimoto, T. Suzuki, T. Yagi and S. Akimoto, Solid State Commun. 50 (1984) 97. [4] W.A. Bassett, T. Takahashi and EW. Stook, Rev. Sci. Instrum. 38 (1967) 37. [5] P.M. Bell and H.K. Mao, Carnegie Inst. Wash. Yearbook 74 (1975) 399. [6] R.W. Vaugham and H.G. Drickamer, J. Chem. Phys. 47 (1967) 1530. [7] C.L. Bruzzone and R. Ingalls, Phys. Rev. B 28 (1983) 2430. [8] A.F. Reid and A.E. Ringwood, J. Geophys. Res. 74 (1969) 3238. [9] S. Ohnishi, Phys. Earth Planet. Inter. 17 (1978) 130.
MOSSBAUER STUDY OF a-Fe203 UNDER ULTRA-HIGH PRESSURE K. KURIMOTO, 1 S. NASU, 2 S. NAGATOMO, 1 S. ENDO 1 and F.E. FUJITA 1 1High Pressure Research Laboratory and 2Department of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan Utilizing the diamond anvil cell, the 57Fe M6ssbauer measurements under high pressure have been performed for a-Fe20 3 up to 72 GPa at room temperature. M6ssbauer spectra above 50 GPa indicated the transition from the usual spin-flip hematite phase to the high pressure phase, which is characterized by the superimposition of the doublet and sextet in the M6ssbauer spectrum, and coincided with the spectrum reported recently by Syono et al. The critical pressure of transformation to this phase was determined to be 52 GPa at room temperature by pressure distribution measurements using the ruby fine powder and laser microscopic systems. At the high pressure phase above 52 GPa the iron has at least two different kinds of electronic states at room temperature, which are shown as the doublet and sextet and a gradual transformation from the magnetically ordered state (sextet) to the paramagnetic state (doublet) occurred with an increase in pressure.
The high pressure phase transition of ct-Fe20 3 (hematite) above 50 GPa was observed in shock compression measurements, X-ray diffraction study and 57Fe M6ssbauer measurements [1-3]. Syono and his co-workers [3] reported recently the 57Fe M6ssbauer spectra of a - F e 2 0 3 under high pressure up to 60 GPa using a diamond anvil cell and found that the spectra of the high pressure phase consisted of at least two components, which were not expected from the simple spin-pairing transition model [9]. 57 In this paper we present the Fe M6ssbauer measurements of a - F e 2 0 3 up to 72 GPa using a diamond anvil cell. The present results agree well with the results reported previously and the critical pressure was determined to be 52 GPa for the transition from the usual spin-flip phase to the new high pressure phase. The pressure dependences of the high pressure phase have been discussed above 60 GPa and it was found that the gradual transformation of the iron states occurred from magnetic (6-line splitting) to paramagnetic (doublet) iron states. 57Fe M6ssbauer measurements were performed in transmission geometry using 57Fe enriched ORNL a - F e 2 0 3 absorbers placed in a diamond anvil cell and a y-ray source of 25 mCi 57Co in Rh matrix. The diamond anvil cell was the Bassett type [4], as shown in fig. 1. Specimen powders
C.
I
I
I cm a
Tungsten carbide half cylinder
b. Diamond anvits c. Adjusting screws d, Piston e. Driving nut
Fig. 1. A modified Bassett-type diamond anvil cell. Specimen powders are placed in the hole of a Waspalloy gasket between the flat parallel faces of two opposing diamond anvils. The path and direction of the "),-ray in M6ssbauer measurements are also shown.
and the mixture of methanol, ethanol and water (16: 3 : 1) were placed in a 0.15-mm diameter hole of a Waspalloy gasket between the flat parallel faces of two opposing diamond anvils. Ruby powders were also inserted into the hole in order to determine the pressure distributions. The Waspalloy gasket contained small amounts of iron as one of the alloying elements (0.35%), but did not
0378-4363/86/$03.50 ~) Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
K. Kurimoto et al. / Study o f c~-Fe20 ~ under ultra-high pressure
496
show any contributions to the M6ssbauer spectrum. The pressure distributions inside the hole were determined using the place scanning method by the ruby fluorescence utilizing the laser microscopic system [5]. Typical size of the ruby powders was about 1 I~m and the diameter of the laser light, which corresponds to the area of pressure measurement, was about 10 t~m. Fig. 2 shows typical results from measuring the distribution of the pressure. The upper part shows a sketch of the specimen and the gasket observed in microscopy indicating the distribution of the pressure. The dotted area shows the Waspalloy gasket and the cross points show the places excited by the laser light. Numerical values indicate the magnitude of the pressure and the solid lines show the contour of the resultant pressure obtained. A histogram of the pressure determined is shown in
,i
/
\
~
iVlax 73.2 GPo
68B 59,;X 7r 1
701N
/,%/ L/
-.4
,./ //
Xx \
gasket
// / < 0.1 mm
\ \\,
AVERAGEPRESSU E -I 130 30 < nn 20 O 10 nO_ 0
692-*25 GPo
~"
65
70
75
PRESSURE (G Pa) Fig. 2. Distribution of the pressure inside a hole of the gasket. Cross points show the positions to measure the pressure by ruby fluorescence in laser microscopic system. Numerical values show the resultant pressure, and solid lines show the contour map of the pressure. The lower figure shows the histogram obtained by the analysis shown in the upper figure. The average pressure was determined to be (69.2 ± 2.5) GPa.
the lower part of fig. 2 and the average pressure was determined to be 69 GPa. All of the M6ssbauer measurements was performed at room t e m p e r a t u r e using a combination of a conventional spectrometer and a multichannel analyser operated by time mode. The pressure dependences on the M6ssbauer spectrum of ~ - F e 2 0 3 were measured from 50 G P a up to 72 GPa. The values were averaged by using the histogram analysis as shown in fig. 1. The spectrum under 5 0 G P a is shown as a superimposition of magnetically split 6-line absorption lines and the paramagnetic absorption at the center portion of the spectrum. The hyperfine field of the 6-line splitting was about 50 T at room temperature and suggested the existence of a pressure induced spin-flip hematite phase [6], which was extensively studied recently by Bruzzone and Ingalls [7]. The contribution of the central paramagnetic c o m p o n e n t to the total absorption was found to be 23% by the least-squares fitting procedure, assuming the existence of the two components. As discussed by Syono et al., the superimposition of the spin-flip hematite spectrum and the paramagnetic central absorption line is due to the pressure gradient within the cell. The pressure distribution analysis using the histogram method showed that 22% of the specimens are under a pressure between 50 and 55 GPa. The spectrum under 56 G P a showed the superimposition of three different spectra, namely the spin-flip hematite, the paramagnetic component and an additional magnetic component. The additional magnetic c o m p o n e n t showed a rather small hyperfine field which was 31.5 T at 56 GPa. At 61 G P a the spectrum consisted of only two components, namely the paramagnetic doublet and the magnetically split 6-line absorption (sextet) whose hyperfine field was 31.0T. The area contribution of the two components at 56 G P a was 87% in total absorption and corresponds to the iron at above 52 G P a in the pressure histogram. At 61 G P a the minimum pressure in the cell was 5 3 G P a and all of the iron transformed into the high pressure phase. The above results suggest that the critical pressure to transform into the high pressure phase was determined
K. Kurimoto et al. / Study of a-Fe203 under ultra-high pressure
497 ! i
1.00
O'3
co
0.98
z
0.96
Q2 C~
LtJ >
69.2 • 2.5 G Pa q
O.94
0.£2 J LJ OC
0.90
-8
I -8
E -4
I -2
VEL~3CITT
I 0 IN
t 2
I 4
t G
~ 8
I
J
MM/SEC
Fig. 3. Typical 57Fe M 6 s s b a u e r s p e c t r u m o b t a i n e d f r o m the high p r e s s u r e p h a s e at 69 G P a of h e m a t i t e . T h e s p e c t r u m was d e c o m p o s e d into two c o m p o n e n t s : one is the magnetically split 6-line absorption and the o t h e r is the p a r a m a g n e t i c doublet.
to be 52GPa at room temperature. From the M6ssbauer spectra the transformation occurred abruptly at this pressure, like a first-order transition. The spectra obtained under 69 and 72 GPa showed only the spectrum of the high pressure phase which is characterized by the superimposition of magnetically split 6-line and paramagnetic doublet. The magnitude of the hyperfine fields of 6-line splitting were about 32 T and did not show any appreciable pressure dependence up to 72 GPa. The magnitudes of the quadrupole splitting observed from the paramagnetic doublet ranged from 0.83 to 1.07 mm/s and did not show any appreciable pressure dependence up to 72 GPa. The results mentioned above suggest that there are two iron states in the high pressure phase. Unfortunately, however, there is no clear evidence for the existence of the structural change in hematite above 50 GPa. A typical M6ssbauer spectrum obtained from the high pressure phase at 69 GPa is shown in fig. 3. The solid line shows the result from the least-squares fit assuming the existence of two components. The dotted lines show the partial components assuming the magnetically split 6-line (sextet) and the paramagnetic doublet, respectively. The central doublet was tried to obtain a fit using a magnetically split 6-line (sextet) having an
extremely small hyperfine field, but a reasonable fit was not obtained. The full widths at halfmaximum of each absorption line are extremely large, ranging from 0.8 to 1.4mm/s, and the reduced chi-square values was also rather large. These broad lines may be due to the distributions of the hyperfine fields and of the isomer shift values, but they did not show any appreciable change by the pressure up to 72 GPa. The area contribution of the sextet to the total absorption was 35% at 69 GPa and gradually decreased from 45% to 25% with an increase in the pressure from 61 GPa to 72 GPa. The iron states at the high pressure phase transformed gradually from the magnetically ordered state to the paramagnetic state, which is quite contrary to the transformation observed abruptly at 52 GPa. One can expect that the gradual transformation is due to a change in the magnetically ordered temperature (decrease of the N6el temperature) with pressure. However, the isomer shift values of the sextet and doublet were 0.31-0.40 mm/s and 0.00-0.14 mm/ s relative to a-Fe, and appreciably different from each other to suggest the difference in charge states for the iron atoms. From the isomer shift values the sextet may be due to the ferrous Fe, as suggested by Syono et al. [3]. Two reasons for the creation of ferrous Fe from
498
K. Kurimoto et al. / Study of a-Fe203 under ultra-high pressure
ferric Fe with pressure may be considered: one is the reduction of the ferric Fe by the electron transfer from ligands to the Fe 3d orbital, and the other is the disproportionate reaction of the ferric Fe to Fe2+Fe4+O3, as proposed by Ringwood et al. [8]. However, the existence of Fe 4÷ is not acceptable, since the area ratio of the sextet and doublet deviated largely from unity and depends on the pressure, something which cannot be explained by the difference in recoil-free fraction. M6ssbauer parameters are not always an unequivocal identification of the state, but the present results suggest that the existence of the complex iron spin and charge states in the high pressure phase above 52 GPa, which might be ferrous, were also changed by a pressure above 60 GPa. In conclusion, we performed the 57Fe M6sbauer measurements of a - F e 2 0 3 (hematite) up to 72 GPa and the spectra of the high pressure phase were obtained above 52 GPa. The spectrum of the high pressure phase showed at least two different kinds of iron state, charcterized by the sextet and doublet. A gradual transformation
from the sextet to the doublet occurred above 60GPa with an increase in pressure, which is contrary to the transformation at 52 GPa observed as an abrupt change in the spectrum.
References [1] R.G. McQueen and S.P. Marsh, in: Handbook of Physical Constants, S.P. Clark Jr., ed. (Geol. Soc. Amer. Memoir 97, New York, 1966) p. 153. [2] T. Yagi and S. Akimoto, in: High-Pressure Research in Geophysics, S. Akimoto and M.H. Manghnani, eds. (Center Acad. Publ. Jpn., Tokyo, 1982) p. 81. [3] Y. Syono, A. Ito, S. Morimoto, T. Suzuki, T. Yagi and S. Akimoto, Solid State Commun. 50 (1984) 97. [4] W.A. Bassett, T. Takahashi and EW. Stook, Rev. Sci. Instrum. 38 (1967) 37. [5] P.M. Bell and H.K. Mao, Carnegie Inst. Wash. Yearbook 74 (1975) 399. [6] R.W. Vaugham and H.G. Drickamer, J. Chem. Phys. 47 (1967) 1530. [7] C.L. Bruzzone and R. Ingalls, Phys. Rev. B 28 (1983) 2430. [8] A.F. Reid and A.E. Ringwood, J. Geophys. Res. 74 (1969) 3238. [9] S. Ohnishi, Phys. Earth Planet. Inter. 17 (1978) 130.