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X-Ray absorption studies of selective site substitution by 3d transition metals in Fe3Si
31
Physica B 158 (1989) 31-33 North-Holland, Amsterdam
X-Ray Absorption in Fe,Si
Studies of Selective Site Substitution
J. I. Budnick, Zhengquan Tan and D. M. Pease Department of Physics and Institute of Materials Connecticut, Storrs, CT 06268
by 3d Transition
Science,
Metals
University
of
Our K-edge XANES spectra of V, Mn, Fe and Co in Fe3_,T,Si (T= V, Mn, and Co) alloys demonstrate straightforwardly and unambiguously that V and Mn selectively substitute for the Fe atoms in the Fe(B) site and that Co atoms substitute for the Fe(A,C) atoms. This is in good agreement with results obtained by other techniques. The EXAFS results reveal quantitatively the selective site substitution and prove that a strong degree of structural order exists up to high concentrations for V and Co substitutions. Introduction Fe3Si is ferromagnetic and crystallographically well ordered with an fee DO, structure. This structure has four sites A, B, C, and D, with the specific near neighbor configurations summarized in table 1. The Fe atoms are located at the two chemically and magnetically distinct (A,C) and (B) sites with magnetic moments of 1.35~~ and 2.20 Brespectively. It was determined from extensive NM R studies [ 21, neutron diffraction fl3 and Mijssbauer experiments 141 that transition metal impurities to the left of Fe in the periodic table (Ti, V, Cr and Mn) selectively substitute for the Fe(B) atoms and those below and to the right of P’e (Co, Ni, Ru, Rh, Pd, OS, and Ir) substitute for the Fe(A,C) atoms. We report here an X-ray absorption study of the site selective substitution in Fe3Si and the local environment of the substituted sites. Table 1. Fej_,T,Si neighbor configurations (from ref. 121). shells R (a)
I
A,C
1 0.43
oT50
0.3705
4 0.83
5 0.86
I.:0
4B 4D
6A,C
L!A,c
12B 12D
8A,C
6A,C
B
8A,C
6D
12B
24A,C
8D
6B
D
8A.C
6B
12D
24A,C
8B
6D
I
Experiments Samples were prepared by arc melting. Details of the preparation method can be found in ref.[21. Transmission and fluorescence data were collected on powdered samples at room temperature. The monochromator used double Si(lI1) crystals. The energy resolution is estimated to be 2.5ev at 9000ev. Results and Discussion Fig. 1 shows the K-edge XANES of Fe in FeCo$i, Mn in Fe, 88Mn0,12Si, V in Fe2.25V0.7& Fe2.96V0.04Sil and in Fe2.98VO.&i . The energy origin is set at the first inflection point. All spectra in Fig. 1 are very similar except that the fine structure features for Mn are less pronounced. This reveals similar local structural environments of Fe, Mn and V in the corresponding samples. The XANES spectra of Fe (Fig. la) and Co (Fig. 2a) of FeCo2Si are much different. Thus the Co atoms do not enter the (A,C) @ Elsevier Science Publishers B.V. Physics Publishing Division)
0921-4526/89/$03.50
(North-Holland
J.I. Budnick
32
et al./Selective
site substitution
and (B) sites randomly. In addition, the Co XANES spectra of FeCozSi and Fe2.1C00,9Si are extremely, similar. Therefore the Co atoms must have substituted for only one Fe site and this site can only be the (A,C) site. The Fe atoms left in FeCO,Si are located at the (B) sites only. By their rescmblancc to the Fe spectra of FcCo,Si, Mn and V spectra are assigned to the (B) site. Therefore our XAhES spectra demonstrate straightforwardly and unambiguously that Co selectively substitute for Fe(A,C) atoms and V and Mn substitute for Fe(B). The XANES spectra can be used as fingerprints of the local environment of a specific atom site. To further illustrate this, wc plot in Fig.2 the XANES spectra of Co in FcCo,Si, Fe in FeCozSi , Fe in bee metal and Fc in Fe0 02Ni0.98. A common energy origin of the bee Fe metal is used for all Fe spectra. No ‘significant edge shift is observed in FeCo2Si and Fe0,0zNi0,98. There are similarities and also differences in the near neighbor configurations around the absorber in these samples. These XANES spectra from well defined structures should be useful for testing theoretical models developed to explain near edge structures. In order to attempt this, the above XANES spectra will be measured with higher energy resolutions. 2.51
2.5
I-
-20
0
20
40
E (ev>
60
-20
0
20
40
60
E (ev>
Fig. 1 (left) K-edge XANES spectra. (a) Fe in FeCo$i, (b) Mn in Fe,.,,MnO.,,Si, (c) V in %.2sV0.+i I (4 V h Fe2.96VcmSi 7(e) V h-~FepaVo.gGi . Fig. 2 (nght) K-edge XANES spectra. (a) Co in FeCo2S1, (b) Fe in FeCo$i, (c) bee Fe, (d) Fe iq Ni0.98Fe0.02. the k2X data of substituted Fe3Si. The Fe in FeCo2Si, Mn in in Fe2.25 V0.75 Si arc in (B) sites and their EXAFS are determined by (B)-site local environment. Similarly the Co EXAFS in FeCo,Si corresponds to the (A,C) atomic environment. The Fourier transform filtered EXAFS were analyzed with non-linear fitting using the single scattering EXAFS formula. Experimentally determined phase and amplitude from CoSi2 (CaF2 structure) and Co foil (hcp structure) arc respectively used for Co-Si and Co-Co, Co-Fe and Fe-Co atom pairs. Fig. 3 presents Fe2.88Mn0.12Si,
v
According to table 1 the Co(A,C) in FeCo*Si has 4 Fe plus 4 Si in the 1st shell and 6 Co in the 2nd shell. The combined 2 shell EXAFS data fit very well to this near neighbor configuration (Fig. 4b). The near neighbor distances all agree with the diffraction values. The Fe EXAFS in FeCo2Si fits the (B)-site configuration excellently. There is no evidence of distortion around the Fe atoms. The complementary fits of Fe to the (B)-site and Co to the (A,C)-site configurations in FeCo2Si reveal that Co atoms substitute exclusively for the (A,C) sites and the DO, structure remains well ordered even up to FeCo2Si. The amount of the Co(A,C)-Si(D) site disorder may be less than 16% as found by X-ray diffraction [5]. The o2 of the Co-Fe and Co-Co pairs (2.45 and 2.83 A) in FeCo2Si are comparable to that of the Co-Co pairs (2.50A) in CO foil. But the M-Si
J.I.
Budnick et al./Selective
site substitution
33
(M = Fe and Co) pairs in FeCo,Si show a o2 larger than that of the Co-Si in CoSi2, suggesting a less tight M-Si bonding in the Fe$i matrix. To fit the V and Mn EXAFS, we used theoretical amplitude and phase 161. The excellent fit (Fig. 4a) reveals a pure (B)-site configuration of V atoms in Fe2.25V0,75Si. A ain no local distortion is found. The u* for V-Fe and V-Si pairs are 0.009 and 0.015 A$ respectively. The Mn EXAFS of Fe2.88Mn0.72 Si also fits well to the (B)-site configuration. The Mn-Fe distance is the same as the diffraction value. But the Mn-Si pair shows a u* = 0.02 - 0.03 A* which indicates large disorder between the Mn and Si. Since the Mn-Fe distance is well defined (R= 2.45 A, u* = 0.005 A*), the Mn-Si pair disorder should be due to the Si atoms. According to an approach to XANES interpretation 171 the peak indicated as 2 in Fig.1 is mainly due to single backscattering from the 2nd shell Si atoms. The damping of the feature 2 in Fe2,88Mn0,12Si is also in support of a disorder in the Si atom locations. The Fe2.88MnD.12Sl sample is as ground without annealing. All other samples are annealed and ordered. 12
5 4
.L ," N : Y
8
._c ," hl : Y
6 4
3 2 1
2
0
0
-1 -2
-2 2.0
5.5
k (inverse
9.0
12.5
angstrom)
16.0
2.0
5.5
k (inverse
9.0
12.5
16.0
angstrom)
Fig. 3 (left) K-edge EXAFS spectra k*X. (a) Fe in FeCo$i, (b) Mn in Fe,.,,Mn,.,$i, (c) V in Fe2.z5V0 T$i, (d) Co in FeCo2Si. Fig. 4 (&ht) Filtered 1st and 2nd shell EXAFS data (solid line) and fit (dashed line). (a) V in Fe2.25V0_75Si,(b) Co in FeCo$i. We wish to thank Xingsheng Ling and Dale Brewe for their help in this work. This work was carried out at the Beam Line X-I I at the National Synchrotron Light Source. We acknowledge the support from the Department of Energy under contract No. DE-ASOS-80-ER10742 and No. DE-AC02-76CH00016.
Acknowledgements:
References [l] W. A. Hines et al. Phys. Rev. B13 (1976) 4060; A. Paoletti and L. Passari, Nuovo
Cimento 32 (1964) 1450. [2] T. 1. Burch, T. L’ltrenta and J. I. Budnick, Phys. Rev. Lett. 33 (1974)421; T. J. Burch et al. Phys. Rev. B24 (198 1) 3866, and references therein. [3] S. Pickart et al. Phys. Lett. A53 (1975) 321. [4] C. Blaauw, G. R. Mackay and W. Leiper, Solid State Commun. 18 (1975) 927. [5] V. Niculescu et al. Phys. Rev. B19 (1979) 452. [6] B. K. Teo and P. A. Lee, J. Amer. Chem. Sot. 101 (1979) 2815. L-77 A. Bianconi et al. in EXAFS and Near-Edge Structure, edited by A. Bianconi, L. Incoccia, and S. Stipcich, (Springer, Berlin, 1983), p.57; F. Lytle et al., Phys. Rev. B37 (1988)1550.
Physica B 158 (1989) 31-33 North-Holland, Amsterdam
X-Ray Absorption in Fe,Si
Studies of Selective Site Substitution
J. I. Budnick, Zhengquan Tan and D. M. Pease Department of Physics and Institute of Materials Connecticut, Storrs, CT 06268
by 3d Transition
Science,
Metals
University
of
Our K-edge XANES spectra of V, Mn, Fe and Co in Fe3_,T,Si (T= V, Mn, and Co) alloys demonstrate straightforwardly and unambiguously that V and Mn selectively substitute for the Fe atoms in the Fe(B) site and that Co atoms substitute for the Fe(A,C) atoms. This is in good agreement with results obtained by other techniques. The EXAFS results reveal quantitatively the selective site substitution and prove that a strong degree of structural order exists up to high concentrations for V and Co substitutions. Introduction Fe3Si is ferromagnetic and crystallographically well ordered with an fee DO, structure. This structure has four sites A, B, C, and D, with the specific near neighbor configurations summarized in table 1. The Fe atoms are located at the two chemically and magnetically distinct (A,C) and (B) sites with magnetic moments of 1.35~~ and 2.20 Brespectively. It was determined from extensive NM R studies [ 21, neutron diffraction fl3 and Mijssbauer experiments 141 that transition metal impurities to the left of Fe in the periodic table (Ti, V, Cr and Mn) selectively substitute for the Fe(B) atoms and those below and to the right of P’e (Co, Ni, Ru, Rh, Pd, OS, and Ir) substitute for the Fe(A,C) atoms. We report here an X-ray absorption study of the site selective substitution in Fe3Si and the local environment of the substituted sites. Table 1. Fej_,T,Si neighbor configurations (from ref. 121). shells R (a)
I
A,C
1 0.43
oT50
0.3705
4 0.83
5 0.86
I.:0
4B 4D
6A,C
L!A,c
12B 12D
8A,C
6A,C
B
8A,C
6D
12B
24A,C
8D
6B
D
8A.C
6B
12D
24A,C
8B
6D
I
Experiments Samples were prepared by arc melting. Details of the preparation method can be found in ref.[21. Transmission and fluorescence data were collected on powdered samples at room temperature. The monochromator used double Si(lI1) crystals. The energy resolution is estimated to be 2.5ev at 9000ev. Results and Discussion Fig. 1 shows the K-edge XANES of Fe in FeCo$i, Mn in Fe, 88Mn0,12Si, V in Fe2.25V0.7& Fe2.96V0.04Sil and in Fe2.98VO.&i . The energy origin is set at the first inflection point. All spectra in Fig. 1 are very similar except that the fine structure features for Mn are less pronounced. This reveals similar local structural environments of Fe, Mn and V in the corresponding samples. The XANES spectra of Fe (Fig. la) and Co (Fig. 2a) of FeCo2Si are much different. Thus the Co atoms do not enter the (A,C) @ Elsevier Science Publishers B.V. Physics Publishing Division)
0921-4526/89/$03.50
(North-Holland
J.I. Budnick
32
et al./Selective
site substitution
and (B) sites randomly. In addition, the Co XANES spectra of FeCozSi and Fe2.1C00,9Si are extremely, similar. Therefore the Co atoms must have substituted for only one Fe site and this site can only be the (A,C) site. The Fe atoms left in FeCO,Si are located at the (B) sites only. By their rescmblancc to the Fe spectra of FcCo,Si, Mn and V spectra are assigned to the (B) site. Therefore our XAhES spectra demonstrate straightforwardly and unambiguously that Co selectively substitute for Fe(A,C) atoms and V and Mn substitute for Fe(B). The XANES spectra can be used as fingerprints of the local environment of a specific atom site. To further illustrate this, wc plot in Fig.2 the XANES spectra of Co in FcCo,Si, Fe in FeCozSi , Fe in bee metal and Fc in Fe0 02Ni0.98. A common energy origin of the bee Fe metal is used for all Fe spectra. No ‘significant edge shift is observed in FeCo2Si and Fe0,0zNi0,98. There are similarities and also differences in the near neighbor configurations around the absorber in these samples. These XANES spectra from well defined structures should be useful for testing theoretical models developed to explain near edge structures. In order to attempt this, the above XANES spectra will be measured with higher energy resolutions. 2.51
2.5
I-
-20
0
20
40
E (ev>
60
-20
0
20
40
60
E (ev>
Fig. 1 (left) K-edge XANES spectra. (a) Fe in FeCo$i, (b) Mn in Fe,.,,MnO.,,Si, (c) V in %.2sV0.+i I (4 V h Fe2.96VcmSi 7(e) V h-~FepaVo.gGi . Fig. 2 (nght) K-edge XANES spectra. (a) Co in FeCo2S1, (b) Fe in FeCo$i, (c) bee Fe, (d) Fe iq Ni0.98Fe0.02. the k2X data of substituted Fe3Si. The Fe in FeCo2Si, Mn in in Fe2.25 V0.75 Si arc in (B) sites and their EXAFS are determined by (B)-site local environment. Similarly the Co EXAFS in FeCo,Si corresponds to the (A,C) atomic environment. The Fourier transform filtered EXAFS were analyzed with non-linear fitting using the single scattering EXAFS formula. Experimentally determined phase and amplitude from CoSi2 (CaF2 structure) and Co foil (hcp structure) arc respectively used for Co-Si and Co-Co, Co-Fe and Fe-Co atom pairs. Fig. 3 presents Fe2.88Mn0.12Si,
v
According to table 1 the Co(A,C) in FeCo*Si has 4 Fe plus 4 Si in the 1st shell and 6 Co in the 2nd shell. The combined 2 shell EXAFS data fit very well to this near neighbor configuration (Fig. 4b). The near neighbor distances all agree with the diffraction values. The Fe EXAFS in FeCo2Si fits the (B)-site configuration excellently. There is no evidence of distortion around the Fe atoms. The complementary fits of Fe to the (B)-site and Co to the (A,C)-site configurations in FeCo2Si reveal that Co atoms substitute exclusively for the (A,C) sites and the DO, structure remains well ordered even up to FeCo2Si. The amount of the Co(A,C)-Si(D) site disorder may be less than 16% as found by X-ray diffraction [5]. The o2 of the Co-Fe and Co-Co pairs (2.45 and 2.83 A) in FeCo2Si are comparable to that of the Co-Co pairs (2.50A) in CO foil. But the M-Si
J.I.
Budnick et al./Selective
site substitution
33
(M = Fe and Co) pairs in FeCo,Si show a o2 larger than that of the Co-Si in CoSi2, suggesting a less tight M-Si bonding in the Fe$i matrix. To fit the V and Mn EXAFS, we used theoretical amplitude and phase 161. The excellent fit (Fig. 4a) reveals a pure (B)-site configuration of V atoms in Fe2.25V0,75Si. A ain no local distortion is found. The u* for V-Fe and V-Si pairs are 0.009 and 0.015 A$ respectively. The Mn EXAFS of Fe2.88Mn0.72 Si also fits well to the (B)-site configuration. The Mn-Fe distance is the same as the diffraction value. But the Mn-Si pair shows a u* = 0.02 - 0.03 A* which indicates large disorder between the Mn and Si. Since the Mn-Fe distance is well defined (R= 2.45 A, u* = 0.005 A*), the Mn-Si pair disorder should be due to the Si atoms. According to an approach to XANES interpretation 171 the peak indicated as 2 in Fig.1 is mainly due to single backscattering from the 2nd shell Si atoms. The damping of the feature 2 in Fe2,88Mn0,12Si is also in support of a disorder in the Si atom locations. The Fe2.88MnD.12Sl sample is as ground without annealing. All other samples are annealed and ordered. 12
5 4
.L ," N : Y
8
._c ," hl : Y
6 4
3 2 1
2
0
0
-1 -2
-2 2.0
5.5
k (inverse
9.0
12.5
angstrom)
16.0
2.0
5.5
k (inverse
9.0
12.5
16.0
angstrom)
Fig. 3 (left) K-edge EXAFS spectra k*X. (a) Fe in FeCo$i, (b) Mn in Fe,.,,Mn,.,$i, (c) V in Fe2.z5V0 T$i, (d) Co in FeCo2Si. Fig. 4 (&ht) Filtered 1st and 2nd shell EXAFS data (solid line) and fit (dashed line). (a) V in Fe2.25V0_75Si,(b) Co in FeCo$i. We wish to thank Xingsheng Ling and Dale Brewe for their help in this work. This work was carried out at the Beam Line X-I I at the National Synchrotron Light Source. We acknowledge the support from the Department of Energy under contract No. DE-ASOS-80-ER10742 and No. DE-AC02-76CH00016.
Acknowledgements:
References [l] W. A. Hines et al. Phys. Rev. B13 (1976) 4060; A. Paoletti and L. Passari, Nuovo
Cimento 32 (1964) 1450. [2] T. 1. Burch, T. L’ltrenta and J. I. Budnick, Phys. Rev. Lett. 33 (1974)421; T. J. Burch et al. Phys. Rev. B24 (198 1) 3866, and references therein. [3] S. Pickart et al. Phys. Lett. A53 (1975) 321. [4] C. Blaauw, G. R. Mackay and W. Leiper, Solid State Commun. 18 (1975) 927. [5] V. Niculescu et al. Phys. Rev. B19 (1979) 452. [6] B. K. Teo and P. A. Lee, J. Amer. Chem. Sot. 101 (1979) 2815. L-77 A. Bianconi et al. in EXAFS and Near-Edge Structure, edited by A. Bianconi, L. Incoccia, and S. Stipcich, (Springer, Berlin, 1983), p.57; F. Lytle et al., Phys. Rev. B37 (1988)1550.