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XRD and Mössbauer investigation of phase segregation in Eu1−xSrxFeO3
ARTICLE IN PRESS
Physica B 393 (2007) 100–104 www.elsevier.com/locate/physb
XRD and Mo¨ssbauer investigation of phase segregation in Eu1xSrxFeO3 Shu-Zhen Li, Yan-Jun Huang, Jing-Bo Zhu, Yuan Zhang, Nan Chen, Yuan-Fu Hsia Department of Physics, Nanjing University, Hankou Road 22, Nanjing, Jiangsu Province 210093, PR China Received 7 December 2006; received in revised form 14 December 2006; accepted 14 December 2006
Abstract Eu1xSrxFeO3 (x ¼ 0, 1/4, 1/3, 1/2, 2/3 and 1) perovskites were prepared using conventional solid-state method. Structure aspects of the Eu1xSrxFeO3 were studied by powder X-ray diffraction. The results showed that Eu1xSrxFeO3 (x ¼ 0, 2/3) was single phase with orthorhombically distorted structure. For samples with x ¼ 1/4, 1/3 and 1/2, phase segregation was observed. The Mo¨ssbauer spectra showed that Fe4+ appeared as the Eu3+ was replaced by Sr2+, EuFeO3-like phase existed in systems with x ¼ 1/4, 1/3 and 1/2. The corresponding oxygen vacancies were also calculated from the Mo¨ssbauer spectra. r 2007 Elsevier B.V. All rights reserved. PACS: 33.45.+x; 61.10.i; 64.75.+g; 81.30Mh Keywords: X-ray diffraction; Mo¨ssbauer spectroscopy; Phase segregation
1. Introduction In recent years, perovskite oxides have been receiving much attention because of their important role in understanding the unique physical properties, such as hightemperature superconductivity in cuprate oxide systems and colossal magnetoresistance in manganite induced by strong electron correlations [1–6]. The perovskite-type ferrite AFeO3 is also focused due to their magnetic properties, which may be attributed to changes in the overage oxidation state and local coordination of iron. For A ¼ La, Pr, Gd, the compounds were found to have a space group of Pbnm with a distorted perovskite structure, and the crystallographic unit cell contains four equivalent iron ions [1]. For A ¼ Sr, the compound showed a cubic structure, which is able to stabilize cations with unusual high oxidation states of Fe atoms. The mixed series of R1xSrxFeO3 (R ¼ rare earth) are widely studied. Depend-
ing on the synthesis conditions, the oxygen vacancies play an important role in the crystal structure and the related physical properties. Varying the content of Sr in the compound results in great change of crystal structures and electrical magnetic properties [2–4]. For instance, for A ¼ La, Pr, the orthorhombic distortion decreases with the increasing Sr content. Rhombohedral phase appears with xE0.5–0.7, while cubic phase is observed with xX0.8 [5,6]. The phase segregation is ascribed to the structural strain resulted from size mismatch of A elements. In view of the complex structure properties of perovskite-type ferrite, we aimed to investigate in detail about the evolution of structure of Eu1xSrxFeO3 with different Sr content. Accordingly, in this paper, we present a systematic XRD and Mo¨ssbauer spectral study on the phase segregation and structural properties of Eu1xSrxFeO3 compounds. 2. Experiment details
Corresponding author. Tel.: +86 25 83594734; fax: +86 25 83594234.
E-mail addresses: [email protected] (S.-Z. Li), [email protected] (Y.-F. Hsia). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.12.077
All compounds of Eu1xSrxFeO3 (x ¼ 0, 1/4, 1/3, 1/2, 2/3 and 1) were synthesized by conventional solid-state
ARTICLE IN PRESS S.-Z. Li et al. / Physica B 393 (2007) 100–104
reaction technique. The raw materials (all analytical reagent), Eu2O3, SrCO3 and Fe2O3 (499%), with corresponding stoichiometric ratio, were fully mixed in an agate mortar and pressed into pellets at 40 MPa, and then prefired at 600 1C in a muffle for about 5 h. Then the pellets were repressed and re-heated at 1200 1C for 10 h for several times in open air to ensure homogeneous solutions. The Xray powder diffraction (XRD) patterns were recorded with Cu-Ka radiation. The program GSAS was used to analyze the crystal structure [7]. The Mo¨ssbauer spectra were collected on a constant acceleration mode Mo¨ssbauer spectrometer with a 57Co/Pd source (25 mCi). The spectra were fitted with Lorentzian lines using MossWinn 3.0 software [8]. All spectra were calibrated by natural a-Fe foil, and the center shifts (CS) were relative to the spectral center of a-Fe at room temperature.
101
3. Results and discussion 3.1. XRD Fig. 1 illustrates the XRD patterns of Eu1xSrxFeO3 with x ¼ 0, 1/4, 1/3, 1/2 and 2/3. For all the samples, it was found that satisfactory refinements could only be achieved with anisotropic temperature factors fitted. The patterns of samples with x ¼ 0 and 2/3 reveal a single phase with orthorhombically distorted structure (space group: Pbnm). A cubic structure (space group: Pm3¯ m) was also proposed for Eu1/3Sr2/3FeO3, but the results were not very ideal. The refinement parameters are listed in Table 1. It shows that with increasing x, the Fe–O length becomes shorter and the bond angle of Fe–O–Fe becomes larger. The decreasing of Fe–O length originates from the substitution of the Sr2+, which results in the formation of Fe4+ and increase of the
500 400
Eu3/4Sr1/4FeO3
300 Counts
Intensity (arb. units)
Eu1-xSrxFeO3 x=0
200 100
x=1/4
0
x=1/3 x=1/2
20
30
x=2/3 20
30
40
50 2-theta (Deg. )
60
70
Fig. 1. X-ray patterns for Eu1xSrxFeO3 (x ¼ 0, 1/4, 1/3, 1/2 and 2/3).
40 50 2-theta (Deg.)
60
70
Fig. 2. Observed and fitted XRD patterns of Eu3/4Sr1/4FeO3. The difference between measured and calculated intensities is plotted at the bottom of the figure. The bars (|) denote the calculated Bragg reflection positions of the EuFeO3-like phase (down) and the Eu1/3Sr2/3FeO3-like phase (up).
Table 1 Refinement parameters of EuFeO3, Eu1/3Sr2/3FeO3 at room temperature x
Lattice constants (A˚)
Lattice coordinate x
0
a ¼ 5.3754 (1) b ¼ 5.6015 (1) c ¼ 7.6876 (2)
Eu (4c) Fe(4a) OI(4c) OII(8d)
0.9843 (3) 0.500 0.086 (2) 0.702 (2)
2/3
a ¼ 5.4768 (1) b ¼ 5.4913 (1) c ¼ 7.7469 (1)
Eu/Sr(4c) Fe(4a) OI(4c) OII(8d)
0.996 (1) 0.500 0.517 (2) 0.241 (1)
y
Fe–O
Fe–O–Fe
Reliability factor (%)
z 0.0573 (2) 0.000 0.471 (2) 0.310 (2)
0.2500 0.000 0.250 0.049 (1)
0.0106 (7) 0.000 0.000 (3) 0.237 (6)
0.250 0.000 0.250 0.009 (2)
Fe–OI 1.983 (3)
Fe–OII 2.08 (1) 1.96 (1)
Fe–OI–Fe 151.4 (6)
Fe–OII–Fe 147.5 (5)
Rwp ¼ 9.84 Rp ¼ 7.08 w2 ¼ 1.767
Fe–OI 1.9390 (6)
Fe–OII 1.93 (2) 1.95 (2)
Fe–OI–Fe 174.4 (7)
Fe–OII–Fe 173.3 (12)
Rwp ¼ 10.01 Rp ¼ 7.02 w2 ¼ 1.562
Atoms are located as follows: Eu/Sr at 4c: (x y 1/4); Fe at 4b: (1/2 0 0); OI at 4c: (x y 1/4); OII at 8d: (x y z).
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Table 2 Structural parameters of the compounds Eu1xSrxFeO3 with x ¼ 1/4, 1/3 and 1/2 from XRD. Parameters
Space group a (A˚) b (A˚) c (A˚) Eu/Sr:x y Fe:x y OI:x y OII:x y z Rwp (%) Rp (%)
x ¼ 1/4
x ¼ 1/3
x ¼ 1/2
A
B
A
B
A
B
Pbnm 5.3804 (2) 5.5790 (2) 7.6846 (3) 1.0044 (6) 0.0559 (2) 0.500 0.000 0.0993 (2) 0.492 (2) 0.762 (2) 0.255 (2) 0.055 (8) 7.79 5.34
Pbnm 5.4568 (6) 5.4789 (5) 7.7164 (4) 0.998 (4) 0.002 (2) 0.500 0.000 0.499 (8) 0.058 (5) 0.216 (3) 0.304 (6) 0.018 (7)
Pbnm 5.3708 (2) 5.5731 (3) 7.6780 (3) 1.0117 (5) 0.0558 (3) 0.500 0.000 0.124 (3) 0.494 (2) 0.287 (2) 0.286 (2) 0.016 (2) 9.68 6.62
Pbnm 5.4685 (2) 5.4757 (3) 7.7489 (3) 1.001 (6) 0.007 (2) 0.500 0.000 0.505 (11) 0.052 (8) 0.271 (6) 0.276 (12) 0.001 (9)
Pbnm 5.3785 (4) 5.5709 (5) 7.6908 (5) 0.9874 (7) 0.0546 (5) 0.500 0.000 0.516 (7) 0.387 (3) 0.854 (3) 0.370 (2) 0.018 (2) 8.64 6.28
Pbnm 5.4632 (2) 5.4855 (2) 7.7239 (2) 1.0072 (9) 0.0076 (9) 0.500 0.000 0.546 (4) 0.029 (3) 0.244 (7) 0.256 (8) 0.007 (3)
Note: A and B indicate the EuFeO3-like and Eu1/3Sr2/3FeO3-like phases. Atoms are located as follows: Eu/Sr at 4c: (x y 1/4); Fe at 4b: (1/2 0 0); OI at 4c: (x y 1/4); OII at 8d: (x y z).
covalency between Fe and O ions. It is well known that for the distorted perovskite structure, the bond angle of Fe–O–Fe deviates from 1801, so while the orthorhombic distortion is weakened, the bond angle of Fe–O–Fe would be more close to 1801 [9]. The orthorhombic distortion is defined as ja bj=ða þ bÞ, where a and b are the crystal axes. The values of orthorhombic distortion for EuFeO3 and Eu1/3Sr2/3FeO3 are 2.1 102 and 1.3 103, respectively. The unit-cell volume of Eu1/3Sr2/3FeO3 is a little larger than that of EuFeO3 since the ionic radii of Sr2+ is bigger than that of Eu3+. For samples with x ¼ 1/4, 1/3 and 1/2, it is observed that with x increasing, the intensities of reflections from (0 2 0), (2 0 0) and (0 2 1) reduce obviously, and the reflection with 2yE40.41 emerges and becomes more prominent. However, it still shows typical features of EuFeO3-like structure properties. We tried to refine the XRD patterns with a single phase, but the value of reliability factor was rather poor. So, two different phases were used to refine the patterns. One was a EuFeO3 type and the other was a Eu1/3Sr2/3FeO3 type [9,10]. Selected XRD pattern of Eu3/4Sr1/4FeO3 is shown in Fig. 2. The structural parameters of the compounds with x ¼ 1/4, 1/3 and 1/2 are listed in Table 2. The weight factors Rwp of these three samples are 7.80%, 9.68% and 9.27%, respectively. And the Rp factors are 5.31%, 6.62% and 6.60%, respectively. The contents of the EuFeO3-like phase are 68.4%, 52.2% and 24.4%, respectively. The XRD pattern of compound SrFeO3 was also measured. A pseudo-cubic structure is found with lattice constant a ¼ 3.8599(5) A˚, and it is expected to have a number of oxygen vacancies due to different synthesis method [11].
3.2. Mo¨ssbauer spectra Fig. 3 shows the 57Fe Mo¨ssbauer spectra of Eu1xSrxFeO3 (x ¼ 0.0–1.0) at room temperature. For EuFeO3, only one sextet is observed with the parameters of CS ¼ 0.37 mm/s, QS ¼ 0.01 mm/s, Bhf ¼ 50.5 T, which is in good agreement with Ref. [12]. For the samples of x ¼ 1/4, 2/3 and 1/2, the magnetic component also appears with the similar parameters as in EuFeO3, which leads us to believe that a EuFeO3-related phase exists in these three compounds. This is consistent with the result of XRD studies. It should be noted that the magnetic component decreases with the value of x, and disappears at x ¼ 2/3, which indicates a single-phase properties of Eu1/3 Sr2/3FeO3. For the compound Eu1/3Sr2/3FeO3, asymmetric paramagnetic components are observed. The spectrum was evolved into two doublets, with parameters CS ¼ 0.11(5) mm/s, QS ¼ 0.23(5) mm/s and CS ¼ 0.63(5) mm/s, QS ¼ 0.18(5) mm/s, respectively. The area ratio is about 57:43. It is known that the Fe4+ ions exist in iron–perovskite compounds, so the two doublets should be assigned to Fe4+ and Fe3+, respectively [4,11,13]. The oxygen vacancies are calculated to be 0.050 from the area ratio of the two paramagnetic components, which indicates a real formula of Eu1/3Sr2/3FeO2.950 for x ¼ 2/3 [3]. For the sample SrFeO3, the analysis method is similar to that of Eu1/3Sr2/3FeO3. The parameters are in good agreement with Refs. [11,13]. Mo¨ssbauer spectra of the compounds with x ¼ 1/4, 1/3 and 1/2 were fitted using a EuFeO3-like phase and a Eu1/3Sr2/3FeO3-like phase, i.e., a magnetic sextet and two paramagnetic doublets. All the Mo¨ssbauer parameters are listed in Table 3. It is indicated that the samples (x ¼ 1/4,
ARTICLE IN PRESS S.-Z. Li et al. / Physica B 393 (2007) 100–104
1/3, 1/2 and 1) showed oxygen vacancy properties. The vacancy numbers are also listed in Table 3. To illustrate the two-phase property of the compounds Eu1xSrxFeO3, the relationship between Sr concentration (x) and the content of EuFeO3-like phase is plotted, which is shown in Fig. 4. It is found that these two experimental methods give almost identical results, and an obvious phase segregation of Eu1xSrxFeO3 system is confirmed.
1.00 0.98 x=0
0.96
103
1.00 0.98 0.96
4. Conclusion
x=1/4
In summary, we studied the Eu1xSrxFeO3 system synthesized by solid-state reaction in open air at high temperature. The phase segregation property was observed clearly from XRD and Mo¨ssbauer spectra. It was found that the EuFeO3-like phase and the Eu1/3Sr2/3FeO3-like phase co-existed in compounds with x ¼ 1/4, 1/3 and 1/2, and the content of the first phase decreased gradually with x.
Relative Transmission
1.00 0.98 x=1/3 0.96 1.00 0.96
x=1/2
0.92 100
XRD
1.00 EuFeO3-like phase (%)
0.96 x=2/3
0.92 0.88 0.96 0.88
x=1
0.80
Mossbauer spectra
80 60 40 20 0
-12
-8
-4
0 4 Velocity (mm/s)
8
12
0.0
Fig. 3. 57Fe Mo¨ssbauer spectra of Eu1xSrxFeO3 measured at room temperature.
0.2
0.4 0.6 Sr concentration X
0.8
1.0
Fig. 4. Sr concentration x dependence of EuFeO3-like phase content.
Table 3 57 Fe Mo¨ssbauer parameters of Eu1xSrxFeO3 at room temperatures x
CS 0 1/4 1/3 1/2 2/3 1
Fe3+
Magnetic sextet
0.37 0.36 0.37 0.37
QS (4) (4) (4) (5)
0.01 0.01 0.01 0.00
HF (4) (4) (4) (5)
50.5 50.4 50.4 50.5
(1) (1) (1) (1)
Area
CS
100 57.4 47.1 16.9 0 0
0.36 0.48 0.47 0.62 0.61
Fe4+ QS
(4) (4) (5) (5) (5)
0.00 0.00 0.00 0.18 0.31
O-Vacancies
CS
(4) (4) (5) (5) (5)
0.00 0.09 0.14 0.11 0.08
QS
(4) (4) (5) (5) (5)
0.38 0.22 0.18 0.23 0.23
(4) (4) (5) (5) (5)
0 0.030 0.063 0.070 0.050 0.177
The parameters of CS, QS (quadruple splitting) are given in mm/s, areas are given in percentage (%), HF indicates the hyperfine field in unit of T. Parameter y indicates the oxygen vacancies.
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Acknowledgment The research was supported by National Natural Science Foundation of China under Grant No. 10575051. References [1] M. Marezio, J.P. Remeika, P.D. Dernier, Acta Crystallogr. B 26 (1970) 2008. [2] S. Yoon, C.S. Kim, J. Appl. Phys. 97 (2005) 10A318. [3] Y.-Q. Liang, N.-L. Di, Z.-H. Cheng, Phys. Rev. B 72 (2005) 134416. [4] J.Q. Li, Y. Matsui, S.K. Park, Y. Tokura, Phys. Rev. Lett. 79 (1997) 297. [5] S.E. Dann, D.B. Currie, M.T. Weller, M.F. Thomas, A.D. AlRawwas, J. Solid State Chem. 109 (1994) 134.
[6] H.W. Brinks, H. Fjellvag, A. Kjekshus, B.C. Hauback, J. Solid State Chem. 150 (2000) 233. [7] A. C. Larsen, R. B. von Dreele, Los Alamos National Laboratory Report LAUR, 1994, pp. 86–748. [8] Z. Klencsar, MossWinn, version 3.0, 2004, unpublished. [9] M. Stange, J. Linden, A. Kjekshus, N. Binsted, M.T. Weller, B.C. Hauback, J. Solid State Chem. 173 (2003) 148. [10] J. Blasco, J. Stankiewicz, J. Garcia, J. Solid State Chem. 179 (2006) 898. [11] P. Adler, A. Lebon, V. Damljanovic, C. Ulrich, C. Bernhard, A.V. Boris, A. Maljuk, C.T. Lin, B. Keimer, Phys. Rev. B 73 (2006) 094451. [12] X. Liu, M. Jin, C. Zhang, J. Appl. Phys. 71 (1992) 5111. [13] P.K. Gallagher, J.B. Macchesney, D.N.E. Buchanan, J. Chem. Phys. 41 (1964) 2429.
Physica B 393 (2007) 100–104 www.elsevier.com/locate/physb
XRD and Mo¨ssbauer investigation of phase segregation in Eu1xSrxFeO3 Shu-Zhen Li, Yan-Jun Huang, Jing-Bo Zhu, Yuan Zhang, Nan Chen, Yuan-Fu Hsia Department of Physics, Nanjing University, Hankou Road 22, Nanjing, Jiangsu Province 210093, PR China Received 7 December 2006; received in revised form 14 December 2006; accepted 14 December 2006
Abstract Eu1xSrxFeO3 (x ¼ 0, 1/4, 1/3, 1/2, 2/3 and 1) perovskites were prepared using conventional solid-state method. Structure aspects of the Eu1xSrxFeO3 were studied by powder X-ray diffraction. The results showed that Eu1xSrxFeO3 (x ¼ 0, 2/3) was single phase with orthorhombically distorted structure. For samples with x ¼ 1/4, 1/3 and 1/2, phase segregation was observed. The Mo¨ssbauer spectra showed that Fe4+ appeared as the Eu3+ was replaced by Sr2+, EuFeO3-like phase existed in systems with x ¼ 1/4, 1/3 and 1/2. The corresponding oxygen vacancies were also calculated from the Mo¨ssbauer spectra. r 2007 Elsevier B.V. All rights reserved. PACS: 33.45.+x; 61.10.i; 64.75.+g; 81.30Mh Keywords: X-ray diffraction; Mo¨ssbauer spectroscopy; Phase segregation
1. Introduction In recent years, perovskite oxides have been receiving much attention because of their important role in understanding the unique physical properties, such as hightemperature superconductivity in cuprate oxide systems and colossal magnetoresistance in manganite induced by strong electron correlations [1–6]. The perovskite-type ferrite AFeO3 is also focused due to their magnetic properties, which may be attributed to changes in the overage oxidation state and local coordination of iron. For A ¼ La, Pr, Gd, the compounds were found to have a space group of Pbnm with a distorted perovskite structure, and the crystallographic unit cell contains four equivalent iron ions [1]. For A ¼ Sr, the compound showed a cubic structure, which is able to stabilize cations with unusual high oxidation states of Fe atoms. The mixed series of R1xSrxFeO3 (R ¼ rare earth) are widely studied. Depend-
ing on the synthesis conditions, the oxygen vacancies play an important role in the crystal structure and the related physical properties. Varying the content of Sr in the compound results in great change of crystal structures and electrical magnetic properties [2–4]. For instance, for A ¼ La, Pr, the orthorhombic distortion decreases with the increasing Sr content. Rhombohedral phase appears with xE0.5–0.7, while cubic phase is observed with xX0.8 [5,6]. The phase segregation is ascribed to the structural strain resulted from size mismatch of A elements. In view of the complex structure properties of perovskite-type ferrite, we aimed to investigate in detail about the evolution of structure of Eu1xSrxFeO3 with different Sr content. Accordingly, in this paper, we present a systematic XRD and Mo¨ssbauer spectral study on the phase segregation and structural properties of Eu1xSrxFeO3 compounds. 2. Experiment details
Corresponding author. Tel.: +86 25 83594734; fax: +86 25 83594234.
E-mail addresses: [email protected] (S.-Z. Li), [email protected] (Y.-F. Hsia). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.12.077
All compounds of Eu1xSrxFeO3 (x ¼ 0, 1/4, 1/3, 1/2, 2/3 and 1) were synthesized by conventional solid-state
ARTICLE IN PRESS S.-Z. Li et al. / Physica B 393 (2007) 100–104
reaction technique. The raw materials (all analytical reagent), Eu2O3, SrCO3 and Fe2O3 (499%), with corresponding stoichiometric ratio, were fully mixed in an agate mortar and pressed into pellets at 40 MPa, and then prefired at 600 1C in a muffle for about 5 h. Then the pellets were repressed and re-heated at 1200 1C for 10 h for several times in open air to ensure homogeneous solutions. The Xray powder diffraction (XRD) patterns were recorded with Cu-Ka radiation. The program GSAS was used to analyze the crystal structure [7]. The Mo¨ssbauer spectra were collected on a constant acceleration mode Mo¨ssbauer spectrometer with a 57Co/Pd source (25 mCi). The spectra were fitted with Lorentzian lines using MossWinn 3.0 software [8]. All spectra were calibrated by natural a-Fe foil, and the center shifts (CS) were relative to the spectral center of a-Fe at room temperature.
101
3. Results and discussion 3.1. XRD Fig. 1 illustrates the XRD patterns of Eu1xSrxFeO3 with x ¼ 0, 1/4, 1/3, 1/2 and 2/3. For all the samples, it was found that satisfactory refinements could only be achieved with anisotropic temperature factors fitted. The patterns of samples with x ¼ 0 and 2/3 reveal a single phase with orthorhombically distorted structure (space group: Pbnm). A cubic structure (space group: Pm3¯ m) was also proposed for Eu1/3Sr2/3FeO3, but the results were not very ideal. The refinement parameters are listed in Table 1. It shows that with increasing x, the Fe–O length becomes shorter and the bond angle of Fe–O–Fe becomes larger. The decreasing of Fe–O length originates from the substitution of the Sr2+, which results in the formation of Fe4+ and increase of the
500 400
Eu3/4Sr1/4FeO3
300 Counts
Intensity (arb. units)
Eu1-xSrxFeO3 x=0
200 100
x=1/4
0
x=1/3 x=1/2
20
30
x=2/3 20
30
40
50 2-theta (Deg. )
60
70
Fig. 1. X-ray patterns for Eu1xSrxFeO3 (x ¼ 0, 1/4, 1/3, 1/2 and 2/3).
40 50 2-theta (Deg.)
60
70
Fig. 2. Observed and fitted XRD patterns of Eu3/4Sr1/4FeO3. The difference between measured and calculated intensities is plotted at the bottom of the figure. The bars (|) denote the calculated Bragg reflection positions of the EuFeO3-like phase (down) and the Eu1/3Sr2/3FeO3-like phase (up).
Table 1 Refinement parameters of EuFeO3, Eu1/3Sr2/3FeO3 at room temperature x
Lattice constants (A˚)
Lattice coordinate x
0
a ¼ 5.3754 (1) b ¼ 5.6015 (1) c ¼ 7.6876 (2)
Eu (4c) Fe(4a) OI(4c) OII(8d)
0.9843 (3) 0.500 0.086 (2) 0.702 (2)
2/3
a ¼ 5.4768 (1) b ¼ 5.4913 (1) c ¼ 7.7469 (1)
Eu/Sr(4c) Fe(4a) OI(4c) OII(8d)
0.996 (1) 0.500 0.517 (2) 0.241 (1)
y
Fe–O
Fe–O–Fe
Reliability factor (%)
z 0.0573 (2) 0.000 0.471 (2) 0.310 (2)
0.2500 0.000 0.250 0.049 (1)
0.0106 (7) 0.000 0.000 (3) 0.237 (6)
0.250 0.000 0.250 0.009 (2)
Fe–OI 1.983 (3)
Fe–OII 2.08 (1) 1.96 (1)
Fe–OI–Fe 151.4 (6)
Fe–OII–Fe 147.5 (5)
Rwp ¼ 9.84 Rp ¼ 7.08 w2 ¼ 1.767
Fe–OI 1.9390 (6)
Fe–OII 1.93 (2) 1.95 (2)
Fe–OI–Fe 174.4 (7)
Fe–OII–Fe 173.3 (12)
Rwp ¼ 10.01 Rp ¼ 7.02 w2 ¼ 1.562
Atoms are located as follows: Eu/Sr at 4c: (x y 1/4); Fe at 4b: (1/2 0 0); OI at 4c: (x y 1/4); OII at 8d: (x y z).
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Table 2 Structural parameters of the compounds Eu1xSrxFeO3 with x ¼ 1/4, 1/3 and 1/2 from XRD. Parameters
Space group a (A˚) b (A˚) c (A˚) Eu/Sr:x y Fe:x y OI:x y OII:x y z Rwp (%) Rp (%)
x ¼ 1/4
x ¼ 1/3
x ¼ 1/2
A
B
A
B
A
B
Pbnm 5.3804 (2) 5.5790 (2) 7.6846 (3) 1.0044 (6) 0.0559 (2) 0.500 0.000 0.0993 (2) 0.492 (2) 0.762 (2) 0.255 (2) 0.055 (8) 7.79 5.34
Pbnm 5.4568 (6) 5.4789 (5) 7.7164 (4) 0.998 (4) 0.002 (2) 0.500 0.000 0.499 (8) 0.058 (5) 0.216 (3) 0.304 (6) 0.018 (7)
Pbnm 5.3708 (2) 5.5731 (3) 7.6780 (3) 1.0117 (5) 0.0558 (3) 0.500 0.000 0.124 (3) 0.494 (2) 0.287 (2) 0.286 (2) 0.016 (2) 9.68 6.62
Pbnm 5.4685 (2) 5.4757 (3) 7.7489 (3) 1.001 (6) 0.007 (2) 0.500 0.000 0.505 (11) 0.052 (8) 0.271 (6) 0.276 (12) 0.001 (9)
Pbnm 5.3785 (4) 5.5709 (5) 7.6908 (5) 0.9874 (7) 0.0546 (5) 0.500 0.000 0.516 (7) 0.387 (3) 0.854 (3) 0.370 (2) 0.018 (2) 8.64 6.28
Pbnm 5.4632 (2) 5.4855 (2) 7.7239 (2) 1.0072 (9) 0.0076 (9) 0.500 0.000 0.546 (4) 0.029 (3) 0.244 (7) 0.256 (8) 0.007 (3)
Note: A and B indicate the EuFeO3-like and Eu1/3Sr2/3FeO3-like phases. Atoms are located as follows: Eu/Sr at 4c: (x y 1/4); Fe at 4b: (1/2 0 0); OI at 4c: (x y 1/4); OII at 8d: (x y z).
covalency between Fe and O ions. It is well known that for the distorted perovskite structure, the bond angle of Fe–O–Fe deviates from 1801, so while the orthorhombic distortion is weakened, the bond angle of Fe–O–Fe would be more close to 1801 [9]. The orthorhombic distortion is defined as ja bj=ða þ bÞ, where a and b are the crystal axes. The values of orthorhombic distortion for EuFeO3 and Eu1/3Sr2/3FeO3 are 2.1 102 and 1.3 103, respectively. The unit-cell volume of Eu1/3Sr2/3FeO3 is a little larger than that of EuFeO3 since the ionic radii of Sr2+ is bigger than that of Eu3+. For samples with x ¼ 1/4, 1/3 and 1/2, it is observed that with x increasing, the intensities of reflections from (0 2 0), (2 0 0) and (0 2 1) reduce obviously, and the reflection with 2yE40.41 emerges and becomes more prominent. However, it still shows typical features of EuFeO3-like structure properties. We tried to refine the XRD patterns with a single phase, but the value of reliability factor was rather poor. So, two different phases were used to refine the patterns. One was a EuFeO3 type and the other was a Eu1/3Sr2/3FeO3 type [9,10]. Selected XRD pattern of Eu3/4Sr1/4FeO3 is shown in Fig. 2. The structural parameters of the compounds with x ¼ 1/4, 1/3 and 1/2 are listed in Table 2. The weight factors Rwp of these three samples are 7.80%, 9.68% and 9.27%, respectively. And the Rp factors are 5.31%, 6.62% and 6.60%, respectively. The contents of the EuFeO3-like phase are 68.4%, 52.2% and 24.4%, respectively. The XRD pattern of compound SrFeO3 was also measured. A pseudo-cubic structure is found with lattice constant a ¼ 3.8599(5) A˚, and it is expected to have a number of oxygen vacancies due to different synthesis method [11].
3.2. Mo¨ssbauer spectra Fig. 3 shows the 57Fe Mo¨ssbauer spectra of Eu1xSrxFeO3 (x ¼ 0.0–1.0) at room temperature. For EuFeO3, only one sextet is observed with the parameters of CS ¼ 0.37 mm/s, QS ¼ 0.01 mm/s, Bhf ¼ 50.5 T, which is in good agreement with Ref. [12]. For the samples of x ¼ 1/4, 2/3 and 1/2, the magnetic component also appears with the similar parameters as in EuFeO3, which leads us to believe that a EuFeO3-related phase exists in these three compounds. This is consistent with the result of XRD studies. It should be noted that the magnetic component decreases with the value of x, and disappears at x ¼ 2/3, which indicates a single-phase properties of Eu1/3 Sr2/3FeO3. For the compound Eu1/3Sr2/3FeO3, asymmetric paramagnetic components are observed. The spectrum was evolved into two doublets, with parameters CS ¼ 0.11(5) mm/s, QS ¼ 0.23(5) mm/s and CS ¼ 0.63(5) mm/s, QS ¼ 0.18(5) mm/s, respectively. The area ratio is about 57:43. It is known that the Fe4+ ions exist in iron–perovskite compounds, so the two doublets should be assigned to Fe4+ and Fe3+, respectively [4,11,13]. The oxygen vacancies are calculated to be 0.050 from the area ratio of the two paramagnetic components, which indicates a real formula of Eu1/3Sr2/3FeO2.950 for x ¼ 2/3 [3]. For the sample SrFeO3, the analysis method is similar to that of Eu1/3Sr2/3FeO3. The parameters are in good agreement with Refs. [11,13]. Mo¨ssbauer spectra of the compounds with x ¼ 1/4, 1/3 and 1/2 were fitted using a EuFeO3-like phase and a Eu1/3Sr2/3FeO3-like phase, i.e., a magnetic sextet and two paramagnetic doublets. All the Mo¨ssbauer parameters are listed in Table 3. It is indicated that the samples (x ¼ 1/4,
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1/3, 1/2 and 1) showed oxygen vacancy properties. The vacancy numbers are also listed in Table 3. To illustrate the two-phase property of the compounds Eu1xSrxFeO3, the relationship between Sr concentration (x) and the content of EuFeO3-like phase is plotted, which is shown in Fig. 4. It is found that these two experimental methods give almost identical results, and an obvious phase segregation of Eu1xSrxFeO3 system is confirmed.
1.00 0.98 x=0
0.96
103
1.00 0.98 0.96
4. Conclusion
x=1/4
In summary, we studied the Eu1xSrxFeO3 system synthesized by solid-state reaction in open air at high temperature. The phase segregation property was observed clearly from XRD and Mo¨ssbauer spectra. It was found that the EuFeO3-like phase and the Eu1/3Sr2/3FeO3-like phase co-existed in compounds with x ¼ 1/4, 1/3 and 1/2, and the content of the first phase decreased gradually with x.
Relative Transmission
1.00 0.98 x=1/3 0.96 1.00 0.96
x=1/2
0.92 100
XRD
1.00 EuFeO3-like phase (%)
0.96 x=2/3
0.92 0.88 0.96 0.88
x=1
0.80
Mossbauer spectra
80 60 40 20 0
-12
-8
-4
0 4 Velocity (mm/s)
8
12
0.0
Fig. 3. 57Fe Mo¨ssbauer spectra of Eu1xSrxFeO3 measured at room temperature.
0.2
0.4 0.6 Sr concentration X
0.8
1.0
Fig. 4. Sr concentration x dependence of EuFeO3-like phase content.
Table 3 57 Fe Mo¨ssbauer parameters of Eu1xSrxFeO3 at room temperatures x
CS 0 1/4 1/3 1/2 2/3 1
Fe3+
Magnetic sextet
0.37 0.36 0.37 0.37
QS (4) (4) (4) (5)
0.01 0.01 0.01 0.00
HF (4) (4) (4) (5)
50.5 50.4 50.4 50.5
(1) (1) (1) (1)
Area
CS
100 57.4 47.1 16.9 0 0
0.36 0.48 0.47 0.62 0.61
Fe4+ QS
(4) (4) (5) (5) (5)
0.00 0.00 0.00 0.18 0.31
O-Vacancies
CS
(4) (4) (5) (5) (5)
0.00 0.09 0.14 0.11 0.08
QS
(4) (4) (5) (5) (5)
0.38 0.22 0.18 0.23 0.23
(4) (4) (5) (5) (5)
0 0.030 0.063 0.070 0.050 0.177
The parameters of CS, QS (quadruple splitting) are given in mm/s, areas are given in percentage (%), HF indicates the hyperfine field in unit of T. Parameter y indicates the oxygen vacancies.
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Acknowledgment The research was supported by National Natural Science Foundation of China under Grant No. 10575051. References [1] M. Marezio, J.P. Remeika, P.D. Dernier, Acta Crystallogr. B 26 (1970) 2008. [2] S. Yoon, C.S. Kim, J. Appl. Phys. 97 (2005) 10A318. [3] Y.-Q. Liang, N.-L. Di, Z.-H. Cheng, Phys. Rev. B 72 (2005) 134416. [4] J.Q. Li, Y. Matsui, S.K. Park, Y. Tokura, Phys. Rev. Lett. 79 (1997) 297. [5] S.E. Dann, D.B. Currie, M.T. Weller, M.F. Thomas, A.D. AlRawwas, J. Solid State Chem. 109 (1994) 134.
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