- Email: [email protected]
Structural studies of SrFeO3 and SrFe0.5Nb0.5O3 by employing XRD and XANES spectroscopic techniques
M A TE RI A L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 0 1 6–1 0 2 0
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Structural studies of SrFeO3 and SrFe0.5Nb0.5O3 by employing XRD and XANES spectroscopic techniques M. Javed Akhtar⁎, R. Tahir Ali Khan Physics Division, PINSTECH, P. O. Nilore, Islamabad 1482, Pakistan
AR TIC LE D ATA
ABSTR ACT
Article history:
The perovskite based SrFeO3 and SrFe0.5Nb0.5O3 materials have been synthesized by solid state
Received 31 March 2011
reaction methods. The structural properties are investigated using a combination of X-ray
Received in revised form 1 July 2011
diffraction and X-ray absorption fine structure spectroscopic techniques. From the Rietveld
Accepted 22 July 2011
refinement of the X-ray diffraction data it has been observed that SrFeO3 has a simple cubic perovskite structure, which is consistent with the previous literature results; whereas
Keywords:
SrFe0.5Nb0.5O3 shows a tetragonal structure within P4mm space group. X-ray absorption
Structure
results demonstrate that the valence state of Fe in SrFeO3 is (IV); however, it changes to (III)
SrFeO3
when 50% Nb5+ is substituted at the Fe sites.
SrFe0.5Nb0.5O3
© 2011 Elsevier Inc. All rights reserved.
X-ray diffraction X-ray absorption
1.
Introduction
Recently, complex iron based perovskite type materials having general formula A(Fe0.5B0.5)O3, where A = Ca, Ba, Sr; B = Nb, Ta, Sb have attracted considerable attention [1–5]. The reason for this interest is that, these materials show high dielectric permittivity over a wide temperature range. These lead-free materials with high dielectric constants are becoming increasingly attractive, because these can replace lead rich relaxor ferroelectrics; which are environmentally pollutants. These materials play imperative role in microelectronics and have various technological applications, such as multilayer capacitors, microwave frequency resonators, sensors, detectors, memory devices and actuators. The complex perovskites, A(B′B″)O3, have many important applications in various fields, such as gas sensors, catalysts, electrical conductors, magnetoresistance, solid oxide fuel cells (SOFCs), memory devices, multilayer capacitors and ferroelectric relaxors [6–10].
In the perovskite based, ABO3, materials where A site cations occupy center of the cube and adopt twelve-fold oxygen coordination and B site cations, having six nearest neighbors oxygen, take up octahedral geometry. In these materials, the physical properties depend on the nature and stoichiometry of the A- and B-site cations. For the complex perovskites, A(B′B″)O3, the properties depend on the cation arrangement at the B sublattice and are controlled primarily by the charge difference between the B site cations and secondly by the ionic size difference between B′ and B″. Saha and Sinha have carried out the structural studies of BaFe0.5Nb0.5O3 and SrFe0.5Nb0.5O3 systems by employing XRD techniques and indexed both systems in monoclinic phases using a standard computer program (POWD) [3,4]. However, the XRD studies by other groups showed that BaFe0.5Nb0.5O3 has a cubic symmetry [5,11,12]; SrFe0.5Nb0.5O3 has also been indexed on the basis of orthorhombic and cubic phases [12,13]. From these studies it is noticed that no detailed structural data
⁎ Corresponding author. Tel.: +92 51 9244801 7; fax: +92 51 9244808. E-mail address: [email protected] (M.J. Akhtar). 1044-5803/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2011.07.014
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 0 1 6–1 0 2 0
Fig. 1 – X-ray diffraction pattern of SrFeO3; crosses are observed intensities, the red line represents calculated pattern and the lower line is the difference between observed and calculated. Vertical lines show the reflection positions. Inset highlights XRD pattern for 65° ≤ 2θ ≤ 100°.
for SrFe0.5Nb0.5O3 is available in the literature; whereas the available data is also contradicting with each other [3,12,13]. A charge disproportionation (2Fe4+ → Fe3+ + Fe5+) with oxygen vacancies has also been reported for perovskite type SrFeO3-δ materials [14]. In case of perovskites, the crystal structure and concentration of oxygen defects are critical parameters, which have pronounced effects on the electrical, transport and mechanical properties of these materials. In the present study, we employ a combination of XRD and XANES spectroscopic techniques to investigate the structural parameters and valence state of Fe in SrFeO3 and SrFe0.5Nb0.5O3.
2.
Experimental Methods
Samples used in the present study were synthesized by solid state reaction procedures [6]. Powders of SrCO3, Fe2O3 and Nb2O5 were mixed thoroughly in appropriate amounts and well ground in acetone. After drying, the mixtures were heated in alumina boats for three days at 1350 °C in air. Following cooling to room temperature these samples were again heated under the same conditions. After the second heat treatment the materials were ground to a fine powder, X-ray diffraction was used to confirm that single phase materials have been prepared. Powder X-ray diffraction measurements were performed, at room temperature, using Philips Vax Rd diffractometer operating with Cu Kα radiation. The XRD data were scanned over the angular range of 15° ≤ 2θ ≤ 140° at a step size of 0.02° and counting time of 10 s per step. The X-ray Absorption Near Edge Structure (XANES) data were collected at the XAFS beam line (11.1) at the ELETTRA Synchrotron Trieste, Italy, with the storage ring running at 2 GeV and a typical current of 200 mA. These data were collected at room temperature and in the transmission mode using Si(111) monochromator. The samples were prepared by deposition from a powder suspension in cyclohexane on a Millipore membrane (type GS 0.22 μm).
3.
Results and Discussion
3.1.
XRD
The X-ray diffraction data have been analyzed using the computer program Rietica [15,16] that uses a full profile Rietveld analysis method [17]. The background was fitted with a simple fourth order polynomial in 2θ and was refined simultaneously with other profile and structural parameters. A pseudo-Voigt peak shape function was employed to model the peak profile. The angular dependence of the peak full width at half maximum was refined with three parameters according to the function described by Caglioti et al. [18]. Instrument peak asymmetry was modeled using a single asymmetry parameter after Howard [19], while preferred orientation was described by one parameter March model [20,21]. The SrFeO3 data was refined using a simple cubic perovskite model in space group Pm-3m [22]. Structural parameters included in the refinement were cell constant, individual temperature factors of Sr, Fe and O; the fractional site occupancy of O was also refined. The final X-ray diffraction pattern and refined crystallographic parameters are presented in Fig. 1 and Table 1, respectively.
Table 1 – Structural parameters for SrFeO3, refined in space group Pm-3m. Atom
Site
Sr Fe O
1b 1a 3d
x
y
z
N⁎
Biso(Å2)
0.5 0 0
0.5 0 0
0.5 0 0.5
1 1 2.94(1)
0.71(1) 0.54(1) 1.65(5)
Note. a = 3.8700(1)Å; *Site occupancy. Rwp = 8.987, Rp = 6.471, χ2 = 3.19
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 0 1 6–1 0 2 0
Fig. 2 – Comparison of shift and splitting of peaks for SrFeO3 and SrFe0.5Nb0.5O3; (a) [(013), (130), (031)] and (b) [(123), (132), (231)] reflections. We observed that when SrFeO3 is doped with Nb5+ at Fe sites there is a shift in the peak positions and some peak split. For elucidation, here we have shown the shift in positions and splinting of two sets of reflections; Fig. 2(a) shows the reflections of (013), (130) and (031) planes; whereas Fig. 2(b) shows (123), (132) and (231) group of reflections. From these results we can infer that there is a change from cubic to some lower symmetry for SrNb0.5Fe0.5O3. Best fit between observed and calculated diffraction pattern was obtained in the tetragonal space group P4mm. The atomic coordinates of isostructural BaTiO3 [23] were used for initial model and were subsequently refined. Temperature factors of Nb and Fe were constrained to a single value. Similarly single temperature factor value was given to both oxygen sites. The final X-ray pattern of SrNb0.5Fe0.5O3 is presented in Fig. 3, while refined crystallographic parameters are presented in Table 2. A
comparison of bond lengths for SrFeO3 and SrNb0.5Fe0.5O3 is presented in Table 3; we note that average bond lengths increase due to Nb5+ doping.
3.2.
XAFS Spectroscopy
X-ray Absorption Fine Structure (XAFS) spectroscopy has revealed itself as a powerful technique for structural characterization of the local atomic environment of individual atomic species, including bond distances, coordination numbers and type of nearest neighbors surrounding the central atom. This technique is particularly useful for materials that show considerable structural and chemical disorder. X-ray absorption edges contain a variety of information on the chemical state and the local structure of the absorbing atom. On the higher energy side of an absorption edge fine
Fig. 3 – X-ray diffraction pattern of SrFe0.5Nb0.5O3; crosses are observed intensities, the red line represents calculated pattern and the lower line is the difference between observed and calculated. Vertical lines show the reflection positions. Inset highlights XRD pattern for 65° ≤ 2θ ≤ 100°.
M A TE RI A L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 0 1 6–1 0 2 0
1019
Table 2 – Structural parameters for SrNb0.5Fe0.5O3, refined in space group P4mm. Atom
Site
Sr Nb Fe O(1) O(2)
1a 1b 1b 1b 2c
x
y
z
N⁎
Biso(Å2)
0 0.5 0.5 0.5 0.5
0 0.5 0.5 0.5 0
0 0.5041 0.5041 0.0340 0.4569
1 0.5 0.5 1 2
0.74(1) 0.46(1) 0.46(1) 1.72(3) 1.72(3)
Note. a = 3.9656(1)Å, c = 3.9795(1)Å; *Site occupancy. Rwp = 7.571, Rp = 5.048, χ2 = 2.14
structure is observed due to backscattering of the emitted photoelectron. The post-edge region can be divided into two parts. Within 50 eV of an absorption edge, the spectrum is interpreted in terms of the appropriate components of the local density of states, which would be expected to be sensitive to the valence state of the atom. The edge position shifts towards higher energy with increase in the oxidation state [24]. In this near-edge region (XANES) absorption modulations are strongly due to multiple scattering [25]. XANES is based on a finger printing technique, where a measured spectrum is identified by comparison with reference spectra obtained for standard materials. A direct comparison of the absorption edges for atoms of interest with known model compounds provides information on the effective charges of atoms in different systems. In this study, the normalization of XANES spectra was carried out by employing standard edge step normalization procedure [26]. The normalized XANES spectra were obtained by subtracting the smooth pre-edge absorption from the experimental spectra and taking edge jump height as unity; further details of the normalization procedure can be found in Ref. [27]. The normalized XANES spectra of the Fe K-edge are shown in Fig. 4; the interpretations of K-edge XANES features for 3d transition metal oxides are well established [28–30]. A small pre-edge peak is observed in all spectra, which is due to 1s to 3d (formally electric dipole forbidden) transition, whereas main peak can be attributed to 1s to 4p transitions [28]. In the present study the Fe K-edge data has been used to investigate the valence state of Fe in SrFe0.5Nb0.5O3 and SrFeO3; we have taken Fe2O3 and Fe3O4 as model compounds. In Fe2O3 the valence state of Fe is III whereas in Fe3O4 it is a mixture of II and III. Fig. 4 shows the normalized XANES spectra of these
Table 3 – Bond lengths (Å) for SrFeO3 and SrNb0.5Fe0.5O3. Bond type
SrFeO3
Fe–O
1.9350 × 6
Fe–Sr
3.3515 × 8
Fe–Fe/Nb
3.8700 × 6
SrNb0.5Fe0.5O3 1.8708 × 1 1.9917 × 4 2.1087 × 1 Av. 1.9910 3.4289 × 4 3.4478 × 4 Av. 3.4384 3.9656 × 4 3.9795 × 2 Av. 3.9702
Fig. 4 – XANES spectra of SrFeO3 and SrFe0.5Nb0.5O3 compared with Fe foil, Fe2O3 and Fe3O4; inset shows the expanded edge region to illustrate energy shift.
materials along with Fe metal foil, which is used for the energy calibration [24,27]; from these results we note that there is a pronounced shift in the edge position. In case of Fe3O4 spectrum, edge is about ~ 1 eV below Fe2O3 and SrFe0.5Nb0.5O3, both these spectra have same energy shift at half of the normalized edge height. However, SrFeO3 edge is about ~2 eV on higher energy side than Fe2O3 and SrFe0.5Nb0.5O3, indicating that valence state of Fe in former is higher than the later ones. In case of Fe2O3 the valence state of Fe is (III), therefore it is evident that SrFe0.5Nb0.5O3 also has a valence state of (III), as both edges have same energy shift at the half of the normalized edge height (as illustrated in the inset of Fig. 4). When we compare the energy shift of SrFeO3 with SrFe0.5Nb0.5O3 a clear edge shift on the higher energy side indicates that the valence state of Fe is higher in SrFeO3 as compared to that of SrFe0.5Nb0.5O3. From these results we can deduce that SrFe0.5Nb0.5O3 has a perovskite type tetragonal structure, where extra positive charge on the lattice, due to Nb5+ doping at Fe site, changes the valence state of Fe from (IV) to (III).
4.
Conclusions
Single phase SrFeO3 and SrFe0.5Nb0.5O3 perovskite based materials were synthesized by solid state reaction procedures. From the analysis of XRD data it has been deduced that pure SrFeO3 has a cubic structure, with space group Pm-3m, consistent with previous reported results; however when 50% Nb5+ is doped at Fe4+ sites the structure changes to tetragonal symmetry having P4mm space group. The electroneutrality of the system is achieved by the conversion of Fe4+ to Fe3+.
Acknowledgements We acknowledge the support of Elettra Synchrotrone, Trieste, Italy, for the provision of beam time and ICTP-Elettra Users Programme for a financial grant. We are grateful to Dr. Luca
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 0 1 6–1 0 2 0
Olivi for his valuable suggestions and help during XAFS data collection.
REFERENCES [1] Wang Z, Chen XM. Ferromagnetic and antiferromagnetic behaviour in Ba(Fe0. 5Ta0. 5)O3 ceramics. J Phys D Appl Phys 2009;42:175005 (5 pp). [2] Raevski IP, Prosandeev SA, Bogatin AS, Malitskaya MA, Jastrabik L. High dielectric permittivity in AFe1/2B1/2O3 nonferroelectric perovskite ceramics (A = Ba, Sr, Ca; B = Nb, Ta, Sb). J Appl Phys 2003;93:4130–6. [3] Saha S, Sinha TP. Dielectric relaxation in SrFe1/2Nb1/2 O3. J Appl Phys 2006;99:014109-1 5 pp. [4] Saha S, Sinha TP. Structural and dielectric studies of BaFe0. 5Nb0. 5O3. J Phys Condens Matter 2002;14:249–58. [5] Intatha U, Eitssayeam S, Wang J, Tunkasiri T. Impedance study of giant dielectric permittivity in BaFe0. 5Nb0. 5O3 perovskite ceramic, Current. Appl Phys 2010;10:21–5. [6] Akhtar MJ, Akhtar ZN, Dragun JP, Catlow CRA. Electrical conductivity and X-ray absorption fine structure studies of SrFe1-xNbxO3 and BaFe1-xNbxO3 systems. Solid State Ionics 1997;104:147–58. [7] Wang Y, Chen J, Wu X. Preparation and gas-sensing properties of perovskite-type SrFeO3 oxide. Mater Lett 2001;49:361–4. [8] Shaula AL, Kharton VV, Vyshatko NP, Tsipis EV, Patrakeev MV, Marques FMB, et al. Oxygen ionic transport in SrFe1-yAlyO3-δ and Sr1-xCaxFe0.5Al0.5O3-δ ceramics. J Eur Ceram Soc 2005;25:489–99. [9] Battle PD, Rosseinsky MJ. Synthesis, structure, and magnetic properties of n = 2 Ruddlesden–Popper manganates. Curr Opin Solid State Mater Sci 1999;4:163–70. [10] Zhao YM, Zhou PF, Yang XJ, Qiu GM, Ping L. Magnetotransport properties of SrFeO2.95 perovskite. Solid State Commun 2001;120:283–7. [11] Intatha U, Eitssayeam S, Pengpat K, MacKenzie KJD, Tunkasiri T. Dielectric properties of low temperature sintered LiF doped BaFe0.5Nb0.5O3. Mater Lett 2007;61:196–200. [12] Rama N, Philipp JB, Opel M, Chandrasekaran K, Sankaranarayanan V, Gross R, et al. Study of magnetic properties of A2B′NbO6 ( A = Ba, Sr, BaSr; and B′ = Fe and Mn) double perovskites. J Appl Phys 2004;95:7528–30. [13] Rodriguez R, Fernandez A, Isalgue A, Rodriguez J, Labarta A, Tejada J, et al. Spin glass behavior in an antiferromagnetic non-frustrated lattice: Sr2FeNbO6 perovskite. J Phys C Solid State Phys 1985;18:L401–5. [14] Berry FJ, Bowfield AF, Coomer FC, Jackson SD, Moore EA, Slater PR, et al. Fluorination of perovskite-related phases of composition SrFe1-xSnxO3-δ. J Phys Condens Matter 2009;21: 256001 8 pp.
[15] Howard CJ, Hill RJ. AAEC/M112; 1986. [16] Wiles DB, Young RA. A new computer program for Rietveld analysis of X-ray powder diffraction patterns. J Appl Cryst 1981;14:149–51. [17] Rietveld HM. A profile refinement for nuclear and magnetic structures. J Appl Cryst 1969;2:65–71. [18] Cagliotti C, Paoletti A, Ricci FP. Choice of collimators for a crystal spectrometer for neutron diffraction. Nucl Instrum Meth 1958;3:223–8. [19] Howard CJ. The approximation of asymmetric neutron powder diffraction peaks by sum of Gaussians. J Appl Cryst 1982;15:615–20. [20] March A. Mathematische theorie der regelung nach der korngestalt bei affiner deformation. Z Kristallogr 1932;81: 285–97. [21] Dollase WA. Correction of intensities for preferred orientation in powder diffractometry: application of the March model. J Appl Cryst 1986;19:267–72. [22] Hodges JP, Short S, Jorgensen JD, Xiong X, Dabrowski B, Mini SM, et al. Evolution of oxygen-vacancy ordered crystal structures in the perovskite series SrnFenO3n-1 (n = 2, 4, 8, and ∞), and the relationship to electronic and magnetic properties. J Solid State Chem 2000;151:190–209. [23] Wyckoff RWG. Crystal structures. 2nd ed. New York: John Wiley & Sons; 1964. p. 401. [24] Olimov K, Falk M, Buse K, Woike T, Hormes J, Modrow H. X-ray absorption near edge spectroscopy investigations of valency and lattice occupation site of Fe in highly iron-doped lithium niobate crystals. J Phys Condens Matter 2006;18: 5135–46. [25] Bianconi A. XANES spectroscopy for local structures in complex systems. In: Bianconi A, Inconccia L, Stipcich S, editors. EXAFS and NEAR Edge Structure. Berlin: Springer; 1983. p. 118–29. [26] Wong J, Lyte FW, Messmer RP, Maylotte DH. K-edge absorption spectra of selected vanadium compounds. Phys Rev B 1984;30:5596–610. [27] Akhtar MJ, Nadeem M, Javed S, Atif M. Cation distribution in nanocrystalline ZnFe2O4 investigated using X-ray absorption fine structure spectroscopy. J Phys Condens Matter 2009;21: 405303–4053011. [28] de Groot F. High-resolution X-ray emission and X-ray absorption spectroscopy. Chem Rev 2001;101:1779–808. [29] Westre TE, Kennepohl P, DeWitt JG, Hedman B, Hodgson KO, Solomon EI. A multiplet analysis of Fe K-edge 1 s → 3 d pre-edge features of iron complexes. J Am Chem Soc 1997;119: 6297–314. [30] Chaurand P, Rose J, Briois V, Salome M, Proux O, Nassif V, et al. New methodological approach for the vanadium K-edge X-ray absorption near-edge structure interpretation: application to the speciation of vanadium in oxide phases from steel slag. J Phys Chem B 2007;111:5101–10.
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Structural studies of SrFeO3 and SrFe0.5Nb0.5O3 by employing XRD and XANES spectroscopic techniques M. Javed Akhtar⁎, R. Tahir Ali Khan Physics Division, PINSTECH, P. O. Nilore, Islamabad 1482, Pakistan
AR TIC LE D ATA
ABSTR ACT
Article history:
The perovskite based SrFeO3 and SrFe0.5Nb0.5O3 materials have been synthesized by solid state
Received 31 March 2011
reaction methods. The structural properties are investigated using a combination of X-ray
Received in revised form 1 July 2011
diffraction and X-ray absorption fine structure spectroscopic techniques. From the Rietveld
Accepted 22 July 2011
refinement of the X-ray diffraction data it has been observed that SrFeO3 has a simple cubic perovskite structure, which is consistent with the previous literature results; whereas
Keywords:
SrFe0.5Nb0.5O3 shows a tetragonal structure within P4mm space group. X-ray absorption
Structure
results demonstrate that the valence state of Fe in SrFeO3 is (IV); however, it changes to (III)
SrFeO3
when 50% Nb5+ is substituted at the Fe sites.
SrFe0.5Nb0.5O3
© 2011 Elsevier Inc. All rights reserved.
X-ray diffraction X-ray absorption
1.
Introduction
Recently, complex iron based perovskite type materials having general formula A(Fe0.5B0.5)O3, where A = Ca, Ba, Sr; B = Nb, Ta, Sb have attracted considerable attention [1–5]. The reason for this interest is that, these materials show high dielectric permittivity over a wide temperature range. These lead-free materials with high dielectric constants are becoming increasingly attractive, because these can replace lead rich relaxor ferroelectrics; which are environmentally pollutants. These materials play imperative role in microelectronics and have various technological applications, such as multilayer capacitors, microwave frequency resonators, sensors, detectors, memory devices and actuators. The complex perovskites, A(B′B″)O3, have many important applications in various fields, such as gas sensors, catalysts, electrical conductors, magnetoresistance, solid oxide fuel cells (SOFCs), memory devices, multilayer capacitors and ferroelectric relaxors [6–10].
In the perovskite based, ABO3, materials where A site cations occupy center of the cube and adopt twelve-fold oxygen coordination and B site cations, having six nearest neighbors oxygen, take up octahedral geometry. In these materials, the physical properties depend on the nature and stoichiometry of the A- and B-site cations. For the complex perovskites, A(B′B″)O3, the properties depend on the cation arrangement at the B sublattice and are controlled primarily by the charge difference between the B site cations and secondly by the ionic size difference between B′ and B″. Saha and Sinha have carried out the structural studies of BaFe0.5Nb0.5O3 and SrFe0.5Nb0.5O3 systems by employing XRD techniques and indexed both systems in monoclinic phases using a standard computer program (POWD) [3,4]. However, the XRD studies by other groups showed that BaFe0.5Nb0.5O3 has a cubic symmetry [5,11,12]; SrFe0.5Nb0.5O3 has also been indexed on the basis of orthorhombic and cubic phases [12,13]. From these studies it is noticed that no detailed structural data
⁎ Corresponding author. Tel.: +92 51 9244801 7; fax: +92 51 9244808. E-mail address: [email protected] (M.J. Akhtar). 1044-5803/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2011.07.014
1017
M A TE RI A L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 0 1 6–1 0 2 0
Fig. 1 – X-ray diffraction pattern of SrFeO3; crosses are observed intensities, the red line represents calculated pattern and the lower line is the difference between observed and calculated. Vertical lines show the reflection positions. Inset highlights XRD pattern for 65° ≤ 2θ ≤ 100°.
for SrFe0.5Nb0.5O3 is available in the literature; whereas the available data is also contradicting with each other [3,12,13]. A charge disproportionation (2Fe4+ → Fe3+ + Fe5+) with oxygen vacancies has also been reported for perovskite type SrFeO3-δ materials [14]. In case of perovskites, the crystal structure and concentration of oxygen defects are critical parameters, which have pronounced effects on the electrical, transport and mechanical properties of these materials. In the present study, we employ a combination of XRD and XANES spectroscopic techniques to investigate the structural parameters and valence state of Fe in SrFeO3 and SrFe0.5Nb0.5O3.
2.
Experimental Methods
Samples used in the present study were synthesized by solid state reaction procedures [6]. Powders of SrCO3, Fe2O3 and Nb2O5 were mixed thoroughly in appropriate amounts and well ground in acetone. After drying, the mixtures were heated in alumina boats for three days at 1350 °C in air. Following cooling to room temperature these samples were again heated under the same conditions. After the second heat treatment the materials were ground to a fine powder, X-ray diffraction was used to confirm that single phase materials have been prepared. Powder X-ray diffraction measurements were performed, at room temperature, using Philips Vax Rd diffractometer operating with Cu Kα radiation. The XRD data were scanned over the angular range of 15° ≤ 2θ ≤ 140° at a step size of 0.02° and counting time of 10 s per step. The X-ray Absorption Near Edge Structure (XANES) data were collected at the XAFS beam line (11.1) at the ELETTRA Synchrotron Trieste, Italy, with the storage ring running at 2 GeV and a typical current of 200 mA. These data were collected at room temperature and in the transmission mode using Si(111) monochromator. The samples were prepared by deposition from a powder suspension in cyclohexane on a Millipore membrane (type GS 0.22 μm).
3.
Results and Discussion
3.1.
XRD
The X-ray diffraction data have been analyzed using the computer program Rietica [15,16] that uses a full profile Rietveld analysis method [17]. The background was fitted with a simple fourth order polynomial in 2θ and was refined simultaneously with other profile and structural parameters. A pseudo-Voigt peak shape function was employed to model the peak profile. The angular dependence of the peak full width at half maximum was refined with three parameters according to the function described by Caglioti et al. [18]. Instrument peak asymmetry was modeled using a single asymmetry parameter after Howard [19], while preferred orientation was described by one parameter March model [20,21]. The SrFeO3 data was refined using a simple cubic perovskite model in space group Pm-3m [22]. Structural parameters included in the refinement were cell constant, individual temperature factors of Sr, Fe and O; the fractional site occupancy of O was also refined. The final X-ray diffraction pattern and refined crystallographic parameters are presented in Fig. 1 and Table 1, respectively.
Table 1 – Structural parameters for SrFeO3, refined in space group Pm-3m. Atom
Site
Sr Fe O
1b 1a 3d
x
y
z
N⁎
Biso(Å2)
0.5 0 0
0.5 0 0
0.5 0 0.5
1 1 2.94(1)
0.71(1) 0.54(1) 1.65(5)
Note. a = 3.8700(1)Å; *Site occupancy. Rwp = 8.987, Rp = 6.471, χ2 = 3.19
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M A TE RI A L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 0 1 6–1 0 2 0
Fig. 2 – Comparison of shift and splitting of peaks for SrFeO3 and SrFe0.5Nb0.5O3; (a) [(013), (130), (031)] and (b) [(123), (132), (231)] reflections. We observed that when SrFeO3 is doped with Nb5+ at Fe sites there is a shift in the peak positions and some peak split. For elucidation, here we have shown the shift in positions and splinting of two sets of reflections; Fig. 2(a) shows the reflections of (013), (130) and (031) planes; whereas Fig. 2(b) shows (123), (132) and (231) group of reflections. From these results we can infer that there is a change from cubic to some lower symmetry for SrNb0.5Fe0.5O3. Best fit between observed and calculated diffraction pattern was obtained in the tetragonal space group P4mm. The atomic coordinates of isostructural BaTiO3 [23] were used for initial model and were subsequently refined. Temperature factors of Nb and Fe were constrained to a single value. Similarly single temperature factor value was given to both oxygen sites. The final X-ray pattern of SrNb0.5Fe0.5O3 is presented in Fig. 3, while refined crystallographic parameters are presented in Table 2. A
comparison of bond lengths for SrFeO3 and SrNb0.5Fe0.5O3 is presented in Table 3; we note that average bond lengths increase due to Nb5+ doping.
3.2.
XAFS Spectroscopy
X-ray Absorption Fine Structure (XAFS) spectroscopy has revealed itself as a powerful technique for structural characterization of the local atomic environment of individual atomic species, including bond distances, coordination numbers and type of nearest neighbors surrounding the central atom. This technique is particularly useful for materials that show considerable structural and chemical disorder. X-ray absorption edges contain a variety of information on the chemical state and the local structure of the absorbing atom. On the higher energy side of an absorption edge fine
Fig. 3 – X-ray diffraction pattern of SrFe0.5Nb0.5O3; crosses are observed intensities, the red line represents calculated pattern and the lower line is the difference between observed and calculated. Vertical lines show the reflection positions. Inset highlights XRD pattern for 65° ≤ 2θ ≤ 100°.
M A TE RI A L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 0 1 6–1 0 2 0
1019
Table 2 – Structural parameters for SrNb0.5Fe0.5O3, refined in space group P4mm. Atom
Site
Sr Nb Fe O(1) O(2)
1a 1b 1b 1b 2c
x
y
z
N⁎
Biso(Å2)
0 0.5 0.5 0.5 0.5
0 0.5 0.5 0.5 0
0 0.5041 0.5041 0.0340 0.4569
1 0.5 0.5 1 2
0.74(1) 0.46(1) 0.46(1) 1.72(3) 1.72(3)
Note. a = 3.9656(1)Å, c = 3.9795(1)Å; *Site occupancy. Rwp = 7.571, Rp = 5.048, χ2 = 2.14
structure is observed due to backscattering of the emitted photoelectron. The post-edge region can be divided into two parts. Within 50 eV of an absorption edge, the spectrum is interpreted in terms of the appropriate components of the local density of states, which would be expected to be sensitive to the valence state of the atom. The edge position shifts towards higher energy with increase in the oxidation state [24]. In this near-edge region (XANES) absorption modulations are strongly due to multiple scattering [25]. XANES is based on a finger printing technique, where a measured spectrum is identified by comparison with reference spectra obtained for standard materials. A direct comparison of the absorption edges for atoms of interest with known model compounds provides information on the effective charges of atoms in different systems. In this study, the normalization of XANES spectra was carried out by employing standard edge step normalization procedure [26]. The normalized XANES spectra were obtained by subtracting the smooth pre-edge absorption from the experimental spectra and taking edge jump height as unity; further details of the normalization procedure can be found in Ref. [27]. The normalized XANES spectra of the Fe K-edge are shown in Fig. 4; the interpretations of K-edge XANES features for 3d transition metal oxides are well established [28–30]. A small pre-edge peak is observed in all spectra, which is due to 1s to 3d (formally electric dipole forbidden) transition, whereas main peak can be attributed to 1s to 4p transitions [28]. In the present study the Fe K-edge data has been used to investigate the valence state of Fe in SrFe0.5Nb0.5O3 and SrFeO3; we have taken Fe2O3 and Fe3O4 as model compounds. In Fe2O3 the valence state of Fe is III whereas in Fe3O4 it is a mixture of II and III. Fig. 4 shows the normalized XANES spectra of these
Table 3 – Bond lengths (Å) for SrFeO3 and SrNb0.5Fe0.5O3. Bond type
SrFeO3
Fe–O
1.9350 × 6
Fe–Sr
3.3515 × 8
Fe–Fe/Nb
3.8700 × 6
SrNb0.5Fe0.5O3 1.8708 × 1 1.9917 × 4 2.1087 × 1 Av. 1.9910 3.4289 × 4 3.4478 × 4 Av. 3.4384 3.9656 × 4 3.9795 × 2 Av. 3.9702
Fig. 4 – XANES spectra of SrFeO3 and SrFe0.5Nb0.5O3 compared with Fe foil, Fe2O3 and Fe3O4; inset shows the expanded edge region to illustrate energy shift.
materials along with Fe metal foil, which is used for the energy calibration [24,27]; from these results we note that there is a pronounced shift in the edge position. In case of Fe3O4 spectrum, edge is about ~ 1 eV below Fe2O3 and SrFe0.5Nb0.5O3, both these spectra have same energy shift at half of the normalized edge height. However, SrFeO3 edge is about ~2 eV on higher energy side than Fe2O3 and SrFe0.5Nb0.5O3, indicating that valence state of Fe in former is higher than the later ones. In case of Fe2O3 the valence state of Fe is (III), therefore it is evident that SrFe0.5Nb0.5O3 also has a valence state of (III), as both edges have same energy shift at the half of the normalized edge height (as illustrated in the inset of Fig. 4). When we compare the energy shift of SrFeO3 with SrFe0.5Nb0.5O3 a clear edge shift on the higher energy side indicates that the valence state of Fe is higher in SrFeO3 as compared to that of SrFe0.5Nb0.5O3. From these results we can deduce that SrFe0.5Nb0.5O3 has a perovskite type tetragonal structure, where extra positive charge on the lattice, due to Nb5+ doping at Fe site, changes the valence state of Fe from (IV) to (III).
4.
Conclusions
Single phase SrFeO3 and SrFe0.5Nb0.5O3 perovskite based materials were synthesized by solid state reaction procedures. From the analysis of XRD data it has been deduced that pure SrFeO3 has a cubic structure, with space group Pm-3m, consistent with previous reported results; however when 50% Nb5+ is doped at Fe4+ sites the structure changes to tetragonal symmetry having P4mm space group. The electroneutrality of the system is achieved by the conversion of Fe4+ to Fe3+.
Acknowledgements We acknowledge the support of Elettra Synchrotrone, Trieste, Italy, for the provision of beam time and ICTP-Elettra Users Programme for a financial grant. We are grateful to Dr. Luca
1020
M A TE RI A L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 0 1 6–1 0 2 0
Olivi for his valuable suggestions and help during XAFS data collection.
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