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A Mössbauer spectroscopy investigation of SrFe1−xScxO3−δ perovskites
Solid State Sciences 12 (2010) 739e744
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
A Mössbauer spectroscopy investigation of SrFe1xScxO3d perovskites Youssef Rizki a, Jean-Marie Le Breton a, *, Emeric Folcke a, Luc Lechevallier a, Yohann Bréard b, Antoine Maignan b a b
Groupe de Physique des Matériaux, UMR CNRS 6634, Université de Rouen, 76801 Saint Etienne du Rouvray, France Laboratoire CRISMAT, UMR 6508 CNRS ENSICAEN, 6 bd du Maréchal Juin, 14050 CAEN Cedex 4, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2009 Received in revised form 28 January 2010 Accepted 13 February 2010 Available online 19 February 2010
Polycrystalline samples of oxygen deficient perovskites SrFe1xScxO3d (0 x 0.5) have been synthesized by direct solid state reactions. Each compound has been stabilized with two different oxygen stoichiometries. The structural study shows, firstly, the good cationic homogeneity of the samples and, secondly, that the Sc and Fe atoms are randomly distributed over the same crystallographic site, whatever the scandium content. This implies that no anionic order is possible. A detailed Mossbauer spectroscopy study clearly shows that the substitution of scandium for iron involves an oxygen content decrease which decreases the tetravalent iron content until its total disappearance for x ¼ 0.5. The evolutions of the isomer shift, the quadrupolar splitting and the relative intensity versus the Sc3þ content are depicted in the present paper. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Perovskites SrFeO3 Mössbauer spectroscopy Valence state of iron Oxygen deficiency
1. Introduction Considerable interest has been focused on the defect structure and transport properties of perovskite-type compounds due to their potential applications. Therefore, numerous studies on the magnetoresistance have been conducted for the purpose of understanding the origin of the phenomena [1,2]. The transition metal of our interest is iron, because of its ability to adopt various oxidation states: Fe2þ, Fe3þ, Fe4þ and conduct different crystallographic structures (tetrahedral, octahedral.). Indeed, the existence of different valence states of iron in the same compound affects the magnetic properties of this compound, and its electronic transport properties. Moreover, iron cations in oxides are very attractive candidates to generate magnetoresistance, especially if one considers the possibility to get the ferromagnetic metallic state in Fe3þ/Fe4þ oxides with mixed valency [3]. This has been observed in the perovskite phase SrFeO3d which exhibited negative magnetoresistance (MR) properties reaching at 20 K about 8% under 7 T [4,5]. However, even if the lack of Fe4þ cations in SrFeO2.5 could explain the absence of MR, it must be mentioned that this compound does not crystallize in a perovskite structure but rather in an orthorhombic brownmillerite one.
* Corresponding author. E-mail address: [email protected] (J.-M. Le Breton). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.02.027
It is well known that partial substitution of Fe3þ/Fe4þ for Sc3þ in SrFeO3d leads to the formation of oxygen vacancies in order to maintain the electroneutrality in the crystal, these vacancies leading to high ionic and electronic conductivities. The distribution of the iron cations thus influences the conductivity of SrFe1xScxO3d compounds and its knowledge is required to characterize its potential implication for the interpretation of the conductivity data. For example, conductivity measurements showed that the ion conduction at 700e950 C is improved for x ¼ 0.1 [6]. Mössbauer spectroscopy is a very useful tool to investigate the distribution of iron cations in Fe-containing compounds. However, there is limited Mössbauer spectroscopy data for SrFe1xScxO3d [7]. In this paper, we report a Mössbauer spectroscopy study of the SrFe1xScxO3d (x 0.5, 0.1 d 0.5) series, in order to investigate the distribution of iron cations.
2. Experimental Polycrystalline samples of oxygen deficient perovskites SrFe1xScxO3d (x ¼ 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5) were synthesized by direct solid state reactions using adequate mixtures of SrO2, Fe2O3, and Sc2O3. These precursors were thoroughly mixed and pressed in the form of bars. The latter were then heated up in
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Y. Rizki et al. / Solid State Sciences 12 (2010) 739e744
air to 1200 C for 12 h. Heating and cooling down to room temperature were performed within 6 h. Oxygen-rich compounds were obtained by annealing the asprepared ones, under high oxygen pressure (15 MPa) at 600 C for 12 h. Phase purity was checked by X-ray diffraction using a Xpert Pro diffractometer working with Cu(Ka) radiation. Data were collected over an angular range 5 < 2q < 120 . X-ray diffraction patterns were used to check for the sample purity and to measure the lattice parameters. The oxygen stoichiometries were determined by the cerimetric titration method described elsewhere [8]. Samples for transmission electron microscopy were crushed in n-butanol and small flakes were deposited on a holey carbon film supported by a copper grid. The electron diffraction investigation was carried out with a JEOL 200CX microscope, tilting around the crystallographic axes. This microscope is equipped with an EDS analyzer. The 57Fe Mössbauer spectra were recorded at room temperature in transmission mode, using a conventional spectrometer operating in the constant acceleration mode from 2.5 to 2.5 mm/s. 57Co/Rh was used as the g-ray source. The isomer shift (relative to metallic a-Fe at room temperature) and the quadrupole splitting are denoted d and D, respectively. Estimated errors for the hyperfine parameters originate from the statistical errors s given by the fitting program, taking 3s [9]. 3. Results 3.1. Structural properties The X-ray diffraction patterns of the as-prepared simples are shown in Fig. 1. The patterns reveal that the samples are singlephase. All the diffraction peaks can be indexed in the cubic Pm3m space group. The same results were obtained for the O2-annealed samples. The unit cell parameters are provided in Table 1 along with the oxygen content determined through cerimetric titration. It is found that for both series of compounds, the oxygen content decreases as the Sc3þ content increases (Fig. 2). Moreover, for a given Sr content, a significant increase of the oxygen content is observed in the O2-annealed samples, compared with the as-
Table 1 Nominal cationic composition, oxygen content and lattice parameter for as-prepared and O2-annealed for ScFe1xScxO3d. Nominal cationic composition
Oxygen content (as-prepared/ O2-annealed)
a (Å) (as-prepared/ O2-annealed)
SrFe SrFe0.95Sc0.05 SrFe0.9Sc0.1 SrFe0.8Sc0.2 SrFe0.7Sc0.3 SrFe0.6Sc0.4 SrFe0.5Sc0.5
2.85/2.95 2.84/2.95 2.78/2.81 2.74/2.77 2.64/2.73 2.59/2.63 2.50/2.50
3.8657(5)/3.8539(2) 3.869(2)/3.8685(4) 3.8768(2)/3.8660(5) 3.8991(3)/3.8916(3) 3.923(3)/3.9118(1) 3.957(4)/3.93306(2) 3.9755(2)/3.9750(5)
prepared samples. These results point out the decreasing ability of the as-prepared samples to uptake oxygen under oxidizing conditions as the Sc3þ content increases; for instance, the post-annealing treatments make the oxygen content increasing from O2.85 to O2.95 in SrFeO3d (x ¼ 0) while the oxygen content keeps a constant O2.5 value in the case of SrFe0.5Sc0.5O3d (x ¼ 0.5) (Table 1). For both series, the increase of the unit cell parameter as the Sc3þ content increases from 0 to 0.5 is due to the larger ionic radius A) compared to those of Fe3þ and Fe4þ of Sc3þ (rSc3þ ¼ 0.745 (rFe3þ ¼ 0.645 A, rFe4þ ¼ 0.585 A) (Fig. 3). Similar results were reported in the case of the SrCo1xScxO3d series [10]. For a given Sc content, the (a) value decreases as the oxygen content increases. The increase of the oxygen content involves an increase of the Fe4þ content to maintain the electroneutrality of the samples. As the ionic radius of Fe4þ is lower than that of Fe3þ, the decrease of the unit cell parameter is directly related to the increase of the Fe4þ content in the sample. The examination of the X-ray diffraction patterns of the asprepared samples initially demonstrated the existence of Sc2O3 impurities for x > 0.5 in SrFe1xScxO3d. Afterwards, the study of the cation compositions by EDX, along with the Electron Diffraction (ED) for all as-prepared compounds corresponding to x < 0.5, displayed a homogenous distribution of the Sc/Fe cations in the microcrystallites, leading to cation contents very near the starting compositions. The lack of additional diffraction spots on the ED pattern of both as-prepared and post-annealed series of samples, as shown in Fig. 4 for SrFe0.5Sc0.5O2.5, points towards a lack of oxygen ordering phenomena in this cubic perovskite structure.
3.0
as-prepared O2 annealed
Oxygen content
2.9 2.8 2.7 2.6 2.5 0.0
0.1
0.2
0.3
0.4
0.5
Sc content Fig. 1. Experimental X-ray powder diffraction patterns of SrFe1xScxO3d samples (0 x 0.5).
the
as-prepared
Fig. 2. Sc content dependence of the oxygen content in both series of SrFe1xScxO3d samples.
Y. Rizki et al. / Solid State Sciences 12 (2010) 739e744
- a paramagnetic contribution fitted with one or two doublets corresponding to Fe(3þ3)þ [4,5], or Fe3.5þ [11] (intermediate valence state of Fe between Fe3þ and Fe4þ, due to a rapid electron exchange between Fe4þ and Fe3þ that occurs in the ferrite structure at ordinary temperature [7]).
3.98
3.96
parameter a (A)
741
3.94
3.92
3.90
3.88
3.86 0.0
0.1
0.2
0.3
0.4
0.5
Sc content Fig. 3. Sc content dependence of the unit cell parameter a of the cubic phase for the as-prepared SrFe1xScxO3d samples. The line is a guide for the eye.
Regardless of the changes of the unit cell due to the oxygen postannealing, the ED patterns of the O2-annealed samples show a deficit of reflections related to the oxygen ordering phenomena as well. These points are related to the disordering effect of the oxygen network supported by the Sc3þ cations, in marked contrast with the Sr2Fe2O5, brownmillerite formation as the oxygen content decreases in SrFeO3d. Therefore, the lack of Fe/Sc ordering is the most significant characteristic in the SrFe1xScxO3d series as shown by the cubic Pm3m perovskite structure of SrFe0.5Sc0.5O2.5. The lack of oxygen uptake under oxygen pressure for the latter is indeed related to the cation disorder mentioned, which prevents the oxygen ordering phenomenon. 3.2. Mössbauer spectroscopy In Fig. 5 are shown the room temperature Mössbauer spectra of the as-prepared and annealed SrFe1xScxO3d (x ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5) samples. For samples with an oxygen content of more than 2.7, satisfactory fittings were obtained with two contributions: - a paramagnetic contribution fitted with one singlet, with an isomer shift typical of Fe4þ [7,11],
For samples with an oxygen content lower than 2.7, satisfactory fittings were obtained with three contributions; the two previous contributions and a paramagnetic contribution with a high quadrupole splitting, assigned to structurally distorded Fe sites [12] namely Fe3þHQS. The refined values of the isomer shift (d), the quadrupolar splitting (D) and the relative intensity (%) of both series are given in Table 2 (as-prepared) and Table 3 (O2-annealed). One paramagnetic doublet is sufficient to fit the Fe(3þ3)þ contribution when its relative intensity is less than 40%. When its relative intensity is higher, this contribution appears to be asymmetric, and several doublets are needed to account for the asymmetry. The Fe(3þ3)þ paramagnetic contribution can be fitted with a distribution of quadrupole doublets, as made by Waerenborgh et al. [13]. However, two doublets allow to take in account the asymmetry, leading to satisfactory fittings. In that case, only the mean d and D values of the contribution are significant, and have thus to be considered. Thus, the d and D values of the Fe(3þ3)þ contribution reported in Tables 2 and 3 are mean values. It is worth to mention that we also fitted the Fe(3þ3)þ paramagnetic contribution with a distribution of quadrupole doublets. This leads to mean d and D values that are equal to the values obtained with two doublets, according to the statistical errors. 3.2.1. The as-prepared series The evolution of both the isomer shift of the Fe4þ and (3þ3)þ contributions and the relative intensities of all the Fe contributions used to fit the spectra are shown in Fig. 6. For all compositions, the isomer shift of the Fe4þ singlet is in the range 0.01e0.05 mm/s. The contribution of Fe(3þ3)þ has an isomer shift between 0.07 and 0.17 mm/s and a quadrupolar shift between 0.56 and 0.80 mm/s. The high value of the quadrupolar splitting of the Fe3þHQS doublet (D z 1.44e1.45 mm/s) is explained by the low oxygen content of the corresponding samples (x ¼ 0.3, 0.4, 0.5) implying ion coordination polyhedra with less than six oxygen atoms. We note that Sc addition decreases the Fe4þ content (Fig. 6) as observed for Al-doped and Ga-doped SrFeO3d
Fig. 4. Electron diffraction patterns of the SrFe0.5Sc0.5O2.5 sample.
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Y. Rizki et al. / Solid State Sciences 12 (2010) 739e744
b-O2-annealed
a- as-prepared 1.00
1.00
x=0 0.93
0.96
1.00
1.00
x = 0.05 0.97
0.96
1.00
1.00
x = 0.1 0.94
0.98
1.00
1.00
x = 0.2 0.98
0.98
1.00
1.00
x = 0.3 0.98
0.98
1.0 1.00
1.00
x = 0.4 0.98
0.66
1.00
x = 0.5
0.94 Fig. 5. Room temperature Mössbauer spectra of (a) the as-prepared samples and (b) the O2-annealed samples of the series SrFe1xScxO3d (0 x 0.5).
based phases [12,13], and decreases the oxygen content from (2.85) to (2.50) (Fig. 2) in order to maintain the electronic neutrality of the samples. The increase of the Sc content produces a deficit of positive charges due to the decrease of oxygen content which is reflected by a decrease in the valence of iron from Fe4þ to Fe3þ. We observe a decrease of the isomer shift of the Fe(3þ3)þ doublet down to the negative values. This can be explained by the change of the environment of Fe(3þ3)þ
ions, as the Sc content increases. The isomer shift of the Fe4þ singlet remains almost constant as the Sc content increases, indicating that the environments around the remaining Fe4þ ions are not perturbed by the reduction of the oxygen content. We note that there are only Fe3þ cations in SrFe0.5Sc0.5O2.5 in relation with the fact that the Sc addition decreases the content of Fe4þ. Hence, the Fe3þ content increases while the Fe4þ content decreases until it disappears completely (Fig. 6).
Y. Rizki et al. / Solid State Sciences 12 (2010) 739e744 Table 2 Fitted room temperature Mössbauer spectroscopy hyperfine parameters (d: isomer shift, D: quadrupole splitting) and relative intensities for the SrFe1xScxO3d (0 x 0.5) as-prepared samples.
0.25 0.20
Samples
Assignment
D (mm/s)
Area (%)
0.15
SrFe
Fe(3þ3)þ Fe4þ
0.16(1) 0.01(2)
0.74(2) 0.12(1)
50 50
0.10
SrFe0.95Sc0.05
Fe(3þ3)þ Fe4þ
0.17(1) 0.02(2)
0.80(6) 0.14(1)
51 49
SrFe0.90Sc0.10
Fe(3þ3)þ Fe4þ
0.04(2) 0.05(2)
0.61(2) 0.09(5)
65 35
SrFe0.80Sc0.20
Fe(3þ3)þ Fe4þ
0.00(2) 0.02(2)
0.68(2) 0.08(2)
90 10
SrFe0.70Sc0.30
Fe(3þ3)þ Fe3þHQS Fe4þ
0.03(2) 0.16(4) 0.02(2)
0.59(1) 1.45(1) 0.08(1)
73 22 5
SrFe0.60Sc0.40
Fe(3þ3)þ Fe3þHQS Fe4þ
0.03(1) 0.14(1) 0.02(2)
0.56(1) 1.45(1) 0.08(1)
68 29 3
Fe3þ Fe3þHQS
0.07(2) 0.13(1)
0.56(3) 1.44(1)
67 33
SrFe0.50Sc0.50
d (mm/s)
743
0.05 0.00 -0.05 -0.10
100 90 80 70 60 50 40 30 20 10 0 0.0
0.1
0.2
0.3
0.4
0.5
Sc content Fig. 6. Isomer shifts (up) and relative intensities (down) versus scandium content of the contributions used to fit the Mössbauer spectra of the as-prepared SrFe1xScxO3d samples.
δ (mm/s)
3.2.2. The O2-annealed series The evolution of both the isomer shift of the Fe4þ and (3þ3)þ contributions and the relative intensities of all the Fe contributions used to fit the spectra are shown in Fig. 7. The post-annealing treatments make the oxygen content increasing for all samples (from O2.85 to O2.95 in SrFeO3d) (Table 1), except for the SrFe0.5Sc0.5O3d sample for which the oxygen content remains constant at O2.5. We note that, like in the as-prepared series, increasing the Sc content decreases the proportion of Fe4þ (Fig. 7), and decreases the oxygen content from (2.92) to (2.50) (Fig. 2). For the same Sc content we observe a very significant difference between the spectra of the two series which reveals the effect of O2 on the samples. Like in the asprepared sample we observe a decrease of the isomer shift of the Fe(3þ3)þ doublet down to negative values. This can be explained by the change of the environment of Fe(3þ3)þ ions, as the Sc content increases, in relation with the decrease of the oxygen content. The isomer shift of the Fe4þ singlet changes slightly, as the Sc content increases, indicating that the
0.15
δ (Fe )
0.10
δ (Fe
4+
(3+ε)+
)
0.05 0.00 -0.05 -0.10
Table 3 Fitted room temperature Mössbauer spectroscopy hyperfine parameters (d: isomer shift, D: quadrupole splitting) and relative intensities for the SrFe1xScxO3d (0 x 0.5) O2-annealed samples. Assignment
SrFe
Fe(3þ3)þ Fe4þ
SrFe0.95Sc0.05
(3þ3)þ
Fe Fe4þ
(3þ3)þ
D (mm/s)
Area (%)
0.15(1) 0.06(2)
0.59(1) 0.06(1)
31 69
0.12(1) 0.04(2)
0.55(1) 0.08(2)
38 62
d (mm/s)
SrFe0.90Sc0.10
Fe Fe4þ
0.07(1) 0.04(2)
0.65(1) 0.08(1)
39 61
SrFe0.80Sc0.20
Fe(3þ3)þ Fe4þ
0.07(1) 0.04(2)
0.68(8) 0.08(1)
64 36
SrFe0.70Sc0.30
Fe(3þ3)þ Fe4þ
0.11(1) 0.04(1)
0.68(1) 0.08(1)
91 9
SrFe0.60Sc0.40
Fe(3þ3)þ Fe3þHQS Fe4þ
0.10(2) 0.11(2) 0.04(2)
0.67(1) 1.49(2) 0.08(1)
89 6 5
Fe3þ Fe3þHQS
0.07(2) 0.13(1)
0.56(3) 1.44(1)
67 33
SrFe0.50Sc0.50
100 90
Fe content ( O 2 annealed )
Samples
80
4+
% Fe (3+ε)+ % Fe 3+ % Fe HSQ
70 60 50 40 30 20 10 0 0.0
0.1
0.2
0.3
0.4
0.5
Sc content Fig. 7. Isomer shifts (up) and relative intensities (down) versus scandium content of the contributions used to fit the Mössbauer spectra of the O2-annealed SrFe1xScxO3d samples.
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Y. Rizki et al. / Solid State Sciences 12 (2010) 739e744
25
present in the compounds whatever x. Secondly, by Mössbauer spectroscopy we highlighted that the progressive substitution of iron by scandium is accompanied with an oxygen content decrease which decreases the Fe4þ content until its complete disappearance for x ¼ 0.5. In the SrFe0.5Sc0.5O2.5 compound, iron is present as Fe3þ only.
20
Acknowledgements
15
Financial support by the French “Agence Nationale de la Recherche” under ANR-08-BLAN-0005-01 project is gratefully acknowledged.
35
3+
% Fe
HQS 3+
HQS
Fe
3+
content
30
% Fe
HQS
as-prepared O2 annealed
10 5
References
0 0.0
0.1
0.2
0.3
0.4
0.5
Sc content Fig. 8. Relative intensity of the Fe3þHQS contribution versus the scandium content.
environments around the remaining Fe4þ ions are weakly perturbed. The appearance of the Fe3þHQS doublet is due to the increase of O2 vacancies when Sc is added. In as-prepared samples, this doublet appears actually with an additional Sc amount of 0.3. Unlikely, in O2-annealed ones, we notice that it appears after a further Sc amount of 0.4 (Fig. 8). We can explain this by the effect of oxygen annealing which leads to the partial filling of the O2 vacancies and thus to their decrease. 4. Conclusion The SrFe1xScxO3d system has been studied for 0 x 0.5, revealing structural properties. Firstly, by combining X-ray and ED investigation it is demonstrated that no cationic nor anionic order is
[1] Y. Tokura (Ed.), Colossal Magnetoresistance Oxides, Gordon and Breach, London, 1999. [2] C.N.R. Rao, B. Raveau (Eds.), Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific, Singapore, 1998. [3] P.D. Battle, M.A. Green, J. Lago, A. Mihut, M.J. Rosseinsky, L.E. Spring, J. Singleton, J.F. Vente, Chem. Commun. 9 (1998) 987. [4] Y.M. Zhao, R. Mahendiran, N. Nguyen, B. Raveau, R.H. Yao, Phys. Rev. B 64 (2001) 024414. [5] A. Maignan, C. Martin, N. Nguyen, B. Raveau, Solid State Sci. 3 (2001) 57. [6] M.M. Patrakeev, A.A. Markov, I.A. Leonidov, V.L. Kozhevnikov, V.V. Kharton, Solid State Ionics 177 (2006) 1757. [7] K.V. Pokholok, D.S. Filimonov, N.A. Pichugin, A.P. Malakho, V.P. Gor'kov, N.N. Savvateev, Russ. J. Inorg. Chem. 50 (2005) 267. [8] T. Motohashi, B. Raveau, M. Hervieu, A. Maignan, V. Pralong, N. Nguyen, V. Caignaert, J. Phys. Condens. Matter 18 (2006) 2157. [9] F. Varret, J. Teillet, unpublished MOSFIT program, 1983. [10] A. Maignan, D. Pelloquin, D. Flahaut, V. Caignaert, J. Solid State Chem. 177 (2004) 3693. [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] M.V. Patrakeev, E.B. Mitberg, A.A. Lakhtin, I.A. Leonidov, V.L. Kozhevnikov, V.V. Kharton, M. Avdeev, F.M.B. Marques, J. Solid State Chem. 167 (2002) 203. [13] J.C. Waerenborgh, D.P. Rojas, A.L. Shaula, G.C. Mather, M.V. Patrakeev, V.V. Kharton, J.R. Frade, Mater. Lett. 59 (2005) 1644.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
A Mössbauer spectroscopy investigation of SrFe1xScxO3d perovskites Youssef Rizki a, Jean-Marie Le Breton a, *, Emeric Folcke a, Luc Lechevallier a, Yohann Bréard b, Antoine Maignan b a b
Groupe de Physique des Matériaux, UMR CNRS 6634, Université de Rouen, 76801 Saint Etienne du Rouvray, France Laboratoire CRISMAT, UMR 6508 CNRS ENSICAEN, 6 bd du Maréchal Juin, 14050 CAEN Cedex 4, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2009 Received in revised form 28 January 2010 Accepted 13 February 2010 Available online 19 February 2010
Polycrystalline samples of oxygen deficient perovskites SrFe1xScxO3d (0 x 0.5) have been synthesized by direct solid state reactions. Each compound has been stabilized with two different oxygen stoichiometries. The structural study shows, firstly, the good cationic homogeneity of the samples and, secondly, that the Sc and Fe atoms are randomly distributed over the same crystallographic site, whatever the scandium content. This implies that no anionic order is possible. A detailed Mossbauer spectroscopy study clearly shows that the substitution of scandium for iron involves an oxygen content decrease which decreases the tetravalent iron content until its total disappearance for x ¼ 0.5. The evolutions of the isomer shift, the quadrupolar splitting and the relative intensity versus the Sc3þ content are depicted in the present paper. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Perovskites SrFeO3 Mössbauer spectroscopy Valence state of iron Oxygen deficiency
1. Introduction Considerable interest has been focused on the defect structure and transport properties of perovskite-type compounds due to their potential applications. Therefore, numerous studies on the magnetoresistance have been conducted for the purpose of understanding the origin of the phenomena [1,2]. The transition metal of our interest is iron, because of its ability to adopt various oxidation states: Fe2þ, Fe3þ, Fe4þ and conduct different crystallographic structures (tetrahedral, octahedral.). Indeed, the existence of different valence states of iron in the same compound affects the magnetic properties of this compound, and its electronic transport properties. Moreover, iron cations in oxides are very attractive candidates to generate magnetoresistance, especially if one considers the possibility to get the ferromagnetic metallic state in Fe3þ/Fe4þ oxides with mixed valency [3]. This has been observed in the perovskite phase SrFeO3d which exhibited negative magnetoresistance (MR) properties reaching at 20 K about 8% under 7 T [4,5]. However, even if the lack of Fe4þ cations in SrFeO2.5 could explain the absence of MR, it must be mentioned that this compound does not crystallize in a perovskite structure but rather in an orthorhombic brownmillerite one.
* Corresponding author. E-mail address: [email protected] (J.-M. Le Breton). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.02.027
It is well known that partial substitution of Fe3þ/Fe4þ for Sc3þ in SrFeO3d leads to the formation of oxygen vacancies in order to maintain the electroneutrality in the crystal, these vacancies leading to high ionic and electronic conductivities. The distribution of the iron cations thus influences the conductivity of SrFe1xScxO3d compounds and its knowledge is required to characterize its potential implication for the interpretation of the conductivity data. For example, conductivity measurements showed that the ion conduction at 700e950 C is improved for x ¼ 0.1 [6]. Mössbauer spectroscopy is a very useful tool to investigate the distribution of iron cations in Fe-containing compounds. However, there is limited Mössbauer spectroscopy data for SrFe1xScxO3d [7]. In this paper, we report a Mössbauer spectroscopy study of the SrFe1xScxO3d (x 0.5, 0.1 d 0.5) series, in order to investigate the distribution of iron cations.
2. Experimental Polycrystalline samples of oxygen deficient perovskites SrFe1xScxO3d (x ¼ 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5) were synthesized by direct solid state reactions using adequate mixtures of SrO2, Fe2O3, and Sc2O3. These precursors were thoroughly mixed and pressed in the form of bars. The latter were then heated up in
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Y. Rizki et al. / Solid State Sciences 12 (2010) 739e744
air to 1200 C for 12 h. Heating and cooling down to room temperature were performed within 6 h. Oxygen-rich compounds were obtained by annealing the asprepared ones, under high oxygen pressure (15 MPa) at 600 C for 12 h. Phase purity was checked by X-ray diffraction using a Xpert Pro diffractometer working with Cu(Ka) radiation. Data were collected over an angular range 5 < 2q < 120 . X-ray diffraction patterns were used to check for the sample purity and to measure the lattice parameters. The oxygen stoichiometries were determined by the cerimetric titration method described elsewhere [8]. Samples for transmission electron microscopy were crushed in n-butanol and small flakes were deposited on a holey carbon film supported by a copper grid. The electron diffraction investigation was carried out with a JEOL 200CX microscope, tilting around the crystallographic axes. This microscope is equipped with an EDS analyzer. The 57Fe Mössbauer spectra were recorded at room temperature in transmission mode, using a conventional spectrometer operating in the constant acceleration mode from 2.5 to 2.5 mm/s. 57Co/Rh was used as the g-ray source. The isomer shift (relative to metallic a-Fe at room temperature) and the quadrupole splitting are denoted d and D, respectively. Estimated errors for the hyperfine parameters originate from the statistical errors s given by the fitting program, taking 3s [9]. 3. Results 3.1. Structural properties The X-ray diffraction patterns of the as-prepared simples are shown in Fig. 1. The patterns reveal that the samples are singlephase. All the diffraction peaks can be indexed in the cubic Pm3m space group. The same results were obtained for the O2-annealed samples. The unit cell parameters are provided in Table 1 along with the oxygen content determined through cerimetric titration. It is found that for both series of compounds, the oxygen content decreases as the Sc3þ content increases (Fig. 2). Moreover, for a given Sr content, a significant increase of the oxygen content is observed in the O2-annealed samples, compared with the as-
Table 1 Nominal cationic composition, oxygen content and lattice parameter for as-prepared and O2-annealed for ScFe1xScxO3d. Nominal cationic composition
Oxygen content (as-prepared/ O2-annealed)
a (Å) (as-prepared/ O2-annealed)
SrFe SrFe0.95Sc0.05 SrFe0.9Sc0.1 SrFe0.8Sc0.2 SrFe0.7Sc0.3 SrFe0.6Sc0.4 SrFe0.5Sc0.5
2.85/2.95 2.84/2.95 2.78/2.81 2.74/2.77 2.64/2.73 2.59/2.63 2.50/2.50
3.8657(5)/3.8539(2) 3.869(2)/3.8685(4) 3.8768(2)/3.8660(5) 3.8991(3)/3.8916(3) 3.923(3)/3.9118(1) 3.957(4)/3.93306(2) 3.9755(2)/3.9750(5)
prepared samples. These results point out the decreasing ability of the as-prepared samples to uptake oxygen under oxidizing conditions as the Sc3þ content increases; for instance, the post-annealing treatments make the oxygen content increasing from O2.85 to O2.95 in SrFeO3d (x ¼ 0) while the oxygen content keeps a constant O2.5 value in the case of SrFe0.5Sc0.5O3d (x ¼ 0.5) (Table 1). For both series, the increase of the unit cell parameter as the Sc3þ content increases from 0 to 0.5 is due to the larger ionic radius A) compared to those of Fe3þ and Fe4þ of Sc3þ (rSc3þ ¼ 0.745 (rFe3þ ¼ 0.645 A, rFe4þ ¼ 0.585 A) (Fig. 3). Similar results were reported in the case of the SrCo1xScxO3d series [10]. For a given Sc content, the (a) value decreases as the oxygen content increases. The increase of the oxygen content involves an increase of the Fe4þ content to maintain the electroneutrality of the samples. As the ionic radius of Fe4þ is lower than that of Fe3þ, the decrease of the unit cell parameter is directly related to the increase of the Fe4þ content in the sample. The examination of the X-ray diffraction patterns of the asprepared samples initially demonstrated the existence of Sc2O3 impurities for x > 0.5 in SrFe1xScxO3d. Afterwards, the study of the cation compositions by EDX, along with the Electron Diffraction (ED) for all as-prepared compounds corresponding to x < 0.5, displayed a homogenous distribution of the Sc/Fe cations in the microcrystallites, leading to cation contents very near the starting compositions. The lack of additional diffraction spots on the ED pattern of both as-prepared and post-annealed series of samples, as shown in Fig. 4 for SrFe0.5Sc0.5O2.5, points towards a lack of oxygen ordering phenomena in this cubic perovskite structure.
3.0
as-prepared O2 annealed
Oxygen content
2.9 2.8 2.7 2.6 2.5 0.0
0.1
0.2
0.3
0.4
0.5
Sc content Fig. 1. Experimental X-ray powder diffraction patterns of SrFe1xScxO3d samples (0 x 0.5).
the
as-prepared
Fig. 2. Sc content dependence of the oxygen content in both series of SrFe1xScxO3d samples.
Y. Rizki et al. / Solid State Sciences 12 (2010) 739e744
- a paramagnetic contribution fitted with one or two doublets corresponding to Fe(3þ3)þ [4,5], or Fe3.5þ [11] (intermediate valence state of Fe between Fe3þ and Fe4þ, due to a rapid electron exchange between Fe4þ and Fe3þ that occurs in the ferrite structure at ordinary temperature [7]).
3.98
3.96
parameter a (A)
741
3.94
3.92
3.90
3.88
3.86 0.0
0.1
0.2
0.3
0.4
0.5
Sc content Fig. 3. Sc content dependence of the unit cell parameter a of the cubic phase for the as-prepared SrFe1xScxO3d samples. The line is a guide for the eye.
Regardless of the changes of the unit cell due to the oxygen postannealing, the ED patterns of the O2-annealed samples show a deficit of reflections related to the oxygen ordering phenomena as well. These points are related to the disordering effect of the oxygen network supported by the Sc3þ cations, in marked contrast with the Sr2Fe2O5, brownmillerite formation as the oxygen content decreases in SrFeO3d. Therefore, the lack of Fe/Sc ordering is the most significant characteristic in the SrFe1xScxO3d series as shown by the cubic Pm3m perovskite structure of SrFe0.5Sc0.5O2.5. The lack of oxygen uptake under oxygen pressure for the latter is indeed related to the cation disorder mentioned, which prevents the oxygen ordering phenomenon. 3.2. Mössbauer spectroscopy In Fig. 5 are shown the room temperature Mössbauer spectra of the as-prepared and annealed SrFe1xScxO3d (x ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5) samples. For samples with an oxygen content of more than 2.7, satisfactory fittings were obtained with two contributions: - a paramagnetic contribution fitted with one singlet, with an isomer shift typical of Fe4þ [7,11],
For samples with an oxygen content lower than 2.7, satisfactory fittings were obtained with three contributions; the two previous contributions and a paramagnetic contribution with a high quadrupole splitting, assigned to structurally distorded Fe sites [12] namely Fe3þHQS. The refined values of the isomer shift (d), the quadrupolar splitting (D) and the relative intensity (%) of both series are given in Table 2 (as-prepared) and Table 3 (O2-annealed). One paramagnetic doublet is sufficient to fit the Fe(3þ3)þ contribution when its relative intensity is less than 40%. When its relative intensity is higher, this contribution appears to be asymmetric, and several doublets are needed to account for the asymmetry. The Fe(3þ3)þ paramagnetic contribution can be fitted with a distribution of quadrupole doublets, as made by Waerenborgh et al. [13]. However, two doublets allow to take in account the asymmetry, leading to satisfactory fittings. In that case, only the mean d and D values of the contribution are significant, and have thus to be considered. Thus, the d and D values of the Fe(3þ3)þ contribution reported in Tables 2 and 3 are mean values. It is worth to mention that we also fitted the Fe(3þ3)þ paramagnetic contribution with a distribution of quadrupole doublets. This leads to mean d and D values that are equal to the values obtained with two doublets, according to the statistical errors. 3.2.1. The as-prepared series The evolution of both the isomer shift of the Fe4þ and (3þ3)þ contributions and the relative intensities of all the Fe contributions used to fit the spectra are shown in Fig. 6. For all compositions, the isomer shift of the Fe4þ singlet is in the range 0.01e0.05 mm/s. The contribution of Fe(3þ3)þ has an isomer shift between 0.07 and 0.17 mm/s and a quadrupolar shift between 0.56 and 0.80 mm/s. The high value of the quadrupolar splitting of the Fe3þHQS doublet (D z 1.44e1.45 mm/s) is explained by the low oxygen content of the corresponding samples (x ¼ 0.3, 0.4, 0.5) implying ion coordination polyhedra with less than six oxygen atoms. We note that Sc addition decreases the Fe4þ content (Fig. 6) as observed for Al-doped and Ga-doped SrFeO3d
Fig. 4. Electron diffraction patterns of the SrFe0.5Sc0.5O2.5 sample.
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Y. Rizki et al. / Solid State Sciences 12 (2010) 739e744
b-O2-annealed
a- as-prepared 1.00
1.00
x=0 0.93
0.96
1.00
1.00
x = 0.05 0.97
0.96
1.00
1.00
x = 0.1 0.94
0.98
1.00
1.00
x = 0.2 0.98
0.98
1.00
1.00
x = 0.3 0.98
0.98
1.0 1.00
1.00
x = 0.4 0.98
0.66
1.00
x = 0.5
0.94 Fig. 5. Room temperature Mössbauer spectra of (a) the as-prepared samples and (b) the O2-annealed samples of the series SrFe1xScxO3d (0 x 0.5).
based phases [12,13], and decreases the oxygen content from (2.85) to (2.50) (Fig. 2) in order to maintain the electronic neutrality of the samples. The increase of the Sc content produces a deficit of positive charges due to the decrease of oxygen content which is reflected by a decrease in the valence of iron from Fe4þ to Fe3þ. We observe a decrease of the isomer shift of the Fe(3þ3)þ doublet down to the negative values. This can be explained by the change of the environment of Fe(3þ3)þ
ions, as the Sc content increases. The isomer shift of the Fe4þ singlet remains almost constant as the Sc content increases, indicating that the environments around the remaining Fe4þ ions are not perturbed by the reduction of the oxygen content. We note that there are only Fe3þ cations in SrFe0.5Sc0.5O2.5 in relation with the fact that the Sc addition decreases the content of Fe4þ. Hence, the Fe3þ content increases while the Fe4þ content decreases until it disappears completely (Fig. 6).
Y. Rizki et al. / Solid State Sciences 12 (2010) 739e744 Table 2 Fitted room temperature Mössbauer spectroscopy hyperfine parameters (d: isomer shift, D: quadrupole splitting) and relative intensities for the SrFe1xScxO3d (0 x 0.5) as-prepared samples.
0.25 0.20
Samples
Assignment
D (mm/s)
Area (%)
0.15
SrFe
Fe(3þ3)þ Fe4þ
0.16(1) 0.01(2)
0.74(2) 0.12(1)
50 50
0.10
SrFe0.95Sc0.05
Fe(3þ3)þ Fe4þ
0.17(1) 0.02(2)
0.80(6) 0.14(1)
51 49
SrFe0.90Sc0.10
Fe(3þ3)þ Fe4þ
0.04(2) 0.05(2)
0.61(2) 0.09(5)
65 35
SrFe0.80Sc0.20
Fe(3þ3)þ Fe4þ
0.00(2) 0.02(2)
0.68(2) 0.08(2)
90 10
SrFe0.70Sc0.30
Fe(3þ3)þ Fe3þHQS Fe4þ
0.03(2) 0.16(4) 0.02(2)
0.59(1) 1.45(1) 0.08(1)
73 22 5
SrFe0.60Sc0.40
Fe(3þ3)þ Fe3þHQS Fe4þ
0.03(1) 0.14(1) 0.02(2)
0.56(1) 1.45(1) 0.08(1)
68 29 3
Fe3þ Fe3þHQS
0.07(2) 0.13(1)
0.56(3) 1.44(1)
67 33
SrFe0.50Sc0.50
d (mm/s)
743
0.05 0.00 -0.05 -0.10
100 90 80 70 60 50 40 30 20 10 0 0.0
0.1
0.2
0.3
0.4
0.5
Sc content Fig. 6. Isomer shifts (up) and relative intensities (down) versus scandium content of the contributions used to fit the Mössbauer spectra of the as-prepared SrFe1xScxO3d samples.
δ (mm/s)
3.2.2. The O2-annealed series The evolution of both the isomer shift of the Fe4þ and (3þ3)þ contributions and the relative intensities of all the Fe contributions used to fit the spectra are shown in Fig. 7. The post-annealing treatments make the oxygen content increasing for all samples (from O2.85 to O2.95 in SrFeO3d) (Table 1), except for the SrFe0.5Sc0.5O3d sample for which the oxygen content remains constant at O2.5. We note that, like in the as-prepared series, increasing the Sc content decreases the proportion of Fe4þ (Fig. 7), and decreases the oxygen content from (2.92) to (2.50) (Fig. 2). For the same Sc content we observe a very significant difference between the spectra of the two series which reveals the effect of O2 on the samples. Like in the asprepared sample we observe a decrease of the isomer shift of the Fe(3þ3)þ doublet down to negative values. This can be explained by the change of the environment of Fe(3þ3)þ ions, as the Sc content increases, in relation with the decrease of the oxygen content. The isomer shift of the Fe4þ singlet changes slightly, as the Sc content increases, indicating that the
0.15
δ (Fe )
0.10
δ (Fe
4+
(3+ε)+
)
0.05 0.00 -0.05 -0.10
Table 3 Fitted room temperature Mössbauer spectroscopy hyperfine parameters (d: isomer shift, D: quadrupole splitting) and relative intensities for the SrFe1xScxO3d (0 x 0.5) O2-annealed samples. Assignment
SrFe
Fe(3þ3)þ Fe4þ
SrFe0.95Sc0.05
(3þ3)þ
Fe Fe4þ
(3þ3)þ
D (mm/s)
Area (%)
0.15(1) 0.06(2)
0.59(1) 0.06(1)
31 69
0.12(1) 0.04(2)
0.55(1) 0.08(2)
38 62
d (mm/s)
SrFe0.90Sc0.10
Fe Fe4þ
0.07(1) 0.04(2)
0.65(1) 0.08(1)
39 61
SrFe0.80Sc0.20
Fe(3þ3)þ Fe4þ
0.07(1) 0.04(2)
0.68(8) 0.08(1)
64 36
SrFe0.70Sc0.30
Fe(3þ3)þ Fe4þ
0.11(1) 0.04(1)
0.68(1) 0.08(1)
91 9
SrFe0.60Sc0.40
Fe(3þ3)þ Fe3þHQS Fe4þ
0.10(2) 0.11(2) 0.04(2)
0.67(1) 1.49(2) 0.08(1)
89 6 5
Fe3þ Fe3þHQS
0.07(2) 0.13(1)
0.56(3) 1.44(1)
67 33
SrFe0.50Sc0.50
100 90
Fe content ( O 2 annealed )
Samples
80
4+
% Fe (3+ε)+ % Fe 3+ % Fe HSQ
70 60 50 40 30 20 10 0 0.0
0.1
0.2
0.3
0.4
0.5
Sc content Fig. 7. Isomer shifts (up) and relative intensities (down) versus scandium content of the contributions used to fit the Mössbauer spectra of the O2-annealed SrFe1xScxO3d samples.
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Y. Rizki et al. / Solid State Sciences 12 (2010) 739e744
25
present in the compounds whatever x. Secondly, by Mössbauer spectroscopy we highlighted that the progressive substitution of iron by scandium is accompanied with an oxygen content decrease which decreases the Fe4þ content until its complete disappearance for x ¼ 0.5. In the SrFe0.5Sc0.5O2.5 compound, iron is present as Fe3þ only.
20
Acknowledgements
15
Financial support by the French “Agence Nationale de la Recherche” under ANR-08-BLAN-0005-01 project is gratefully acknowledged.
35
3+
% Fe
HQS 3+
HQS
Fe
3+
content
30
% Fe
HQS
as-prepared O2 annealed
10 5
References
0 0.0
0.1
0.2
0.3
0.4
0.5
Sc content Fig. 8. Relative intensity of the Fe3þHQS contribution versus the scandium content.
environments around the remaining Fe4þ ions are weakly perturbed. The appearance of the Fe3þHQS doublet is due to the increase of O2 vacancies when Sc is added. In as-prepared samples, this doublet appears actually with an additional Sc amount of 0.3. Unlikely, in O2-annealed ones, we notice that it appears after a further Sc amount of 0.4 (Fig. 8). We can explain this by the effect of oxygen annealing which leads to the partial filling of the O2 vacancies and thus to their decrease. 4. Conclusion The SrFe1xScxO3d system has been studied for 0 x 0.5, revealing structural properties. Firstly, by combining X-ray and ED investigation it is demonstrated that no cationic nor anionic order is
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